Note: Descriptions are shown in the official language in which they were submitted.
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Radiation detector of very high performance
and planispherical parallax-free X-ray imager
comprising such a radiation detector
The present invention relates to an ~mproved
technique for embodying a radiation detector of very high
performance that can be used for detecting in position
ionizing radiations such as charged particles, photons,
X-rays and neutrons.
Radiation detectors exploiting the process of
ionization and charge multiplication in gases have been in
use with continued improvements since hundred years.
Methods for obtaining large "stable" proportional aains in
gaseous detectors are a continuing subject of investiaation
in the detectors community.
Several years ago, G.CHARPAK and F.SAULI introduced
the multistep chamber, thereafter designated as MSC, as a
way to overcome on limitations of gain in parallel plate
and multiwire proportional chambers, thereafter designated
as MWPC.
In MSC chambers, two parallel grid eiectrodes
mounted in the drift region of a conventional gas detector
and operated as parallel plate multipliers allow to pream-
plify drifting electrons and transfer them into the main
25 detection element. Operated with a photosensitive gas
mixture, the MSC chamber allows to reach gains large enough
for single photodetection in ring-imaging CHERENKOV detec-
tors, thereafter designated as RICH. For more details with
respect to MSC chambers and RICH chambers, we refer to the
_ following publications:
- G.CHARPAK and F.SAULI, Physics Letters, vol.78B,
1978, p.523, and
- M.ADAMS and al., Nuclear Instrumentation Methods,
217, 1983, 237.
More recently, G.CHARPAK and Y.GIOMATARIS have
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2
developed an improved radiation detector device thereafter
designated as MICROMEGAS which is a high gain gas detector
-_~sing as multiplying element a narrow gap parallel plate
avalanche chamber.
In a general point of view, such a detector
consists of a gap in the range 50 to 100 m which is
realized by stretching a thin metal micromesh electrode
oarallel to a read-out plane. G.CHARPAK and Y.GIOMATARIS
have demonstrated very high gain and rate capabilities
ic which are understood to result from the special properties
of electrode avalanches in very high electric fields. For
-nore details concerning the MICROMEGAS detector, we refer
-o the publication edited by Y.GIOMATARIS, P.RFBOUGEARD,
J.P.ROBERT and G.CHARPAK in Nuclear Instruments Methods,
~5 A376, 1996, 29.
The major point of inconvenience of both described
detectors lies in the necessity of stretching and maintai-
ning parallel meshes with very good accuracy. The presence
of strong electrostatic attraction forces adds to the
2~ oroblem particularly for large size of the detectors. To
overcome this drawback, heavy support frames are required
and in the case of the MICROMEGAS detector the introduction
in the gap of closely spaced insulating lines or pins with
~:he ensuing complication of assembly and loss of efficiency
25 is necessary.
Another radiation detector device was recently
developed and proposed by F.BARTOL and al. Journal of
Physics III 6 (1996), 337.
This detector device, thereafter designated as CAT,
30 Ifor Compteur a trous, substantially consists of a matrix of
holes which are drilled through a cathode foil. The inser-
tion of an insulating sheet between cathode and buried
anodes allows thus to guaranty a good gap uniformity and to
obtain high gains.
3= Radiation detectors more particularly directed to
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3
planispherical X-ray imaging devices have been up to now
also investigated. Most important worK concerning that
particular subject matter was developed ;y Georges CHARPAK
at the EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH in
Geneva (Switzerland).
A first development concerned the properties of
proportional chambers with spherical drift spaces.
A proportional wire chamber equipped with a resistive
divider adapted to generate appropriate spherical
equipotential surfaces within the drift space of the wire
chamber has been first disclosed by G.CHARPAK, Z.HAJDUK,
A.JEAVONS, R.STUBBS - CERN, Geneva, Switzerland, and
R.KAHN, Centre Multidisciplinaire Paris XII, av. General
de Gaulle, Creteil, France, and edited by NUCLEAR
INSTRUMENTS AND METHODS 307 (1974) - Geneva, 29 July 1974.
A proportional wire chamber embodied as a large
aperture X-ray imaging chamber equipped with a spherical
drift space has been also disclosed by G.CHARPAK,
C.DEMIERRE, R.KAHN, J-C.STANDIARD and F.SAULI at the CERN
2C in Geneva. See NUCLEAR INSTRUMENTS AND METHODS 141 (1977)
449-455, North-Holland Publishing Co.
A spherical drift space is disclosed as to embodying
entrance and exit electrodes of spherical shape with an
angular acceptance for X-rays to 90 . Coupling of
25 spherical drift space and readout proportional chamber is
disclosed to consist of a transfer space T, the lateral
wall of which comprises a resistive divider adapted to
generate spherical equipotential surfaces of increasing
radius up to the first cathode electrode of the readout
30 proportional chamber.
A general survey on various methods of correction
for parallax errors on gaseous detectors for X-rays and UV
has been published by G.CHARPAK, CERN, Geneva,
Switzerland. See NUCLEAR INSTRUMENTS AND METHODS 201
35 (1982) 181-192, North Holland Publishing Company.
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4
More recently, P.REHAK, G.C.SMITH and B.YU,
Brookhaven National Laboratory, Uptown N.Y. 11973
presented a method for reduction of parallax broadening in
gas-based position sensitive detectors at the 1996 IEEE
Nuclear Science Symposium, Anaheim, CA, November 2-9, 1996
and published as IEEE Transactions on Nuclear Science,
vol.44, No. 3, 1997, 651-655.
Although the drift space for photons is confined within an
entrance electrode and the cathode wire plane of the
io readout chamber are plane and parallel, entrance window of
the readout chamber is further provided with a particular
conductive pattern adapted to introduce progressive
bending of the equipotential surfaces, electric field
lines crossing thus this equipotential surfaces at right
i5 angle, whichever the impinging direction of X-rays
emanating from the focal point, so as to correct and
reduce any parallax error.
In a general point of view, the above mentioned X-
ray imagers may prove satisfactory to the extent that the
20 parallax error is now reduced to a few percent. Embodying
the entrance window of the readout chamber with conductive
pattern adapted to provide full correction of parallax
error is quite difficult to implement, since actual
pattern and corresponding voltage which is to be applied
25 to these conductive patterns are such that the electric
field is approximately radial only close to the ring
patterned entrance window, while it becomes substantially
parallel in approaching the equipotential second electrode
which defines the conversion volume. As a consequence,
30 parallax error is thus increasing with penetration of the
converting X-rays.
An object of the present invention is therefore to
provide a radiation detector of very high performance that
overcomes the above-mentioned drawbacks of the radiation
_~ detectors of the prior art.
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In accordance with another broad aspect, the present
invention provides a radiation detector of very high
performance that appears to hold both the simplicity of the
5 MSC chamber and the high field advantages of the MICROMEGAS
and CAT radiation detectors however mechanically much simpler
to implement and more versatile in use.
In accordance with another broad aspect, the present
invention provides a radiation detector of very high
performance in which a very high degree of accuracy and
resolution is obtained thanks to an electric charges transfer
coefficient which substantially equals unity.
In accordance with another broad aspect, the present
invention provides a radiation detector with substantially
constant amplifying factor for counting rates up to 105 Hz/mm2.
More particularly, in accordance with the present
invention, there is provided a radiation detector in which
primary electrons are released into a gas by ionizing
radiations and drift to a collecting electrode by means of an
electric field. The radiation detector of the invention
includes a gas electron multiplier comprising at least one
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6
matrix of electric field condensing areas with these electric
field condensing areas being distributed within a solid
surface which is substantially perpendicular to the electric
field. Each of the electric field condensing areas is adapted
to produce a local electric field amplitude enhancement proper
to generate in the gas an electron avalanche from each one of
the primary electrons. The gas electron multiplier operates
thus as an amplifier of give gain for the primary electrons,
said solid surface being planar, spherical or cylindrical in
shape, said collecting electrode being adapted to operate at
unity gain, in ionization mode, said collecting electrode (CE)
at least consisting in a plurality of elementary anodes (STi)
allowing an electronic detection of each electron avalanche.
In accordance with a further broad aspect, the present
invention provides a radiation detector for photons emitted by
an external source. The radiation detector comprises at least,
in a vessel containing a gas adapted to generate an electron
avalanche from a primary electron through an electric field: an
inlet window (IW) and a transparent electrode (C) laid onto the
inner face of the inlet window (IW), the inlet window and
transparent electrode (C) being adapted to transmit the photons
within the gas; a photocathode layer (Phc) facing the
transparent electrode (C), the photocathode layer (Phc) being
adapted to generate one photo-electron as a primary electron
under impingement of each one of the photons thereof; a gas
electron multiplier (1) which comprises at least one matrix of
electric field condensing areas (li). The matrix of electric
field condensing areas comprises a foil metal-clad insulator
(10) on each of its faces so as to form a first (11) and second
(12) metal cladding onto the foil insulator (10), the
photocathode layer (Phc) being laid onto the first metal
cladding (11) so as to face the transparent electrode (C), the
photocathode layer (Phc), first (11) and second (12) metal
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6a
cladding forming thus a regular sandwich structure with the
insulator foil (10), a plurality of bored-through holes
traversing said regular sandwich structure, each of said bored-
through holes allowing thus free flowing therethrough for the
gas and any electrically charged particle generated therein;
first biasing means (B1) adapted to maintain said transparent
electrode (C) and first metal cladding (11) substantially to
the same voltage potential value, so as to allow extraction of
any photo-electron generated by said photocathode layer under
impingement thereof of each one of said photons; second
biasing means (B2) adapted to develop a bias voltage potential
which is applied between said first (11) and said second (12)
metal cladding, so as to form at the level of each of said
bored-through holes one of said electric field condensing areas
(1i), in which a condensed electric field (9') is generated,
said condensed electric field operating thus so as to convey
each of said photo-electrons to one given electric field
condensing area (li) and then to generate from said photo-
electron regarded as a primary electron one electron avalanche
which is passed through said bored-through hole forming said
given electric field condensing area (li); a collecting
electrode (FCE) consisting at least of a plurality of
elementary anodes (STi), said collecting electrode facing said
second metal cladding (12) and being spaced apart thereof, so
as to define a detection region within said vessel; third
biasing means (B3) adapted to develop a bias voltage potential
which is applied to said collecting electrode (FCE) so as to
allow the detection of said electron avalanche.
The features of the present invention will become more
apparent upon reading of the following non-restrictive
description of preferred embodiments thereof which are given by
way of example only with reference to the accompanying
drawings.
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6b
In the appended drawings:
- Figure la is a perspective view of a preferred
embodiment of a radiation detector in accordance with the
present invention which is cylindrical in shape;
- Figure lb is a perspective view of a particular
embodiment of a radiation detector in accordance with the
present invention which is planar in shape;
- Figure ic is a perspective view of a particular
embodiment of a radiation detector in accordance with the
present invention which is spherical in shape;
- Figure 2a is a cross-section view along a section
plane designated as plane P which is represented in phantom
line for figures la and lb;
- Figure 2b is a cross-section view along a section
plane designated as plane P which is represented in phantom
line at figure ic;
- Figure 3a is a diagram representing the electric
field lines for figure 2a;
- Figure 3b is a diagram representing the electric
field lines for figure 2b;
- Figure 4a is a front view representing a detail
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'7
of figure lb, such a detail consisting of a gas electron
multiplier comprising one matrix of electric fie~u conden-
sing areas;
- Figure 4b is a front view of a detail of figure
~ 4a in which the matrix of electric field condensing areas
is shown in a non-limitative way to consist of' circular
bored- through holes;
- Figures 4c, 4d, 4e and 4f show particular embodi-
ments of matrices provided with bored-through holes of
~c different shapes and pitch;
- Figure 5a is a perspective view of a detail of
figure 4b in which the mode of operation of the gas
electron multiplier in a radiation detector in accordance
with the invention operates to generate an electron
:5 avalanche from a primary electron;
- Figure 5b is a cross-section view along a section
plane designating as plane R represented in phantcm line at
figure 5a, in which the electric field lines and electric
potential lines are represented at the level o= a local
20 electric field condensing area with the poten-:ial lines
being represented in solid lines and the electric field
line being represented in phantom lines;
- Figure 5c is a diagram representing the electric
field distribution within the local condensing area shown
25 at figure 5b, the electric field being plotted with
reference to a symmetry axis XIX shown at figure 5b;
- Figures 6a and 6b are each a schematic view of a
radiation detector in accordance with the invention in
which more than one matrix of electric field condensing
30 areas are used so as to embody such a radiation detector;
- Figure 7a is a schematic view of a gas electron
multiplier in accordance with the present invention which
is inserted into a particular radiation detector, the gas
electron multiplier of the invention operating thus as a
35 preamplifier for primary electrons;
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8
- Figure 7b is a schematic view representi-.g
successive gas electron ~lultiplier i-: accordance wi=h
present invention which are integratea. within a parr:cu_ar
host radiation detector, the successive gas eiectrcn
multipliers operating thus as separate preamplifiers f,:r
the primary electrons;
- Figure 8a is a diagram representing t-_e
amplification factor which is obtained for several gas
mixtures filling a radiation detector in accordance wi--n
the invention, with this amplification factor being plotted
with respect to the voltage potential which is applied tc a
matrix of local electric field condensing areas;
- Figure 8b is a diagram representing the relat;-,-e
pulse height obtained from a radiation detector in accc:-
is dance with the invention which is formed from a MS -
chamber in which a gas electron multiplier is inserted as
shown at figure 7a wit:: the relative pulse height be'_ng
plotted with respect to the count-rate expressed in Hz/m~,-;
- Figure 8c is a diagram of comparative measures cf
the preamplifying or amplifying factor of a gas eiectr~,n
multiplier in accordance with the invention in case c.-y
mixture of argon and carbon dioxide and a wet mixture zf
the latter is used as a gas filling the radiation deteczzr
in accordance with the invention, with the amplifying or
preamplifying factor being plotted with respect to time
expressed in minutes;
- Figure 8d is a preferred embodiment for one local
electric field condensing area in which enhancement of t=~
electric field along the central axis of symmetry of this
local electric field condensing area is furthermore increa-
sed thanks to permanent electric charges which are
implanted into particular zones of this local electr'_c
field condensing area;
- Figure 9a is a front view of a radiation detector
in accordance with the present invention which is partic.-
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9
larly adapted to be used for crystallography experiments;
- Figures 9b and 9c are front views representing a
preferred embodiment of a radiation detector in accordance
with the present invention which is more particularly
adapted for the detection of ionizing radiations which are
generated by colliding particles accelerated within the
colliding ring path of an accelerator of the synchrotron-
type, this accelerated particles having thus very high
energy levels;
io - Figure 10 is a cross-section view like figure 3a,
of a non limitative embodiment of the radiation detector of
the invention which is more particularly directed to
photons detection.
- Figure lla is a section view of a preferred
embodiment of a parallax-free X-ray imager in accordance
with the present invention;
- Figure 11b is a section view of a gas electron
multiplier structure integrated within the parallax-free
X-ray imager of the invention particularly adapted to
operate as an amplifier of given gain for primary
electrons generated within the spherical conversion volume
chamber, amplification of these primary electrons taking
place through an avalanche phenomenon;
- Figure llc is a partial perspective view of
Fig.la in which the mechanical structure of the entrance
window and the gas electron multiplier structure and their
relative position adapted to embodying the parallax-free
X-ray imager in accordance with the present invention is
represented;
- Figure lid is a voltage potential distribution
representation of the voltage potentials which are
successively applied to the electrodes forming the
entrance window and the gas electron multiplier structure
embodying the parallax-free X-ray imager in accordance of
the present invention;
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- Figure 12a is a partial section view of the
spherical conversion volume chamber, the gas electron
multiplier structure and transfer and induction volume
embodying the parallax-free X-ray imager of the invention
5 in which relative voltage potential values applied to
corresponding electrodes and corresponding electrical
equipotential surfaces are shown;
- Figure 12b is a detail of Fig.2a in which local
deformations of the electrical equipotential surfaces and
io corresponding electric field lines in the vicinity of the
electric field condensing areas forming the gas electron
multiplier structure are shown for better comprehension;
- Figure 12c is a section view of a gas electrcr.
multiplier structure integrated within the parallax-free
X-ray imager of the invention more particularly adapted to
allow a proper electrical voltage potential feeding of the
successive conductive rings in the absence of substantia'
degradation of the image through masking of the feeding
connecting lines.
The radiation detector according to the invention
is now disclosed as a non-limitative example in the presen;.
specification. Particularly, it should be kept in mind tha-L.
the radiation detector in accordance with the invention can
be used with the same advantages in many types of applica-
tions such as radiography, imaging medicine, and in a more
general sense any kind of radiation which comes to effect
to release primary electrons in a gas.
The radiation detector in accordance with the
invention is thus disclosed with reference to figures ia,
lb and lc.
In the accompanying drawings, the same references
designate the same elements while relative dimensions of
these elements are not represented for the sake of better
comprehension of the whole.
As shown at figure la, the radiation detector in
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accordance to the invention is a detector of the type in
which primary elec:.rons are released into a gas by ionizing
radiations with these primary electrons being drifted to a
collecting electrode by means of an electric field. In the
above-mentioned figures, vector E designates the electric
field, CE designates the collecting electrode.
Generally, the radiation detector of the invention
may comprise a vessel referred to as V containing the gas
in which the primary electrons are released by an incident
:o ionizing radiation. In figures la, lb and lc, the ionizing
radiation is designated as X-rays or gamma-rays which are
generated from a source referred -.c as S. The X-rays or
gamma-rays generated by the source S enter thus the radia-
tion detector of the invention through an inlet window
referred to as IW and generate primary electrons which are
released into the aas contained within the vessel V. The
inlet window IW has a metal clad inner surface generally
consisting of a thin metal film which, in operation, is put
at a drift potential thereafter designated as VD. As shown
2o at figure la for example, the collecting electrode CE, and
the inlet window IW and drift electrode DE may well form
the vessel V so as to contain the gas in which the primary
electrons are thus released on inpingement of the ionizing
radiation. Light frames referred to as F1, F2 may be used
25 to build up the vessel V.
As further shown at figures la, lb or lc, the
vessel V is further provided with a gas inlet thereafter
designated as GI, and a gas outlet designated as GO, both
consisting of a threaded tiny tube allowing the filling of
30 the vessel V with a particular gas mixture or dedicated gas
as it will be disclosed in more details later in the
description. Gas inlet GI and gas outlet GO may well be
located onto opposite sides of the vessel V so as to insure
proper gas filling and circulation.
3 5 As clearly shown at figures la, lb and lc, the
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_G
radiation detector in accordance with the invention further
4ncludes a gas electron mulr-iplier, thereafter designated
as GEM and bearing reference sign 1, this gas electrcr,
multiplier 1 comprising at least one matrix of electric
field condensing areas with these electric field condensing
areas being each designated as 1i.
- In the above-mentioned figures, the electric field
condensing areas are distributed within a solid surface
which is substantially perpendicular to the electric field
~o vector E. Each of the electric field condensing areas 1; is
adapted to produce a local electric field amplitude
enhancement which is proper to generate in the gas an
electron avalanche from each one of the primary electrons.
The gas electron multiplier 1 operates thus as an amplifier
of given gain for these primary electrons while the
collecting electrode CE allows a detection of the electron
avalanche to be performed, as it is disclosed later in the
specification. As shown at figures la, lb and 1c, the solid
surface forming the matrix of electric field condensing
20 areas may well have different shapes with the shape of the
vessel V containing the gas being adapted accordingly as
shown in the above-mentioned figures. Thus, at figure ia,
zhe solid surface embodying the gas electron multiplier is
cylindrical in shape with both the inlet window IW and
25 associated drift electrode DE together with collecting
electrode CE being of same cylindrical shape so as to
develop a radial electric field vector E which is
substantially perpendicular to this cylindrical solid
surface forming the gas electron multiplier 1.
30 At figure Zb, to the contrary to figure la, the gas
electron multiplier is formed by a solid surface which is
planar in shape with the inlet window IW and its associated
drift electrode DE together with collecting electrode CE
being parallel to one another so as to form a planar
structure. As a consequence, the electric field vector,
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13
vector E,which is developed between collecting e'.ec-:rccie
CE and inlet window and drift electrode DE,
substantially perpendicular to the planar solid surface
embodying the gas electron multiplier 1.
At figure lc, the solid surface embodying the gas
electron multiplier 1 is spherical in shape with this solid
surface being delimited by planar intersections of this
solid surface. In the same way as to figures la and lb,
collecting electrode CE and inlet window IW and its
lo associated drift electrode DE are spherical in shape so as
to develop an electric field vector E which is
substantialiy perpendicular to corresponding spherical
solid surface embodying the gas electron multiplier 1.
As shown at figures la, lb and lc, each electric
field condensing area 1: is represented for better compre-
hension as to consist of a hole in which the local electric
field amplitude enhancement generated thereto is substan-
tially symmetrical in relation to an axis of symmetry of
this condensing local area. This local electric field
amplitude enhancement is thus substantially at a maximum at
the center of symmetry of each condensing local area 1.. In
accordance with one particular aspect of the radiation
detector of the invention, the electric field condensing
areas 1i are substantially identical in shape and regularly
distributed within the solid surface whichever its shape as
shown at figure la to ic so as to form the gas electron
multiplier 1.
More details relative to the structure and the mode
of operation of the gas electron multiplier 1 embodying the
radiation detector of the invention will be given now with
reference to figures 2a, 2b and 3a, 3b.
Figure 2a represents a cross-section view of the
radiation detector in accordance with the invention as
shown at figure la or figure lb with this cross-section
view being taken along intersecting plane P which is shown
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14
in phantom line at figures la and lb while figure 2b is a
cross-section view along corresponding intersecting plane P
shown in phantom line at figure lc.
Figures 2a and 2b differ only in the extent that
the same elements of figure 2b are bent owing to the
spherical shape of the solid surface embodying the gas
electron multiplier 1 and the collecting electrode CE, the
inlet window IW and its associated drift electrode DE. In
any case, collecting electrode CE is deemed to consist as
an example of metal pads or strips which are laid onto a
printed circuit board so as to allow detection of the
electrode avalanches as previously mentioned in the
specification.
As shown at figures 2a and 2b in a preferred
~ embodiment of the gas electron multiplier forming the
radiation detector of the invention, the matrix of electric
field condensing areas 1; may comprise a foil metal clad
insulator, referred to as 10, on each of its faces so as to
form a first and second metal-cladding, referred to as 11
and 12 respectively, with these metal-cladding sandwiching
the insulator foil 10 to form a regular sandwich structure.
The matrix of electric field condensing areas further
comprises a plurality of bored-through holes, referred to
as 1;, traversing the regular sandwich structure as shown
at figures 2a and 2b so as to form these electric field
condensing areas.
In addition, biasing means are adapted to develop a
bias voltage potential which is applied to the first and
second metal cladding 11, 12, so as to generate at the
level of each of the bored-through holes one electric field
condensing area li. At figures 2a and 2b, the biasing means
are referred 2 and adapted to develop a difference
potential denoted VGEM.
The mode of operation of the radiation detector in
accordance with the invention and more particularly the
*rB
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mode of operation of the gas multiplier 1 which is shown at
figures 2a and 2b is now disclosed wi-:h reference to fioure
3a and figure 3b.
Generally speaking, with the regular sandwich
~ structure being put in operation substantially
perpendicular to the electric field vector E, the first
metal-cladding 11 forms thus an input face for the drift
electrons while the second metal-cladding 12 forms an
output face for any electron avalanche which is generated
lo at the level of each bored-through hole forming one of the
electric field condensing areas 1;.
With reference to figure 3a, the electric field
lines bearing the electric field vec,~or E are represented
between drift electrode DE and the gas electron multiplier
15 1, respectively the latter and collecting electrode CE
while the electric field lines bearing the electric field
vector E" are represented betwee-: the gas electron
multiplier 1 and the coliecting electrode CE. With the
first 11 and second 12 metal-cladding being put at a
convenient voltage potential, i.e. a continuous voltage
potential difference value, each of the local electric
field condensing area 1i, i.e. each bored-through hole,
behaves as a dipole which in fact super-imposes a further
electric field vector E' with this further electric field
being substantially directed along a symmetry axis of each
bored-through hole. It should be bcrne in mind that the
electric field lines are thus distorted as shown at figure
3a or 3b at the level of each of the local electric field
condensing areas li.
For the sake of clarity and better comprehension,
figures 3a and 3b are shown in the absence of electric
charges within the drift region and the detection region
that in such a case fully corresponds to the absence of
ionizing radiations. For instance, any virtual solid
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16
surface thereafter designated as FT which is delimited by
t~e outermost electric field lines reaching one given local
e_ectric field condensing area, as shown at figure 3a for
example, delineates an electric field tube FT in which the
electric field flux presents a preservative character. As a
consequence, it is clear to any person of ordinary skill in
the corresponding art that the enhancement of the electric
field at the level of each local electric field condensing
area li is thus given accordingly with any surface being
passed through by the condensed electric field vector E'
being in direct relation to the enhancement of the
resulting electric field which is thus equal to the sum of
original electric field vector E and superimposed electric
field vector E'.
Owing to the symmetrical character of the sandwich
structure with respect to the symmetry plane referred to as
plane Q at figure 3a, any virtual solid surface formed by
the outermost electric field lines reaching a corresponding
local electric field condensing area li is substantially
2 0 transferred as a symmetrical virtual solid surface formed
by the electric field line leaving the same local electric
field condensing area in the detection region, as shown at
figure 3a with respect to the same electric field tube FT.
As a consequence, provided given relations between voltage
difference potential which is applied to the first 11 and
the second 12 metal-cladding sandwiching the insulator foil
10 which will be explained later in the specification are
fulfilled, it is thus clear that the distorted solid
surface of electric field lines of the drift region is
fully restored within the detection region as shown at
figure 3a. It is furthermore emphasized that while the
electric field E within the drift region and the electric
field E" within the detection region are substantially
parallel, they may well have amplitude of different value.
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17
As an example, the detection region elec-:ric field
amplitude IE"l may be set up at a larger value than the
drift region electric field amplitude lEl so as to increase
the transfer velocity to the collecting electrode to get
thus faster signals. The same situation occurs at figure 3b
with the general form of the electric field lines being
modified only by the spherical shape of the sandwich
structure and more particularly its circular shape as
represented at figure 3b.
A preferred embodiment of the gas electron
mutiplier embodying a radiation detector in accordance with
the present invention is now disclosed with reference to
figures 4a, 4b and more generally figures 4c to 4f. As
shown for example at figure 4a, the gas electron multiplier
1~ 1 may consist of a thin insulator foil referred to as 10
which is metal clad on each of its faces, the metal
cladding being thus referred to as 11 and 12 with reference
to figures 2a and 2b, the sandwich structure t::us formed
being further traversed by a regular matrix of =i ny holes
referred to as li. Typical values are 25 to 500 m c-f
thickness for the foil with the centre of the ziny holes
being separated at a distance comprised between 50 and 300
m. The tiny holes may well have a diameter which is
comprised between 20 and 100 m. The matrix of tiny holes
1= is generally formed in the central area of an insulator
foil of regular shape as shown at figure 4a. The insulator
foil 10 is thus provided with electrodes on each of its
faces which are referred to as 120 and 110, these
electrodes being thus adapted so as to apply a potential
difference between the two metal sides of the mesh
embodying the matrix of tiny holes. The composite mesh can
thus be manufactured with conventional technologies which
will be described later in the description, is simple to
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18
install rugged and resistant to accidental discharges.
The mesh as shown at figure 4a can be realized c,;
conventional printed circuit technology. As an example, tk--,
identical films or masks are imprinted with the desired
pattern of holes and overlaid on each side of the meta'
clad insulator foil 10 which is previously coated with a
light sensitive resin. The insulator foil 10 may consist of
a polymer such as KAPTON or the like, KAPTON being a
registered trade-mark to DUPONT DE NEMOURS. Exposure to
lo ultra-violet light and development of the resin exposes
thus the metal to acid etching only in the regions to be
removed, i.e. the tiny holes. The foils are then immerse:
into an adequate solvent for the polymer used and holes dig
within the foils from the two sides by chemical etching.
The whole processing uses common and well-known industriaI
procedures as though a precise control of the etching
parameter are essential to obtain a reproducible mesh. The
above-mentioned method is proper to allow the manufacturing
of mesh from an insulator foil of thickness comprise~
between 20 to 100 m for example. For insulator foils of,
greater thickness, i.e. of a thickness comprised betweer:
about 100 to 500 gm, alternative standard methods of
manufacturing like plasma etching or laser drilling can
also be used and provide similar results. One method o--"
particular interest appears to be laser drilling since the
process of drilling holes can be computed and controlled
accordingly so as to obtain matrices of tiny holes o'
adapted shape with respect to corresponding application.
A detail of the mesh thus obtained is represented
at figure 4b. Although the tiny holes shown at figure 4b
are circular in shape, they may well be of different shape
as it will be thus disclosed with reference to figures 4c,
4d and 4e.
These figures consist of a front view of the mesh
3 5 together with a cross-section view of this front view alono
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19
a plane containing the center of symmetry of two successive
tiny holes forming the matrix of tiny holes in the corres-
ponding front view. With reference to figures 4b, 4c, 4d
and 4e, each tiny hole is deemed to be included within an
opening aperture diameter which is comprised between 20 and
100 m. While the tiny holes as shown at figure 4b are
circular in shape with the outermost dimension of the holes
fully corresponding to its aperture diameter, to the
contrary, the tiny holes which are shown at figures 4c and
4d fully correspond to square holes with rounded angles
with the rounded angles corresponding to the opening
aperture diameter of the hole.
The rounded angles allow to reduce the erratic
electric discharges phenomenon.
At figure 4e, the tiny holes are represented so as
to fully correspond to the tiny holes which are shown at
figure 4b. In figures 4c, 4d and 4e, parameters P. D, d, T
and S designate:
P the distance separating two successive tiny holes
centers;
D the outermost dimension of any square tiny hole;
d the innermost dimension of any square tiny hole;
T the thickness of the insulator foil 10,
S the thickness of the first 11 and second 12 metal
cladding embodying the sandwich structure.
Corresponding values of the above-mentioned parameters P,
D, d, T and S are thus given for figures 4c and 4d with
these dimensions being expressed in micrometers.
As shown as an example at figures 4c and 4d, each
bored-through hole 1i consists of a bored-through hole
which is formed by a first and a second frusto-conical
bored hole. The first frusto-conical bored hole extends
from the first metal-cladding 11 to an intermediate surface
of the regular sandwich structure which is referred to as
plane Q at figure 3a, 3b and 4c, 4e. The second frusto-
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conical bored hole extends from the second metal-claddi::;
12 to the same intermediate surface referred to as plane
both frusto-conical bored-holes ::aving a first circular
opening of a diameter of a given value as previously
5 mentioned in the description at the level of the
corresponding metal-cladding 11 or 12. Both of the frusto-
conical bored holes join together at the level of the
intermediate surface Q of the regular sandwich structure
forming thus the corresponding bored-through hole 1i as
io shown at figures 4c and 4e. With the same pitch P of given
value as previously mentioned in the description, the
bored-through holes i; which are identical in shape and
regularly distributed over all ti-:e metal clad faces of tne
insulator foil 10 form thus the matrix of tiny holes
?5 embodying the matrix of local electric field condensi::c-
areas in operation.
At figure 4d, a further Darticular embodiment c=
the matrix of tiny holes of the invention is shown in whic::
each of the bored-through holes 11 has a cross-secticr:
20 along a longitudinal plane of symmetry of this bored-
through hole which is conical in s.-iape.
Corresponding parameters are given now with respec-:
to figures 4c to 4e in which:
P, T and S fully designate the same parameters as
per figures 4c and 4e, and
D1 designates the outermost dimension of one tiny
hole formed at the level of first cladding 11, for example;
D2 designates the outermost dimension for a square
tiny hole which is formed at the level of the second
cladding 12;
dl designates the outermost dimension for the
bored-through hole within the insulator foil 10 at the
level of first cladding 11;
d2 designates the outermost dimension for the
square bored-through hole through the insulator foil and at
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2_
the level of second metal cladc-'ng 12.
These dimensions are given in micr---meters. These
parameters values are given :hereafter as sizes example
only with reference to tables I, II and III which are
related to Fig. 4c, Fig. 4d and Fig. 4e, 4f respectively.
Table I
p D D T s
140 110 6G 50 15
200 130 70 50 18
Table II
P D1 D2 dl d2 T s
200 160 120 75 60 50 5
Table III
p D D T d
200 130 10c 50 18
Each of the bored-through holes 1i as shown at
figure 4d comprises thus a first and a second circular
opening or substantially circular opening for given values
which are different from each other and thus form a first
and a second opening aperture diameter of different value
at the level of the first 11 ar.:,; the second cladding 12.
Figure 4f refers to another particular embodiment
in which each of the bored-through holes is fully circular
in shape, all the way through. The dime-isions given at
2S figure 4f may thus well corres=ond to those given at table
*rB
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22
III, with d being thus equal to D. Such a matrix as shown
at figure 4f can be obtained by laser drilling.
A more detailed mode of operation of the gas
electron multiplier 1 embodying the radiation detector of
the invention is now disclosed with reference to figures
5a, 5b and 5c.
In operation, when a potential difference is
applied between the first and the second metal cladding 11
and 12 of the mesh, very high localized electric fields as
lo vector E' previously mentioned in the description are
created within the open channel in the tiny holes, as shown
at figures 3a, 3b and 5a, 5b, 5c.
The electric field enhancement as shown at figures
3a or 5a, 5b is large enough to induce an avalanche multi-
plication from any primary electron entering one of the
field tube FT of the drift region as shown at figures 3a,
3b or 5a.
Figure 5b represents the distribution of the
electric field lines and the potential lines at the level
of one electric field condensing area of the gas electron
multiplier 1 embodying a radiation detector in accordance
with the object of the invention, with the electric field
lines being represented in solid lines and the potential
lines in phantom lines. It is particularly emphasized that
provided a given potential difference VGEM is applied to
the first 11 and second 12 metal-cladding of the gas
electron multiplier 1 embodying a radiation detector in
accordance with the present invention, no electric field
lines do reach either the first and second metal-cladding
11 and 12 or the insulator foil 10 as it is clearly shown
at figure 5b.
It is also emphasized with reference to figure Sc
that the electric field distribution along an axis of
symmetry designated as X'X at figure 5b or 3a, 3b is
substantially symmetrical with respect to the intermediate
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23
surface Q which is the plane of symmetry with respect to
figure 5b as shown at figure 5c. It s%:ould be borne in mind
that since no field line from the drift region except. for
the mathematical boundary between cells or field tube FT
terminates on the upper electrode, any local electric field
condensing area 1; provides thus a full transmission of any
drift electron as an electron avalanche, the gas electron
multiplier 1 embodying the radiation detection of the
invention providing thus a full electrical charges
_ transmission and, as a consequence, an electrical
transparency that substantially equais 1. This electricai.
transparency should be distinguished over the optical
transparency of the mesh embodyin; the gas electron
multiplier 1 since this electrical transparency
1= substantially equal to 1 is obta=~ned for an optical
transparency of the mesh which is defined as the ratio
between the total surface of all the tiny holes embodying
the local electric field condensing areas over the total
surface of the metal clad insulator foil and thus is
20 comprised between 10% and 50%. It Is further emphasized
that the high density of channels, i.e. of tiny holes,
reduces thus the image distorsions to values which are
comparable to the intrinsic spread due to diffusion.
A particular embodiment of the radiation detector
25 of the invention is now disclosed with reference to figure
6a.
The gain or the amplifying factor of the radiation
is in a direct relationship to the amplifying factor yield
by the gas electron multiplier as disclosed in the descrip-
30 tion. This amplifying factor is in a direct relationship to
the electric field enhancement and more particularly to the
electric field amplitude value along the symmetrical axis
of symmetry X'X of each tiny hole embodying one electric
field condensing area together with the path length of the
3 electron avalanche within one of the local electric field
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24
condensing area, and as a consequence, the thickness of the
metal clad insulator foil 10. Insofar as the thickness is
open to reach 100 m with the tiny holes being drilled
thanks to a laser processing as previously mentioned in the
description, the amplifying factor which is defined as a
ratio of the number of electrons of the electron avalanche
entering the detection region to one primary electron
yields those values to above 1000. With such a gain, or
amplifying factor, the collecting electrode CE is adapted
ic to operate at unity gain in ionization mode for example. In
such a case, this electrode may consist of a plurality of
elementary anodes as shown for example at figures la to lc,
each elementary anode consisting for example of one strip
or one pad of conductive material which allows an
electronic detection of each electron avalanche. Each
elementary anode as shown for example at figures 2a and 2b
is put at a reference potential such as a ground potentiai
and is connected thanks to a capacitor CA to an amplifier A
adapted to deliver a detection signal to a detection device
20 which is not shown in the above-mentioned figures. The
detection device is not disclosed for it is well-known per
se to any person of ordinary skill in the corresponding
art.
Thanks to its above mentioned electrical
25 transparency that substantially equals one, the radiation
detector of the invention may well be adapted to perform
either monodimensional or bidimensional position detection.
For such a purpose, as shown as a non-limitative example at
figure 2a, the collecting electrode CE may be provided with
3o elementary anodes STi which are laid onto the face of an
insulator foil or printed circuit board facing the gas
electron multiplier 1, in case of monodimensional
detection, with these elementary anodes each consisting of
one electric conductive strip, these strips being thus
paralle'_ and extending along a first direction.
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In case of bidimensional detection however further
elementary anodes STj may be provided on -he other side of
the insulator fo-'l, and separated from t::= firs= ones, so
as to form parallel electric conductive strips extending
5 along a second ciirection transverse to the first one. The
conductive strips ST; facing the gas electron multiplier 1
are preferably regularly spaced apart from each other so as
to cover 50% onlv of the total surface of the collecting
electrode CE, so as to allow any electron avalanche genera-
io ted in front of any elementary anode ST.. facing the gas
electron multiplier 1 to also induce a corresponding
detection signal onto corresponding elementary anodes STj
which are partia-~~~ly masked by the latters. The gain of
detection amplifiers A embodying each detection circuit
;.~ with capacitor C~% and resistor RA may we'1 be set up to
different adapted values for each set of elementary
electrodes, so as to introduce a good balance of the
induced detection signal onto each set of elementary
electrodes.
20 In order to improve the gain yie'Ld from the gas
electron multiplier embodying a radiat'Lon de-r-ector in
accordance with the invention as shown at figure 6a, a
plurality of successive matrices of electric field conden-
sing areas can be used, these matrices being in a cascade
23 relationship over the primary electron stream, two matrices
referred to as GEM1 and GEM2 being shown cnly for the sake
of better comprehension at figure 6a. These successive
matrices are put parallel to one another, i.e. in the
absence of intersection, to define homothetic matrices over
a common centre C forming the radiation detector as shown
at figure 6a. As shown at this figure, two successive
matrices are spaced apart from each other at a given
separating distance value in a direction which is parallel
to the corresponding electric field. As a consequence, the
drift electrode DE, the first matrix or gas electron
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26
multiplier GEM1, the second matrix or second gas electror,
multiplier GEM2 and successive ma-:rices together with the
ccllecting electrode CE de-fine therebetween successive
electric fields which are referred to as vector EIo, vector
E21, vector E02 and the like, each successive electric
field allowing any primary electron or electron of one
electron avalanche to drift as a primary electron along the
separating distance thanks to its corresponding electric
field.
1n The gas electron multiplier formed by successive
matrices as shown at figures 6a and 6b cooperates thus as
an amplifier, the gain of which is the product cf the gai:-.
yield for each successive matrix. Figure 6b actually
represents a planar embodiment of the radiation detectcr
shown at figure 6a. It is further recalled that for planar
embodiments as shown at figure 6b, the commor, center C
actually lies at an infinite distance.
The radiation detector of the invention as it has
been disclosed up to now with reference to figures la to 6b
2 fully operates as an amplifier, t'r.e ccllecting electrode :.E
of which operates at unity gain and can thus be made of a
simple and very cheap stripped printed circuit for which
the total gain or amplifying factor is obtained from the
gas electron multiplier only, either single or multiple gas
25 electron multiplier as shown at figures 6a and 6b.
Another way to embodying the radiation detector c-LP
the invention is now disclosed in which the gas electron
multiplier 1 is inserted into a host detector which has its
proper gain with reference to figures 7a and 7b. The host
3G detector, in a general way, may consist as a non-limitative
example, as a well-known micro-strip gas chamber,
thereafter designated as MSGC, or a multiwire proportional
chamber. As shown at figure 7a in case of a MSGC, the
collecting electrode CE consists now of successive anode
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27
electrodes designated as AN and cathode electrodes,
referred to as CO, which are inter"Leaved and distributed
over a dielectric support so as to form the collecting
electrode CE. Each of the anode electrodes AN is connected
to the reference potential referred to as the ground
potential through resistor RA and to an amplifier A so as
to allow detection while each of the cathode electrodes Co
is connected to a bias potential generator VC, the MSGC
chamber having thus its own gain depending on the gain
Lc which is yield through amplification between each of the
cathode electrodes and anode electrodes. As further shown
at figure 7a, one gas electron multiplier 1 is further
inserted between the drift electrode DE and the collecting
electrode CE so as to define a first drift region, drift:,
and a second drift region, drift2, which are separated from
each other by the gas electron multiplier 1.
While proportional counters, multiwire chambers,
and microstrip gas chambers, all exploit the basic
amplification process of electron avalanche multiplication
20 but differ only in their geometry and their performances,
the maximum amplification factor that can be safely reached
depends on many parameters and is limited by the
probability of a catastrophic hazardous discharge in case
too large gains, i.e. too large voltages, are used.
25 As an example, the microstrip gas chamber which is
made with its thin and fragile metal strips appears
particularly exposed to discharge damages. The
sophisticated electronic circuits connected to the strips
such as amplifier A as shown at figure 7a, can also be
30 irreversibly damaged by these discharges.
Inserting a gas electron multiplier 1 as shown at
figure 7a within for example a microstrip gas chamber with
the gas electron multiplier being inserted on the path of
electrons drifting in the gas under the effect of a
35 moderate electric field comes to effect to pull the primary
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28
electrons which are generated in the first drift region,
driftl, into the tiny holes forming the local electric
field condensing areas and multiply them in an avalanche in
the high local electric field and thus push them out from
the other side, i.e. in the second drift region, drift2,
with the primary electrons being multiplied by a factor of
many hundreds.
The gas electron multiplier 1 of the invention
operates thus as a preamplifier of given gain for the
1 o primary electrons upstream the collecting electrode CE of
the radiation detector.
Provided the bias potentials which are put to the
drift electrode DE and the collecting electrode CE,
particularly to the cathode electrode CO and the first and
second metal-cladding 11 and 12 of the gas electron
multiplier 1 as shown at figure 7a are independent, such a
configuration allows independent operation of the gas
electron multiplier 1 and the microstrip gas chamber or
multiwire proportional chamber as well as a controlled
2C injection of ionization electrons into the preamplifying
gas electron multiplier 1.
Such mode of operation is called preamplification
mode and can be used to largely increase the electric
charges to be detected. Combined with a multiwire or a
microstrip gas chamber, it makes much easier and safer to
detect small amounts of electric charges. While the
combination of a gas electron multiplier 1 adapted to a
multiwire proportional chamber or a microstrip gas chamber
of corresponding shape can be performed with these shapes
corresponding to spherical or cylindrical ones, the
preamplification mode of operation of the gas electron
module 1 of the invention appears of highest interest in
case of multiwire proportional chamber or microstrip gas
chamber of planar structure, the gas electron multiplier 1
in such a case corresponding also to a planar structure as
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29
shown at figure 7a.
As per figures 6a or 6b to which the gas electron
multiplier operates in amplification mode, combining
several successive gas electron multipliers as shown at
figure 7b appears of outmost interest so far these gas
electron multipliers are adapted to operate independently
since it is thus possible to achieve increasing large gains
in a succession of elements with each of the elements being
individually set at moderate amplification factor and
=0 therefore intrinsically safer to operate. As shown at
figure 7b, two successive gas electron multipliers,
referred to as GEM1 and GEM2, are shown to embody a
resulting gas electron multiplier with each gas electron
multiplier GEM1, GEM2 being set to yield a gain or
amplifying factor to 100. The resulting amplifying factor
is thus the product of each gain, then, as a consequence,
has a value that equals 10 000.
Irrespective to its mode of operation, in order to
operate the radiation detector of the invention which is
LJ shown at figures 6a, 6b or 7a, 7b, the voltage potentials
can be set up at the following values:
- conducting strips of the collecting electrode CE
of figures 6a or 6b at the reference potential referred to
as the ground potential;
25 - anode AN of the collecting electrode CE of
figures 7a or 7b at the reference potential.
All the other voltage potentials set up with
respect to the reference or ground potential. The following
potential values are given as a non-limitative example for
30 a given A-COZ (argon-carbon dioxide) gas mixture, as shown
at figure 8a, given gas electron multiplier geometry
embodying an insulator foil 10 of thickness 50 m and tiny
holes of diameter 100 m, this gas electron multiplier
being operated with this gas mixture being at atmospheric
35 pressure. Change of any parameter would imply correlative
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changes in the ranges of voltage potential values.
- cathode potential VC to each cathode electrode CO ar-
figure 7a or 7b, Vc = -500 V;
- V4 set up between -100 V and -1000 V;
5 - V3 set up between -600 V and -1500 V with VGEM _-500 V;
- V2 set up between -1600 V and -2300 V;
- V1 set up between -2100 V and -2800 V with VGEM _-500 V.
The distances separating the gas electron
multiplier from the drift electrode, or the successive
lo electrode CE were set up to 3 mm.
A multistage detector in accordance with the inven-
tion operating in either amplification or preamplification
mode is thus functionnaly equivalent to a multidynode
photomultiplier except it operates in a gaseous environment
15 while each matrix element of local electric field
condensing areas has a much larger gain.
As compared to similar gas devices realized wit:n:
stretched parallel metal meshes, the so-called parallel
plate and multistep chambers, the gas electron multiplier
20 which is the object of the invention is fully self-suppor-
ting since the multiplying gap and therefore the gair. are
kept substantially constant by the fixed thickness of the
insulating foil regardless of the precise location of the
gas electron multiplier within the detector or the host
25 detector. Furthermore, heavy support frames are not
necessary, this greatly simplifying construction and
increasing reliability while reducing costs.
Extensive experimental measurements were realized
with several types and models of gas electron multipliers,
30 meshes as self-standing one's operating in amplification
mode or in combination with host detectors and have been
described in papers which are listed thereafter :
= Nuclear Instrum. Methods, Methods in Phys.Res.
A386(1997)531; F.SAULI;
= IEEE Trans.Nucl.Sci. NS-(1997); R.BOUCLIER,
*rB
CA 02275159 1999-06-09
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31
M.CAPEANS, W.DOMINIK, M.HOCH, J-C.LABBE, G.MILLION, L.ROPE-
LEWSKI, F.SAULI and A.SHARMA;
= CERN-PPE/97-32; R.BOUCLIER, W.DOMINIK, M.HOCH,
J-C.LABBE, G.MILLION, L.ROPELEWSKI, F.SAULI, A.SHARMA and
G.MANZIN;
= Progress with the Gas Electron Multiplier,
CERN-PPE/97-73; C.BUETTNER, M.CAPEANS, W.DOMINIK, M.HOCH,
J-C.LABBE, G.MANZIN, G.MILLION, L.ROPELEWSKI, F.SAULI,
A.SHARMA.
During those experimental measurements, preamplifi-
cation factors above 100 have been observed in many gases
and gas mixtures of noble gases such as helium, argon,
xenon or the like with organic or inorganic quenchers like
carbon dioxide, methane and dimethylether. Figure 8a gives
some examples of the gas electron multiplier amplification
factor which is plotted in relation to the potential
difference which is applied to the first and second metal-
cladding 11 and 12 embodying one gas electron multiplier 1
in accordance with the invention. Experimental results as
shown in figure 8a are given for a first mixture of:
Argon and dimethylether, thereafter designated as
A DME with 90% argon and 10% DME;
Argon and carbon-dioxide thereafter designated as
A CO2 with a ratio of 90% argon and 10% C02;
Helium and methane, thereafter designated as He_CHy
with a ration of 70% helium and 30% methane;
Argon and dimethylether, thereafter designated as
A DME with a ratio to 50% argon and 50% DME.
Preceding ratios are given as volume ratios.
The voltage difference which was applied to the
first 11 and second metal-cladding 12 was comprised between
200 and about 600 volts, thereafter designated as VGEM.
Most measurements have been realized at atmospheric
pressure convenient for the manufacture and operation of
light and safe detectors but correct performance at
*rB
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32
pressure between few millibars and 10 bars revealed satis-
factory.
A fundamental property of the gas electron multi-
plier embodying one radiation detector in accordance with
the invention appears to be the wide range of electric
fields strengths that can be applied above the mesh forming
the matrix of local electric field condensing areas without
affecting the gain actually yield. Such a property appears
of highest importance because it makes the gas electron
multiplier of the invention almost insensitive to large
mechanical variations in the surrounding electrodes. As a
consequence, such a property allows the choice of the drift
field for optimal physical requirements as the value of the
electrons'drift velocity, diffusion and collection time.
i5 A concern of high-rate applications is the behavior
of the gas electron multiplier embodying the radiation
detector in accordance with the present invention under
condition of large detected currents. While most of the
electric charges, electrons and positive ions, smoothly
drift in the open gas channel without affecting the opera-
tion, some stray charges may collect on the surface of the
insulator with these stray charges distorting the field and
therefore the gain thus obtained. It has been however
demonstrated that a very small surface conductivity in the
channel which is obtained very simply by the addition to
the gas of a small amount not exceeding 1% of water vapor
completely stabilizes the operation up to detected X-ray
fluxes of 10' Hz cm 2 or more.
Other methods of increasing the surface
conductivity to the desired value have been investigated
such as ion implantation or vacuum evaporation of semi-
conducting layers. It has thus been observed that using a
polymeric foil embodying the insulator foil 10 with an
intrinsic resistivity between 1012 and 1013 n x cm would
3_5 properly solve the charging up problem in a natural way.
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33
As a consequence, as it is shown at figure 8d, each
tiny hole or bored-through hole 1, is provided with an
internal lateral surface which is delimited by the
insulator foil 10. As clearly shown at figure 8d, this
lateral surface comprises preferably one local zone with
intrinsic resistivity between 1012 and 1013 C, x cm. In a
non-limitative way, as shown at figure 8d, this local zone
is deemed to cover the extremal portion of the frusto-
conical bored-through hole in which electric charges such
~o as positive ions have been introduced through ion
implantation for example.
With reference to figure 8d, it is clear to one of
ordinary skill in the corresponding art that, thanks to the
presence of the positive electric charges which are implan-
:~ ted at the extreme part of the frusto-conical profile of
the insulator foil with these electric charges being
distributed substantially with the same concentration all
around the periphery of the tiny hole, i.e. in the vicinity
of the medium plane or symmetry plane Q which was already
2c mentioned with reference to figure 5b, the electric field
lines are made very tight at the level of the intermediate
plane or symmetry plane Q shown at figure 8d with the
electric field being thus accordingly increased thanks to
the preservative character of its flux within the modified
25 solid surface or field tube FT through the presence of the
implanted electric charges.
To detect the amount of the electrical charges
which are released into a gas by soft X-rays or fast
particles, about 100 electrons, amplification factors of 10
30 000 or so are necessary, given the limitations of modern
highly integrated electronics. This can be achieved safely
by combining one gas electron multiplier mesh with an
amplifying factor of 100 together with a multiwire or
microstrip gas chamber safely operated also at a gain of
_ 100. The discrete nature of the electrodes in the host
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34
detector which are wires or strips allows then to achieve
the electron avalanche localization.
It is also clear to one of ordinary skill in the
corresponding art that this can also be achieved thanks to
a radiation detector operating as an amplifier in which the
collecting electrode CE is put at unity gain so far the gas
electron multiplier 1 is enough thick to yield
corresponding value of amplifying factor equal to 10 000
with the thickness of the sandwich structure being thus
io open to reach a thickness substantially equal to 500 m, or
by a multistage gas electron multiplier as shown at figure
6a or 6b for example.
Another fundamental property of the gas electron
multiplier embodying the radiation detector of the
invention is its high-rate capability while the gain or the
relative pulse height of the radiation detector is
substantially maintained at a constant value over its full
rate range.
While the gain of the gas electron multiplier in
accordance with the present invention has been defined as
the ratio of the electrons number in the electron avalanche
leaving the output face to the number of electrons of the
primary electrons or the electrons entering the input face
at the level of each local condensing area of the matrix
embodying the gas electron multiplier, one mode of
operation to evaluate such a gain may consist as an example
to measure the preamplification factor or the amplification
factor which is defined as a ratio of the most probable
pulse height between transferred and direct spectra for the
3c 5.9 keV line radiated by an external 55Fe source.
As shown at figure 8b, the relative pulse height PH
is plotted with respect to the rate expressed in Hz/mm 2 in
three modes of operation of a gas electron multiplier
inserted within a host detector which consists of a micro-
strip gas chamber in the following situations:
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- micro-strip gas chamber only,
- gas electron multiplier only, and
- multi-strip gas chamber and gas electron multi-
plier joined together.
, The results which are shown at figure 8b clearly
confirm the high-rate capability for the charge gain
remains essentially constant within few percent up to the
maximum rate that could be achieved, around 105 Hz/mmz,
regardless of the mode of operation thus demonstrating the
_ absence of short-term ion induced charging up or charge
space effects in the local electric field condensing areas.
One should also note that the fraction of ions
receding into and through the gas electron multiplier local
electric field condensing areas depends on the applied
1.7 voltages. In the mode of operation of unity gain of the
micro-strip gas chamber with the gas electron multiplier
being ooerative only, there are no positive ions produced
in the lower gas volume and presumably no substrate
charging up and ageing problems.
2~ Another fundamental property of the radiation
detector in accordance with the present invention which is
embodied through a gas electron multiplier fully concerns
the absence of time-dependent gain shifts.
While the presence of an insulator material close
25 to the multiplication channels or the tiny holes is open to
introduce the possibility of dynamic gain shifts due to the
deposition of electric charges and the consequent modifica-
tion of electric fields, this drawback can thus be fully
overcome as already mentioned previously in the
30 description, either by using a wet gas mixture in which a
given proportion of water vapor is introduced or by giving
particular values of electric conductivity to given zones
of the internal part of each tiny hole forming a
corresponding local electric field condensing area, as
3:~ previously mentioned in the description.
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36
With respect to this last solution consisting for
example in implanting positive ions as it is shown at
figure 8d, it is also emphasized that it comes to effect to
repell the positive charges which are possibly generated by
the electron avalanche towards the symmetry axis X'X as
shown at figure 8d thereby allowing to reduce the charging
up phenomenon of the insulator foil internal lateral
surface while the electrons of the electron avalanche are
quite unaffected by the presence of the implanted ions. The
~C) residual electric charges which are charged up by the
internal lateral surface of the insulator foil has thus its
contribution to the total electric field distorsion drasti-
cally reduced, the charging up phenomenon being thus
overcome.
Figure 8c shows the variation of the pulse ampli-
fying factor of one gas electron multiplier 1 in accordance
of the object of the present invention, with this ampli-
fying factor being plotted over the time during which the
gas electron multiplier 1 is actually on, the time being
expressed in minutes.
Corresponding curve I is given for a gas electron
multiplier operated with a potential difference applied to
the first 11 and second 12 metal-cladding of the sandwich
structure which was put to 420 volts with the radiation
detector being filled with a gas mixture of argon and
carbon dioxide to a ratio 72% / 28%.
The charging up phenomenon comes up to effect to
increase the pulse amplifying factor for an initial value
that equals 40 to a value greater than or substantially
equal to 52 after 20 minutes the radiation detector is on.
Corresponding curve II is given for the same radia-
tion detector as it was used to get curve 1 except that the
gas mixture is further provided with water vapor to 0.35%
in addition.
Curve II clearly shows the full constant character
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~7
of the pulse amplifying factor which substantially equa-_s
40 all over the time the radiation detector of t:r.--
invention is on, that is from the very beginning to the er.d
of the experiment 50 minutes later.
It should be thus understood that after the
addition of water vapor, the inter-electrode resistivity cf
the gas electron multiplier mesh decreases gradually by a
factor of 10, from 100 to 10 Gn, and then remains
constant. Taking into account the total area of the
110 channels and particularly of the tiny holes embodying the
latters, this clearly indicates that a surface resistivity
around 1016 Q /square is sufficient to eliminate t:-.e
charging up phenomenon as the highest rates. The original
value of resistivity as well as the final one after
1:~ introduction of water depend on the total area and the
number of tiny holes. Preceding values refer to a 10 x 1-0
cm' gas electron multiplier 1 provided with about 5 x
tiny holes.
Particular embodiments well adapted to specific
23 applications are now described with reference to figures
9a, 9b and 9c.
Each of the above-mentioned embodiments is well
adapted to operate either on amplification or preamplifica-
tion mode as previously disclosed in the description. It is
25 furthermore emphasized that the amplification mode may well
be preferred for applications in which ionizing radiations
of very high energy level are to be investigated.
Accordingly, figure 9a shows the radiation detector
of the invention in which the sandwich structure forming a
3o gas electron multiplier 1 is provided which is spherical in
shape. This radiation detector may well correspond to tha:
which is shown at figure lc with the external form of the
detector being circular in shape as shown at the front view
of figure 9a. This radiation detector is adapted to
_ crystallography trials in which X rays are directed t--
~
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38
crystal, the radiation detector of the invention being thus
adapted to allow a full detec~:ion of the diffraction
pattern generated by the impingement of the X-rays onto the
crystal. As clearly shown at figure 9a, the bored-through
holes forming the electric field condensing areas are
regularly distributed over a part only of the metal-clad
faces of the insulator foil so as to form at least one
blind detection zone which is referred to as BZ for the
radiation detector. The blind detection zone is thus
substantially spherical in shape and located at the center
part of the sandwich structure with the bored-through holes
being distributed all around this blind detection zone so
as to allow detection of the di=fraction pattern out of
this blind detection zone only. Particularly in case the
radiation detector of the invention as shown at figure 9a
is used in amplification mode, that is in the absence of
micro-strip or multiwire chamber as final amplifier, it
allows to adapt the collecting electrode CE shape to the
needs with this electrode for example consisting of strips,
pads or rings, the rings being particularly adapted in case
of crystal diffraction measurements. At figure 9a, the
rings forming the collecting electrode CE are shown in
phantom line for better comprehension and clarity of the
drawings.
Figures 9b and 9c are concerned with radiation
detectors in accordance with the present invention which
are more particularly adapted and suited for colliding
beams accelerators or very high energy particles colliding
ring accelerators like that which is in operation at the
CERN (Centre Europeen de Recherche Nucleaire) in Geneva,
Switzerland. At figures 9b and 9c, the colliding ring
accelerator, owing to its very high curvature radius, is
represented as a straight portion. As shown at figures 9b
and 9c, the gas electron multiplier embodying the radiation
detector in accordance with the invention consists of a
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39
solid surface made of adjacent elementary solid surfaces,
each elementary solid surface forming one elementary gas
electron multiplier which comprises at least one matrix of
electric field condensing area so as to form elementary
detectors which are referred to as RD1 to RD9. The
elementary detectors are joined to one another so as to
form a three-dimensional radiation detector which surrounds
the colliding ring accelerator as shown at figures 9b and
9c.
The three-dimensional detector shown at figure 9b
is spherical in shape and formed from elementary radiation
detectors which are each spherical in shape and fully
correspond to the radiation detector in accordance with the
present invention which is shown at figure lc with
elementary detectors RD:, RD2, RD3 and RD4 being designed so
as to form a skullcap while the other elementary detectors
are design as a part of a corresponding volume spherical in
shape. Elementary detectors RD2 and RD3 may well be
provided with a central blind detection zone, as already
shown at figure 9a, this blind detection zone being further
drilled so as to allow the colliding ring acceleraLor to
pass through. Each elementary radiation detector may be
manufactured as the radiation detector shown at figure lc
by thermo-forming all its constituting parts such as the
input window and drift electrode, the sandwich structure
and the collecting electrode CE together with the
intermediate frames which are necessary to build up any
radiation detector or elementary radiation detector in
accordance of the present invention. As shown at figure la
or lc, in order to embody one elementary radiation detector
as shown at figure 9b or 9c, the gas inlet and gas outlet
GI and GO may be removed and replaced by bored-through
holes with the bored-through holes forming the gas inlet
and gas outlet of two neighbouring adjacent elementary
radiation detectors, such as RD2 and RD5 at figure 9b,
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these bored-through :-:oles being put to face each other and
to be sealed thanks to 0 rings. The electrodes ter:-.=nals
which are adapted to apply the difference potential -:s the
input and output face formed by the first and second r..etal-
5 cladding 11 and 12 as shown at figures la and ic, are
reduced and adapted to further allow the interconnect_ng of
the first and second metal-cladding respectively of two
successive adjacent elementary radiation detectors, the
same difference potential voltage being thus applied to
lo each gas electron multiplier embodying each elementary
radiation detector which as a consequence yield the same
gain.
As further shown at figure 9a, one genera: gas
inlet GI and gas outlet GO only are provided whic' are
15 preferably located close the blind zone in the vicini=y of
the colliding ring accelerator. The same for the elec==odes
110 and 120, one of these electrodes only being thus
provided to allow a same difference voltage potentia~ VGEM
to be applied to each elementary first 11 and secc-:---4 12
20 metal-cladding.
Figure 9c :s directed to a three-dimens_onal
radiation detector which is substantially cylindrical in
shape at the extremities of which two elementary -:alf-
spherical radiation detectors are abutted. The eleme-tary
25 half-spherical radiations detectors may well consist cff one
or several elementary radiation detectors thereafter
designated as RD1, RD2, RDBr RD9 with elementary radiation
detectors RD1 and RD9 playing the same role as the
elementary detectors as RD2 and RD3 at figure 9b. The
30 length of the cylindrical part as shown at figure 9c may
extend along the colliding ring accelerator for several
meters with this cylindrical part consisting of several
adjacent elementary radiation detectors thereafter
designated as RD3 to RD?. In order to allow t:;ree-
35 dimensional radiation detectors of great dimensions -:o be
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41
operated, the inner part of these detectors as shown at
figures 9b and 9c may well be filled outside the inlet
window of each elementary radiation detector with a foam,
which is substantially transparent to the X or gamma rays
of very high energy.
A radiation detector of very high efficiency, in
accordance with the present invention, has thus been
disclosed in which a gas electron multiplier may be used in
the field of elementary particle experiments.
Generally speaking, embodying a radiation detector
in accordance with the invention operating in the preampli-
fication mode with the gas electron multiplier mounted
within a micro-strip gas chamber for example, allows to
operate such a sophisticated but fragile device in much
safer conditions.
Several new experiments embodying a gas electron
multiplier in accordance with the object of the invention
were actually conducted.
one first new approved experiment, thereafter
designated as HERA-B at DESY in Hamburg, Germany (DESY, for
Deutsche Elektron Synchrotron) qualified and adopted the
gas electron multiplier of the invention, in order to
improve the reliability of the high rate host tracking
detector.
One second new approved experiment, thereafter
designated as COMPASS at CERN, came to adopt the gas
electron multiplier technology in accordance with the
invention for similar reasons.
Another proposed new experiment designated as FELIX
and conducted at the CERN (Centre Europeen de Recherche
Nucleaire) in Geneva is carried out so as to improve
radiation detectors operating in the amplification mode in
the cylindrical geometry.
Another detector, thereafter designated as HELLAZ,
is proposed for large cosmic rays experiment in the Italian
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42
Laboratory under the GRAN SASSO with the aim of achieving
large enough gains to detect single electrons.
A further particular use of the gas electron
multiplier of the invention may also consist to prevent
transmission of electrons and/or ions through the control
of external voltages. As shown for example at figure 2a or
2b, the biasing source 2 may well consist of two detening
voltage generators of opposite polarity that can be
switched through a common switch K. Operating the switch K
io allows the difference voltage potential VGEM to be reversed
so as to allow to prevent transmission of electrons and/or
ions, the sandwich structure operating thus as an active
gate, the enhanced electric field being thus strong enough
to repell given electric charges ions or electrons.
A further embodiment of the radiation detector in
accordance with the object of the present invention is now
disclosed with reference to figure 10.
This embodiment is more particularly directed to a
radiation detector for photons which are emitted by an
externa= source.
The operating principle of the gas electron multi-
plier I which is the object of the present invention
operating as a photon detector relies on the following
specific properties of its structure:
- a controlled electrical transparency, from 0 to
1, actually depending on the voltage potentials which are
applied on the various electrodes of a composite structure
operating either as an amplifier or a preamplifier and
including thus a gas electron multiplier as previously
3o disclosed in the description;
- a geometry controlled optical transparency from
about 10% to 50% which is obtained by appropriate
patterning during manufacturing;
- a demonstrated operation with gain in pure and
inert gases which actually proved harmless to photocathode
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;3
materials, and the existence of photocathode materials
operating in many particular wavelengths either visible or
invisible ones that have large quantu:n efficiency and long
survivability in a gaseous environment.
The schematics of a reverse photocathode, gas
electron multiplier, photon detector in accordance with the
object of the present invention is shown at figure 10
together with its corresponding features and electric field
lines.
As previously disclosed in the description with
reference to figure 3a for example, the radiation detector
for photons which is the object of the present invention
consists of a vessel, which is not shown at figure 10 for
the sake of better comprehension, with this vessel being
ls filled with a gas adapted to generate an electron avalanche
from a primary electron through an electric field.
An inlet window IW is further provided which is
associated with a transparent electrode denoted as C, this
inlet window and transparent electrode being adapted to
transmit the photons within the gas contained by the
vessel. The inlet window IW and transparent electrode C are
made of a material which is substantially transparent to
the photons wavelength. Well-known technology may be used
so as to put the inlet window IW and the transparent
electrode C together, the transparent electrode for this
reason being represented with phantom line only at figure
10.
As further shown at the above-mentioned figure, a
photocathode layer, denoted as PhC, faces the transparent
electrode C with this photocathode layer being adapted to
generate one photo-electron as a primary electron under
impingement of each one of the photons onto this photoca-
thode layer.
A gas electron multiplier 1 is further provided so
as to include at least, as previously mentioned in the
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description, one matrix of electric field condensing areas
which is formed from the foil metal clad insulator 10
nrovided with metal cladding 11 and 12 onto its faces, with
metal cladding 11 facing the transparent electrode C.
As clearly shown at figure 10, the photocathode layer PhC,
the metal claddings 11 and 12 together with the insulator
foil 10 form thus a regular sandwich structure as
previously mentioned in the description. Furthermore, a
plurality of bored-through holes denoted li traverse thus
_ the regular sandwich structure with each of the bored-
through holes being adapted to allow a free flowing
therethrough for the gas and any electrically charged
particle generated within the latter. As a matter of fact,
in order to embody the electron gas multiplier 1 as shown
at figure 10, one may well have first a metal clad
insulator provided with metal claddings 11 and 12 onto one
of the faces of which a layer of photosensitive material is
deposited so as to build up the photocathode layer PhC. The
bored-through holes may thus be drilled according to anyone
2 of the technologies which are actually disclosed in the
description.
As shown at figure 10, inlet window IW and transpa-
rent electrode C are spaced apart to form a convey region
which operates in a similar way as the drift region of
2~ figure 3a, as it will be disclosed in more details later in
the description.
On the bottom side of the vessel, the detector of
the invention further includes a detection unit adapted to
perform a position detection of any electron avalanche
3C generated within the detection region which is formed
between the gas electron multiplier 1 and the detection
unit as shown at figure 10. For the sake of better
comprehension, the detection unit is represented as a
collecting electrode CE as previously mentioned with
3c: reference to figures 2a or 3a. It is further emphasized,
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although not represented for the sake of better
comprehension at figure 10, that the detection unit may
well include another gas electron mu_tiplier so as to form
a multistage gas electron multiplier as previously
5 mentioned in the description or a microstrip chamber or
even a multiwire chamber for example.
To the contrary, as shown at figure 10, the top
electrode of the collecting electrode CE is provided with
elementary anodes, each of which is denoted STi, whith
io these elementary anodes consisting for example as parallel
electric conductive strips which are laid onto an insulator
foil denoted CEF. Electronic circuits consisting of
resistor RA, capacitor CA and amplificator A, are further
provided as previously mentioned in the description.
15 As further shown at figure 10, a biasing circuit
referred to as B1, is provided and adapted so as to
maintain the transparent electrode C and the first metal
cladding 11 substantially to the same voltage potential
value with respect to the reference potential value so as
20 to allow extraction of any photo-electron which is
generated by the photocathode layer PhC under impingement
onto the latter of each one of the emitted photons. Biasing
circuit B, is represented thus as a short-circuit
conductor.
25 A further biasing circuit, referred to as B2, is
provided so as to develop a bias voltage potential referred
to as VGEM which is applied between the metal claddings 11
and 12 so as to form at the level of each of the bored-
through holes one of the electric field condensing areas l;
30 as previously mentioned in the description. Applying such a
voltage allows thus to generate a condensed electric field
denoted as vector E' within each of the electric field
condensing area.
Another biasing circuit, referred to as B3, is
35 further provided so as to develop a bias voltage potential
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46
which is actually applied between metal-cladding 12 and
collecting electrode CE and more particularly elementary
anodes referred to as STi at fiaure 10 so as to allow the
detection of the electron avalanche as it will be explained
thereafter.
At first, it is recalled that each elementary anode
STi forming part of the collecting electrode CE is substan-
tially set up as a reference potential thanks to resistor
RA which is a resistor of very high value connecting each
lo corresponding elementary anode to the reference potential.
The mode of operation of the radiation detector for
photons as shown at figure 10 is now explained with refe-
rence to this figure.
Maintaining the transparent electrode C and the
metal-cladding 11 which faces the transparent electrode
substantially to the same voltage potential value thanks to
biasing means Bi comes to effect to put the electric field
vector E as shown at figure 3a to a value that
substantially equals 0.
As a consequence, each condensed electric field
vector E' generated within each electric field condensing
area, which is thus an electric field of very high
amplitude value, operates thus within the region delimited
between the transparent electrode C and the metal-cladding
11 and photocathode layer PhC as to convey each of the
photo-electron to one given electric field condensing area
which is the closest of the impingement region of this
photon within the fill tube FT which is actually generated
between metal-cladding 11 and collecting electrode CE, as
shown at figure 10. Cancelling the electric field vector E
with its amplitude being quite set up to zero value in the
vicinity of transparent electrode C as shown at figure 10
comes thus to the effect of substituting a convey region to
the drift region which is represented at figure 3a. As a
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consequence, the field tube FT is thus folded back to the
metal-cladding 11 with any photo-electron being thus
conveyed to within a corresponding electric fie_u
condensing area 1i. The condensed electric field vector E'
operates thus to generate from this photo-electron regarded
as a primary electron one electron avalanche within
corresponding bored-through hole with this electron
avalanche being thus passed through this bored-through hole
to the detection region, as shown at figure 10. The
lo electron avalanche is thus submitted to detection thanks to
electric field vector E" and elementary anodes ST.. of the
collecting electrode CE.
For distances separating on the one hand the
transparent electrode C from the photocathode layer PhC and
on the other hand metal-cladding 12 from elementary anodes
STi defining thus the convey region and the detecticn
region, which have quite the same values as those
previously mentioned with reference to figure 3a,
corresponding voltage potential values may well be set up
to similar values. As a consequence, potential value VG=M
may well be set up to 500 volts while potential value
applied between metal-cladding 12 and elementary anodes S--:
may be set up to 1000 volts, with this values being thus
given as an example.
As further shown at figure 10, position detecticn
of any avalanche which is passed through any electric field
condensing area li may preferably be performed as a
bidimensional detection. In such a case, while the inner
face of the collecting electrode CE is provided with a
first set of elementary anodes STi, the outer face of same
collecting electrode CE is thus provided with another set-
of elementary anodes referred to as STj consisting also of
parallel electric conductive strips, with each of the sets
of elementary anodes STi and STj extending along distinct
transverse directions so as to allow bidimensiona{
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detection in corresponding directions.
In case a further electron gas multiplier is used
so as to embody a multistage radiation detector for
photons, the multiplied electrons by the high field in the
hole in avalanche process drift to the second element of
amplification for further amplification.
A fundamental property of the radiation detector
for photons either as single stage or multistage version,
which cannot be obtained with any other known gas detector,
?o is that secondary photons produced during the electron
avalanche process, both primary in the bored-through holes
forming each electric field condensing area of the gas
electron multiplier and secondary in the second stage
element, cannot heat the photocathode layer PhC thereby
preventing to induce secondary emission.
The high dipole field which is created within the
bored-through holes allow thus to obtain a collection
efficiency, i.e. electrical transparency close to unity
with an optical transparency close to zero.
The large ratio of the total area to the holes area
implies also that most of the surface of the metal-cladding
11 can thus be coated by photosensitive material with a
geometrical quantum efficiency close to 1. The field
configuration which is obtained with a large difference of
potential between metal-cladding 12 and elementary anodes
STi is such that only a small fraction of the positive ions
which are produced at the final amplification stage can
thus actually reach the photocathode layer PhC reducing
thus the damage effects.
The radiation detector for photons in accordance
with the object of the present invention permits thus to
obtain simultaneously :
- large quantum efficiency of over extended areas,
- large gains without photons feed-back and very
reduced ions feed-back.
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The total combined gain of the two amplification
elements in case of a multistage gas electron multiplier
may thus be set up to a value sufficient enough for the
detection and localization of single photo-electrons
opening thus the way to numerous scientific, technical or
industrial applications like CHERENKOV ring imaging, image
intensifiers, fluorescence analysis in the visible and near
ultraviolet range, or any applications requiring detection
and localization of photons over extended areas.
The rigid and simple construction of gas electron
multiplier detectors in accordance with the object of the
present invention, either in prearnplification or amplifi-
cation mode, makes them interesting for applications in
many fields where low and high rate detection and
localization of radiation can be exploited for industrial
or medical diagnosis.
At end, while present technologies which are used
to manufacturing the gas electron multiplier embodying the
radiation detectors of the invention do consist in drilling
holes on metal clad by chemical etching, plasma etching or
laser drilling, future developments may consist in coating
with conductors an insulating mesh with narrow holes like
for example micropore filters.
Medical diagnosis covers corresponding medical
fields as large as:
- Radio and beta-chromatography, electrophoresis in
which anatomical preparations or blot paper diffusions
contain molecules labelled with electron emitting isotopes,
the two-dimensional activity distribution measured on slide
samples which provides information on the tissue in take
off labelled molecules or on the molecular weight of
substances diffusing on a support under the effect of
electric field;
- Position-dependent fluorescent analysis in which
the capability of simultaneous obtention of information on
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the energy and the emission point of soft X-rays over
extended areas can be exploited for material analysis in
Archeology and Art certification;
- Protein crystallography which is realized in a
5 spherical geometry for which gas electron multipliers
detectors can map without parallax distorsions both
position an intensity of the diffraction pattern of
crystallized molecules. High rates are achievable at the
dedicated synchrotron radiation facilities;
10 - Mammography in which a gas electron multiplier in
accordance with the invention when coupled to a secondary
electron emitted converter can effectively map the absorp-
tion profile of X-rays which are used for soft Lissue
radiography, with a sub-millimeter resolution;
15 - High flux beam diagnosis which is used for
therapy in which high flux charged particle beams can be
fully certified in spatial and energy loss profiles before
or during exposure. In such an application, the dynamic
control of the beam characteristics is thus possible.
20 One further possibility of the radiation detector
of the invention also concerns the possibility for the gas
electron multiplier to be tailored to applications or
specific needs and particularly its shape with special cut
outs as for approaching vacuum beam tubes in accelerators
25 or the like.
Among the above-mentioned large medical fields, the
gas electron multiplier of the invention appears of highest
interest for embodying parallax-free X-ray imagers.
More particularly, in accordance with the present
30 invention, there is provided a planispherical parallax-
free X-ray imager in which a parallel X-ray beam is
directed to a crystal so as to generate a conical X-ray
beam for illuminating an entrance window of the X-ray
imager. The X-ray imager at least comprises a vessel
35 containing a ionizing gas through the entrance window.
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The X-ray imager further comprises within the vessel, a
spherical conversion volume chamber which is associated
with the entrance window. The conversion v:,lume chamber
comprises a first and a second parallel electrodes adapted
to generate in operation electrical equipotential surfaces
of spherical shape and corresponding radial electric field
lines within this spherical conversion volume chamber with
these electrical equipotential surfaces of spherical shape
being thus each centred at a focus common centre point
substantially corresponding to the location of the crystal
so as to allow any primary electron generated within the
spherical conversion volume chamber to drift along the
radial field lines. A third electrode substantially
parallel with the second electrode is provided so as to
:5 form together a gas electron multiplier structure which
comprises at least one matrix of electric field condensing
areas distributed within a solid surface. Each of the
electric field condensing areas is adapted to produce a
local electric field amplitude enhancement proper to
generate within the gas an electron avalanche from one of
the primary electrons so as to allow the gas electron
multiplier structure to operate as an amplifier of given
gain for the primary electrons. A readout electrode is
further provided with an array of elementary electrodes
which is formed onto a wall of the vessel and is laid
parallel to the third electrode.
The X-ray imager also comprises, outside the
vessel, an electrical bias circuit which is connected to
the first, second and third electrodes and thus adapted to
deliver adequate voltage potentials so as to drift the
primary electrons within the spherical conversion volume
chamber and then multiply corresponding drifted primary
electrons through an avalanche phenomenon within the gas
electron multiplier structure. A detection circuit is
further provided and connected to the readout electrode so
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as to allow a bi-dimensional readout of the position of
any generated avalanche phenomenon thanks to the gas
electron multiplier structure in the absence of a
substantial parallax readout phenomenon.
A parallax-free X-ray imager embcdy'ing a specific
gas electron multiplier in accordance with the present
invention is now disclosed as a non limitative example.
Particularly, it should be kept in mind that the
planispherical parallax-free X-ray imager in accordance
with the invention can be used with speciffic advantages in
various types of applications such as imaging of the
diffraction patterns of X-rays diffused from a crystal
used for proteins structural analysis and genome
characterization, low dose absorption radiography for
medical diagnosis for mammography, indus-z~rial absorptive
and back-scattering radiography with X-rays, and focused
imaging of specific regions within a body with blurring of
the photons emitted from surrounding materials.
More particularly, any kind of radiations which
come to effect to release primary electrons in gas with
these radiations emanating as a conical X-ray beam
illuminating an entrance window can thus be detected
thanks to the planispherical parallax-free X-ray imager of
the invention.
The planispherical parallax-free X-ray imager in
accordance with the invention is thus disclosed with
reference to Figures lla, lib and llc.
In the accompanying drawings, relative dimensions of
corresponding elements are not represented for the sake of
better comprehension of the whole.
Figure lla shows a section view of the
planispherical parallax-free X-ray imager of the
invention, this section view being thus represented within
a symmetry plane corresponding to the plane of Fig. lla.
The parallax-free X-ray imager of the invention is more
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preferably embodied as cylindrical in shape, this symmetry
plane corresponding thus to a radial symmetry plane of
this cylinder, as it will be disclosed in more detail
later in the specification.
As shown at Fig. lla, the planispherical parallax-
free X-ray imager of the invention is used with a parallel
X-ray beam which is directed to a crystal so as to
generate a conical X-ray beam for illuminating an entrance
window, referred to as IW, of the X-ray imager.
As further shown at Fig.la, the X-ray imager of the
invention comprises a vessel V containing a ionizing gas
for generating primary electrons under impingement of the
X-ray beam and particularly the conical X-ray beam, as
further mentioned in the specification, within the
ionizing gas through the entrance or input window IW.
As previously mentioned in the specification, the vessel V
is cylindrical in shape with its entrance window IW being
thus circular, plane and oriented towards the impinging
conical X-ray beam.
The X-ray imager of the invention as shown at Fig.
lia further comprises within the vessel V a spherical
conversion volume chamber, referred to as SPC, which is
associated with the entrance window IW. This conversion
volume chamber SPC comprises a first 1 and a second 2
parallel electrodes which are adapted to generate in
operation electrical equipotential surfaces of spherical
shape and corresponding radial electric field lines FL
within this spherical conversion volume chamber SPC.
As a consequence, according to one feature of
3o highest interest of the parallax-free X-ray imager of the
invention, the conversion volume chamber SPC fully
operates as a spherical conversion volume chamber, since
its equipotential surfaces are spherical in shape while it
has a full planar or rectangular structure only. It should
thus be born in mind that while such a rectangular or
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planar structure is quite easy to implement a fine control
of the spherical equipotential surfaces shapes can thus be
performed through adequate voltage potentials applied to
the electrodes embodying such rectangular or planar
~ structure as will be explained later in the specification.
According to one essential feature of the
parallax-free X-ray imager in accordance with the
invention, the electrical equipotential surfaces are each
centred at a focus common centred point, referred to as
1o FP, which in operation substantially corresponds to the
location of the crystal in order to allow anv primary
electrons generated within the spherical conversion volume
chamber SPC to drift along the radial field lines.
In Fig. lla, one radial field line only is
15 represented with this field line being fully orthogonal to
the spherical electrical equipotential surfaces which are
represented in dotted lines within the conversion volume
chamber SPC. The field line is referred to as FL at
Fig.11a.
20 Further to the first 1 and second 2 electrodes,
the vessel embodying the parallax-free X-ray imager in
accordance with the invention further comprises a third
electrode 3 which is substantially parallel with the
second electrode 2 with these second 2 and third 3
25 electrodes forming thus a gas electron multiplier
structure, referred to as GEM, which is adapted to thus
operate as an amplifier of given gain for the primary
electrons.
In a general sense, the gas electron multiplier
30 structure GEM comprises one matrix of electric field
condensing areas, referred to as Ci. These electrical
field condensing areas Ci are thus distributed within a
solid surface with this solid surface being delimited by
the above mentioned second 2 and third 3 electrodes
35 contained within the vessel V.
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The structure is shown at Fia. lib and its mode of
operation substantially correspond t3 that of Fig. 3a. In
Fig. 11b however a drift electrode DE is referred to as
first electrode 1, first and second metal-cladding as
5 second and third electrodes 2 and 3 respectively and
collecting electrode CE as readout electrode 4.
As shown in more detail a~ Fig.llb, the above
mentioned solid surface may be thus embodied through a
printed circuit board and preferably may consist of a thin
10 insulator foil which is metal clad on each of its faces,
the metal cladding being thus referred to as 2 and 3 so as
to embody the second 2 and third 3 electrodes contained
within the vessel. The sandwich st~---cture thus formed is
further traversed by a regularly matrix of tiny holes,
15 referred to as Ci at Fig. lib. Typical values are 25 to
500 m of thickness for the foil with the centre of the
tiny holes being thus separated at a distance comprised
between 50 and 300 m. The tiny holes may well have a
diameter which is comprised between 20 and 100 m. The
20 matrix of tiny holes is generally formed in all or most of
the area of an insulator foil of regular shape. The
insulator foil is thus provided with=electrodes on each of
its faces, these electrodes being thus adapted so as to
form the second 2 and third 3 elec-~:rodes and to apply a
25 potential difference between the metal sides of the mesh
embodying thus the matrix of tiny holes.
The composite mesh can thus be manufactured with
conventional technologies as mentioned earlier in the
present specification, and appear simple to install rigid
30 and resistant to accidental discharges.
The mesh embodying the matrix of tiny holes can be
thus released by conventional printed circuit technology.
The structure of the matrix of tiny holes,
dimension and shapes of the holes, type of gas or gas
35 mixture and corresponding mode of operation of the GEM
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structure are disclosed earlier in the present
specification.
The second 2 and third 3 electrodes are thus
adapted to be set at a convenient voltage potential, i.e.
a continuous voltage potential difference value so as to
form at the level of each of the tiny holes forming the
matrix of tiny holes within this solid surface to form a
corresponding electric field condensing area Ci. It should
be thus understood that each tiny hole or through hole
lo traversing the sandwich structure behaves thus as a dipole
which in fact superimposes a further electric field vector
E' with this further electric field being substantially
directed along a symmetry axis of each tiny hole, as
disclosed earlier in the present specification.
As a consequence, each of the electric field
condensing area is thus adapted to produce a local
electric field amplitude enhancement, referred to E',
which is proper to generate within the gas an electron
avalanche from the primary electrons generated within the
2o spherical conversion volume, referred to as SPC, under
impingement of one ray of the conical X-ray beam.
For the sake of clarity and better comprehension,
Fig.llb is shown in the absence of electric charges within
the drift region, i.e. the spherical conversion volume
SPC, and the transfer and induction volume, referred to as
TIVC, which corresponds to a detection region, this case
fully corresponding as an example to the absence of
ionizing radiations. With reference to Fig.llb, any
virtual solid surface, thereafter designated as FT, which
is delimited by the outermost electric field lines
reaching one local electric field condensing area as shown
at Fig.lla for example, delineates thus an electric field
tube FT in which the electric field flux presents a
preservative character. As a consequence, it is clear to
any person of ordinary skill in the corresponding art that
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the enhancement of the electric field at the level of each
local electric field condensing area C; is thus given
accordingly with any surface being passed through by the
condensing electric field vector E' being in direct
relation to the enhancement for the resulting electric
field which is thus equal to the sum of original electric
field vector E and superimposed electric field vector E'.
it is further emphasized that the sandwich
structure embodying the matrix of electric field
lo condensing areas Ci is of symmetrical character with
respect to a symmetry plane, referred to as plane Q at
Fig.lb. As a consequence, any virtual solid surface fcrrned
by the outermost electric field lines reaching a
corresponding local electric field condensing area C: is
substantially transferred as a symmetrical virtual solid
surface formed by the electric field line leaving the same
local electric field condensing area Ci in the detection
region, as shown at Fig.la with respect to the same
electric field tube FT.
As further shown at Fi.g.lla, the parallax-free X-
ray imager in accordance with the present invention is
further provided within the vessel V with a signal readout
electrode 4 preferably formed onto a wall of the vessel V
and which is parallel to the third electrode 3. The signal
readout electrode 4 may for example consist of elementary
electrodes, referred to as 4jk, each elementary electrode
consisting for example of parallel conductive strips or
pads in case bidimensional readout is performed.
In a general sense, the readout electrode 4 and
corresponding elementary electrodes 4jk form a transfer
and induction volume, referred to as TIVC, with the third
electrode 3. This transfer and induction volume chamber
TIVC fully corresponds to a detection region as previously
mentioned with reference to Fig.llb. For this reason, the
3~ electrical equipotential surfaces of the transfer and
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induction volume chamber TIVC are represented parallel to
the signal readout electrode 4 as shown at Fig.lla. As it
will be disclosed in more details in the specificatic:-:,
electrical equipotential surfaces of the TIVC chamber may
even be slightly bent through appropriate electrodes in
order to have a full transfer of" the avalanche phenomenons
which are generated within each electric field condensing
areas Ci in the absence of any substantial parallax error.
As further shown at Fig.lla, the planispherical
lo parallax-free X-ray imager in accordance with the present
invention is further provided, cutside the vessel V, with
electrical bias means 5 which are connected to the first
1, the second 2 and the third ? electrodes and which are
adapted to deliver adequate vo-tage potentials so as to
drift the primary electrons within the spherical
conversion volume chamber SPC, multiply corresponding
drifted primary electrons through the above mentioned
avalanche phenomenon within the gas electron multiplier
structure GEM and then transfer this avalanche phenomenon
within the TIVC chamber up to the signal readout electrode
4 in proper conditions. For the sake of comprehension, the
electrical bias circuit 5 is represented in a conventional
manner at Fig.la as a D.C. or voltage source feeding an
adequate resistor adapted to deliver necessary potentials
to the first 1, the second 2 and the third 3 electrodes as
known in a conventional manner. It should be born in mind
that the signal readout electrode 4, or in other words the
elementary electrodes 4jk embodying the latter, are put at
the reference potential with the difference voltage
potential applied to the third, the second and the first
electrodes being thus decreasing negative potentials.
Further to the electrical bias circuit 5,
detection circuits 6 are provided outside the vessel V and
connected to the readout electrode 4. The detection
circuits 6 may consist of elementary amplifiers 6jk, each
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connected to one of the elementary electrode embodying the
signal readout electrode 4 in a well-known manne=. In case
the elementary electrodes associated with their own
elementary operational amplifier are provided, the
position of any generated avalanche phenomenon can be thus
readout in a bidimensional readout thanks to the index j
and k which are allotted to each elementary electrode and
associated operational amplifier.
As further shown at Fig.lla, the first 1, second 2
and third 3 electrodes are each provided with electrical
conductive field rings or surfaces which are engraved onto
these electrodes. The electrical conductive field rings of
first electrode 1 are referred to as lo to 1H, those of
electrode 2 are referred to as 2o to 2N and those of
electrode 3 are referred to as 3o to 3N. These electrical
conductive field rings have a common centre, referred to
as lo, 2o and 3o respectively and are each distributed over
the external surface of their corresponding electrodes.
A general perspective view of the parallax-free X-
2o ray imager of the invention is shown at Fig.lc for a
vessel V which is cylindrical in shape. in such a case,
the entrance window IW, the first 1, second 2 and third 3
and readout 4 electrodes are shaped as a disk with each of
this disks being thus joined together thanks to a lateral
curve surface so as to form the cylindrical vessel V. As
shown in more detail in connection with Fig.lc, the common
centre lo, 2o and 3o of first 1, second 2 and third 3
electrodes may thus consist of a single disk of conductive
material while the rings of upper rank have their own
common centre and are each distributed over the external
surface of the corresponding electrode.
As shown in more details in connection with
Fig.lla, lic and lld, the second 2 and third 3 electrodes
are each provided with concentric electrical field rings
which are spaced apart from one another on one face of its
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corresponding electrode by a circular grove, one grove and
one electrical field ring of same rank of the second
electrode 2 facing cne corresponding grove and electric
conductive field ring of same rank of the third electrode
3 so as to allow, on the one hand, the electrical
conductive field rings of the second electrode 2, when
these are set at an adequate electrical potential, to
define corresponding limit electrical potential values for
the electrical equipotential surfaces in a direction which
io is parallel to the surface of the second electrode 2 and,
on the other hand, to allow the second 2 and the third 3
electrodes to perform the gas electrode multiplier
function in the absence of any substantial distortion.
More particularly, it will thus be understood that
the same ring pattern is realized on both sides of the gas
electron multiplier structure by second etching the foils
after implementation of the matrix of tiny holes for
example, as described earlier in the present
specification. A fine segmentation is thus performed
allowing thus the local difference of potential within
second electrode 2 and third electrode 3 embodying the gas
electron multiplier structure to remain roughly constant
and thus ensure a good gain uniformity.
As a matter of fact, the lateral curved surfaces
joining the first and second electrodes or even the third
and the signal readout electrode 4 are further provided
with edge-shaping electrodes, referred to as ES1 to ESh.
The first 1, second 2 and corresponding lateral curved
surface and edge-shaping electrodes ES1 to ESN form thus
the spherical conversion volume chamber SPC, with the
edge-shaping electrodes ES1 to ESN being set at an
adequate electrical potential so as to generate adapted
limit electrical potential values for the electrical
equipotential surfaces of spherical shape, as shown at
Fig.lla. The same corresponding feature can be provided at
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the level of the TIVC chamber so as to give to the
electrical potential surfaces cr the TIVC chamber a s1ight
bend, as it will be disclosed in more detail later in the
specification.
As shown in more details at Fig.lld, the signal
readout electrode 4 is set in operation at a reference
potential while the central electrical conductive ring of
the third, second and first electrodes, referred to as 3o,
20, lo respectively, are set at relative decreasing bias
electrical potential with respect to the reference
potential. Accordingly, each of the electrical conductive
ring belonging to one of the third 3, second 2 and first 1
electrodes are further set to successive increasing bias
electrical potential with respect to the corresponding
bias electrical potential of its corresponding central
electrical conductive ring 30, 2c, lo respectively, thanks
to the electrical bias circuit 5.
As a consequence, the potential gradient between
two electrical conductive rings facing each other onto
these second 2 and three electrode 3 have substantially
the same value between conjugate rings 2o, 3o to 2N, 3N,
these gradients of same value generating thus a
substantially same amplifying electric field E' within the
whole gas electron multiplier structure GEM.
As further shown at Fig.lla, the electrical bias
circuits 5 may be provided with adjustable bias voltage
potential device, feeding resistors referred to as R12, R23
and R34, this device being adapted to deliver a bias
voltage potential of adjusted value within a given voltage
range value which is applied to the first and second
electrodes 1, 2, so as to vary the focus location along
the symmetry axis shown at Fig.lla. Operating the
adjustable bias voltage potential device, or even
adjusting one or several of the resistors values, allows
thus to dynamically vary the focal length in a given range
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by adjusting externally the voltage potentials which are
applied to the main nodes and then to the conductive
rings.
A full representation of the electrical
equipotential surfaces of spherical shape within the
spherical conversion volume chamber SPC and corresponding
electrical equipotential surfaces within the TIVC chamber,
or in other words within the drift region and the
detection region respectively, is shown at Fig.12a for
given electrical potential values applied to the
successive rings forming the first 1, second 2 and third 3
electrodes and corresponding edge-shaping electrodes ES:
to ESM of the above mentioned chambers.
At Fig.12a, half part of these chambers are shown,
i.e. the left part as referred to at Fig.lla with respect
to the symmetry axis Y'Y.
Potential values are indicated in kV as an example
only.
In order to have the electrical equipotential
2c surfaces of the TIVC chamber slightly bent as shown at
Fig.12a, given steps of voltage potentials to 100 volts
may be spread along the edge-shaping electrodes referred
to as ESi to ESp as shown at Fig. 12a.
The most external conductive ring, referred to as
3N, of electrode 3, is thus preferably set at a voltage
potential decreased of one voltage's step with respect to
the last shaping-electrode ESP while successive inner
rings are set at voltage potentials which are decreased by
the same voltage's step, i.e. 100 volts, with the central
ring 3o being set at -1.3 kV.
Corresponding conjugate conductive rings are set
with reference to Fig.lld at corresponding potentials so
as to generate the same voltage gradient between conjugate
rings 20, 30 to 2N, 3N. The most external conductive ring
2N is thus put at a voltage potential to -1.0 kV as shown
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at Fig.2a. Successive edge-shaping electrodes, referred to
as ESp+I to ESM which are distributed over the lateral
surface af the spherical conversion volume SPC, as shown
at Fig.12a, are set at successive step potentials of 100
~ volts with the last edge-shaping electrode referred to as
ESM being thus put to -2.6 kV.
Successive conductive rings of the first electrode
1 from the outermost conductive ring 1N are thus set at
stepped potentials decreasing from corresponding step
io value with respect to last potential value applied to the
last edge-shaping electrode ESM, central conductive disk
lo being thus put to the most negative voltage potential
to -3.7 kV.
As shown at Fig.12a, it is thus emphasized that
i5 applying successive decreasing step voltages to the edge-
shaping electrodes ES1 to ESP, then to conjugate
conductive rings 3N, 2N to 30, 2o and then to edge-shaping
electrode ESp+1 to ESM and successive conductive rings of
the first electrode 1N to lo allows thus to generate
20 voltage equipotential surfaces of spherical shape within
the drift region of the spherical conversion volume
chamber SPC and then to transform these electrical
equipotential surfaces to slightly bent equipotential
surfaces which are then modified to planar electrical
25 equipotential surfaces in the vicinity of the readout
electrode 4 without introducing any substantial distortion
of the image read on this readout electrode.
A representation of the electrical equipotential
surface, referred to as EPS, and the field lines, referred
30 to as FL, in the vicinity of the electrical field
condensing area C; of two conjugate conductive rings, for
example conductive ring 32 of electrode 3 and conductive
ring 22 of electrode 2, is now disclosed with reference to
Fig. 12b.
35 As a matter of fact, Fig. 12b fully corresponds to
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Fig.llb in which the electrical equipotential surfaces are
bent in the drift region, as shown fcr example at Fig.12a,
while corresponding electrical equipotential surfaces of
the detection region are also slightly bent to correspond
to those of the TIVC chamber in the detection region.
As shown at Fig.12b, the electrical equipotential
surfaces EPS are slightly bent and distorted in the
vicinity of each electrical field condensing area Ci only.
As a consequence, any corresponding field lines FL is thus
submitted to a local distortion only while each of them is
maintained in orthogonal relationship to the distorted
electrical equipotential surface EPS. Consequently, any
field tube FT is preserved, in the same manner as in
Fig.llb, as shown at Fig.12b, in the absence of any
substantial distortion of the image introduced by the
transfer of the electrons from the drift region to the
detection region after amplification through avalanche
phenomenon.
Adequate electric potential bias voltages feeding
the successive conductive rings 2o to 2N and 3o to 3N may
thus take place either by direct feeding of the
appropriate voltage potentials to each conductive ring
from an external resistive partition network, using
insulating conductors, or thanks to surface mount
resistors of appropriate values directly soldered and thus
connected between adjacent rings, while feeding adequate
voltage potentials to the central rings 2o and 3o through
single insulated conductors.
A sectional view of the GEM structure is shown at
Fig. 12c in a preferred embodiment in which a special
sandwich structure has been developed to allow a proper
electrical voltage potential feeding of the conductive
rings in the absence of a substantial degradation of the
image through masking introduced by the feeding connecting
lines.
..__..~.~
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As shown at Fig. 12c, the sandwich structure consists of
the second electrode 2 and its rings 2o to 2N, a resistive
layer l0a covering the insulator foil 10 and a further
resistive layer lOb and the third electrode 3 and its
5 rings 3o to 3N. The whole structure is traversed by tiny
holes embodying the electric field condensing areas, which
are not shown at Fig. 12c. Connecting each resistive layer
10a, 105 through adequate resistors Rloal, Rioa2 and Rlobl,
R1ob2 to adapted voltage potential values -VU1, -VU2 and
10 -VD1r -VD2 respectively allow thus to put corresponding
conductive rings to adaptive voltage potential values, as
shown in Fig. 12a, while smoothing the electric field
transition from one ring to the adjacent one, the voltage
gradient between two conjugate rings being preserved and,
i5 as a consequence, the GEM structure amplification factor
or gain over the whole surface of the latter.
