Note: Descriptions are shown in the official language in which they were submitted.
22
1 PHD. 77-126
The invention relates to a radiation detection
device, comprising a housing which comprises an entrance
window and in which there is arranged at least one ioniza-
tion chamber detector which comprises a flat high-voltage
electrode which is directed transversely of the entrance
window and a flat collector electrode which is arranged to
be substantially parallel thereto and which comprises an
end face which faces the entrance window and which is
electrically conductively connected to a contact face which
is situated opposite the end face and which forms part of a
flat auxiliary electrode which extends parallel to the
entrance window.
A radiation detection device of this kind is par-
ticularly suitable for use in a computer tomography
apparatus in which a body is irradiated from a large number
of directions by means of a fan-shaped radiation beam. A
- radiation detection device comprising a large number of
detectors serves to measure the radiation absorption of the
body along a large number of radiation paths; on the basis
of these measuring data, the density distribution of the
- irradiated part of the body is calculated and displayed on,
for example, a television monitor by means of a computer.
A known radiation detection device of the des-
cribed kind has an auxiliary electrode, which is made of an
electrically suitably conductive material, extending as far
as the vicinity of the high-voltage electrode. This auxi-
liary electrode intercepts charge carriers formed by
ionization which, in the case of a detector without such
an auxiliary electrode, would be incident on the entrance
window instead of on the collector electrode, so that they
would not contribute to a detector output signal. Because
the auxiliary electrode is electrically conductively con-
nected to the collector electrode, the charge carriers
intercepted by the collector electrode do contribute to
the detector output signal. As a result, the detection
efficiency of the detector is high in co-mparison with a
detector which does not include such an auxiliary elec-
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6~
2 PHD~ 77-126
trode. Radiation which enters the detector vla the
entrance window produces a comparatively large number of
charge carriers in the immediate vicinity of the entrance
window, because the intensity of the radiation, and the
associated number of ionizations caused thereby, exponen-
tially decreases as a function of the distance from the
entrance window in the direction transversely of this
window.
In the known detector, the distance between the
auxiliary electrode and the high-voltage electrode is
smaller than the distance between the collector electrode
and the high-voltage electrode. As a result, after appli-
cation of a high voltage between the electrodes, the
electrical field arising between the auxiliary electrode
and the high-voltage electrode will be larger than the
electrical field arising between the collector electrode
and the high-voltage electrode. The electrical field in
the detector thus exhibits a spatial inhomogeneity, with
the result that the detector is comparatively slow, be-
cause the measuring speed, being proportional to thedrift speed of charge carriers formed by ionization, is
limited in the case of high electrical field strengths by
space charges caused by avalanches of secondary ioniza-
tions and in the case of low electrical field strengths
by the value of this field strength.
The invention has for its object to provide a
radiation detection device which offers a high detection
efficiency and in which said drawback is mitigated. To
this end, a radiation detection device of the kind set
forth in accordance with the invention is characterized in
that a second contact face of the auxiliary electrode
having a comparatively high electrical resistance measured
in the direction transversely of the collector electrode,
is situated opposite an end face of the high-voltage
electrode facing the entrance window and is electrically
conductively connected to the high-voltage electrode.
~ uring operation of the detec~or, a compara-
3;~2
3 PHD. 77-126
tively small direct current flows from the high-voltage
electrode through the auxiliary electrode to ~he collector
electrode, with the result that the voltage difference
between a point of the auxiliary electrode and the collec-
tor electrode linearly increases with the distance betweensaid point and the collector electrode. The electrical
field between the high-voltage electrode and the collector
electrode is thus homogeneous as far as in the auxiliary
electrode as if it were. Since the electrical field in
the ionization chamber will be as homogeneous as described
before too when the first and the second contact face of
the auxiliary electrode are electrically conductively con-
nected to a potential which equals the high-voltage elec-
trode potential and the collector electrode potential
respectively, whereby the contact faces may be situated on
~he side of the auxiliary electrode facing the electrodes
or on the side of the auxiliary electrode facing the
entrance window as well, the meaning of "electrically con-
ductively connected to a contact face" has to be under-
stood in a broad sense. When this field is chosen to beso large that avalanches of secondary ionizations are just
precluded, the adjustment of the detector is optimum in
respect of measuring speed.
A preferred embodiment of the radiation detec-
tion device in accordance with the invention is character~
ized in that the collector electrode consists of a flat,
electrically insulating support, a flat side of which is
provided with an electrically conductive layer in order to
intercept charge carriers formed by ionizations in the de-
tector, an end face of said support being provided with anearthed metal strip which is electrically conductively
connected to the first contact face of the auxiliary elec-
trode.
Because the collector electrode of an ionization
chamber detector is on average at least substantially at
earth potential during operation, the electrical field
between the high-voltage electrode and the collector elec-
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26.10.78 L~ P~ID 77-126
trode is also-homogeneous as far as in the auxiliary elec-
trode as if it were in this preferred embodiment of the
radiation detection device. Because, moreover, the signal
current generated between the electrodes by ionization
during operation and the direct curren-t through the
auxiliary electrode are isolated from each other, noise on
the direct current through the auxiliary electrode, having
a high resistance, cannot have an adverse effeet on the
signal current.
A very simple radiation detection device in
accordance with the invention is characterized in that the
auxiliary electrode is formed by a resistance layer which
directly contacts the end face of the collector electrode
and the end face of the high-voltage electrode.
When the auxiliary electrode is brought into
direct contact with the end faces, moreover, vibrations of
the high-voltage electrode with respect to the collector
electrode during operation are attenuated.
~ The invention will be described in detail here-
inafter, by way of example, wi-th reference to the accompa-
nying diagrammatic drawing~
Figure 1 is a sectional view of a radiation de-
tection device in accordance with the invention,
Figure 2 is a sectional view of a preferred em-
~odiment of a radiation detection device in accordancewith the invention, and
` Figure 3 is a cross-sectional view of a radia-
tion detection device in accordance with the invention,
taken along the line III-III in Figure 1, illustrating a
3~ special embodiment of the auxiliary electrode.
Figure 1 is a sectional view of a radiation de-
tection device in accordance with the invention, compri-
sing a housing 1 which is provided with an entrance window
2 wherethrough radiation to be detected, diagrammatically
denoted by arrows 3, can enter the housing. In the housing
1 there is arranged an ionization chamber detector 4 which
- comp ises a flat high-voltage electrode 5, directed trans-
versely of the entrance window 2, and a flat collector
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26.10.78 5 PHD 77-126
electrode 6 which is arranged to be parallel thereto. Via
connection wires 7, passed through electrically insul.ated
passages 8 through the housing, a high-voltage source 9
can be connected between the high-voltage electrode 5 and
the collector electrode 6, said source generating an homo-
geneous electrical field, diagrammatically denoted by
arrows 10, in the ionization chamber 4. The ionization
chamber detector 4 is filled with a medium which can be
ionized, for example Xe-gas, and in which free charge
carriers are formed by the incident radiation by ioniza-
tionO In the ionization chamber detector 4 these chargecarriers cause an electrical current which causes a vol-
tage difference across a resistor 11, said difference
being applied to an amplifier 12 in order to generate a
detector output signal which can be derived from the out-
put 13 of the amplifier 12.
The hi.gh-voltage electrode 5 and the collector
electrode 6 are secured in the housing by means of insula-
ting holders 14.. End faces 15 and 16, facing the entrance
window 2, of the collector electrode 6 and the high-
voltage electrode 5, respectively, are electrically con-
ductively connected in direct contact.to contact faces 17
and 18, situated opposite the end faces 15 and 16, of a
flat auxiliary electrode 19 which extends parallel to the
entrance windo~ 2.
The auxiliary electrode 19 consists of an homo-
: geneous resistance layer and exhibits, measured in the di-
rection transversely of the collector electrode 6, a com-
paratively high electrical resistance of, for example, 10
to 10 ohms per cm. As a result, during operation of the
detector, a comparatively small direct current flows from
the high-voltage electrode 5 to the collector electrode
~, said current causing the voltage difference between a
point of the auxiliary electrode 19 and the collector
electrode 6 to increase linearly with the distance between
this point and the collector electrode 6. The electrical
field between the high-voltage electrode 5 and the collec-
$or electrode 6 is thus homogeneous as far as in the
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26.10.78 6 PHD 77-126
auxiliary electrode 1CJ as if it were. This f`ield is then
adjusted to be so large that the appearance of avalanches
Or secondary ionizations in the ionization chamber detec-
tor is just precluded. As a result, the adjustment of the
detector is optimum in respect of measuring speed. In
order to isolate the direct current through the auxiliary
electrode 19 from the signal current which is generated by
the free charge carriers formed during the ionizations and
which, consequently, varies pulse-li~e in the time, the
voltage difference appearing across the resistor 11 due to
the two currents is applied, via a capacitor 20, to the
amplifier 12, with the result that the direct voltage
component is blocked.
The space Z1 between the entrance window 2 and
the auxiliary electrode 19 and the thickness of the
auxiliary electrode 19 are shown in exaggerated form in
the drawing, but should be as small as possible, because
in the housing 1 filled with medium which can be ionized,
a~comparatively large number of free charge carriers is
formed in the immediate vicinity of the entrance window 2
by the incoming radiation 3, because the intensity of the
radiation and the associated number of ioni~ations caused
thereby decreases exponentially in the direction trans-
versely of the entrance window 2.
Figure 2 is a sectional view of a preferred
embodiment of a radiation detection device in accordance
with the invention in which reference numerals correspond-
ing to Figure 1 are used for corresponding parts. The
co~lector electrode 6 consists of a flat, electrically in-
sulating support 25, a flat sicle of which is provided with
an electrically conductive layer 26 for intercepting free
charge carriers formed by ionization in the detector. The
othe~ flat side may also be provided with an electrically
conductive layer 27 in order to intercept free charge
3~ carriers formed by ionizations in a neighbouring detector.
The end face 15 of the collector electrode 6 is formed by
a metal strip 28 which is electrically conductively
connected to the contact face 17 of the auxiliary electro-
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26.10~78 7 PHD 77-126
de 19 and which is isolated from the electrically conduc-
tive layers 26 and 27 by a space 29. The metal strip 28
is connected to earth potential in the same manner as the
connection of the collector electrode 6 and the high-
voltage electrode 5. Because the collector electrode 6 ofthe ionization chamber detector 4 is on average sub-
stantially at earth potential during operation, in this
preferred embodiment the electrical field 10 is also homo-
geneous as far as in the auxiliary electrode 19 as if it
were. Because, moreover, the signal current generated by
ionizations and the direct current through the auxiliary
electrode are isolated from each other by the space 29,
noise on the direct current cannot have an adverse effect
on the signal current. Moreover, the capacitor 20 in the
circuit shown in ~igure 1 can then be omitted.
~ igure 3 is a cross-sectional view of a radia-
tion detection device in accordance with the invention,
~taken along the line III-III in Figur-e 1) illustrating a
special embodiment of the auxiliary electrode 19. In
~igure 3, parts which correspond to parts in ~igure 1 are
again denoted by corresponding reference numerals. The
auxiliary electrode 19 consists o~ a flat, electrically
insulating support 30 on which a meandered resistance
track is provided. Parts 31 of the resistance track which
extend in the direction transversely o~ the collector
- electrode 6 exhibit a comparatively high electrical re-
sistance of, ~or example, from 10 to 10 ohms per cm,
whilst parts 32 thereof which extend in the direction
parallel to the collector electrode 6 exhibit a low elec-
trical resistance. An auxiliary electrode of this kind can
be simply and accurately manufactured, for example, by
vapour deposition.
A particularly compact radiation detection
device which, moreover, offers a high radiation detection
efficïency is characterized in that the resistance layer
at the same time forms the entrance window of the radia-
tion detection device. Because the window 2 and the space
21 are omitted, radiation 3 entering the ionization
L!322
26.10.70 8 PHD 77-126
chamber detector 4 will have been attenuated to a small
degree`ol~ly.
Preferably, the end faces 16 and 15 of the high-
voltage electrode 5 and the collector electrode 6, respec-
tively, are electrically conductively connected to theauxiliary electrode 19 by means of an electrically conduc-
tive adhesive (not shown in the Figures), so that vibra-
tions are additionally attenuated during operation.
