Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OF THE INVENTION
The present invention pertains to an improvement in
electron multiplier devices and, more particularly, to such
type which comprises an insulating body pierced by a channel
of a very small diameter, the inner wall of which is coated
with a very thin electrically resistive layer having
properties of secondary electron emission.
BACKGROUND OF THE INVENTION
It may be recalled that, with such electron multiplier
devices of the type described, an electric field is produced in
the channel by establishing a suitable potential difference
between the extremities and by causing a beam of primary
electrons to penetrate into the channel under different angles
thereby resulting in a series of secondary electron emissions
on the internal wall of the channel. At the output, the number
of electrons is greatly increased as compared to the primary
beam at the input. An electron multiplier device can be used
in the form of a single tube, or a plurality of such tubes can
be bundled together.
Typical prior art secondary electron multipliers
consist of straight or curved tubes of insulating material, such
as glass, coated on its entire inside surface with a conductive
coating made of secondary electron emissive material. There
exists also another type of secondary electron multiplier using
a tube made only of ceramic such as barium titanate or zinc
titanate which may be found described in U.S. patent No.
3,612,946 issued October 12, 1971 to Toyoda.
While the ceramic type of electron multiplier devices
overcome the many disadvantages associated with electron
multiplier devices wherein the insulating material used is
glass (little resistance to impact of charged particles or
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mechanical injuries, expensive to mass produce, unstable in
operation, etc.), they are, however, limited in construction
and in use. Indeed, since the ceramic materials used in these
electron multiplier devices have semi-conducting properties,
it is practically impossible to maintain a uniform potential
gradient inside a curved channel machined or molded in a bulk
piece of semi-conducting ceramic. The polarising current in
semi-conducting ceramic electron multiplier is distributed
inside the whole body of the multiplier this characteristic
imposes a limitation on their construction. For such
multipliers to operate satisfactorily, the thickness of the
ceramic must be constant throughout the length of the multiplier
channel. The selection of ceramic to be used is extremely
limited and as described in the above mentioned Toyoda patent,
the advantages obtained with ceramic is best achieved with a
barium titanate or zinc titanate family semi-conducting ceramic
material having positive or zero-resistance temperature charac-
teristics. Such ceramic is not one which is easily machinable
or moldable with high accuracy thereby limiting the shapes
which can be given to an electron multiplier device.
OBJECTS OF THE INYENTION
It is an object of this invent-i-on to provide an
electron multiplier device which is capable of providing
a great variety of shapes or geometrical configurations.
It is another object of the present invention to
provide an electron multiplier device which is simple in
construction and which is capable of receiving the great
precision inherent to machining or molding processes.
It is another object of the present invention to
provide an electron multiplier device which is capable of
being recycled when its emissive surfaces become exhausted
or deteriorated.
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STATEMENT OF THE INVENTION
The present invention therefore relates to an electron
multiplier device which comprises: in combination, a support
made of high temperature-resisting electrically-insulating
ceramic material, the support having an internal wall defining
a channel therethrough; a layer of secondary electron emitting
semi-conducting glass material fused to the internal wall; the
ceramic material and the glass material having the same coef-
ficient of expansion.
This invention is therefore concerned with the
construction of an electron multiplier device whereby glass
material is fused to an insulating ceramic. This realization
enables a great variety of ceramic material; for example,
a machinable ceramic can be selected which can be worked
with conventional steel working equipment. Also, a ceramic
can be chosen independently of any constraint imposed by
the semi-conductivity and the secondary emission which must
be satisfied in the case of a multiplier entirely made of
ceramic. However, to achieve this, the coefficient of
expansion of the glass material must be matched to that of
the ceramic material so that minimum strain be produced on
the multiplier formed. The semi-conducting glass is formed
from an appropriate mixture of oxydes, for example, SiO2, PbO,
BaO, Bi203, which are intimately mixed together and fused in
a crucible to 1000C. The mixture is poured in the channel
molded or machined in the ceramic support. The emissive and
semi-conducting surfaces in the electron multiplier are
obtained by depositing a layer of semi-conducting glass on the
internal wall of the channel. The ceramic selected must be
capable of support temperatures in the 1000C without
deformation and this stability permits to the semi-conducting
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glass to flow easily in the channel. During the flowing process
(which may be by gas pressure), the semi-conducting glass
adheres to the wall of the channel in the ceramic support by
surface tension; when the semi-conducting glass has flown through
the channel, the apparatus is cooled down and the gas pressure
is removed. This process leaves a semi-conducting layer on the
wall of the channel.
The entire mass of glass material may be rendered
semi-conducting, in such embodiment, the thickness of the layer
should be made constant as much as possible so as to obtain a
uniform potential gradient throughout the channel. On the
other hand, in a preferred form of the invention, the conduction
properties of the layer are stimulated and enhanced by reduction
of the metallic oxydes contained in the glass which covers the
wall of the channel. The reducing agent, for instance hydrogen,
is allowed to flow in the channel for a given time and tempera-
ture (for example, at 450C for about 4 to 6 minutes). This
produces a semi-conducting and emissive film on the layer of
glass, thus simplifying the construction of the electron
multiplier device. Indeed, few machining imprecisions,
fissures or loss of material are acceptable in the ceramic
without affecting the properties of the finished-product. These
defects are filled by the glass in fusion and, since the latter
is only superficially conducting, they have no effect on the
potential gradient inside the multiplier device. The film
8 10
obtained has a resistance ranging from 10 to 10 ohms.
In practice, after long periods of electron
bombardment, the emissive surfaces of the multiplier deteriorate
irreversibly. For example, exposure to oil vapors from pumping
systems can lead to polymer formation at the end of the channel
or a pressure accident in a vacuum system can cause destruction
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by ion bombardment. A treatment which does not depend on the
particular surface state reached is thus needed. Rebuilding
the semi-conductive layer, which is only 1 ~m thick, is one
solution to the problem and the present invention makes it
easily realizable. First, the electrical contacts are dissolved
with appropriate acid solutions and the reduced layer and
foreign materials, if present, are etched in an appropriate
solution, for example, a mixture of HF, HN03, H20 with ultrasonic
agitation. By using less concentrated solutions, one can etch
only a small thickness of the glass layer and remove the contami-
nants. For more serious damage, rebuilding of the semi-conduct-
ing layer is done by adding a small quantity of semi-conducting
glass through the channel and flowing and reducing in the manner
described above. This recycling cannot be done with multipliers
of the prior art because of the high temperatures involved in
this process which can only be supported by pure silica or
ceramic. However, to achieve this recycling feature, the fusion
point of the ceramic material must be higher than that of the
glass material thereby ensuring that the ceramic material will
not be affected during the recycling process.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 ;s a longitudinal cross-section of a straight
continuous dynode electron multiplier made in accordance with
the present invention;
Figure la is an enlarged view of the internal channel
showing the soldered ceramic and glass materialsi
Figure 2 is a cross-sectional view showing another
embodiment of the present invention;
Figure 3 is a cross-sectional view showing a further
embodiment of the present invention; and
Figure 4 is a schematic illustration of a device
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for making an electron multiplier in accordance with the present
invention.
GENERAL DESCRIPTION OF THE INVENTION
The electron multiplier device according to the
present invention is principally characterized by the
combination of layer of semi-conducting glass that has the
proper composition to possess the same coefficient of expansion
as that of the ceramic material which acts as the envelope or
supporting frame of the multiplier device. The ceramic
material is first machined or molded to provide a channel
therethrough. Then, the layer of glass material is formed
by flowing the glass material under gas pressure through the
channel and cooling.
EMBODIMENTS OF THE INVENTION
Referring to Figure 1, a straight continuous dynode
electron multiplier 10 is shown as being formed of a solid
body having a central channel that includes a funnel entrance 12
followed by an elongate straight channel section 14 and an exit
section 16. As described above, a semi-conductive and emissive
film 18, with a thickness of about 1 ~m for example, is produced
on the glass layer 20 intimately soldered to the ceramic support
22; an interdiffusion zone between the two materials is shown at
24. The thickness of the glass layer is about 200 ~m. Two
peripheral grooves 26 and 28 are provided at opposite ends of
the multiplier to facilitate wire attachments. Electrical
contacts between the reduced layer 18 and the wire attachment
can be performed with silver paint; however, for ultra-high
vacuum applications, vacuum deposited chromium contacts are
preferred. Grooves 26 and 28 are molded or machined in the
cerdii,ic material and may receive stainless steel wires which
are fixed at both ends of the ceramic frame after the chromium
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vacuum deposition. Shoulder 30 on the outer wall of the body
provides a stop for the ceramic frame ~hen gas pressure is
applied at entrance 12 during the fabrication process and, also,
provides fixation facilities for utilization of the multiplier
in other apparatus. Typical dimensions of such an electron
multiplier are 1 mm for the inner diameter and 69 mm for the
length of the straight section 14.
As mentioned above, an important feature of the
present invention is that both the ceramic material and the
glass material have substantially the same coefficient of
expansion. It has been found that differences in the range up
8% can be allowed between both coefficients of expansion
without affecting the composite multiplier device. In one
example, for instance, the glass may have a coefficient of
expansion of 9.2 x 10 6/C with an annealing point at about
400C. From this temperature to room temperature, the expansion
of glass is about 3500 ppm. Hence, for a combination of glass
and ceramic, 300 ppm can be tolerated, which is about 8%.
Referring to Figure 2, there is shown another
embodiment of the present invention wherein an electron
multiplier 40 is formed of an inner cylinder 41 made of ceramic
and having a funnel entrance 42 which is machined. A hole 44
is bored from the bottom of entrance 42 to the outer part 46
of cylinder 41. An helical groove 62 is made from the output
of hole 44 to the other end 48 of the cylinder. Cylinder 41
is then fitted inside an outer shell 50 and is fixed thereto
by a stem 52, preferably made of platinum. During the fusion
process, a small portion of the semi-conducting glass flows
between the inner cylinder and the outer shell soldering both
parts together. The outer shell 50 is provided with a ring 54
which serves as a fixation collar when the multiplier is used
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as a detector in other apparatus and as a stop when the
multiplier is in the forming device (as described hereinbelow)
and subject to the application of gas pressure. Grooves 56
and 58 are similar to grooves 26 and 28 as described above with
respect to the first embodiment of the invention. Typical
dimensions of the present embodiment are: 2.4 cm length and
1.9 cm overall diameter. The semi-conducting glass layer 60
is deposited inside the cone 42, the channel 44, the helical
passage 62 and the output 48. A metallic film is deposited at
the input to electrically connect the semi-conducting layer 64
to the groove 56 and the exit 48 to the groove 58.
In Figure 3, another embodiment of an electron
multiplier device made in accordance with the present invention
is shown as 70. A parallelepipedic piece of ceramic 72 includes
an S-shaped groove 74 molded, or machined by pentographic
methods, on the largest face of piece 72; typical dimensions
of element 72 are 1.8 cm x 1.6 cm x 4 mm. A half-cone
configuration 76 is machined in piece 72 to form the entrance
of the electron multiplier. Another piece (not shown), which
is a mirror image of piece 72, is machined with a groove and
half-cone entrance which will fit exactly over the channel 74
and the half cone 76. Platinum stems may be inserted in
alignment holes 78,79,80 and 81, thus making a semi-conducting
glass-ceramic continuous dynode electron multiplier of the
sandwich-type in which the section of the dynode may be circular,
elliptical or otherwise shaped. The stems selected must match
~he coefficient of expansion of the ceramic and the glass.
Sho~!der 82 serves the same function as shoulder 30 and ring 54
in ~igures 1 and 2, respectively. Grooves 84,86 serve the same
funct-Son as grooves 26 and 28, and grooves 56 and 58 of Figures
1 and 2, respectively. The semi-conducting surface 88 is formed
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of the glass layer 90 with a soldering zone at 92. Metallic
Tilms 94,96 are provided for electrical contacts and are
deposited between the cone entrance and groove 84 and between
the exit 98 and groove 86.
Figure 4 is a schematic illustration of a semi-
conducting glass ceramic continuous dynode electron multiplier
forming device lO0. It is constituted by a tubular member 102
which can be rotated about its axis 104 and is orientable at
a variable angle. The tubular member 102 is closed around a
ceramic frame 106 by a ring 108 of refractory and gas-tight -
material. A cover 110 which retains the ring 108 prevents the
frame 106 from sliding out of the tubular member 102 when air
pressure is applied in chamber 112. The semi-conducting glass
114, which is first melted in the cone-shaped entrance 116 at
the appropriate temperature, is forced in channel 118 by the
pressure difference existing between chamber 112 and a second
chamber 120 delimited by a cap member 122, the latter serving
to control the gas flow and composition in the areas 112,118
; and 120.
Although the invention has been described above with
respect to specific forms of realization, it is evident that
it may be-refined-and mod-ified.in vario.us ways. For examp.le,
a series of these devices may be connected in a parallel
arrangement and stacked to form an electron multiplier device
to obtain superior effects. Therefore, it is wished to have
it understood that the present invention should not be limited
in scope except by the terms of the following claims.
