Proximity focusing image intensifier tube with spacer shims

The disclosure relates to image intensifier tubes of the proximity focusing type, wherein it especially concerns the positioning of a primary screen with respect to a slab of microchannels. An image intensifier tube comprises a sealed chamber containing a primary screen and a slab of microchannels. The slab of microchannels is fixed to the body of the chamber. According to one characteristic, the primary screen is fixed to the slab, from which it is kept at a distance by means of at least one insulating shim. The result thereof is greater precision and greater uniformity of the spacing between the primary screen and the slab of microchannels.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to image intensifier tubes of the type wherein, 
firstly, an incident ionizing radiation is converted into photons in the 
visible or near-visible range and wherein, secondly, a slab comprising 
microchannels is used to ensure a gain in electrons. 
2. Description of the Prior Art 
Image intensifier tubes such as these are often called "proximity focusing" 
tubes. They are used, for example, in radiology. The principle of 
radiological image intensifier tubes ( IIR tubes in short ) using slabs of 
microchannels is well known. It is described notably by J. Adams in 
"Advances in Electronics and Electron Physics", volume 22A, pp. 139-153, 
Academic Press, 1966. 
FIG. 1 gives a schematic view of the structure of a standard IIR tube using 
a slab of microchannels such as this. 
The IIR tube 1 comprises a vacuum-tight chamber, constituted by a tube body 
2 positioned about a longitudinal axis 13 of the tube. The body 2 is 
closed at one end by an input window 3 and at the other end by an output 
window 14. 
The X-rays penetrate the IIR tube through the input window, which should be 
as transparent as possible to these rays: the input window 3 is generally 
constituted by a thin metal foil (aluminium, tantalum, etc.). 
The X-rays then encounter a layer 4 of scintillating material in which they 
are absorbed and give rise to a local emission of light proportional to 
the quantity of X-radiation absorbed. The scintillator material may be, 
for example, caesium iodide forming the layer 4 with a thickness of the 
order of 0.1 to 0.8 nm. The layer 4 of scintillator material is supported 
by a single plate 5 transparent to X-rays, formed for example by a thin 
metal foil (for example made of aluminium alloy) or else a silica-based 
glass plate etc. The supporting plate 5 is located towards the input 
window. 
The scintillator 4 bears a photocathode 6. The photocathode 6 is 
constituted by a very small thickness (often smaller than one micrometer) 
of a photo-emissive material. This layer is deposited on a face of the 
scintillator 4 that is opposite the supporting plate 5. The photocathode 6 
absorbs the light emitted by the scintillator 4 and, in response, sends 
out electrons locally into the surrounding vacuum, in proportion to this 
light. The set constituted by the supporting plate 5 bearing the 
scintillator 4 which itself bears the photocathode 6 constitutes a primary 
screen 15. 
The electrons (not shown ) emitted by the photocathode 6 are directed by an 
electrical field towards the input face 8 of a slab 7 of microchannels. To 
this effect, a first potential and a second potential V1, V2 are applied 
respectively to the photocathode 6 and to the input face 8, the second 
potential V2 being more positive than the first potential V1. 
The slab 7 of microchannels is an assembly of a multitude of small parallel 
channels 12 assembled in the form of a rigid plate. Each primary electron 
(sent out by the photocathode) that penetrates a channel is multiplied by 
a phenomenon of secondary emission in cascade on the walls of the channel, 
so that the flow of electrons at the output of the slab can be more than a 
thousand times greater than the input flow. The diameter d1 of the 
channels may range from 10 to 100 micrometers. The channels 12 are 
inclined with respect to the normal to the plane of the slab so that the 
electrons which are emitted by the photocathode 6 in parallel to this 
normal cannot emerge from a channel without giving rise to a phenomenon of 
secondary emission. In order to reduce the number of electrons that strike 
the input face of the slab 7 outside the channels 12, it is the usual 
practice to make a widened portion 35 at the input to these channels and 
hence to reduce the thickness of their walls. The thickness E of the plate 
that forms the slab 7 of microchannels is typically between 1 and 5 mm. 
The electronic gain of the slab may be adjusted over a wide range of 
values, for example between 1 and 5000, as a function of the voltage 
developed between the input face 8 and an output face 9 of this slab 7, 
namely an output face 9 to which a third potential V3 is applied. 
The electrons at output of the slab of microchannels are accelerated and 
focused by an electrical field, on a luminescent screen (10) positioned so 
as to be facing the slab, parallel to this slab, and at a distance D of 
the order of 1 to 5 mm. The luminescent screen 10 locally emits a quantity 
of light proportional to the incident electron current. The luminescent 
screen therefore restores a visible and intensified image of the X-ray 
image projected on the scintillator, through the input window of the tube. 
The luminescent screen, which is a layer with a thickness of some microns, 
constituted by grains of luminophor material, is deposited on a glass port 
which may constitute the output window 14 of the tube. The face of the 
luminescent screen 10, pointed towards the slab 7 of microchannels, is 
coated with a very thin metal layer 18, made of alumininum for example. 
The metallization enables the electrical polarization of the screen (by 
the application of a fourth potential V4 that is more positive than the 
third potential V3) and acts as a reflector for the light reflected 
rearwards by this screen. The port 14 supporting the screen 10 may be made 
of glass, or may be constituted for example by a fiber-optic system. The 
screen 10 may be deposited directly on this port or on an intermediate 
transparent support if it is desired to insulate the screen 10 from the 
port because of constraints of use. 
The primary screen 15 and the slab 7 of microchannels are fixedly joined to 
the body 2 of the tube, for example by means of lugs 21, 22, 23 sealed to 
this body. To these lugs, there are furthermore applied the polarizing 
potentials V1, V2, V3. The polarizing of the input and output faces 8, 9 
is furthermore ensured by means of a metallization (not shown) with which, 
as a rule, these input and output faces of the slab are generally coated 
except, naturally, in positions facing the channels 12. The primary screen 
15 and the slab 7 are thus fixed so as to be electrically insulated from 
each other while, at the same time, being separated by a relatively small 
distance D1 of the order of some tens of millimeters (it must be noted 
that for greater clarity, FIG. 1 has not been drawn to scale). 
These conditions are necessary to obtain, between the photocathode 6 and 
the input face 8 of the slab, an electrical field suited to the task of 
accelerating the electrons emitted by the photocathode 6 towards the input 
of the microchannels of the slab 7; this electrical field should be 
intense enough to limit the angular dispersion of the electrons which 
tends to reduce the spatial dispersion of the IIR tube. 
Furthermore, the distance D1 between the photocathode 6 and the slab 7 
should be maintained uniformly to obtain high image resolution on the 
entire field. 
Under these conditions, the accurate positioning of the primary screen 15 
and, especially, of the photocathode 6 with respect to the slab 7, is a 
lengthy and delicate operation that is made even more difficult by the low 
mechanical rigidity of the supporting plate 5 (bearing the scintillator 4) 
in order to absorb the X-radiation to the minimum extent. 
An additional complexity is provided by a difference between the expansion 
coefficients of the scintillator 4 and of its support 5. The result of 
this difference is that the primary screen 15 structure tends to get 
deformed, and that it is difficult to limit this deformation to less than 
some tens of millimeters when it takes effect over lengths close to 
several centimenters. Furthermore, if the primary screen 15 is moved away 
from the slab 7 to minimize the influence of the deformations, the result 
is an unacceptable loss of resolution. 
Now, what is sought is the industrial-scale manufacture of IIR tubes with 
proximity focusing, capable of picking up large-sized images as is the 
case with IIR tubes in which the image, formed on the output screen by the 
electrons emitted by the photocathode, results from a focusing of these 
electrons by means of an electronic optical device. In IIR tubes using 
electronic optical devices, the primary screen may commonly attain a 
diameter of up to about 50 centimeters. 
It is clear that, with such dimensions, the positioning of a primary screen 
with respect to a slab of microchannels raises serious problems. At 
present, this constitutes one of the major drawbacks of IIR tubes with 
proximity focusing. However, this type of tube has advantages as compared 
with those using an electronic optical device. Thus, for example, this 
type of tube may be much flatter than the latter type of tube (with a 
smaller distance between the primary screen and the output screen); 
furthermore, it can be made more easily to receive and form a rectangular 
image. 
SUMMARY OF THE INVENTION 
The present invention relates to image intensifier tubes wherein there is 
used, firstly, a scintillator to convert an ionizing radiation into light 
radiation or radiation close to the visible range, and wherein there is 
used, secondly, a slab of microchannels positioned in the vicinity of the 
primary screen and, more specifically, in the vicinity of the 
photocathode. The invention is aimed at enabling a relative positioning 
that is precise and reliable between the primary screen and the slab of 
microchannels, with a very small distance which may be smaller than 0.2 
millimeters. 
To this end, the invention proposes to fixedly join the primary screen and 
the slab of microchannels, by means of electrically insulating shims. The 
number and distribution of these shims are chosen notably as a function of 
the surfaces that face each other, so as to obtain the most efficient 
compromise between mechanical rigidity and minimum absorption of the 
electrons emitted by the photocathode. 
The invention therefore relates to an image intensifier tube comprising a 
primary screen, a slab of microchannels fixed in the intensifier tube, the 
primary screen comprising a scintillator borne by a supporting plate, a 
photocathode borne by the scintillator, the photocathode facing an input 
face of the slab, wherein the primary screen is fixedly to the slab by 
means of insulating shims.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 2 shows an IIR tube 20 according to the invention. The tube 20 has a 
general structure similar to that of the IIR tube shown in FIG. 1. 
However, the tube 20 differs from the one shown in FIG. 1 essentially by 
the way in which its primary screen is fastened. 
The tube 20 comprises a vacuum-tight chamber, constituted by a tube body 2 
closed at one end by an input window 3 and at the other end by an output 
window 14. This chamber contains a primary screen 19 and a slab 7 of 
microchannels positioned between the primary screen 19 and the output 
window 3. 
The primary screen 19 is formed by a thin foil or plate 5 acting as a 
support for a scintillator 4; the scintillator is constituted for example 
by a layer of caesium iodide. The supporting plate 5 is oriented towards 
the input window 3 and the scintillator 4 is oriented towards the slab 7 
of microchannels. On a face oriented towards the slab 7, the scintillator 
4 bears a fine layer of a photo-emissive material forming a photocathode 
6. 
The slab 7 of microchannels is fixed into the body 2 of the tube by means 
of fixing lugs 22, 23 which, firstly, are sealed into the body 2 which 
they cross and, secondly, are soldered to the two opposite large faces 8,9 
which respectively constitute the input face and the output face of the 
slab 7. The fastening lugs 22, 23 may thus serve, furthermore, to apply 
the potentials V2,V3 necessary for the operation of the slab 7 as .already 
explained here above. 
According to one characteristic of the invention, the primary screen 19 
rests on the input face 8 of the slab 7 of microchannels by means of one 
or more electrically insulating shims 25. The height of the shims 25 
defines the spacing between the photocathode 6 and the input face 8 of the 
slab 7, i.e. the distance D1 between these elements. 
In the non-restrictive example shown in FIG. 2, the shims 25 are glass 
beads having, for example, a diameter d2 of 100 micrometers which forms 
the height of the shims. Beads such as these are commonly available in the 
market with a small variation of diameters around the nominal value. 
Since the slab 7 of microchannels is fixed to the body 2 of the tube, it 
constitutes the support of the primary screen 19 which is kept resting on 
this screen under the thrust force exerted by one or more thrustor 
elements 26. 
The primary screen 19 is thus mechanically fixed to the slab 7 of 
microchannels, and not to the body 2 of the tube as is the case in the 
prior art. 
The thrustor elements 26 may be constituted in different ways, notably as a 
function of the modes of manufacture proper to each IIR tube. In the 
non-restrictive example of the description, these pressure devices rest on 
an internal peripheral part 27 of the input window 3, this peripheral part 
being more massive than the central part which, for its part, must absorb 
the incident X-radiation to the least possible extent. 
In the example shown in FIG. 2, these thrustor elements 26 comprise: a 
rigid spacer 28 and a spring washer 29. The spring washer 29 is placed on 
the supporting plate 5 (in a peripheral zone of this plate 5) and the 
spacer 28 is placed between the input window 3 and the spring washer 29. 
The spacers 28 have a height H that is suited to keeping the primary 
screen 19 applied to the shims 25 by means of the spring washers 29. 
Several thrustor elements such as these may be used, distributed about the 
primary screen 15. 
The first potential V1 is brought to the tube 20 by a crossing or 
lead-through element 31, to be applied to the photocathode 6, without 
thereby setting up any rigid link between the body 2 and the primary 
screen 19. The electrical link between the lead-through element 31 and the 
photocathode may be set up in different ways through the use of means that 
are simple per se. In the non-restrictive example described, this is 
obtained, firstly, by connecting the lead-through element 31 to the spring 
washer 29, by a flexible conductive wire 32, the spring washer 29 being 
itself in contact with the supporting plate 5 bearing the scintillator 
(the supporting plate 5 is then preferably made of an electrically 
conductive material); furthermore, the spring washer 29 is electrically 
connected to the photocathode 6 through a conductive layer 33, and a 
metallization layer 34 made between the scintillator 4 and the 
photocathode 6 in a peripheral zone of the primary screen 19 (this 
metallization 24 clearly does not overlap the useful central surface of 
the primary screen). 
The metallization 34 is made, for example, by vacuum evaporation of a thin 
layer (for example with a thickness of 0.1 to 1 micrometer) of chromium or 
aluminium or of another metal deposited on the periphery of the 
scintillator 4. 
This metallization 34 is then covered partially by the photocathode, in 
such a way that the electrical connection with the photocathode is set up 
while, at the same time, the most peripheral part of the metallization 34 
is kept clear. This most peripheral part of the metallization 34 is then 
covered with the conductive layer 33 which is also in contact with the 
supporting plate 5 and the spring washer or washers 29, and also with the 
edge of the scintillator 4. In fact, the conductive layer 33 may cover the 
entire perimeter of the primary screen 19, i.e. the edge of this primary 
screen, the edge on which it can be deposited simply: for example, it may 
be result from the application, by means of a brush, of a paste containing 
metal granules. Suspensions of silver granules enabling a use such as this 
are commonly available in the market. 
In the exemplary embodiment shown in FIG. 2, where the shims 25 are 
constituted by beads, these beads may be fixedly joined to the input face 
8 of the slab 7 of microchannels by bonding. The bonder used may be a 
photosetting or thermosetting bonder and may be compatible, in its set 
condition, with use under vacuum. The bonder used for this purpose may be, 
for example, the one known as Araldite, the polymerization of which is 
accelerated by heating. 
The beads or shims 25 are distributed and fixed to the input face 8 in a 
pitch p in the range of 2 centimeters for example. This can be 
accomplished in a simple way, for example by the deposition, on the input 
face 8 of the slab, of the spots of bonder with a spacing pitch p of two 
centimeters. Once the spots of bonder are deposited, the input face 8 of 
the slab are covered with a layer of glass beads and then the bonder is 
made to set by insolation or by heating. The glass beads are then 
eliminated except for those that have been in contact with a spot of 
bonder and have been consequently fixed to the slab 7 by these spots of 
bonder. The laying of these spots of bonder can be done by hand, or by 
means of automatic laying machines that are standard per se. 
Since the beads 25 are fixedly joined to the slab 7, said slab is fixed 
mechanically into the tube by means of standard techniques. 
The primary screen 19 is then placed in the slab 7 and fixed to this slab 
as explained further above through the application of pressure, at regular 
intervals, on the small glass beads or shims 25. Clearly, the primary 
screen 19 can itself be made in a conventional way. 
The diameter of the beads may be chosen as a function of the desired image 
resolution: it should be small enough for the beads not to be visible in 
the image. The pitch p of the beads is matched to the deformability of the 
primary screen 19, i.e. the greater the deformability, the smaller is this 
pitch. 
To obtain a situation where the photocathode 6 rests more evenly on the 
shims 25, it is also possible to give the primary screen a slightly 
non-plane shape, notably a concave shape (as seen from the input window 3) 
before it is fixed to the slab 7. 
FIG. 3 is a sectional view similar to that of FIG. 2, showing the primary 
screen 19 before it is fixed to the slab 7 of microchannels. 
The primary screen 19 has a slightly concave shape such that, when it is 
placed above the slab 7 before being fastened to said slab 7, it is first 
of all by its central zone 30 that it is in contact with the shims 25. By 
then providing for regular pressure on the periphery 36 of the primary 
screen 19, when it is being fixed, by means of thrustor elements 26 (shown 
in FIG. 2), a uniform pressure of the primary screen on the shims 25 is 
obtained, by bringing the elasticity of the primary screen and, 
especially, of the supporting plate 5 into play. 
A shape such as this, notably a concave shape, of the primary screen 15 may 
result from an internal mechanical tension of the primary screen 19. This 
mechanical tension may itself result from the concave shape initially 
given to the supporting plate or support 5 before the deposition of the 
scintillator 4 on this support. The coefficient of expansion of caesium 
iodide is generally higher than that of the support, and this scintillator 
is deposited hot on this support. As a result, the tension exerted by the 
scintillator 4 tends to reduce the initial concavity, and the support 5 
should be given a concavity slightly greater than the one that is finally 
necessary. It is possible, for example, to give an initial deflection that 
is close to one millimeter for a support 5 made of an aluminium alloy with 
a 0.5 millimeter thickness and a diameter of 15 to 25 centimeters. 
By thus fixing the primary screen 19 to the slab 7, the uniformity of the 
spacing between this slab 7 and the photocathode 6 depends to a greater 
extent on the diameters of the beads that constitute the shims 25 than on 
the mechanical rigidity of the support or supporting plate 5. 
Consequently, the thickness of the supporting plate 5 may be reduced so as 
to absorb the incident radiation to a smaller extent. 
It must be noted that, by giving a concave shape such as this to the 
primary screen 19, resulting from an internal mechanical tension as 
explained here above, it is possible not only to obtain the most efficient 
fastening of the primary screen but also to restrict or even cancel the 
mechanical deformations of this primary screen, during operation, caused 
by differences between the heat expansion coefficient of the scintillator 
4 and that of its support 5. This can be obtained, of course, on condition 
that the prior mechanical tension, on the one hand, and the cases of heat 
expansion, on the other, cause deformations in opposite directions. 
FIG. 4 gives a schematic view of another way of making the insulating shims 
25 which separate the photocathode 6 from the slab 7 of microchannels. 
FIG. 4 shows a partial view of the slab 7 of microchannels in a sectional 
view that is similar to that of FIG. 3, but is enlarged with respect to 
this figure. In this other version, these insulating shims (referenced 
25a) are constituted by a deposit or deposits of electrically insulating 
material, these deposits being formed by one or more layers 40 deposited 
on the input face 8 of the slab 7, between the inputs of certain channels 
12 or all of them. These deposits or shims 25a should preferably (but not 
imperatively) obstruct the channels 12 to the least possible extent. 
The deposits 25a can be obtained, for example, by a vacuum evaporation type 
of method for the deposition of an insulating material such as silica 
SiO.sub.2, alumina A1.sub.2 O.sub.3 0 or any other material compatible 
with techniques using vacuums and photocathodes. The insulator material 
may be evaporated at an incidence that is highly oblique with respect to 
the surface of the slab, so as not to overlap the wall of the channels 12 
in depth. The use of microchannels with a widened input 35 limits the 
surface area made available for the deposition of the insulator, and thus 
limits the obstruction of these channels 12. The penetration of the 
insulator material into the channels may be limited to the depth of the 
widened portion 35. 
With a method such as this, it is possible to deposit a single layer 40 of 
insulating material on the input face 8 of the slab 7. This input face is 
pierced in the part facing each channel 12. However, it is also possible 
to make several localized deposits that do not constitute a single 
interrupted layer. 
After the shims 25a are made, the slab 7 is fixed into the tube and the 
primary screen 19 is fixed to the slab 7 in a manner similar to that 
explained here above with reference to FIGS. 2 and 3. Naturally, this 
embodiment of insulating shims is applicable also when the primary screen 
19 comprises an internal mechanical tension that gives it a concave shape.