Method and device for determining the distribution of .beta.rays emerging from a surface

A method and device are disclosed for determining the distribution of the .beta. rays emerging from a surface. The device comprises an enclosure (3) filled with a gas mixture having a first preamplification chamber (6) between two grids (9 and 10), a second transfer chamber (7) between two grids (10 and (11) and a third multiplication chamber (8) between two grids (11 and 12); as well as three DC voltage sources (13, 14 15); and an impeding assembly (22, 23, 17, 18, 19, 20 and 21).

The present invention relates to the determination of the distribution of 
.beta. rays emerging from the surface of a body, particularly of the inner 
layer, containing .beta. ray emitting radio elements. 
Numerous techniques, particularly in biology, biochemistry, medicine and 
chemistry, use the determination of the distribution of .beta. rays 
emitted by the radio elements present in the bodies to be studied. 
The oldest and most widely used techniques consist in applying a 
photographic film on the .beta. rays emitting surface and to observe, 
after development, the darkening of the film caused by the .beta. rays 
received thereby. But this technique requires very long exposure times, 
several days, even several weeks or even several months. 
To speed up the measurements, a .beta. ray detector may be used comprising 
an avalanche chamber containing a gas between two electrodes formed by 
parallel grids, one of which forms the anode and the other the cathode 
against which the surface to be observed is applied. In this chamber the 
.beta. rays emitted by the surface release electrons by ionization of the 
gas between the electrodes and form avalanches which ionize some atoms of 
the gas. The atoms thus ionized produce a light emission. After brightness 
amplification by brightness amplifiers (necessary because of its low 
intensity), the light emission is detected by video cameras, particularly 
of the CCD (charge coupling device) type, which makes it possible to 
localize in the gas volume of the chamber the points of entry of the 
.beta. rays emitted by the radio elements of the surface to be observed. 
Such a technique is described in a document CERN-EP/88-165 of Nov. 25, 1988 
by Messrs. CHARPAK, DOMINIK and ZAGANIDIS. 
Although this technique makes it possible to determine the distribution of 
.beta. rays coming from a surface more rapidly than the photographic 
method, on the other hand it requires costly equipment, particularly the 
costs of the high price of brightness amplifiers and of CCD type cameras. 
Also, the sensitivity of the CCD type cameras is not used to a maximum, 
because these cameras are particularly sensitive to visible light, whereas 
the light emitted by the avalanches of the device described in the above 
article is generally not concentrated in the visible spectral range and is 
of relatively low intensity which requires a light amplifier. 
It is further known that in the chamber containing a gas between two 
parallel electrode grids between which a DC voltage A is applied, there 
exists a threshold value Vs of this voltage V (depending on the gas 
filling the chamber) from which, following ionization of the gas under the 
effect of a passage of ionizing particle, a spark is produced and this 
spark short-circuits the two electrodes. 
Such a phenomenon may be used for constructing a detector or counter for 
detecting highly ionizing particles, such as .alpha. particles. Each 
particle causes a spark which short-circuits the electrode grids whilst 
stopping the detector for a short instance; then the latter is again ready 
for counting a new particle. 
On the other hand, such a type of detector is very difficult to perfect for 
detecting the .beta. particles which have a low ionizing power. It is very 
difficult to adjust the DC voltage V to a threshold Vs' such that only the 
.beta. rays produce sparks which are detected, without reproduction of a 
spark in the ionizing chamber by the cosmic rays or by the electrons 
emitted by the electrode forming the cathode. 
It has however been proposed to construct avalanche chambers, having 
several stages, so as to form .beta. ray (and possibly slow neutron) 
detectors. For examples see the following articles: 
PETERSON, CHARPAK, MELCHART and SAULI in Nuclear Instruments and Methods 
176 (1980), p. 239-244; and 
CHARPAK, MELCHART, PETERSON and SAULI in IEEE Transactions on Nuclear 
Sciences, Vol. NS-28 No. 1, Feb. 1989, p. 849-851. 
In this type of detector with a multistage structure, the following are 
provided: 
a first preamplification stage which comprises a first chamber between two 
parallel electrode grids--a first grid of which forms the cathode against 
which the .beta. ray emitting surface to be examined is applied and a 
second grid, biased positively with respect to the first one--between 
which a DC voltage is applied, capable of providing low gain 
preamplification, of the order of 10.sup.3 to 10.sup.4 ; 
a second transfer stage which comprises a second chamber defined by 
auxiliary electrode grids; and 
a third multiplication stage which comprises a third chamber in which the 
electrons, which have passed through the second said grid of the first 
stage of the transfer stage, are again multiplied between two parallel 
electrode grids and give rise to avalanches which may be localized by 
different electronic means. 
If the electric field between the two grids of the chamber of the third 
stage is such that the ions produced in the avalanche cannot return to the 
chamber of the first stage through the transfer stage because of auxiliary 
electrode grids subject to an appropriate pulse field, for a certain time 
the transfer of ions to the input cathode of the first chamber is 
prevented. Such a result is desired as such an ion might risk causing a 
new emission of electrons by this cathode and so a new avalanche in the 
third chamber, which would cause a permanent self-sustained discharge. 
Such an arrangement with three stages or chambers, with pulsed electrode 
grids in the second stage, has the drawback of requiring adjusting means. 
Such adjusting means are difficult to control, for avoiding the return to 
the first chamber of ions resulting from the avalanches. This makes it 
possible however to increase the gain of the third chamber to a level such 
that visible sparks are produced there, but with the drawback that the 
sparks thus produced in the third chamber are relatively extended 
filaments. The observation of such filaments does not make possible a 
precise localization of the points at which the .beta. rays emerge from 
the emissives to be observed, which is applied against the input cathode. 
The object of the present invention is to overcome the above drawbacks of 
prior techniques by providing a simple method and device for very 
accurately determining the distribution of .beta. rays emitted by a 
surface, advantageously by means of optical reading, without complicated 
adjustment means and without using a light detector. 
For this purpose the invention provides: 
on the one hand, coupling by electric impedances (capacitors, resistances 
and/or self-inductances) between stages of the detector; and 
on the other hand, an appropriate gas filling allowing us to obtain, in the 
multiplication stage, sparks in the form of localized brilliant points. 
The object of the invention is first of all a method for determining the 
distribution of the .beta. rays emerging from a surface, which methods 
consists in causing these .beta. rays to pass through: 
a first substantial proportional preamplification stage, formed by a first 
chamber defined by two electrode grids, parallel to each other, between 
which the DC voltage is applied, in which the electrode grids forming the 
cathode is that against which said .beta. ray emitter surface is applied, 
a second transfer stage, formed by a second chamber disposed between that 
one of the two above electrode grids which forms the anode and a third 
electrode grid parallel to the latter and biased positively with respect 
thereto; and 
a third multiplication stage, formed by a third chamber disposed between 
this third electrode grid and a fourth electrode grid biased anodically by 
a DC voltage relatively to the third grid; 
these three chambers being filled with at least one gas; 
and which is characterized in that 
on the one hand, impedance coupling is provided between these four 
electrode grids, and 
on the other hand, said at least one gas is chosen so that, in an automatic 
way through said impedance coupling, the sparks produced by multiplication 
in the third chamber cannot, under the effect of the return of ions 
through the transfer stage to the cathode of the first chamber, cause a 
permanent discharge, and in the sparks are in the form of brilliant 
localized points on the surface of the cathode thereof. 
In the first stage, under the effect of the electric field created between 
these two electrode grids, an avalanche of electrons is produced. This 
avalanche is initiated preferably by the ionization electrons released in 
the gas contained in this first stage, close to the cathode grid thereof, 
by the .beta. rays which penetrate into this stage. In the second stage, 
the electrons of said avalanche are transferred from the first to the 
third stage. 
Another object of the invention is to provide a device for implementing 
said method, comprising: 
an envelope filled with at least one gas and which comprises 
a first proportional preamplification chamber, defined by a first electrode 
grid and a second electrode grid permanent to the first one; 
a second transfer chamber, defined by said second electrode grid and a 
third electrode grid parallel thereto; and 
a third electrode grid parallel thereto; and 
a third multiplication chamber, defined by the third electrode grid and a 
fourth electrode grid parallel thereto; 
a first DC voltage source, connected between the first and second electrode 
grid, the first one being biased so as to form with respect to the second 
a cathode for receiving substantially against it the .beta. ray emitting 
surface to be examined; 
a second DC voltage source for biasing the third electrode grid positively 
with respect to the second electrode grid; and 
a third DC voltage source for biasing the fourth electrode grid positively 
with respect to the third electrode grid; 
and characterized 
in that it further comprises a set of impedances, formed by capacitors, 
resistor and/or self inductance network, connected to said electrode 
grids, and 
in that said at least one gas is formed by at least a noble gas 
(particularly, argon, neon and/or helium) to which is added a small 
proportion of at least one organic gas (particularly triethylamine, 
tetramethylpentane and/or methylal). 
It may be noted that the method and device of the invention make it 
possible to neutralize all the delayed secondary electron sources ejected 
by the cathode of the first stage. 
The light which leaves the device, in fact produced by the bright localized 
points, can be observed by a camera (particularly a small aperture camera 
or the CCD type camera) or even with the naked eye. 
The video camera may be connected directly to a computer. 
Such a detector, which is relatively inexpensive and requires neither 
complicated adjustment, nor expensive observation equipment, has numerous 
applications, particularly: 
formation of the image of the .beta. rays emitted by an emitting surface 
applied against the input face of the first chamber; 
the counting, with a very low background noise, of the .beta. rays emitted 
by the samples comprising radio elements, even if their radioactivity is 
very low; in fact, the background noise of the detector is about bright 
light point or spot per minute and per cm.sup.2, which makes it possible 
to detect practically without a noise the .beta. rays coming from a zone 
of 1 square mm.

On FIG. 1, at 1 has been shown a body comprising radio elements and having 
a substantially flat surface 2 which emits .beta. rays (references .beta. 
in the figure) whose distribution is to be determined. 
The detector as such comprises an envelope 3 with an input face 4 
transparent to the .beta. rays. Input face 4 is formed for example by an 
aluminized "MYLAR" foil 6 microns thick, against which surface 2 is 
applied. The detector also includes an output face 5 formed by an optical 
window allowing light in the visible range to pass through. 
This enclosure comprises three chambers 6, 7 and 8 forming three successive 
stages I, II and III. 
The first chamber 6 of stage I is a chamber for preamplification of the 
electrons e formed on the entry into chamber 3 of .beta. rays having 
passed through the input face 2, with a gain of about 10.sup.3 to 
10.sup.4. Chamber 6 is defined by a first electrode grid 9 and a second 
electrode grid 10 parallel to, separated by a gap of 4.3 mm. 
The second chamber 7 of stage II forms a transfer chamber for the 
multiplied electrons e and is defined by the second electrode grid 10 and 
a third electrode grid 11 parallel thereto, spaced apart by 12 mm. 
The third chamber 8 forming the third stage III is a multiplication or 
ionization chamber in which the transferred electrons e' cause, at the 
level of the third grid 11, and particularly at the intersection points of 
the wires forming this grid, sparks E which have the form of bright light 
points. The light L coming from these light points propagates through this 
chamber 8 and passes through the optical window forming the output face 5. 
This chamber 8 is defined by the third electrode grid 11 and the fourth 
electrode grid 12 parallel thereto, spaced apart by 5 mm. 
Three DC volt sources 13, 14 and 15 apply, to points 13a, 14a and 15a 
respectively, positive DC voltages +V.sub.1, +V.sub.2 and +V.sub.3 
respectively with respect to ground, to which points 16a and 16b are also 
connected. 
Resistors 17, 18, 19, 20 and 21, having values of R.sub.1, R.sub.2, 
R.sub.3, R.sub.4 and R.sub.5 respectively, are connected between point 16a 
and electrode grid 9, point 13a and electrode grid 10, point 16b and point 
16c, point 14a and electrode grid 11, and the point 15a and the electrode 
grid 12, respectively. Two capacitors 22 and 23 of capacities C.sub.1 and 
C.sub.2 respectively, are connected the first one between point 16c and 
grid 10 and the second between point 16c and grid 12. The assembly 17, 18, 
19, 20, 21, 22, 23 forms an impedance regulation device automatically 
preventing the production and amplification of secondary electrons 
produced in particular by the return to the cathode grid 9 of the positive 
ions emitted by a spark E in the third chamber 8. 
Finally, the envelope 3 is filled, substantially at atmospheric pressure, 
with a gas mixture comprising a noble gas, such as argon, neon or helium, 
and an organic gas, such as triethylamine, tetramethylpentane or methylal. 
In this example a mixture of argon and 2% of triethylamine has in 
particular been used. The following values for the voltage, resistors and 
capacitors, respectively involved, (in megohms and picofarads), are given 
by way of non-limiting examples: 
______________________________________ 
V.sub.1 
V.sub.2 
V.sub.3 R.sub.1 
R.sub.2 
R.sub.3 
R.sub.4 
R.sub.5 
C.sub.1 
C.sub.2 
______________________________________ 
3125 4000 7325 2 100 2 10 100 470 470. 
______________________________________ 
The electrode grids, particularly the third grid at the level of which the 
sparks are produced in the form of bright localized points, are formed by 
metal wires disposed in two directions perpendicular to each other. The 
wires have a diameter of 10 to 200 microns, for example about 50 microns. 
Two successive wires are spaced apart from each other by a distance of 100 
to 2000 microns, for example about 500 microns, in each direction. 
In the figure, there has also been shown the direction of the electric 
fields E.sub.1, E.sub.2 and E.sub.3 produced by the assembly of the 
voltage sources, resistors and capacitors. 
The light L', essentially in the visible range, which passes through the 
output face 5, can be observed by the naked eye or by means of a camera K. 
Camera K can be, for example, a small aperture photographic camera or a 
CCD type video camera, without requiring a light amplifier. 
The operation of the detector, which has just been described with reference 
to the two figures is the following: 
A .beta. ray emerging from surface 2 of the radio-active body 1 passes 
through the input face 4 and appears in enclosure 3, at the level of the 
first electrode grid 9 forming the cathode, in the form of one or more 
ionization electrons e which are amplified or multiplied in the first 
chamber 6 to give a few thousands electrons e' which move towards the 
electrode grid 10 forming the anode, under the first electric field 
E.sub.1. 
It will be noted that it is the electron e closest to cathode 9, namely the 
one from the .beta. ray at its input, which gives the largest avalanche or 
cloud electrons e' and which will produce the succession of the pheonomena 
described hereafter. This is what will make it possible to localize 
accurately the .beta. ray arriving from the surface 2 and passing through 
the face 4 at the level where it strikes the electrode grid 9. 
A fraction of the electrons e' (about 10%) then passes through the second 
electrode grid 10 and moves, under the effect of the electric field 
E.sub.2, towards the third electrode grid 11 while passing through the 
transfer chamber 7. 
Finally, the electrons e' pass through grid 11 and, after a second 
multiplication at the level thereof and more precisely at the intersection 
of the wires forming this grid, cause discharges or sparks in the form of 
bright light points E. It will be appreciated that in prior spark 
detectors, a spark was obtained formed by a filament connecting together 
the two electrode grids defined in the third chamber. 
It should be noted that this light point or cathode spot E is situated 
exactly in a line (in the direction perpendicular to the parallel 
electrode grids) with the impact on the cathode forming electrode grid g 
of all the .beta. ray be emitted by surface 2 and represented by electron 
e, which makes it possible to accurately localize this impact. Furthermore 
the light L emitted is in the visible range, and easy to observe with the 
naked eye or to photograph with an ordinary film camera and/or to film 
with a video camera, for example of CCD type, possibly connected to a 
computer (not shown). 
The avalanche forming light point E in the third chamber 8 causes an 
electric pulse between the electrode grids 11 and 12. This pulse, through 
the impedance networks 17, 18, 19, 20, 21, 22, 23, reduces the gain in the 
first chamber 6 between electrode grids 9 and 10 by applying a negative 
voltage pulse to the electrode grid 10. This prevents amplification of 
secondary electrons produced by the impact on the electrode grid 9 of 
positive ions generated by spark E, and so ultimately emission of 
electrons by this cathode forming electrode grid. 
Possiblly, if the .beta. rays emitted by surface 2 of body 1 are not very 
penetrating (for example if body 1 comprises tritium as radio element), it 
is possible to make this surface 2 conductive by evaporating thereon a 
thin conducting layer and use this surface 2 for forming the assembly 
constituted by the input face 4 and cathode 9 of the detector. 
In case .beta. rays emitting surface 2 is introduced into the enclosure so 
as to operate as a cathode, as previously mentioned, it is appropriate to 
make it conductive through a thin conducting coating layer 100, such as 
gold for example. 
It is nevertheless possible to overcome the drawback of such an operation 
by placing said emitting surface 2 back at cathode 90, as shown at FIG. 2, 
with cathode 90 being transparent to electrons. 
In such a case, as shown in FIG. 3, cathode 90 may consist of parallel 
wires of diameter comprised between 20 and 60 .mu.m, with two adjacent 
wires being spaced apart from one another from 300 to 600 .mu.m. Cathode 
90 may consist of knitted wires constituting a grid or of an 
electro-eroded grid. 
Preferably the thin conducting layer 100 of body 1 is placed spaced apart 
back from cathode 90 with a distance d comprised between 1 mm and 5 mm. A 
bias electric field E0 is generated with E0.ltoreq.0.2.times.E1 between 
the thin conducting layer 100 and cathode 90. Bias electric field E0 is of 
feeble magnitude with respect to electric field E1 established into first 
proportional preamplification chamber I. 
Most of the electrons generated through ionization in between body 1 and 
cathode 90 are transferred into the first proportional preamplification 
chamber I along the lines of force of the electric field, and penetrate 
into the first proportional preamplification chamber I with an accuracy of 
99%. 
More particularly in the case where the emitting radio element contained in 
body 1 is tritium, the electrons of which have a penetration length about 
100 .mu.m into a mixture essentially made of xenon under atmospheric 
pressure, such a penetration length is to be reduced to a few tens microns 
under higher pressure. Thus, electrons produced by ionisation are 
generated very near the thin conducting layer 100, and after their 
transfer along the lines of force they come to first proportional 
preamplification chamber I. They thereby constitute true images of .beta. 
rays emitted by the thin conducting layer 100 of body 1, with a distortion 
corresponding to the electric field E0 lines of force paths. Such paths 
are easy to compute with reference to the electrostatic known relations. 
Corresponding computing thus allows an image cathode without distortion of 
the actual distribution of .beta. rays as emitted by body 1 to be 
obtained. 
As shown at FIG. 2 in a non-limiting way, in order to generate the electric 
field E0, the emitting surface 2 can be coated with a thin conducting 
coating 100, made of gold, thus allowing a potential V0 to be applied with 
respect to cathode 90. As an example potential, V0 may have a value of 300 
Volts. 
In operation, emitting surface of body 1 is placed spaced apart from 
cathode 90 with a distance d comprised between 1 and 5 mm. Electric field 
E0 is thereafter generated between thin conducting layer 100 and cathode 
90 so as to establish a transfer field for the .beta. rays electrons 
emitted towards the first proportional preamplification chamber I. 
For such a purpose, thin coating 100 can be set to the ground potential, 
cathode 90 being set to a potential of 300 V through a DC generator 22. 
Potential of subsequent grids 10, 11, 12 is this shifted accordingly to 
corresponding values V1+300 Volts; V2+300 Volts; and V3+300 Volts. 
With respect to the utilization of naked thin conducting coating 100 as a 
cathode within the first proportional preamplification chamber, one of the 
most important advantages in utilizing cathode 90 appears to consist of 
the fact that the device is no longer sensitive to roughness and 
asperities of the emitting surface 2. 
The embodiment of the device of the invention as shown at FIG. 2 is more 
particularly directed to bodies or samples 1 of great dimensions, in case 
of measurement and establishment of cross section views of entire animals 
for example. 
As is evident, the invention is in no way limited to the modes of 
application and embodiments which have been more especially envisaged; it 
embraces, on the contrary, all variants thereof.