US3717766A - Spark chamber radiation-detecting device - Google Patents

Abstract

A DEVICE FOR DETECTING AND VISUALIZING THE SPATIAL DISTRIBUTION OF RADIATION COMPRISES THREE PARALLEL ELECTRODES DISPOSED WITHIN AN ENCLOSURE WHICH CONTAINS AN IONIZABLE GAS MIXTURE AND CONSISTING OF A GRID BETWEEN A CATHODE WHICH IS TRANSPARENT TO SAID RADIATION AND AN ANODE. THE GRID DEFINES WITH THE CATHODE A DETECTION SPACE IN WHICH THE INCIDENT RADIATION CAUSES IONIZATION OF THE GAS MIXTURE AND DEFINES WITH THE ANODE A SPACE FOR THE MULTIPLICATION OF ELECTRONS WHICH ARE PRODUCED IN THE DETECTION SPACE AND PASS THROUGH THE GRID, THEREBY INITIATING THE PRODUCTION OF SPARKS. THE DETECTING DEVICE FURTHER COMPRISES AN EXTERNAL CIRCUIT WHICH PROVIDES A CONNECTION BETWEEN THE ANODE AND THE GRID AND INCLUDES MEANS WHICH ARE TRIGGERED BY THE SPARKS PRODUCED WITHIN THE MULTIPLICATION SPACE IN ORDER TO SHORT-CIRCUIT SAID MULTIPLICATION SPACE TEMPORARILY AT THE TIME OF APPEARANCE OF EACH SPARK. THE ANODE IS ESSENTIALLY PROVIDED WITH TWO CONDUCTIVE COATINGS WHICH ARE ELECTRICALLY CONNECTED TO EACH OTHER RESPECTIVELY ON THE TWO FACES OF AN OPTICALLY TRANSPARENT SUPPORT PLATE. ONE OF THE TWO COATINGS HAS HIGH SURFACE RESISTANCE AND LIMITS THE ELECTRON MULTIPLICATION SPACE WHILST THE OTHER COATING HAS LOW SURFACE RESISTANCE.

D R A W I N G

Description

Feb; 20; 1973' c. ALLARD AL 3,717,766

SPARK CHAMBER RADIATION-DETECTING DEVICE Filed Oct. 22, 1970' 8 ELEC TRON/I 4: U/ 7' c4 0 e 5e fis wmgmmmmz United States Patent France Filed Oct. 22, 1970, Ser. No. 83,060 Claims priority, application France, Oct. 30, 1969,

37, Int. (:1. G01t 1/18 U.S. Cl. 25083.6 R 3 Claims ABSTRACT OF THE DISCLOSURE A device for detecting and visualizing the spatial distribution of radiation comprises three parallel electrodes disposed within an enclosure which contains an ionizable gas mixture and consisting of a grid between a cathode which is transparent to said radiation and an anode. The grid defines with the cathode a detection Space in whlch the incident radiation causes ionization of the gas mixture and defines with the anode a space for the multiplication of electrons which are produced in the detection space and pass through the grid, thereby initiating'the production of sparks. The detecting device further comprises an external circuit which provides a connection between the anode and the grid and includes means which are triggered by the sparks produced within the multiplication space in order to short-circuit said multiplication space temporarily at the time of appearance of each spark. The anode is essentially provided with two conductive coatings which are electrically connected to each other respectively on the two faces of an optically transparent support plate. One of the two coatings has high surface resistance and limits the electron multiplication space Whilst the other coating has low surface resistance.

The present invention relates to a radiation-detecting device of the gas type which can be employed especially as a spark chamber for permitting visualization of the spatial distribution of X- or v-radiation. The invention is more particularly concerned with a device of the threeelectrode type such as those employed in the field of medicine in order to produce .scanning records or maps showing the distribution of a radioactive tracer in a given organ. However, it is wholly apparent that this application is not at all limitative.

Three-electrode radiation-detecting devices of this type comprise within an enclosure containing an ionizable gas mixture and successively in the direction of radiations to be detected a cathode which is transparent to said radiations, a grid and an anode as well as means for biassing said electrodes in order that the grid should define with the cathode a detection space in which the incident radiation causes ionization of the gas mixture and that it should define with the anode a space for the multiplication of electrons which are produced within the detection space and pass through the grid. Within the multiplication space, localized electron avalanches thus take place at points defined by the path of primary electrons which pass through the grid and these avalanches result in the production of sparks if the voltage between grid and anode is sufiiciently high. By positioning a photographic film in such a manner as to produce an image of the sparks thereon, records of the type mentioned above are obtained. However, and in spite of the expression spark chamber which is commonly employed to designate them, these devices can also be utilized without production of sparks proper, in which case the spatial information supplied by the electron avalanches is processed by means of a light amplifier.

Since the appearance of the first three-electrode spark chambers, research has constantly been directed towards a reduction in the dead time, namely the period which must necessarily elapse between two consecutive sparks in order that these latter should be effectively generated by ionization of the gas between cathode and grid, the result achieved at the same time being to lengthen the service life of the chamber. The life of a spark chamber is essentially a function of the deterioration on the one hand of the anode and on the other hand of the filling gas. To this end, it has been proposed to incorporate with the spark chamber an external electronic circuit having a fast response which makes it possible by temporarily short-circuiting the multiplication space at the time of production of each spark to reduce the energy which is discharged in the sparks inasmuch as these latter are the chief cause of deterioration both of the anode and of the filling gas.

The present applicant has already described different alternative forms of the aforesaid external circuit which is connected to the anode and to the grid and comprises means triggered by the sparks produced within the multiplication space in order to short-circuit said multiplication space temporarily at the time of production of each spark. In particular, a description of this circuit already appears in the U.S. application Ser. No. 786,339 filed Dec. 23, 1968, now Pat. No. 3,612,880 for Method of Spark Control and Systems for Utilization of Said Method in Spark Chambers.

In the forms of construction which were described in the cited literature, steps were taken at the same time to increase the surface resistance of the conductive metallic coating which constitutes the anode. The low current which passes in the spark nevertheless produces a powerful signal at the grid of the spark chamber and this signal serves to initiate closure of the external short-circuit in order to discharge the greater part of the anode-grid capacitance outside the spark.

This invention is essentially intended to permit a further reduction of the energy expended by each spark in the filling gas of the multiplication space in order that the life of the spark chamber may consequently be increased while reducing the dead time.

Accordingly, the invention proposes a detecting device of the type defined above which can be employed in particular in spark chambers for visualization of the spatial distribution of radiation and which comprises within an enclosure containing an ionizable gas mixture three parallel electrodes constituted by a grid between a cathode which is transparent to said radiation and an anode, said grid being such as to define with said cathode a detection space in which the incident radiation causes ionization of said gas mixture and with said anode a space for the multiplication of electrons which are generated in said detection space and which pass through the grid in order that said electron multiplication should give rise to the production of sparks. The detecting device further comprises an external circuit which provides a connection between the anode and the grid and includes means which are triggered by the sparks produced within the multiplication space in order to short-circuit said multiplication space temporarily at the time of appearance of each spark. Said device is characterized in that the anode is provided with two conductive coatings which are electrically connected to each other respectively on the two faces of a same optically transparent support plate, a first coating being endowed with high surface resistance and limiting said multiplication space whilst a second coating has low surface resistance.

The two coatings may in particular be electrically connected to each other by any means on the periphery of the support plate which is preferably made of glass, whether said means consist of a simple connecting wire. for example, or of a number of wires distributed around the plate or alternatively an annular ring placed around the lateral periphery of the plate.

Under conditions of direct current supply, these two coatings behave as one and the same electrode. However, in the device according to the invention, the signal which is applied to the anode by the external short-circuit after the appearance of a spark is sufiicient to produce a capacitive effect between the coating which possesses good con ductivity and in which the imposed potential is established instantaneously and the coating which possesses low conductivity (that is to say the above-mentioned coating which has high surface resistance). The voltage of lowconductivity coating then drops sufficiently to quench the spark. This immediate interruption of the spark therefore makes it possible to produce a further increase in that portion of the energy which passes through the external short-circuit. The use of photographic films having high sensitivity nevertheless permits utilization of the sparks from the instant of ignition in order to produce cartographic records, for example.

A further advantage of the device according to the present invention lies in the removal of problems which are concerned especially with adjustment of surface resistance of the anode and which, in the case of conventional devices, arose from the need to obtain sparks exhibiting uniform luminosity over the entire surface of the anode from the center to the periphery. The reason for this is that the spark discharge involves only a very small surface area of the electrodes.

Further properties and advantages of the invention will become apparent from the description which now follows below. The description relates to one particular embodiment of the device which is illustrated in the single accompanying figure and given by way of example without any limitation being implied. In the case under consideration, the device is employed as a spark chamber for the visualization of the spatial distribution of X- or 'y-radiation.

As shown in the figure, the spark chamber as described is essentially constituted by a cylindrical enclosure 1 which is filled with an ionizable gas mixture consisting of a rare gas doped with an organic compound in vapor phase such as methylal or diethylamine. On the bottom face, the enclosure is fitted with a collimator 2, the assembly being oriented so that said collimator should be disposed on the side nearest the radiations to be detected. The top face is formed of transparent material in order to permit viewing of the sparks produced within the chamber and recording of these latter by means of a camera.

Three fiat electrodes are disposed in parallel relation within the assembly 1, namely successively in the direction of radiation: a cathode 3, grid 4, anode 5. The cathode is a disc of aluminum or beryllium which limits the enclosure 1 and is transparent to radiation. The grid is constituted by a stainless steel screen. The anode is transparent in order to permit observation of the sparks.

The grid 4 and the anode 5 are connected to voltage sources which serve to apply a bias thereto with respect to the cathode, respectively at a lower and a higher voltage. There then appears within the so-called detection space 6 which is formed between the cathode 3 and the grid 4 a relatively weak electric field in which the radiation is detected mainly by photoelectric effect on the atoms of the gas whereas a strong electric'field prevails between the grid 4 and the anode 5 within the so-called multiplication space 7, with the result that those primary electrons which are produced within the detection space and pass through the grid are subjected within said space 7 to electron multiplication until a spark is produced. Each spark is localized immediately above the trace of the pri-v mary electron which has given rise to the spark.

The device further comprises a circuit which is located outside the enclosure and is in parallel with the grid-anode multiplication space. This external circuit consists of an electronic assembly 8 which serves to close the circuit for a short time interval, closure being initiated by the signal produced at the grid at the time of appearance of a spark. The electronic assembly has not been shown in detail in the figure but can be in accordance with either of the descriptions contained in the prior patents granted to the present applicant, in particular in the US. application Ser. No. 786,339.

As was the case with the spark chambers which were previously described by the present applicant, the external circuit together with the electronic assembly which serves to close said circuit temporarily at the time of appearance of each spark permits the possibility of limiting the discharge produced within the interior of the multiplication space by short-circuiting this latter. On the other hand, the chamber which is described differs from chambers of the prior art in the design of the anode which permits a further reduction of energy expended in the spark.

In this design, the anode 5 consists of a transparent support plate which is made or glass and provided with two conductive coatings of doped tin oxide. The coating 10 which is deposited on the bottom face is a so-called lowconductivity coating having a relatively high surface resistance; this coating limits the multiplication space and is similar to the single coating of spark chambers of the prior art. The coating 11 which is deposited on the opposite face of the transparent plate is, on the contrary, a highconductivity coating which has a low surface resistance. These two coatings are electrically connected to each other at one or a number of points along the edge of the glass plate.

During operation, the high-conductivity coating 11 is capable of transmitting a voltage established by the external circuit more rapidly than the low-conductivity coating 10. In consequence, each time a spark is produced within the multiplication space, the temporary closure of the external short-circuit causes the appearance of a voltage signal at the anode and the voltage which is immediately imposed on the high-conductivity coating 11 p oduces a reduction in the voltage of the low-conductivity coating 10 by virtue of a capacitive effect. This temporary voltage drop is sufficient to quench the spark and can attain one-half the value of the anode-bias high voltage.

In the spark chamber which has thus been described, the fact that the spark is extinguished within a very short time interval makes it possible to discharge in the Spark only a small proportion of the energy expended whereas the greater part of this energy passes through the external circuit. Moreover, the discharge calls for the presence of only one capacitive element having a small surface area around the point at which the spark is produced so that variations in luminosity as a function of the location of the sparks on the anode surface are practically non-existent.

By way of example, the spark chamber in accordance with the design illustrated in the accompanying figure and described in the foregoing had the following characteristics in one particular embodiment:

useful diameter of the electrodes: 15 cm.

thickness of the detection space: 24 mm.

thickness of the multiplication space: 2 mm.

filling gas mixture: xenon 720 torr, diethylamine 46 torr.

thickness of the anode (glass plate) 6 mm.

surface resistance of the high-conductivity coating: approx.

1009 square.

resistance of the semi-conducting coating: 11K!) square.

The external circuits comprised an avalanche transistor and a vacuum tube which is capable of withstanding 10 kv. and of delivering 10 A. instantaneously. The time which elapses between the signal produced at the grid by a spark and the passage of the current within the vacuum tube f the short-circuit is of the order of 27 nsecs.

The chamber as thus constructed has been employed for visualizing the distribution of X- or 'y-radiations with photographic recording on a Polaroid film. The sparks can be observed by the naked eye only after dark adaptation but can be perceived individually by recording on a film having a sensitivity of 3000 ASA with a camera-lens aperture of f/2.5. However, the luminosity of the spark can be increased by introducing a time-delay in the triggering of the vacuum tube which closes the short-circuit. In order to increase the count rate by reduction of the dea time, the value of the electric resistance which connects the anode to the high-voltage source can be reduced to 1M9 instead of 10M!) as was the case in designs of the prior art. The chamber is then capable of recording over 2000 sparks per second.

It is readily apparent that the invention is not limited in any sense to the embodiment as hereinabove described with the reference to the accompanying figure but extends on the contrary to all alternative forms. In particular, the device which is constructed in accordance with the invention with an anode having two conductive coatings as de scribed in the foregoing can also be employed in the avalanche condition. In this case the device permits a greater degree of operational safety and the sparks produced cause lower disturbance.

What is claimed is:

1. A radiation-detecting device especially for visualization of the spatial distribution of radiation which comprises within an enclosure containing an ionizable gas mixture three parallel electrodes constituted by a grid between a cathode transparent to said radiation and an anode, said grid defining with said cathode a detection space in which the incident radiation causes ionization of said gas mixture and defining with said anode a space for the multiplication of electronis generated in said detection space and passing through the grid in order that said electron multiplication cause production of sparks, an external circuit including a connection between the anode and the grid and further including means triggered by the sparks produced Within the multiplication space to short-circuit said multiplication space temporarily at the time of appearance of each spark, said anode having two conductive coatings electrically connected to each other and located respectively on the two faces of an optically transparent support plate, one of said coating having high surface resistance and limiting said multiplication space and the other of said coatings having low surface resistance.

2. A device according to claim 1, said one of said coatings having a surface resistance within the range of 6 to 30KQ square.

3. A device according to claim 1, said other of said surface coatings having a surface resistance which is lower than 3009 square.

References Cited UNITED STATES PATENTS 3,461,293 8/1969 Horwitz 25083.6 R 3,449,573 6/1969 Lansiart et a1 250-83.6 R

ARCHIE R. B'ORCHELT, Primary Examiner US. Cl. X.R.

Images