IL26894A - Amplification of gas-detector images - Google Patents

Description

711011 ]Π3 ΤϊΙΠΙ"! 'ΊΊ PATENT ATTORNEYS O'Q IDE) 1 □ T 11) ^ PATENTS AND DESIGNS ORDINANCE SPECIFICATION TA '"» »Λ'? τρηηο m^apnon nia sn ® mam I/We COMMISSARIAT A L'EHERGIE ATOMIQUE, of 29, rue de la FldeVation, Paris 15e, Prance do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in. and by the following statement :- This invention relates to a novel method of utilization of a gas detector which permits visual detection of β particles as well as X-ray photons or γ-ray photons and to a device for the practical application of said method.

A detector of known type which is described in Patent specification No. 23354 ffifciu Patont Application ac filod by tho prooont A li diib on April I th8 196! undor tho title "Par-tiolo and as illustrated in Fig„ 1 comprises a leak- tight chamber formed of a cylindrical casing 2, the top end of which is closed by a transparent glass plate 4 and the bottom end of which is constituted by a thin metallic cathode 6 such as a strip of aluminum foil for example „ Said thin strip is supported by a plate 10 through which are formed parallel apertures and which performs the function of collimato 0 The plate referred-to can be made either of lead or of stainless steel „ An auxiliary electrode in the form of a fine- mesh metallic grid 12 and a transparent anode 14 having a conducting surface (which may also be constituted by a fine-mesh grid or formed of conductive glass, for example) are placed within the gas-filled chamber above the cathode and parallel thereto.

If the detector is filled with an inert gas, it is not possible to obtain the image of a radioactive source On the other hand, if there is added a suitable quantity of organic vapor which absorbs the ultraviolet radiations of the gas, it is possible to produce discharges and even sparks in. the presence of said source provided that the value of the auxiliary-electrode/anode voltage is judiciously determined. time a particle is detected by the detector. But the production of sparks results in much greater destruction of these molecules and consequently in' a limitation of the service life of gas detectors which operate on the spark discharge principle , Further reference will now be made to the triggering process which results in the appearance of spark discharges . When X-ray photons or γ-ray photons are transmitted by the collimator 10 and reach the cathode 6, they give rise to primary ionizing electrons between the cathode and the gridc The secondary electrons which are produced pass through the grid and penetrate into the field which exists between this electrode and the anode, said field being of substantially greater density than the field which exists between the cathode and the grido In the field just referred-to, the electrons initiate the process of cumulative ionization or electron avalanche which usually results in the formation of a spark and in the visualization of the particle .

When a spark is produced between the grid and the anode, the interelectrode capacitance which is a function of the electrode dimensions accordingly discharges 5 the value of resistance is so determined that the time constant of the recharging circuit is sufficiently high to make it unlikely that a further spark will again appear at the same point .

This recharging process consequently has a given time duration which corresponds to a relatively long dead time, thereby slowing down the triggering rate of the detecto „ The present invention proposes a method of the detectors with a view to obtaining images caused by the formation of the electron avalanches aforesaid in such a manner as to circumvent the disadvantages which have been briefly outlined above and which arise from the formation of sparks The method in accordance with the invention for the utilization of a gas detector of known type for obtaining images resulting from the formation of electron avalanches consists in adjusting the voltage developed between the auxiliary electrode and the anode to the maximum permissible value for the prevention of spark formation under the action of β particles as well as X-ray photons or γ-ray photons, in amplifying the images of glow discharges caused by electron avalanches, then in recording the resultant images.

The invention is also directed to an assembly for the practical application of the above-mentioned method, characterized in that it comprises a gas detector comprising three electrodes, an image amplifier of known type which is adapted to transmit the images of glow discharges caused by electron avalanches and an apparatus for recording said images (such as, for example, a photographic camera) „ The main advantages of this novel application lies in the fact that there is no discharge of the anode-grid capacitance and that the dead time does not appear, with the result that there are no longer any disadvantages involved in increasing the pressure with a view to enhancing the quantum efficiency of detection of particles „ In addition to these main arrangements, the invention is also concerned with a number of different which relate to modes of applicatio of the method according to the inventio „ The technical characteristics of the invention will become more readily apparent from the following description of one exemplified embodiment which, as will be understood, is not intended to imply any limitation either in the modes of execution of the invention or in the potential uses thereof „ Reference is had to the accompanying drawings, in which · - Fig, 1 is a sectional view of a gas detector as employed in accordance with the invention ; - FigQ 2 is a general arrangement diagram of an assembly for the practical execution of the method according to the invention, - In order that the detector of Fig, 1 should operate as a spark chamber, the filling-gas pressure and the value of the anode voltage which has to be stabilized are adjusted in such a manner as to obtain a sufficient number of sparks per second in the presence of a radioactive source Under these conditions, an image caused by collected radioactive particles can be produced in a time interval of a few minutes „ The sparks produced are in that case due to the breakdown of the dielectric strength of the gasu The quantity and distribution of the electrons which pass through the grid are sufficiently rich to ensure that a reduction of a few per cent in the grid-anode voltage makes it possible to obtain only those sparks which are caused by primary tracks of ionizing particles. Said sparks can readily be observed and Applicant has sought to obtain similar results by making use of the same gas detector in such a manner as to obviate the grave disadvantages which arise from the dead time „ The Applicant has found that, by reducing the amplitude of the grid-anode field to a slight extent in the presence of the same radioactive source, it is possible to avoid the formation of sparks. There nevertheless take place electron avalanches which result in emission of light „ And while the quantity of light emitted is smaller than it would have been otherwise, it nevertheless remains larger than in the case in which the particles are transmitted directly to a scintillator of the thallium-activated sodium iodide type, for example A study of the variations, as a function of the grid-anode voltage, of the ratio p of the amplitude of the light signals obtained in studies of the same source by means of a gas detector which operates in the avalanche condition and by means of a scintillator of the type already defined makes it possible to draw up the following table : : grid-anode Avalanche condition : voltage 6, 900 6, 875 6, 850 6,800 6, 750 i : p : 122 99 80 36 i It has also been found as a result of these studies that there was no longer any dead time and that increase the ratio p0 It will be noted by way of example that, when the pressure is doubled, said ratio is multiplied by a factor which can vary between 1.5 and 2.

The utilization of a gas detector in the avalanche condition no longer permits of direct observation of the image or of direct recording by means of a camera, with the result that said image must undergo intermediate amplification „ Fig. 2 shows the complete assembly comprising a gas detector A which operates in the avalanche condition and an image amplifier which must be employed in order to obtain an accurate and visible image.

Similar elements which appear in both figures are designated by the same reference numerals. This assembly makes it possible to study a radioactive source 16 which is surmounted by a shield 18 „ These two elements are located above a gas detector of the same type as that which is shown in Fig. 1.

The cathode 6 and the stainless steel collimator 10 are brought to ground potential whilst the auxiliary electrode 12 and the anode 14 are respectively coupled to high-voltage and low-voltage sources HT and BT by means of resistors 20 and 22.

The image formed by the glow discharges caused by electron avalanches is transmitted to the camera 24 by the multi-stage secondary-emission image amplifier 26, the image observed is formed on the photocathode 28 of the image amplifier by means of an object-lens 30, and the image which is intended to be recorded by the camera appears on the screen 32 of the tube. A focusing The present Applicant has designed a number of assemblies such as the unit which is shown in Fig„ 2, in which the gaseous mixture employed was xenon and methylal, the partial pressures of which were respectively 730 torr and 30 torr for a total filling pressure of 760 torr„ These units make it possible to obtain in approximately ten seconds images which exhibit strong contrasts and in which there can clearly be seen the useful components of images caused by the avalanche phenomenon of stray components which are due to amplifier noise „

Claims (4)

HAVIHG ϊϊθΐΐ particularly described and ascertained the nature of our said invention and in what manner the sjame is to he performed, we declare that what we claim is:

1. A method of utilization of a gas detector of known type for obtaining images resulting from the formation of electron avalanches which consists in adjusting the voltage developed between the auxiliary- electrode and the anode to the maximum permissible value for the prevention of spark formation under the action o β particles as well as X-ray or γ-ray photons, in amplifying the images of glow discharges caused by electron avalanche phenomena, then in recording the resultant image s„

2. „ A device for the practical execution of the method defined in Claim 1, characterized in that it comprises' a gas detector having three electrodes, an image amplifier of known type which is adapted to transmit the images of glow discharges caused by avalanches and an apparatus for recording the said images (such as, for example, a photographic camera) „

3. A device in accordance with Claim 2, characterized in that the gas employed for filling the gas detector is a xenon-methylal mixture and that the respective partial pressures of xenon and methylal are in the vicinity of 50 torr and 30 torr. . A method of utilization of gas detectors for producing and amplifying images caused by electron avalanche phenomena, substantially as hereinbefore described „

4. A device for the practical application of the aforesaid method, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings „