US 3612880 A
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United States Patent  Inventors Alain Lansiart 34, avenue Salnt-Laurent-9l, Orsay; Jean Leloup, 4, rue du Clos-9l, Git-sur- Yvette; Jean-Pierre Morucci, 5, Residence de Villebou-9l, Villebon-sur-Yvette; Georges Roux, 95, avenue Foch-78, Chatou, all of France [2 1] Appl. No. 786,339
 Filed Dec. 23, 1968  Patented Oct. 12, I971  Priority Feb. 26, 1968  France Continuation-impart of application Ser. No. 592,828, Nov. 8 1966, now abandoned.
 METHOD OF SPARK CONTROL AND SYSTEMS FOR THE UTILIZATION OF SAID METHOD IN SPARK CHAMBERS Claims, 8 Drawing Figs.
[52 use! 250/83.6R,
51 1111.01 3 G0lt 1/205 50 FieldoiSearcb 250/83.6; 313/93; 315/340  References Cited UNITED STATES PATENTS 2,924,720 2 1960 Hamelink 250/83.6 3,373,283 3/1968 Lansiartetal. 250/83.6
Primary Examiner-Raymond F. Hossfeld Attorney-Cameron, Kerkam & Sutton ABSTRACT: A spark quenching device for a three-electrode spark chamber in which a discharge device is associated in parallel with the intermediate-electrode/anode space of the spark chamber and a trigger device is controlled by the spark chamber current.
PAIENTEDHBT 12 Ian 3, 5 12.8 8 0 SHEET 1 UF 8 PAIENTEBum 12 ISTI SHEET 2 BF 8 SVHEET 30F 23 PATENTEDUET 12 um E .Hi N2 ll mm PATENTEDum 12 I9Tl 3.612.880
SHEET w 8 FIG 4 PATENTEUUDT 12 IHYI SHEET 5 [IF 8 PATENTEDUCT 12 197! SHEET 8 [IF 8 PAIENTEnnm 12 Ian SHEET 7 0F 8 PAIENTEUum 12 Ian SHEET 8 [IF 8 METHOD OF SPARK CONTROL AND SYSTEMS FOR THE UTILIZATION OF SAID METHOD IN SPARK CHAMBERS CROSS REFERENCE TO RELATED APPLICATIONS This is a continuation-in-part of application Ser. No. 592,828 filed on Nov. 8th, 1966, now abandoned disclosing a method of spark quenching which is intended to be employed in spark chambers for visualizing beta particles as well as X- ray photons or gammaray photons and to systems for the practical application of said method.
BACKGROUND OF THE INVENTION It will be recalled that a detector of this type consists of a chamber which is filled with a mixture o'frare gas and organic vapor and has two geometrical spaced limited by flat and parallel electrodes: a detector space, said space being defined between a cathode and an auxiliary electrode or grid in which a fairly low electric field is maintained and in which the radiation is mainly detected by photoelectric effect on the atoms of gas, and an electron multiplication space between the grid and an anode in which a high electric field is maintained. By virtue of the electron transparency of the grid which separates the detection space from the electron multiplication space, an electron avalanche which can result in a localized spark discharge is produced immediately above the ionized track of each primary photoelectron.
The leaktight chamber is constituted by a cylindrical sleeve, the upper end of which is closed by a transparent glass plate and the lower end of which is constituted by the thin metallic cathode. There can be placed against said electrode, for example, a plate in which are formed parallel apertures and which performs the function of collimator, said plate being made either of lead or of stainless steel.
If the detector is filled with a rare gas, it is not possible to obtain images'of a radioactive source. On the other hand, if there is added a suitable quantity of organic vapor which absorbs ultraviolet radiation, it is possible to produce discharges and especially sparks which correspond to the image of the source, provided that the value of the grid-anode voltage is judiciously determined.
Further consideration will now be given to the process involved in initiating the appearance of discharges. When X-ray or gamma-ray photons are transmitted by the collimator and reach the cathode, they give rise to primary ionizing electrons between cathode and grid. 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 grid. In this electron multiplication space, the electrons initiate a process of cumulative ionization or electron avalanche which can result in the formation of a spark.
Each time a particle is detected by the detector, an electron avalanche or spark discharge takes place and molecules of organic vapor are destroyed; it should be noted that the production of sparks results in much greater destruction of these molecules and consequently in a limitation of the service life of particle detectors which operate in the spark-discharge condition.
When a spark is produced between the auxiliary electrode and the anode, the interelectrode capacitance which is a function of the electrode dimensions accordingly discharges. It should be noted that the resistor which is in series with the anode 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 process will consequently have a time duration corresponding to a relatively long dead time, thereby slowing down the triggering rate of the detector.
A number of different gaseous mixtures have been employed for the purpose of filling gas detector of the type considered. The atmosphere at the outset consisted of a mixture of argon and methane but has been replaced by the combination of xenon and methylal. A particle detector in which this second mixture is employed exhibits a countingplateau and makes it possible to obtain satisfactory images in the sparkdischarge condition and in the electron-avalanche condition but the operating voltage is of a relatively high order.
The use of a gaseous mixture of xenon and diethylamine as filling gas in a detector which operates either in the spark discharge condition or in the electron avalanche condition described in this continuation-in-part application Ser. No. 752,637 entitled Particle Detector now U.S. Pat. No. 3,560,746.
SUMMARY OF THE INVENTION The present invention proposes the use of a spark control system for improving the operation of spark chambers of the type herein described by reducing the dead time and increasing the service life of the tube.
The method of spark control for spark chambers in accordance with the present invention is also intended to improve the operation of this type of apparatus by reducing the dead time and increasing its useful life.
In accordance with this method, the auxiliary-electrode/anode space of a spark chamber is associated in parallel with a discharge device and a fast reduction of the impedance of said discharge device is initiated by means of the current which passes through the auxiliary-electrode/anode space of the spark chamber for the purpose of discharging the auxiliary-electrode/anode capacitance of said spark chamber when the formation of a spark has been initiated and in rapidly quenching said spark.
The invention is also directed to devices for the practical application of said method. In accordance with a first embodiment of an apparatus of this type, the discharge device of the spark chamber quenching system is a spark gap and the triggering element is a pulse transformer in which the magnetic core is a ferrite torus. By virtue of this quenching system, the charged particles no longer pass through the detector and flow externally.
The main advantage attached to the use of this control system lies in the fact that the process of discharge of the auxiliary-electrode/anode capacitance as well as the dead time are shortened and, in addition, the destruction of the organic vapor which forms part of the filling gas is slowed down, with the result that the service life of the tube is extended.
The present applicant has found that the operation of the detector can be even further improved to an appreciable degree and in particular that the number of sparks which can appear in a given time can be increased.
The ionization time of a spark gap causes a too large minimum dead time and the rate of rise of the triggering pulse produced by the ferrite transformer, which triggers said spark gap, is too low, with the result that the instant of short circuit of the anode-grid space of the spark chamber is too late, thus further increasing the dead time. Further, after a relatively limited number of discharge (for example [0), the triggering characteristics 'of the spark gap deteriorate and the operation of the spark gap becomes erratic.
In accordance with a second embodiment, the spark chamber quenching device for the practical application of the method according to the invention comprises by way of discharge devicea vacuum tube which has at least three electrodes, and in which the anode-cathode space short-circuits the series-connected assembly comprising the auxiliary-electrode/anode space of the spark chamber and a resistor of small value after said vacuum tube has been rendered highly conductive by an avalanche-connected transistor followed by a secondary-emission amplifier tube, said transistor being controlled by the voltage developed across the terminals of said resistor at the moment of appearance of a spark which discharges the anode-grid space.
Among the properties of the last-mentioned device according to the invention, there can be mentioned the high speed of response and the extremely short dead time, thereby making it possible for the detector to supply a large number of sparks in a short time interval.
It has also been found by the applicant that the optimum characteristics of the triggering element are related to the surface conductivity of the anode.
For example, in the case of a spark chamber in which the anode has a surface conductivity of 3 kilohms per square centimeter, the signal which appears at the auxiliary electrode corresponds to a maximum spark current of 3.6 amps which is attained after 150 nanoseconds whereas, when said surface resistance is of the order of 7 kilohms, the same signal corresponds to a maximum current of 1.5 amps which appears alter 200 nanoseconds.
Depending on the characteristics of the signal which appears at the auxiliary electrode of the spark chamber, the triggering element must have a sufficiently high gain to trigger the short-circuit discharge device. When the resistance of the glass envelope is sufficiently high, use can be made of a vacuum tube as in the second embodiment.
The present applicant has given close study to the operation of the spark quenching circuits referred-to and has observed that it was possible to improve them in such a manner as to limit the dead time of the detector even further as well as to extend its useful life.
It has also been proposed that the gas detector of the type hereinabove described should be employed for the purpose of obtaining images resulting from the formation of electron avalanches so as to eliminate the dead time and in particular to remove the limitation attached to the service life of gas detectors in the type of operation which has been considered in the foregoing as described in patent application Ser. No. 595,714 now US. Pat No. 3,449,573.
Further improvements in the discharge control device permit the possibility of employing this latter for the operation of the detector in the avalanche condition and for improving the luminous intensity of the images.
A discharge-quenching circuit for an improved particle detector in accordance with the invention which can be employed either in the spark-discharge condition or in the electron-avalanche condition, is characterized in that it comprises a short-circuit device which comprises a transistor stage operating in the avalanche condition, a first triode powerstage and a second tetrode power-stage in which the anodecathode space is associated in parallel with the grid-anode space of the detector.
In the case of the spark discharge condition, the electronic shunt circuit which comprises the short-circuit device produced effective action less than 40 nanoseconds after the occurrence of a spark discharge within the electron multiplication space. It would be an advantage to retard the transition of charged particles which are stored in the grid-anode capacitance of the detector across the spark which is produced within this latter in order to pass out into the electronic shunt circuit; this would reduce the dead time of the detector and at the same time increase its life.
In accordance with the invention, this result is achieved by providing the detector tube of one anode with a high surface resistance. Preferably, the value of said resistance varies between 5,000 ohms per square centimeter and 9,000 ohms per square centimeter.
From a study of the operation of the detector which consists in measuring the grid current and the intensity of the light emitted by the spark, it is permissible to assert that, when the mixture of xenon and diethylamine is employed, two stages can be distinguished in the formation of a spark.
During a first stage, there exists a relatively weak electric field on the path of the spark, which results in the formation of an initial spark passage having a high electrical resistance. Only a low value of current passes through said passage.
During a second stage, a sufficiently strong emission of the grid results in the formation of the main discharge.
The present applicant has found that the energy expended in the spark is considerably reduced when the spark is extinguished by means of the electronic shunt circuit after a delay of a few tens of nanoseconds and during the first conduction stage. Under these conditions, the dead time is reduced to 1.2 nanoseconds without affecting stability and the life of the detector is increased.
The timelag which is caused by the electronic shunt circuit must consequently be determined so that the short-circuit phenomenon occurs only after the beginning of the second spark-formation stage.
This time-lag is of the order of a few tens of nanoseconds as mentioned above. Under these conditions, the useful light energy becomes very low and the photographic object-lenses which are employed for the purpose of recording these spark discharges must have very wide apertures.
When the xenon-diethylamine mixture is employed, a propagation of the discharge of the grid-anode capacitance under the action of the electronic shunt circuit is carried out at a relatively slow rate from the edges of the electrodes, the connections being made form the grid and anode towards the center. This phenomenon causes the appearance of images which lack uniformity and, in the case of a single phenomenon, results in spots having different degrees of brightness according to the distance between the point at which the spark discharge occurs and the center of the detector.
Under these conditions, the sparks transfer a larger number of charged particles to the center of the detector where the light intensity of the images is greater.
in order to overcome these disadvantages and in accordance with the invention, a series resistor is placed between the electronic shunt circuit and one of the electrodes which limits the multiplication space. I
The value of the resistor which is associated in series with the electronic shunt circuit is preferably comprised between 500 and 1,500 ohms.
According to a further aspect of the invention, the discharge control device of the detector is adapted to operation in the avalanche condition.
The study of the avalanche signal which is supplied by a particle detector reveals that said signal exhibits a rapid rise, a plateau which results from the displacements of the ions followed by a sharp bend or knee which results in the appearance of a few sparks.
The reference 7,, will designate the time which has elapsed between the appearance of a grid current equal to 20 percent of the stable value and the appearance of knee which results from the formation of a spark In accordance with the invention, the electronic shunt circuit which is associated with the detector must reduce the electric field to zero within said multiplication space before the time 'r after the appearance of a grid current which is equal to 20 percent of the plateau.
In other words, in accordance with the invention, the sum of time-lags arising from the different devices which form the electronic shunt circuit must be less than 1 If an electronic shunt circuit of this type is employed, the primary avalanche can have a number of carriers which is substantially higher than [0 without resulting in a spark discharge.
Maximum light intensity is accordingly obtained with the minimum dead time if said sum of time-lags is sufficiently close to 1,, although smaller than this latter.
When the sum of time-lags caused by the different elements of the electronic shunt circuit is substantially smaller than the time-lag 1, a delay device is inserted in series in said shunt circuit.
In regard to the weakness of the signal resulting from the avalanches and to the need to prevent the appearance of sparks, it has proved necessary to add elements for the purpose of increasing the power of the shunt circuit and to add a further number of elements for the purpose of increasing its operating speed.
In accordance with the invention, the short circuit device which has already been employed in the spark-discharge condition is mounted in series with a wide-band field-effect transistor preamplifier having a high input impedance, a wideband high-gain amplifier and a reversing cable.
Since the field-effect amplifier is particularly delicate, said amplifier is preferably protected by an assembly of diodes mounted in top-to-tail relation between the grid and ground.
Apart from these main arrangements, the invention is also concerned with a number of different secondary arrangements which will be mentioned hereinafter and relate to the mode of application of the spark-control system in accordance with the invention.
In order that the technical characteristics of the invention may be more readily understood, two examples of execution of the method according to the invention will now be described, it being understood that these examples do not imply any limitation either in the modes of operation of the invention or in the potential applications thereof.
BRIEF DESCRIPTION OF THE DRAWINGS Reference is made to the accompanying drawings, in which:
FIG. 1 is a sectional view of a spark chamber as employed in accordance with the invention;
FIG. 2 is a general arrangement diagram of a first spark control system for spark chambers in which the method of the invention is carried into effect;
FIG. 3 is a general arrangement diagram of a second system of this type as also employed in the practical application of the invention with a view to permitting of more efi'ective utilization of a spark chamber;
FIGS. 4 and 5 are diagrams of two details of FIG. 3;
FIG. 6 illustrates the short circuit device which is employed both in the spark-discharge condition and in the electronavalanche condition;
FIG. 7 is a diagram representing the electronic shunt circuit as associated with the particle detector which operate in the spark-discharge condition;
FIG. 8 is a diagram representing the electronic shunt circuit as associated with the detector which operates in the electronavalanche condition.
DESCRIPTION OF THE PREFERRED EMBODIMENTS 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 in the presence of a radioactive source a sufficient number of sparks to permit the forming an image in a time interval of a few minutes.
It is known that molecules of organic vapor are destroyed each time a particle is detected but that a spark discharge results in considerably greater destruction of said vapor. As long as the total number of spark discharges which have occurred is below a predetermined number (which depends on the size of the spark chamber), it is possible to reestablish the initial spark rate by slightly increasing the auxiliary-electrode/anode voltage. if the spark chamber operates after having supplied said number of sparks, the images exhibit a few stray points having a high degree of brightness and the dead time must accordingly be increased in order to prevent the appearance of such points. In one case in which the organic vapor employed was methylal, the composition of the gaseous mixture contained in the spark chamber A was studied by means of a mass spectrometer. It was observed that the products of decomposition of the vapor are essentially hydrogen, carbon monoxide and in smaller proportions ethane, ethylene, methane and carbon dioxide.
The prolonged operation of a spark chamber is accompanied by an increase in pressure, and the reduction in the count rate at constant voltage is due to a lack of absorption of the ultraviolet rays which are emitted at the time of production of a spark.
When the conductive deposit on the anode has a resistance of 3 kilohms per square centimeter, the charged particles in the grid-anode capacitance pass out in a time interval of approximately 1 microsecond.
Researches carried out by the present applicant have shown that it is possible to reduce the harmful effect of destruction of the organic vapor by causing these charged particles to pass through an external circuit as soon as a spark is produced. It is as a result of these findings that the use of a spark gap has been adopted for the purpose of shunting the auxiliary-electrode/anode space of the spark chamber as soon as the discharge appears.
There will now be described a circuit for controlling a spark chamber and comprising a spark gap as shown in FIG. 2.
As will be clear from the accompanying drawings, similar elements which appear in difi'erent figures are given the same reference numerals.
Whereas the cathode 6 of the tube A is connected to ground, it will be observed that the auxiliary electrode 12 and the anode 14 are respectively connected to two voltage sources BT (low-tension) and I-IT (high-tension) through two resistors 18-20.
The control circuit which is illustrated has a spark gap 22 which shunts the auxiliary-electrodelanode space of the spark chamber when this latter has been triggered by the trigger unit 24 which is controlled by the anode-grid discharge current of said spark chamber.-
The current pulse which appears in the tube when a spark is produced is transmitted to the spark gap 22 by means of a pulse transformer 26, the magnetic core of which is ferrite torus 28 on which are wound a primary winging 30 and a secondary winging 32.
The spark gap comprises two solid main electrodes 37 and 38, provision being made between said main electrodes for intermediate electrodes 40 and 42 through each of which is pierced a central passageway. The metallic point 43 is an electrode of the corona-discharge type which is connected to a high-tension supply HT 2 through a resistor 44.
One of the ends of the primary winding 30 is connected directly to the grid 12 of the spark chamber while the other end is connected to the anode 14 through a capacitor 34. It should be noted that the first end of said primary winding is also connected to ground through a parallel-connected assembly comprising a resistor 35 and a capacitor 36 which has a high capacitance compared with that of the capacitor 34. One end of the secondary winding 32 of the transformer is connected to the intermediate electrode 40 of the spark gap and the other end of said secondary winding is connected to one of the tappings of a chain of resistors 46, 48, 50 which serves to equalize the voltage applied to the terminals of the spark gap electrodes. One of the ends of the resistor 50 is connected to the end electrode 38 of the spark gap and also to the anode of the spark chamber, the common point of the resistors 48 and 50 is connected to the second intermediate electrode 42 of the spark gap. The common point of the resistors 46 and 48 is connected to the secondary winding 32 while the other end electrode 37 is connected to the second end of the resistor 46.
The operation of the above-described control circuit can be summarized as follows:
When a spark is produced in the spark chamber A, the current which passes through said chamber produces at the secondary winding 32 of the transformer 26 a voltage pulse of large amplitude. In approximately I00 nanoseconds, said pulse amplitude is sufiicient to trigger the vacuum tube 22 during the pulse rise time.
It has proved possible to obtain images of radioactive sources fairly rapidly by making use of the system which is shown in FIG. 2.
A second embodiment of the invention is illustrated in FIG. 3. This figure again shows the spark chamber A which is similar to that of FIG. 1. However, it should be pointed out that the anode 14 is made up of a glass disc and is preferably provided on the underface thereof with a conductive and transparent deposit of doped oxide of tin. The anode can be connected into the circuit which is shown by means of a conductive annulus 17 which is secured to the peripheral portion of the underface of the anode.
The grid 12 is connected to ground through a resistor 110 which has a low value of resistance (not more than a few tens of ohms). The cathode 6 is connected to the movable contact 116 of a potentiometer 118 which serves to connect to ground the negative pole of a low voltage source BT, The potential adjustment of the movable contact 116 serves to regulate the value of the voltage which is applied to the space between the cathode 6 and the auxiliary electrode 12 of the spark chamber. Finally, the anode 14 is connected through a resistor 122 having a value of resistance which is higher than I megohm to a very-high-voltage source 124.
The value of resistance of the resistor 122 is determined according to the spark frequency which is desired.
The anode-cathode space of a vacuum tube 126 of the tetrode type is associated in parallel with the series-connected assembly comprising the space between anode 14 and auxiliary electrode 12 of the spark chamber and the resistor 1 10. The anode 128 of the tetrode is in fact connected through a resistor 130 to the anode l4 and the cathode 132 of the tetrode is connected to ground. The screen grid 134 of said vacuum tube is connected through a resistor 135 to the positive pole of a high-voltage source l-IT The control grid 138 is connected to the negative pole of the source BT through the resistor 140 and a portion of the potentiometer 142 through which the said pole of the source BT is connected to ground. Said grid 138 is also connected to a control device through a capacitor 144. The control device referred-to is constituted by the assembly of an avalanche-connected transistor 146 and a secondaryemission amplifier tube 148.
When a spark is produced in the chamber A, there appears between the point of connection of the resistor 110 to the grid 12 and ground a pulse B having a steep leading edge. Said pulse is successively amplified by the avalanche-connected transistor 146 and the secondary-emission amplifier tube 148. There then appears at the grid 138 a pulse C having a large amplitude and a duration of less than lpS. The impedance of the tube is of very low order and the assembly comprising the space 12-14 of the spark chamber and the resistor 110 is short-circuited.
FIG. 4 shows the avalanche connected transistor 146 as essentially made up of an avalanche-connected transistor 54.
The common point of the grid 12 (not shown) and of the resistor 110 (FIG. 3) is connected in FIG. 4 through a smallcapacitance capacitor 51 and a rectifier 52 to the base of said transistor 54.
It will be noted that the base and emitter circuits of transistor 54 are isolated from ground by means of blocking inductance devices 56 and 58. The collector of transistor 54 is connected to ground through capacitors 60-62-64 and to a potentiometer chain 66 which connects the ground to the positive pole of a high-voltage source TI-IT The avalanche operation of transistor 54 of FIG. 4 is made possible by the judicious adjustment of the position of the sliding contact of the potentiometer 70. The bias voltage applied to the base of the transistor can be regulated by means of a second potentiometer chain 72 which connects the same positive terminal of the high-voltage source HT; to ground. Said bias voltage determines the sensitivity of the transistor 54. The voltage pulse which is amplified by the transistor appears between the terminal 74 which is connected to the emitter and ground, said terminal being connected to the control grid 76 (as shown in FIG. 5) of a secondary-emission amplifier tube.
FIG. 5 shows the amplifier 148, the essential element of which is a secondary emission tube 78. Said tube comprises a cathode 80, a control grid 76 which is connected, as has previously been explained, to the emitter of the avalanche-connected transistor 54 and which is connected to ground through a resistor 82, a Zener diode 84 being associated in parallel with said resistor. Said tube additionally comprises a screen grid 83 which is biased through the potentiometer chain 85, a suppressor grid 86 which is connected to the cathode, a dynode 88 which is connected to an intermediate point of the potentiometer chain and an anode 90 which is connected through a resistor 94 to the positive terminal of a high-voltage source HT and to ground through a capacitor 96.
Signals which are amplified by the assembly of devices 146 and 148 of FIG. 3, 4 and 5 appear on the one hand between the dynode 88 and ground and on the other hand between the anode and ground. One of these two elements can therefore be connected to the control grid 138 of the tetrode tube 126 of FIG. 3.
It should be noted that, depending on the circuit arrangement which is employed in this case, the dynode 88 of FIG. 5 is connected to the control electrode 138 of FIG. 3 of the tetrode tube 126 through the capacitor 144.
The short circuit device 220 as shown in FIG. 6 can be employed either in the spark-discharge condition or in the electron-avalanche condition and comprises a stage 222 of avalanche-connected NPN transistors, a first triode power stage 224 and a second output power stage 226 comprising a tetrode 228, the anode-cathode space of which is associated in parallel with the grid-anode space of the detector or spark chamber A of FIG. 2.
The amplified signal is transmitted to the base of one of the transistors 230 via a diode 240 (FIG. 6). The amplifier 222 comprises two avalanche-connected transistors 230, 232 of the NPN type, the collector-emitter spaces of the two transistors being in series. The collector of transistor 232 is connected to the positive terminal of a supply voltage source A through a resistor 234, the base and the emitter of the transistor 232 are connected together. The emitter of the transistor 230 is connected to ground through a resistor 236 and to the control grid of the triode of the following stage through a capacitor 238.
The cathode of the triode 242 of the stage 224 (FIG. 6) is connected to ground while the anode of said triode is connected to the positive terminal of the high-voltage source B through the primary winding of a coupling transformer 244 and a resistor 248. w
The cathode of the tetrode 228 (FIG. 6) of the output stage 226 is also connected to ground while its anode is connected to the anode 6 of the particle detector of FIG. 7 and to the positive terminal of a high-voltage source AC through a resistor 246 (as shown in FIGS. 7 and 8). The control grid of said tube 228 of FIG. 6 is connected to one end of the secondary winding of the transformer 244, the second end of which is connected to ground. The screen-electrode of said tetrode 228 is brought to a fixed potential in the usual manner.
It is worthy of note that the signal to be amplified is derived directly from the grid of the detector in the case of operation in the spark-discharge condition but that it is amplified before being applied to the short circuit device in the case of operation in the electron-avalanche condition as will become apparent hereinafter with reference to FIG. 8.
The assembly comprising the particle detector and electronic shunt circuit which is coupled therewith will now be described in the case of operation in the spark-discharge condition.
It is usually preferred to retard the flow of charged particles across the spark discharge by providing the detector with an anode 6 (FIG. 7) of conductive glass which has a high surface resistance. This anode is connected to the positive terminal of the high-voltage source AC through a resistor 246 while the grid 4 is connected to the same source through a resistor 250, the cathode 2 of the detector being connected to ground.
In order that the grid-anode voltage should be made substantially constant, said grid is brought to a stabilized potential by the Zener diodes 256 and 258 (FIG. 7). The assembly which consists of resistor 252 and capacitor 254 has the intended function of transmitting the grid signal (spark signal) to the shunt circuit.
The series assembly of the short circuit device 220 which has already been described with respect to FIG. 6 and of the resistor 253 which serves to ensure uniformity of the image which couples them to the anode 6.completes the electronic shunt circuit.
There is also-shown a photomultiplier 261 (FIGS. 7 and 8) which serves to measure the luminous intensity of the images produced inasmuch as the circuit arrangement of FIG. 7 has also'served for the 'study of the properties of the detector.
In one device constructed by the present applicant the surface resistance of the anode 6 (FIG. 7) has a value of 7,000 ohms per square centimeter, the gaseous mixture consists of xenon and diethylamine maintained at respective pressures of 720 torr. There has been clearly observed a counting plateau which is obtained in respect of mean electricfields of 13,000 v. cm. which represents a marked improvement compared with the use of methylal.
The foregoing description has already served to demonstrate the manner in which the production of a spark discharge in two stages can be turned to useful account as well as the advantages tobe gained by determining the time-lag resulting from the operation of the short circuit device 220.
Stress should also be laid on the fact that the value of the resistor 253 (FIG. 7) must be chosen in order to obtain good unifonnity of intensity of the images over the entire surface of the anode of the detector.
In the case of the arrangement described for FIG. 7, theme of a resistor having a value of resistance of 500 ohms attenuated the intensity of the images located' next to the edge of the anode to a considerable extent while a resistor having a value of l,l ohms served to increase the light intensities of the lateral images with respect to those of the central region.
After various attempts, the optimum value of the series resistance of the electronic shunt proved to be 1,000 ohms.
I In the caseof the study of an iodine-125 source which emits X-rays having a principal-energy equal to 27 Kev., it has been found that the length of the plateau was in the vicinity of 4 percent of the mean operating voltage.
The detector is still serviceable after 3X10 spark discharges although the efficiency decreases at this value. Thelength. of life has been determined with operating rates of several millionspark discharges per day.
In the case of a low daily count rate in the vicinity of 200,000 spark discharges, the lengths of life are extended and are likely to attain 1 year.
As has already been described in the copending application Pat. No. 595,714 now US. Pat. No. 3,449,573 referred to above, the particle detector has been employed in the electron-avalanche condition. As is already known, the principal advantages of this mode of operation are a considerable reduction of the dead time and a very considerable increase in the useful life.
By making use of the electronic shunt circuit in order to prevent the appearance of spark discharges, this mode of operation can be improved still further.
The discharge quenching circuit for avalanche operation is shown in FIG. 8 and is somewhat similar to the circuit of FIG. 7 (for operation in the spark-discharge condition). The essential differences are the absence of a series resistor (resistor 253), the use of additional amplifier stages by reason of 'the weakness of the drive signal and the use of a reversing device.
In FIG. 8, the grid 4 is connected to the input of the short circuit device 220 of FIG. 6 through anassembly which consists of a field-effect transistor preamplifier 258 having a high input impedance, a wide passband and low noise, a second wide-band amplifier 260 having a gain which is comprised between 100 and 1,000 and a reversing cable 262. In this embodiment, in order to protect the field-effect transistors of the amplifier 258 when the electric field in the multiplication space is reduced to zero, use is parallel made of a series 264 of diodes mounted in parallelrelation.
There is also shown in FIG. 8 a photomultiplier 261 which serves to measure the intensity of the images which appear at the anode since the circuit arrangement has been employed in order to study the properties of the detector.
For the same reason, the winding 266 (FIG. 8) of a current transformer has been disposed around the connecting-wire of the anode 6.
The present applicant has found that it was possible to increase the quantity of light emitted by the detector by delaying by a time interval r, the instant of nullification of the electric field within the multiplication space. With this objective, an additional delay device 270 is employed, for example, as shown in FIG. 5.
If the reference 1', designates the sum of time-lags resulting from the normal components of the electronic shunt circuit, it is necessary to satisfy the equation:
In addition to the increase in emitted light resulting from the increase in the time-lag 1,, it has been observed that the spectrum of distribution of luminous amplitude of the avalanches broadens and that the operation of the detector becomes increasingly less stable when 'r increases. However, a small fluctuation in the time-lag involved in the nullification of the electric field does not disturb the operation of the detector to any appreciable extent at the low energies of the X-photons which are employed.
What is claimed is:
l. A spark quenching device for a three-electrode spark chamber having a detection space between a cathode an intermediate electrode and a multiplication space between said intermediate electrode and an anode of transparent conductive glass comprising a discharge device outside said chamber connected in parallel with the intermediate electrode-anode space of the spark chamber and a trigger device for amplifying the current from said intermediate electrode when a discharge occurs in the-multiplication space and triggering said discharge device to short circuit said multiplication space by said discharge device, said discharge being visualized through said transparent anode before the multiplication space is short-circuited.
2. A spark quenching device for a three-electrode spark chamber having an intermediate electrode anode space comprising a discharge device connected in parallel with the intermediate electrode-anode space of the spark chamber and a trigger device for the discharge device controlled by the amplified spark chamber current said discharge device including a spark gap tube, a potentiometric chain of resistors connected in parallelwith said intermediate electrode-anode of the spark chamber and said trigger device including a pulse transformer having torus-shaped ferrite magnetic core, a primary for said transformer connected across said space and a secondary for said transformer connected between an intermediate electrode of said spark gap tube and a point of said potentiometric chain.
'3. A spark quenching device for a three-electrode spark chamber having an intermediate electrode-anode space comprising a-discharge device connected in parallel with the intermediate electrode-anode space of the spark chamber and a trigger device for the discharge device controlled by the amplified spark chamber current from the anode, said discharge device including a series assembly of a resistor and said intermediate electrode-anode space of the spark chamber, a gridcontrolled vacuum tube, a cathode-anode space for said tube which short-circuits said series assembly and said trigger device comprising a control circuit for the control grid of said vacuum tube, an amplifier stage for said control circuit including an avalanche connected transistor and a secondary-emission amplifier tube.
4. A spark quenching device according to claim 3, including a dynode for the secondary-emission amplifier tube coupled through a capacitor to said control grid of said short circuit vacuum tube.
5. A spark quenching device according to claim 1 for a three-electrode spark chamber operating in the sparkdischarge condition, said quenching device including in the trigger device an avalanche-connected transistor stage and a first triode power stage and, in the discharge device, a second tetrode power stage with an anode-cathode space connected to the intermediate-electrode/anode space.
6. An avalanche quenching device for a three-electrode chamber having a detection space between a cathode and an intermediate electrode and a multiplication space between said intermediate electrode and an anode of transparent con ductive glass and operating in the electron-avalanche condition including a discharge device outside said chamber connected in parallel with said multiplication space, said discharge device including a tetrode power stage with an anode-cathode space coupled with the intermediate electrode/anode space of said chamber, and a trigger device including an avalanche transistor stage and a first triode power stage for amplifying the current from said intermediate electrode when a discharge occurs in the multiplication space and triggering said discharge device to short circuit said multiplication space, said discharge being visualized through said transparent anode before the multiplication space is short-circuited.
7. A spark quenching device according to claim comprising a resistor connected in series between said discharge device and an electrode of the multiplication space of said chamber.
8. A spark quenching device according to claim 7, wherein the surface resistance of the anode is within the range of 5,000
ohms per square centimeter to 9,000 ohms per square centimeter.
9. A spark quenching device according to claim 7, wherein said resistor has a value within the range of 500 to L500 ohms.
10. An avalanche quenching device according to 6, wherein the sum of time-lags caused by the amplifiers and the discharge device is smaller than the time t of development of the sparks after the appearance in said multiplication space of a grid current equal to 20 percent of the plateau and said discharge device is connected in series with a field-effect transistor amplifier having a high input impedance, a wideband a low noise, a wide-band high-gain amplifier and a reversing cable.
11. An avalanche quenching device according to claim 10, wherein the field-effect transistor amplifier is protected by an assembly of diodes mounted in parallel relation connected between the intermediate electrode of the chamber and ground.
12 An avalanche quenching device for a three-electrode chamber according to claim 10, including an additional delay element.