On the multiple efficiency of current limited spark chambers

NUCLEAR

INSTRUMENTS

AND

METHODS

II 4

(I974) 375-380;

©

NORTH-HOLLAND

PUBLISHING

CO.

O N T H E M U L T I P L E E F F I C I E N C Y OF C U R R E N T L I M I T E D SPARK C H A M B E R S R. M. B U L L , D. L. H E N S H A W

a n d J. H. Y O U L E

University of Nottingham, Nottingham, England Received 15 M a y 1973 Current limited spark c h a m b e r s have the particular a d v a n t a g e tha,L they record events involving m a n y particle tracks. This property has been investigated using indirect m e t h o d s a n d it is concluded that the external experimental conditions (rather than

the intrinsic properties of the chambers) provide the upper limit to the particle densities that can be recorded. It is suggested that, at particle densities in excess of 10 6 m -2, a n o m a l o u s behaviour o f the c h a m b e r s might be expected.

1. Introduction

situations can be isolated each suggesting its own special definition of multiple efficiency: (i) When a more or less homogeneous distribution of tracks occurs throughout the chamber as might be expected in some EAS experiments. (ii) When interest is focussed on single isolated particles which are expected in the vicinity of regions of high track density. Thus, for example, the detection of a charged pion may be influenced by the high density of tracks associated with an accompanying n ° initiated shower. (iii) Interest may alternatively focus on the high density track region itself when the relevant parameter is the fraction of tracks registered by sparks in that localised region.

Spark chambers have established themselves as most powerful nuclear particle detectors. Their basic simplicity, their versatility and precision and, not least, their modest cost have led to considerable developments in their design, resulting in the production of many devices, each having its own particular advantages together with its inevitable drawbacks. Certainly one of the main drawbacks of the basic conventional spark chamber is that the efficiency with which it detects several simultaneous particles is suspect and this, no doubt, motivated the development of the discharge chamber by Fukui and Miyamoto l'z). This chamber differed from the conventional by having a layer of insulator between the electrodes, thereby reducing the current drain on the high voltage pulse generator. Such a "current limited" spark chamber has many of the characteristics of the conventional chamber, but differs in the efficiency with which it detects multiple particle events. It is this crucial characteristic that is considered here. 2. De~nitions and scope

'The most straightforward definition of the multiple efficiency of a spark chamber is the fraction of particle tracks that are registered by sparks. However, for a current limited spark chamber, such a definition is misleading. If it could be assumed that a substantially uniform density of tracks traversed the entire chamber this definition would be quite meaningful: but this is rarely the practical situation, where wide fluctuations in track density are the rule. Unlike a conventional chamber, in a current limited device it is certain that the probability of a track being registered by a spark will depend on the local environment of that track as well as on the overall number or average density of tracks. Thus, at least three different experimental 375

Clearly a simple definition of "multiple efficiency" is not possible. It may be defined as the probability of a particular track causing a spark when it is accompanied by other particle tracks; but it is then a complicated function of the local track density distribution, the total number of tracks and possibly other factors as well. In particular, it is clear that the multiple efficiency derived from one experimental situation may well be quite meaningless when applied to another. In this work no attempt has been made to quote a multiple efficiency; rather is it the intention to consider the various factors that influence the multiple efficiency, bearing in mind that the relevance of a particular factor often depends on the use to which the cMmber is to be put. This work also has technological limitations. It is not easy to obtain a well-known high density of tracks through a chamber. In order to obtain sufficiently high particle densities for inefficiency to be noticeable, it is necessary to use cascade showers, and then it is not possible to know exactly how many tracks are available to cause sparks. In consequence such techniques were

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nct used in this experiment and the factors determining the multiple efficiency uere sought by more indirect means. We have considered four factors which have relevance to the multiple efficiency of current limited chambers: (a) The applied pulse, in conventional chambers the dominant limitation to the multiple efficiency occurs because the first few sparks short circuit the external electronics and consequently inhibit further potential sparks from developing. Since the current limited chamber is designed specifically to overcome this defect by isolating the discharges from the external electronics, this defect is obviated to a large extent and, consequently, other less obvious limitations become significant. (b) The spark mechanism. The gas discharge mechanism operative in these chambers is not well known. It is conceivable, houever, that the presence of nearby sparks may effect tLe development of a particular discharge. For example, if secondary photons are an essential component of the normal discharge mechanism, adjacent tracks would contribute additional photons to each other. It is probable that such a factor would effect the characteristics of the discharges (such as their brightr.ess and speed of development) rather than their occurrence. (c) Field distortion. The existence of a spark channel in tke gas ~.ill clearly cause field distortion in the gap and thus affect the development of other discharges. In particular, this may be of extreme importance in very high density events, when the gas gap tends towards a localised conducting plasma which may severely impede the development of other tracks. (d) Technical d!fifculties. Although a chamber may be capable of producing multiple sparks, it may not be possible to photographically record them. By photographing through one of the electrodes such that sparks are viewed end-on it is possible to record densities that would result in a uniform "glow" when photographed through the side of a chamber. Nevertheless, there is a limit to the proximity of two sparks that can be recorded as two resolvable photographic images. This is of enhanced importance when angled tracks are involved.

et al.

glass plates and had dimensions 2 5 x 2 5 x I l c m 3. Particular care was taken to ensure that the gap widths were identical such that both chambers operated satisfactorily when pulsed by identical external circuitry. The chambers were arranged between a fourfold Geiger counter telescope such that S1 was operated in projection mode and $2 in the normal mode. The chambers were connected electrically in parallel such that they constituted a single chamber from an electronic circuitry point of view. Since the optimum operating conditions of normal and projection mode chambers are nearly identical, the application of an appropriate high voltage pulse to both chambers will lead to an average of 68 projection mode sparks in SI and a single normal mode spark in $2. If the sparks in S1 were to affect the high voltage applied pulse sufficiently to reduce the chamber efficiency, the efficiency of $2 would be expected to decrease. By displacing the detection system it was possible to measure the efficiency of $2 with and without sparks occurring in SI. It was found that, when a multiplicity of sparks occurred in SI, there was no decrease in efficiency or spark brightness in $2. It was concluded that the occurrence of up to 70 sparks in a single chamber does not lead to a decrease in efficiency. This conclusion is applicable to a relatively low number of sparks. However, in the next section an apparatus is described which employed 2272 simultaneous discharges, and the applied pulse was not observably altered whether the discharges occurred or not. It was consequently concluded that this factor can be ignored for normal track densities. When ultra high , Particle I' trajectory

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3. The effect of the applied pulse

In order to be certain that spark formation is not influenced by self-induced pulse variations, an experiment was performed using two spark chambers, S1 and $2, as shown in fig. 1. The clqambers were current limited by

S2

Fig. 1. The apparatus and pulsing system used to investigate self-induced pulse variation.

THE M U L T I P L E

EFFICIENCY

OF C U R R E N T

densities occur this may n o t be valid, and this extreme case is considered in section 6.

4. The multiple efficiency at moderate track densities ]In order to investigate the remaining factors influencing the multiple efficiency, a special apparatus was constructed, as shown in fig. 2. A photon source is situated at the top of the apparatus and consists of a spark gap SG attached to the metal electrode of a vertical current limited spark chamber SC1. The spark gap is sited adjacent to a hole in this electrode such that photons produced by a discharge in SG can travel into SCI. SG1 is triggered after SG has discharged, resulting in the complete electrical breakdown of SCI throughout its entire volume. The chamber SC1 is atlached to a square metal plate such that part of the plate constitutes one of the chamber walls. Two holes in this wall allow photons to travel downwards from SC1 into the volume enclosed by a 20 cm diameter, 30 cm long glass cylinder, which is rigidly sealed to the upFer metal plate. The lower cylinder edge fits into a circular, 3 cm deep, groove on the upper side of the metal electrode of another 22 x22 x 2 cm s current limited spark chamber SC2. Vacuum oil in the groove forms a gas seal and allows the glass cylinder to be easily separated from SC2. The top electrode of SC2 has a regular array of 2272 small holes drilled in it to form a rectangular matrix of 32 x71 holes, with a hole separation of 2.5 ram. This matrix may be considered in two parts: one part is the test area (referred to as " A " ) and consists of 32 x32 holes; and the remainder (referred to a s " B") is used solely to simulate a large density of tracks in the vicinity of the test area. The matrix is adjusted relative to the top chamber SC1

glass cylinder oil seal / b o l e matrix

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sc 2 Fig. 2. The apparatus used to simulate and investigate a current limited spark chamber operating with moderate track densities.

LIMITED SPARK CHAMBERS

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such that the holes in the wall of SC1 are symmetrically placed over the two parts of the matrix. A perspex shield (not shown in the figure) ensured that the test area received photons from SCI through only one of the two holes. Thus, photons produced in SC1 pass through the two holes in its wall and are incident on the matrix of holes in the electrode of SC2. Each hole in this matrix is 2 mm long and 0.5 mm in diameter: thus, a finely collimated beam of photons will cross the gap of SC2 through each of the matrix holes. These photons in turn will produce photoelectrons in the gap of SC2, forming "ion tracks" adjacent to each hole. If then SC1 is pulsed I ps before SC2, the latter chamber, prior to being pulsed, is in a state simulating a large density of near vertical particle tracks, By making the lowest electrode of SC2 of positive polarity the photoelectrons produced at the glass surface do not contribute to the discharge. The discharges occurring in SC2 were photographed by a camera placed at least 60 cm from the chamber utilizing a 45 '~ mirror placed below the grid covered glass insulator of SC2. The camera was aligned to view the matrix A. The experiment was carried out in several parts, In the first instance part B of the matrix was covered such that only matrix A was irradiated. This corresponded to a uniform density of 16 x 104 particles m -2. This was repeated with matrix B uncovered. By appropriately covering some of the holes in the test area, the experiment was again repeated at a density corresponding to 104 particles m 2 both with and without matrix B oFerative. Finally, as a crude check of the simulation of particle tracks, all but three of the holes of matrix A were covered and the apparatus was triggered by tLe traversal of a cosmic ray through the test area. Thus, three simulated sparks were observed together with one genuine particle track. No visual difference between the tracks was observed. In fact, for vertical tracks it is most improbable that the spark mechanism operative in these chambers is dependent upon the number of initial ions providing a linear density ~ 4 c m -1 is exceeded. A cosmic ray will produce ~ 2 0 ions cm -I, which is well above this minimum. Consequently, all that is required of SC1 is to produce sufficient photons to provide more than 4 photoelectrons cm -1 in the gap of SC2. From other experiments conducted with similar photon sources (to be published) this requirement is almost certainly satisfied. It is safe to conclude that this apparatus does simulate particle tracks satisfactorily at least in the context of the present experiment; that is, the "particles" are nearly vertical and there is no significant

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delay between the production of the electron trail and the application of the high voltage pulse to SC2. Considering first the results obtained from the low density matrix of 64 holes corresponding to a density of 104 particles m -2. It was found that the spark efficiency was 0.94+0.02. However, it was found that two of the holes very rarely had sparks adjacent to them and it is most probable that these were constricted by dust. If these holes were omitted from the analysis, the spark efficiency was 0.97+_0.02, which is consistent with the single particle efficiency of these chambers. This result was anticipated from the other characteristics of current limited spark chambers, and it is reasonable to conclude that, at (uniform) densities below 104 particles m 2 the existence of multiple tracks has no effect on the operation of the chamber. Furthermore, it was found that the presence of matrix B had no effect on the spark characteristics or on the efficiency. At the higher density (16<104 particles m -z) extreme difficulty was encountered in analysing the films. Although it is possible to isolate individual spark ends near the centre of the matrix, at the edges the inclination of the tracks (never greater than 7.5 ° to the vertical) and the off-axis camera position combine to cause the spark images to overlap and become unresolved. However, using the small area of matrix A that facilitated individual spark identification ( ~ 6 cm2), the efficiency of spark production remained very close to unity. 5. Conclusions of the uniform track density experiments The results of the previous work has shown unequivocally that very high densities can be recorded. It is concluded with some confidence that at these densities the probability of occurrence of a spark is influenced very little by the presence of other nearby sparks. Clearly there is a limit to the validity of this conclusion when very small ( 4 3 ram) track separations are involved. However, we have resisted the temptation to determine this limiting separation of tracks, since it is apparent that the intrinsic properties of the chambers will not limit the multiple efficiency. Certainly, even in the controlled situation of a sFecially designed experiment, it is difficult to photograph spark densities of 16x 104 sparks m -z except over a very limited area of chamber and for vertical sparks. It is our belief that the photographic difficulties will present the dominant limitation in any real experimental environment.

In a practical case the efficiency would also be limited by the non-uniform nature of the particle tracks. Assuming that N statistically independent tracks are produced in some area A, it is easy to show that the mean number of resolvable tracks n = (A/~)(1

--e-~U/a),

where it is assumed that two or more tracks falling within an area ~ will result in a single spark (either as two unresolved sparks or as a single channelled spark). It is evident that ~ will be determined primarily by the photographic system and the angular distribution of the incident particles. 6. Ultra high track densities The track densities registered in real experiments normally fall well below the track densities discussed previously. However, exceptions occur with events involving the core of energetic cascade showers and, particularly, in some Extensive Air Shower research. It is evident that this detector cannot (nor indeed can any other) detect individual particles at track densities exceeding 20 × 104 tracks m 2. This does not necessarily imply that the chamber ceases to be of use, for it may still record individual particles in other, less populated regions of the chamber. In the remainder of this work therefore, interest is centred not on the efficiency of recording tracks in a localised region of extremely high particle density, but on the effect of such a region on the registration of an individual track in its vicinity. Even with single track use the current limited spark chamber exhibits a characteristic glow around the spark. Although not completely understood, this glow undoubtedly originates from photo-electrons forming small avalanches in the vicinity of the spark. Consequently, it can be expected that, as the multiplicity of tracks increases, the background glow becomes very considerable. Thus, in the exFeriment described in section 4, even with a density of 104 tracks m 2 the individual sparks are difficult to extricate from the background glow when the chamber is viewed parallel to the electrodes. Presumably the luminosity of the glow is related directly to the local ion density; thus, as higher track densities are envisaged, it is probable that the gas can be considered as a homogeneous conducting plasma. Such a plasma would considerably reduce the potential across the gas gap thus causing gross field distortions within the chamber. Such an effect would certainly be expected to influence the development of sparks in adjacent regions of more moderate track density.

THE MULTIPLE EFFICIENCY OF CURRENT LIMITED SPARK CHAMBERS In fact it is easy to simulate such a situation. This has, been done by partially or wholly irradiating a chamber with UV photons prior to pulsing it. When thi,~is done, intense glow is observed and, most important, a very fast reduction in the applied pulse is apparent, Thus, when extremely high track densities are involved, not only is gross field distortion possible but the applied pulse itself may be severely affected. 7. The mechanism of a current limited spark chamber at ultra high track densities

It is anticipated that an ultra high track density will cause intense ionisation and glow rather than identifiable individual sparks. In fact, the most reasonable expectation is that the gas becomes a conducting plasma, which will cause not only gross field distortion, but an alteration in the capacity of the chamber. In the limiting case of glow throughout the entire chamber one would expect the chamber capacity to alter from C~,~A/9 to C ' ~ e A / d , where A is the area of the chamber, g the gap width, and d and e the insulator thickness and dielectric constant respectively. Such a capacity change would clearly cause the applied voltage pulse across the chamber to fall markedly. An experiment has been devised to verify this process. If a very long pulse length is used with a current limited chamber, the applied voltage pulse (before significant decay has occurred) will be Vr = V o C p ( C p + C ) - 1

(I)

where V0 is the dc high voltage potential and Cp the capacitance of the pulsing capacitor. Subsequently the voltage will decay slowly to zero. It is thus possible, by measuring Vf to find C, since both V0 and Cp are known. In general the spark chamber capacitance is given by C = [C;'+C;-1] -' +Cw+C,, where the suffixes g, i, w and t refer to the gas gap, glass insulator, glass walls, and strays, respectively. Normally, of course, Cg is the dominant factor; however, if the chamber is operated under conditions in which the entire gas volume behaves like a conducting plasma, then the chamber capacitance becomes C' = C~+Cw+C,.

(2)

Of these terms, Ci = eA/d and Ct is very small and constant. The capacitance due to the glass walls is not easy to compute, since the field is certainly not uniform in this region. However, if it is assumed that the gas behaves like a conductor then the field in the glass

379

walls will also be contained in a thickness of the order of d. Thus, Cw < ~'A'/d, where A' is the area of the glass wall capacitor and e' is the dielectric constant of glass. Normally Cw is only a small corrective capacitance to the dominant Ci. It is possible, therefore, to operate a chamber under conditions of excessive ionisation and measure the capacitance C' using eq. (i). This can be compared with the value expected from eq. (2) on the assumption that the gas gap of such a chamber is conducting. A conventional spark chamber (that is, without a solid insulator between the electrodes) was constructed with dimensions 2 1 . 2 x 1 6 x 2 . 6 5 c m 3. A spark gap was placed adjacent to a hole in the top electrode of the spark chamber such that, when the gap discharged, the spark chamber gas was irradiated by UV photons. The spark chamber was placed accurately horizontal and the interior filled to a depth d with rotary vacuum pump oil, which acted as a variable thickness insulator of known dielectric constant. The chamber was pulsed 1 ps after the photon spark gap had discharged. Over a wide range of Vo (from 10 to 18 kV) the chamber invariably glowed intensely and the applied voltage pulse showed a very fast reduction in potential. Thus, even though the CR time of the pulsing network was several milliseconds, the effective pulse length was much less than 100 ns which is shorter than the pulse length normally used with these chambers. It was considered, therefore, that the system simulated a small current limited spark chamber operating under quite normal conditions of pulse height and length when subjected to a very high density of incident particles such that the initial ionisation of the entire gas gap was almost homogeneous. The experiment consisted of measuring the ratio Vo/Vr for a variety of insulator thicknesses and dc potential Vo. In fact, no change was observed in the ratio Vo/V f over the entire range of V0 used. Using 80O

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R.M. BULL et al.

eq. (1), fig. 3 shows the variation of C' with insulator thickness d. From eq. (2), the gradient of this line should be (cA +e'A') leading to a measured value of the dielectric constant of oil of 2.26+_0.1 compared with the known value of 2.1 +0.1. This good agreement is excellent evidence that the gas in such a discharge is conducting. Although it is recognised that the simulation of an extremely high particle density is not perfect in these experiments, it is concluded that in these chambers and probably in a practical chamber operating with ultra high track densities, the glow regions behave like conducting mediums with a consequential reduction in the applied voltage pulse. It is very reasonable to assume that, if only a localised region of area ~ was subjected to such high track densities, the capacitance of the chamber would increase to

C' -~ (A - ~ ) / g + e:~/d, causing an appropriate rapid drop in the applied voltage pulse.

8. Conclusions

This work is but a preliminary study of the effects of ultra high track densities. It is clear that both field distortion and pulse reduction may and probably will occur. To what extent and with what effect is still uninvestigated. That the effects might be quite striking is anticipated. Let us suppose a region of the chamber glows as a result of a high track density. It would seem probable that the development of this "glow" discharge will occur quite quickly; possibly much quicker than the development of a single spark. In fact, there is some evidence for this since the applied pulse falls abruptly

within about 5 ns suggesting that the transition between normal avalanching and the final conducting plasma is very fast. This is a much shorter time scale than is normal with a single spark. If, in fact, the "glow discharge" precedes the development of single spark channels in neighbouring low track density regions, it may be expected that these sparks may n o t reach maturity due to field distortion. Using simple field sketching techniques it is estimated that such an effect might be noticeable to distances 4 d from the glow region. Even more severe effects may occur in the case of tracks situated between two glow regions. Furthermore, if the high particle density is sufficiently extensive the voltage on the whole chamber will fall tending to inhibit the development of sparks throughout the entire chamber. In a large chamber even more complicated effects might occur due both to the finite time that pulses are propagated over the chamber and, more important, to the variation of pulse height over the area of the chamber. It is not useful to hypothesise the effects that might be observed: it is probable that they will depend upon the dimensions of the particular chamber, its construction details and storage capacitor. It is more useful to emphasise that the performance of large current limited spark chambers working with a "glow region" is suspect. The authors would thank the Science Research Council for the provision of an oscilloscope, without which much of this work would have been impossible. One of us (J.H.Y.) would thank the same council for the provision of a CAPS award. References

1) S. Fukui and S. Miyamoto, Nuovo Cimento ll (1959) 113. 2) S. Fukui and S. Miyamoto, J. Phys. Soc. Japan 16 (1961) 2574.