Detection and energy measurement of electrons above 1 GeV in a multiplate spark chamber

NUCLEAR

INSTRUMENTS

AND

METHODS

88

(I97O)

IO9-II8;

©

NORTH-HOLLAND

PUBLISHING

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D E T E C T I O N AND E N E R G Y M E A S U R E M E N T OF E L E C T R O N S ABOVE 1 GeV I N A MULTIPLATE SPARK CHAMBER B. A G R I N I E R , Y. K O E C H L I N , B. P A R L I E R , J. P A U L and J. V A S S E U R

Service d'Electronique Physique, Centre d'Etudes Nuch;aires, Saclay, France G. B O E L L A and G. S I R O N I

CNR, Laboratorio di Fisica Cosmica e Tecnologie Relative, Istituto di Scienze Fisiche dell' Universitc~, Milano, ItaO' A. R U S S O and L. S C A R S I

CNR, Laboratorio di Fisica Cosmica e Tecnologie Relative, Istituto di Fisica dell' Universit&, Palermo, Italy Received 6 July 1970 The response o f a multiplate spark chamber to electron shower events has been investigated, for primary electron energy in the range 1 - 14 GeV, with exposures to m o m e n t u m analyzed beams at the DESY electron-synchrotron and at the proton-synchrotron

of C E R N . The detector discrimination properties between electron showers and nuclear interactions are discussed; experimental relations are given between the shower image brightness, measured photometrically, and the energy of the primary electron.

1. Introduction

discriminate, for momenta above ~ l GeV/c, an electron component of a relative intensity as low as a fraction of a percent in a flux of nuclear active particles, such as the cosmic rays, allowing the identification of the single events6). For the events identified as electron cascades, spark counting or total width measurement on the groups of sparks and photometric measurements on the spark images have been used to relate the shower observed to the energy of the generating electron; the results indicate interesting possibilities in the use of the spark chamber as an electron energy measuring device, with a precision comparable to that obtainable with conventional calorimeters or with nuclear emulsions of equivalent depth in radiation lengths. The data obtained with the J-I detector, already published3'6), will be recalled when relevant to the discussion: in the following we will be mainly concerned with the calibration of J-II.

A small multiplate spark chamber controlled by the coincidences between the elements of a plastic scintillator telescope has been used as a detector in a series of balloonborne experiments carried out by the Milano-Palermo and Saclay cosmic ray groups with the aim of measuring the flux and energy spectrum of the primary cosmic ray electrons in the region above few GeVI"5). Two types of apparatus have been employed, differing in the number and nature of the plates in the spark chamber, but otherwise essentially equivalent. A detailed description of the second version, (J-II)4'5), is given in section 2; the first version, (J-I) 1,2,3,6), differed only for the total radiation lengths contained in the spark chamber (5 X0 of lead instead of 7 X 0 of tungsten) and the number of plates (9 instead of i1). In order to investigate their discriminating power between electron showers and nuclear interactions and to establish an experimental relation between the energy of the incident electron and the characteristics of the shower developped in the spark chamber, the detectors have been exposed to momentum analyzed beams of known particles. The analysis of the events is made on the picture recorded on 16 mm K o d a k tri-X film by a stereo camera. The results from calibration runs with mixed beams of electrons, muons, pions and protons in the momentum range 450 MeV/c14 GeV/c at the proton-synchrotrons of Saclay and C E R N and with electron beams in the 1-5.8 GeV energy range at the electron-synchrotron of Hamburg, together with the data obtained in the balloon flights, have shown that by visual criteria it is possible to 109

2. Experimental technique 2.1.

THE SPARK CHAMBER

The spark chamber of the detector J-II consists of 11 plates, 110 × 110 mm square, separated by 10 m m gaps, enclosed in a stainless steel vessel filled with a gas mixture of 99.5% neon and 0.5% argon, at 800 mm Hg pressure. The first three electrodes on-top and the last one are made with aluminium, 3.3 mm thick (0.03 rad. length); the remaining 7 electrodes are of tungsten, 3.3 mm thick (1 rad. length). The spark chamber layout. is shown in fig. la; the interelectrode gaps are labeled, from top to bottom: A l, A2, A3, 1, 2, 3, 4, 5, 6, 7.

110

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,

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DETECTION 2.2.

AND

ENERGY

MEASUREMENT

COINCIDENCE TRIGGER AND HIGH VOLTAGE SUPPLY TO THE S P A R K C H A M B E R

The schematic diagram of the high voltage circuit of the spark chamber is shown in fig. lb. The 10 gaps are in parallel connection and receive power from the same charged capacitor: the odd electrodes are connected to the ground line, the even electrodes to the high-voltage feeding capacitor. The control counter telescope consists of two plastic scintillators, one above (counter A: 100 x 100 × 10 m m 3) and one below the chamber (counter B: 120x 120× × 30 mm 3), fig, I c. The coincidence signals trigger the high voltage system; the overall time delay between the passage of the particles in the spark chamber volume and the appearance of the maximum high voltage value on the electrodes is in the range 3 0 0 - 3 5 0 nsec. The memory time of the chamber is limited to about 1ysec by the application of a clearing field of 50 V/cm. 2.3. EVENT RECORDINGAND ANALYSIS The events occurring in the spark chamber are photographed, through a glass viewing window, by a stereo camera at l meter distance on 16-ram K o d a k tri-X

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OF ELECTRONS

111

film. The processing of the film is carried out always in the same controlled conditions. The analysis of the events recorded on film involves three stages: a. "Pattern recognition" by visual inspection, on a magnified image, of the distribution of sparks in the spark chamber gaps, in order to classify the type of event. b. "Incoming direction" of the primary particle in the reference frame of the spark chamber, determined by measuring the coordinates of the sparks in the two stereo views with a digitized coordinatometer. c. "Electron energy determination" through measurements of the shower size in the various gaps of the chamber. Different methods are available: - - Below 1 GeV, at low particle density in the shower, straightforward counting of the number of sparks is possible6). - - For electron energies above 1 GeV, the increased density of particles in the shower core lowers the efficiency of formation of single sparks from each individual particle. The energy dependent parameter is now related to the brightness and size of the light bands appearing along the shower axis, and is determined photometrically. The photometric measurements of the shower image on the film have been made with a microdensitometer (CDC-Vassy type M D3); a schematic drawing of the apparatus is given in fig. 2. The film is scanned along the gap images with a slit, 1.5 mm high and 0.06 m m wide; for each gap a density profile of the spark images is obtained, which is then translated into a spark brightness profile through a calibration curve determined with light sources of known intensity and color simulating the sparks in the chamber and photographed through the same optical system. 3. Experimental results

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In the calibration tests with J-II, four classes of events have been used to investigate the response of the spark chamber in the various conditions: a. Relativistic singly charged particles crossing the chamber plates in straight line without producing secondary particles (#-mesons or non-interacting protons and n-mesons) (fig. 3a). b. Multiply charged particles, mainly alfa particles, crossing the chamber in straight line without interacting (fig. 3b). c. Nuclear interactions from protons or n-mesons, from momentum analyzed beams at the C E R N proton-synch rotron.

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Fig. 3a. Singly charged relativistic particle.

d. Electron showers, with exposures to momentum analyzed beams at the DESY electron-synchrotron in H a m b u r g (1, 3.2, 5 and 5.8 GeV) and at the C E R N proton-synchrotron (6, 8, 10 and 14 GeV), fig. 4a, b, c. Class (a) events were obtained both at accelerators and in the balloon flight exposures; class (b) events in the balloon flights only. Distinction between (a) and (b) events was based on pulse height analysis of the signals from the upper scintillator of the triggering telescope. The events, recorded on film, were analyzed both by visual inspection and with photometric measurements. 3.1. VISUAL INSPECTIONANALYSIS

3.1.1. Visible sparks in the spark chamber gaps from single non interacting particles By analyzing the class (a) events, the efficiency of the various gaps in giving a visible spark shows small systematic differences, the absolute values ranging from 90% to 95%; the probability of finding at least one spark in the three gaps AI, A2, A3, is 99%. For class (b) events, the efficiency is better than 98% for each gap, reaching ,-~ 100% for one spark at least in one of the three gaps AI, A2, A3. 3.1.2. Visual structure of electron showers An high-energy electron normally crosses the gaps

Fig. 3b. Multiply charged particle.

AI, A2, A3 as a single particle and starts the multiplication process in the first or second tungsten plate; a multiparticle event is therefore present in the gaps 1 or 2 to 7, the number of particles and their density along the shower axis in each gap being dependent on the primary electron energy. The gaps AI, A2, A3 are crossed by a relativistic singly-charged particle, such as for class (a), but now the probability of having one spark at least in these three gaps, is found to decrease systematically with increasing primary electron energy, going from 98% at 1 GeV to 2% at 14 GeV (fig. 5). In addition, in the multiplication gaps (1 to 7), at high density of particles near the shower axis, more than one particle contribute with ions along the path of the spark discharge; multiparticle sparks get wider and brighter, like light bands, while single-particle sparks become thinner and fainter; the event tends to lose its widened multispark structure, reaching, at the highest electron energies, the aspect of a narrow, compact, pencil-like beam (fig. 4a, b, c.) 3.2. DISCRIMINATION BETWEEN EI_ECTRON SHOWERS AND NUCLEAR INTERACTIONS

In exposure to a mixed beam of electrons and minimum ionizing nuclear-interaction particles giving rise to multispark events in the spark chamber, electron showers and nuclear interactions are distinguishable,

DETECTION

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A N D E N E R G Y M E A S U R E M E N T OF E L E C T R O N S

(a)

(c) Fig. 4. P h o t o g r a p h s o f electron shower events. Shower initiated by an electron of: 1 (a); 6 ( b ) a n d 14(c) GeV.

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on the single event basis, by the following visual criteria, defined after examination of events recorded in exposures to accelerator beams in the momentum range (1-14 GeV/c).

a. Backward moving rescondaries, frequently present in the nuclear interactions, do not occur in electron showers. b. At equal track multiplicity, the angular dispersion of the secondaries is broader for nuclear interactions than for electron showers. c. Most of the secondaries from nuclear interactions

114

B. AGRINIER et al.

cross the chamber plates in straight lines, easily identified. d. In electron showers, at low energy the sparks are somewhat dispersed; at high energies the sparks in the shower head (gaps A1, A2, A3) are very faint or absent, while in the multiplication gaps (1 to 7) the shower loses its multispark structure, assuming the shape of a narrow compact beam. The visual separation of electron shower from nuclear interactions is then controlled by studying the statistical distribution of the events, that is, shower origins and interaction vertexes, along the chamber plates. Since each of the 7 tungsten plates covers one radiation length for electrons, but only 1/30 of nuclear interaction length for protons or n-mesons, the distribution of the total number of events (showers and interactions) in each plate vs plate of origin shows a marked excess in the first two radiation lengths essentially due to electron showers. From such a distribution the fraction of electron showers can be calculated. This control, carried out during the calibration runs and for the balloon flight exposures, shows regular consistency with the number obtained by visual selection. For particles of momentum above 1 GeV/c, a fraction of a percent of electrons, present in a mixed beam of protons and pi-mesons, can be discriminated. 3.3. PHOTOMETRICMEASUREMENTS Examples of densitogrammes are given in figs. 6 and 7. By analyzing the various classes of events, the following general results have been obtained.

3.3.1.

Spark brightness

- - For the electron showers, the peak brightness values observed in the gaps 3 to 7, where the shower development is well under way, do not vary appreciably with the primary electron energy in the entire energy range explored (1-14 GeV): they are equal to those observed for the z~>2 single particle events; the spark brightness appears therefore to be independent from the particle density in the shower core (above a given limit) reaching a saturation level already for 1 GeV electrons. - - The average brightness B A = al-(BA1-I-BA2nt- BA3 ) of the "shower head" shows a marked dependence on the primary electron energy, decreasing systematically with increasing electron energy; fig. 8 shows the results obtained over a sample of 42 electrons of 1 GeV, 50 of 3.2 GeV and 31 of 5 GeV: in the same scale of fig. 8, the average brightness B== 1 for relativistic singly charged particles crossing the chamber without multiplication is 100 brightness units and B=~>2 = 150 brightness units. - - Several examples of two electron showers recorded simultaneously by the spark chamber have been observed in the calibration runs at DESY. The charac-

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DETECTION

A N D E N E R G Y M E A S U R E M E N T OF E L E C T R O N S

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116

s. AGRINIER et al.

teristics of each of the two showers appears very nearly the same as those of the events recorded individually; it follows that the "brightness saturation" phenomena have to be ascribed to the structure of the shower itself and not to an overall saturation effect of the spark chamber.

For a single track crossing the spark chamber, the fluctuation of the brightness value B/ in the iH gap, around the average B along the track in the chamber, follows with good approximation a gaussian law with a standard deviation characteristic of this type of event. The average of l0

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= O. l l .

The value of ~b is lower for the more ionizing particles. In a similar way the behaviour of the brightness fluctuation has been investigated for the electron shower events on the gaps 3 to 7. The results are given in table 1 and fig. 9. In the energy range explored the mean value ~5~ appears to be dependent on the primary electron energy E, following a law (~p oc E - ( ° ' 4 ° - + ° ' ° 5 ) In an electron cascade, at a depth i, the number N~ of i

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7 Fig. 9. q~# = ½ Z IBi-B371/B3v vs p r i m a r y e l e c t r o n e n e r g y i=3 (the e r r o r b a r s c o r r e s p o n d t o -~ o n e s t a n d a r d d e v i a t i o n in t h e d i s t r i b u t i o n o f ~be).

7

q~ = ~ }2 IB~- B3~!/B~ ; B,~ -- ~ y~ B,. i=3 Primary electron energy (GeV)

3.3.2. Fluctuation of the spark brightness value

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electrons is very nearly proportional to the primary electron energy; the fluctuations on N i is given by a(y,) ~ E -b~ 7), with b i depending on the depth along the shower ( near shower maximum is ~ 0.5). The similar power law dependence of qSp and acN,) with energy suggests that the fluctuations in brightness are related to the fluctuations in the number of particles which provide ions for the spark. A simple model can be proposed to account for the behaviour of the spark chamber used in our experiments; the main points are the following: - - Up to the saturation level, the spark brightness is related, at a given interelectrode voltage, to the ionization density present along the spark path at the instant of the hv appearance on the electrodes. For J-lI, the saturation level is reached between l and 4 minimum-ionizing particles in the shower core; the saturation is not reached for one minimum ionizing particle. - - The time delay (At)~, between the appearance of the high voltage on the electrodes and the reaching of the local sparking conditions in a gap, is a function of the initial ionization density along the spark path, being shorter for higher initial ionization densityS). - - Local sparking occurs with delays (At)~ increasing with decreasing initial ionization density. - - When many differently ionized paths are simultaneously present, like in a shower, another phenomenon is superimposed: the more ionized paths will spark earlier than the less ionized ones, starting the discharge of the common feeding capacitor; consequently due to the progressive lowering of the hv supply, less ionized paths will spark with an additionally increased delay or even will not spark at all. Of course, this interdependence is common to all the gaps. Because the shower core gives rise in the spark chamber to an increasing local ionization with increasing shower energy, the core sparks will have short

117

D E T E C T I O N A N D E N E R G Y M E A S U R E M E N T OF E L E C T R O N S

(At)s and will originate strong light bands, while side

TABLE 2

sparks in all gaps together with the shower head sparks in the A1, A2, A3 gaps will be progressively weakened and eventually will disappear (§ 3.1 and 3.2). Fluctuations in ionization density give rise to fluctuations in (At)s and therefore to fluctuations in the spark brightness (§ 3.3.1).

Primary electron energy (GeV) N u m b e r of events measured

50

3.4. ELECTRONENERGY MEARUREMENTS

2 ~x

The results described above have been used to derive practical relations, in order to evaluate the primary electron energy from measurements on the electron shower observed in the spark chamber. 3.4.1. Energy range 1-6 GeV A parameter 2 is defined as the ratio between the sum of the width of the brightness profiles in each gap and the mean brighness value BA=½(BA~ + B A z + B 3 A ) of the shower head. Table 2 and fig. 10 show the values of 2 obtained from the calibration runs at accelerators: over the 1-6 GeV interval the standard deviation on the energy measurements, corresponding to a2, is ~ 40%. 3.4.2. Energy > 6 GeV For electron energies greater than 6 GeV, the probability of visible sparks on the "shower head"becomes too low (fig. 5) and the 2-method is practically ruled out. The shower parameter 7

(~ = ~ Y~ IBI-BzTI/Bz7, i=3

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Fig. 10. 2 vs primary electron energy (the error bars correspond to ~z one standard deviation in the distribution of 2).

Calibration values of 2 in the energy range 1 - 6 GeV.

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6

42

37

30

25

75

105

120

10

20

28

30

~40%

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~z40%

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defined in section (3.3) can be used for energy measurements. The calibration made covers the range 6 - 1 4 G e V (table 1 and fig. 9); on the other hand, following the given interpretation, ~b~ is still far from being saturated and an extrapolation of the power law dependence to higher energies as given in fig. 9, seems reasonable. In the calibration range the standard error in the energy evaluation is roughly (+~oo%~ 50%J of the measured value; for higher energies the error is difficult to evaluate, but we estimate it should remain of the same order up to several tens of GeV. 4. Conclusions In investigating the primary cosmic ray electron flux, two main problems are present: (a) the separation of the electron component, mixed in a flux of protons and other nuclear active particles about two orders of magnitude more abundant; (b) the energy measurement of the identified electrons. Purely electronic detectors, although having good possibilities for the energy measurement, suffer from the disadvantage of a rather poor discriminating power between electron-events and nuclear interactions simulating " p u r e electron s h o w e r ' ; the selection is often based on subtraction methods between samples with different enrichment in the two components. At the other extreme, the nuclear emulsion, a detector with potentially excellent properties in both discrimination and energy measurement precision, is handicapped by the lack of time resolution and by efficiency problems, connected to the difficulty of estimating the scanning efficiency as a function of the electron energy. The multiplate spark chamber lies in an intermediate position and it appears to be a good compromise in the energy region of the GeV's and tens of GeV. Obviously composite apparatus, combining in a convenient way different types of detectors, can

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be devised and have been employed, a l t h o u g h the a d d e d c o m p l i c a t i o n often does n o t p r o p o r t i o n a l l y p a y for the better overall response. A multiplate s p a r k c h a m b e r o f the type described in this p a p e r (§2), when used as an electron detector for energies .~ 1 GeV, shows the following interesting features: - - The use o f a parallel hv supply to the c h a m b e r , o b t a i n e d connecting the o d d electrodes to the g r o u n d line and the even electrodes to a single hv feeding c a p a citor, gives the a d v a n t a g e o f correlating the shower images in the different gaps, p r o v i d i n g usefull p a r a meters for the identification o f the events and the m e a s u r e m e n t o f the energy on each electron event. -By visual analysis o f the spark d i s t r i b u t i o n in the recorded events, a relative p r o p o r t i o n o f a fraction o f a percent o f electrons present in the cosmic ray flux, for m o m e n t a ~ 1 GeV/c, can be d i s c r i m i n a t e d ; electron showers are individually identified and a b o v e ~ 1 GeV, energy m e a s u r e m e n t on the electron showers can be m a d e by analyzing the s p a r k brightness distribution in the gaps. F o r a 10 plates spark c h a m b e r as J-II, with the first 3 electrodes only a small fraction o f r a d i a t i o n length thick and the following 7 o f 1 r a d i a t i o n length each, in the 1-6 G e V interval, the s t a n d a r d error in the energy m e a s u r e m e n t s for individual electron showers is ~ 40%. F o r energies ~ 6 G e V the error increases to A E / E ~ (+ loo%~. 5o%/, although the calibration was limited to 14 GeV, an e x t r a p o l a t i o n o f the experimental curve to t o w a r d s the 100 G e V region seems possible, w i t h o u t loosing t o o m u c h in energy resolution. A n increase in the n u m b e r o f r a d i a t i o n lengths present

in the s p a r k c h a m b e r , together with an increase in the n u m b e r o f gaps by using thinner plates, should offer a d v a n t a g e s for the precision in the energy measurement. This w o r k has been m a d e possible by the generous help o f the G r o u p s o f D E S Y and C E R N , who allowed us the use o f their b e a m s ; we wish to t h a n k particularly Drs. D. Degele, K. Lutz, H. D. Shultz, U. T i m m , G. W e b e r and all the Leiden C o s m i c R a y G r o u p s for their friendly c o o p e r a t i o n . O u r gratitude goes to Prof. C. Dilworth, Prof. J. L a b e y r i e and Prof. G. Occhialini for their c o n s t a n t s u p p o r t and criticism. References

1) B. Agrinier, Y. Koechlin, B. Parlier, G. Boella, G. Degli Antoni, C. Dilworth, L. Scarsi and G. Sironi, Phys. Rev. Letters 13 (1964) 377. 2) C. J. Bland, G. Boella, G. Degli Antoni, C. Dilworth, L. Scarsi, G. Sironi, B. Agrinier, Y. Koechlin, B. Parlier and J. Vasseur, Phys. Rev. Letters 1'7 (1966) 813. a) C. J. Bland, G. Boella, G. Degli Antoni, C. Dilworth, L. Scarsi, G. Sironi, B. Agrinier, Y. Koechlin, B. Parlier and J. Vasseur, Nuovo Cimento 55A (1968) 451. 4) B. Agrinier, Y. Koechlin, B. Parlier, J. Paul, J. Vasseur, G. Boella, C. Dilworth, L. Scarsi, G. Sironi and A. Russo, Lettere Nuovo Cimento 1 (1969) 53. 5) B. Agrinier, Y. Koechlin, B. Parlier, J. Paul, J. Vasseur, G. Boella, G. Sironi, A. Russo and L. Scarsi, Proc. X1 Intern. Conf. Cosmic ray (Budapest, 1969) to be published. 6) B, Agrinier, Y. Koechlin, B. Parlier, G. Boella, G. Degli Antoni, C. Dilworth, L. Scarsi and G. Sironi, Nuovo Cimento 36 (1965) 1077. 7) N. M. Gerasimova, Soviet Phys. JETP 16 (1963) 358. 8) L. Caris, B. Kniper and E. M. Williams, Nucl. Instr. and Meth. 59 (1968) 145.