Accelerator evaluation of a spark-chamber experiment for gamma-ray astronomy

NUCLEAR INSTRUMENTS AND METHODS IO8 (I973) 257-268; © NORTH-HOLLAND PUBLISHING CO.

ACCELERATOR E V A L U A T I O N OF A SPARK-CHAMBER EXPERIMENT FOR GAMMA-RAY A S T R O N O M Y G . F . BIGNAMI, J. PAUL, E. PFEFFERMANN, B.N. SWANENBURG, A. SCHEEPMAKER*, W. H. VOGES and R. D. WILLS THE C A R A V A N E C O L L A B O R A T I O N Cosmic-Ray Working Group, Kamerlingh Onnes Laboratorium, Leiden, The Netherlands Laboratorio di Fisica Cosmica e Tecnologie Relative del CNR, lstituto di Scienze Fisiche dell' Universith di Milano, Italy M a x Planck Institut fiir Physik und Astrophysik, Institut fiir Extraterrestrische Physik, Garching near Munich, Germany Service d'l~lectronique Physique, Centre d'l~tudes Nucl~aires de Saclay, Gif-sur-Yvette, France Space Science Department, European Space Research and Technology Centre, Noordwijk, The Netherlands

Received 17 July 1972 and in revised form 11 December 1972 The ESRO satellite COS-B will carry a single experiment, aiming at the detection and energy measurement of cosmic gamma radiation at energies above 25 MeV. The instrument incorporates a wire matrix spark chamber and a caesium-iodide energy spectrometer. The response of the instrument to gamma rays has been determined using a tagged gamma-ray beam (25300 MeV). A first estimate of the susceptibility to charged-

particle background has been obtained from an exposure of the instrument to protons of 1450 MeV/c. The measured performance characteristics of the experiment are presented and discussed. As a result of the tests improvements in the design of the flight instrument, leading to increased sensitivity to gamma rays and a lower susceptibility to background, could be incorporated.

1. Introduction

rays by a b o u t f o u r orders o f m a g n i t u d e - the ability to reject this b a c k g r o u n d m u s t be extremely good. 3) The requirements o f efficiency a n d precision are n a t u r a l l y conflicting - the thick plates needed to optimise conversion d e g r a d e the a n g u l a r resolution a n d energy m e a s u r e m e n t . The design o f a g a m m a - r a y a s t r o n o m y e x p e r i m e n t s h o u l d aim at the best possible c o m b i n a t i o n o f efficiency, a n g u l a r resolution a n d energy resolution. It is i m p r a c t i c a b l e to evaluate theoretically the detailed p e r f o r m a n c e o f a p a r t i c u l a r instrument. I n s t e a d p h o t o n b e a m s a n d charged particle b e a m s can be used as efficient tools in o p t i m i z i n g the design and perform i n g an a b s o l u t e calibration. This is especially i m p o r tant since the experiments so far c o n d u c t e d have not all been in full agreement. K r a u s h a a r 2) has described the use o f a tagged g a m m a - r a y b e a m to p r o v i d e comp a r a t i v e m e a s u r e m e n t s o f two different experiments which resulted in a significant i m p r o v e m e n t in the consistency o f their results. One o f the experiments now being d e v e l o p e d for g a m m a - r a y a s t r o n o m y in the energy range a b o v e 25 M e V will f o r m the p a y l o a d o f the E u r o p e a n Space R e s e a r c h O r g a n i s a t i o n ' s satellite COS-B3'4). It is designed a n d built by a c o l l a b o r a t i o n o f l a b o r a t o r i e s a n d the m e a s u r e m e n t s described in this p a p e r f o r m a p a r t o f the c o l l a b o r a t i v e effort. In o r d e r to assess the p e r f o r m a n c e o f the p r o p o s e d design 3'5) before the

G a m m a - r a y a s t r o n o m y is a y o u n g b r a n c h o f astrophysics, a i m i n g at the identification o f high energy processes in discrete sources, in the galaxy or in intergalactic space. High-energy g a m m a rays (above a b o u t 20 M e V ) m a y be p r o d u c e d b y inelastic nuclear interactions, b r e m s s t r a h l u n g o r the inverse C o m p t o n process. I n this energy range a b s o r p t i o n m a y be neglected, even on a c o s m o l o g i c a l scale. T h e observ a t i o n o f these g a m m a rays, i.e. the m e a s u r e m e n t o f arrival direction a n d energy, w o u l d enhance o u r k n o w l e d g e o f the d i s t r i b u t i o n s o f cosmic r a d i a t i o n , m a t t e r a n d m a g n e t i c fields in the galaxy and in intergalactic space a n d w o u l d thus have an i m p a c t on a n u m b e r o f fields o f a s t r o p h y s i c a l interest. T h e present g e n e r a t i o n o f experiments 1) are b a s e d a l m o s t exclusively on s p a r k c h a m b e r s in which the p h o t o n s are c o n v e r t e d to electron pairs a n d the electron directions are m e a s u r e d . Three m a j o r factors influence the design o f e x p e r i m e n t s for g a m m a - r a y astronomy. 1) Extraterrestrial g a m m a rays are very r a p i d l y o v e r w h e l m e d by a t m o s p h e r i c g a m m a rays - therefore the experiments have to be suitable for inclusion in the p a y l o a d s o f b a l l o o n s or satellites. 2) P r i m a r y c h a r g e d particles o u t n u m b e r the g a m m a * Present address: Centre for Space Research, Massachusetts Institute of Technology, Boston, Massachusetts, U.S.A. 257

G.F. BIGNAMIet al.

258

configuration of the flight hardware was frozen a scientifically representative model of the experiment (the Scientific Model) was constructed for use in conjunction with accelerator beams. This model, which is described below, used detectors of the same basic design as the proposed flight models but was constructed of standard laboratory materials and components and operated via N I M standard electronics. The four properties of a gamma-ray experiment which should be assessed by means of beam calibrations are detection efficiency, angular resolution of direction measurement, energy resolution and background rejection capability. The first three of these require exposure to a beam of g a m m a rays of appropriate energy but the fourth investigation requires beams representative of the background radiation to which the instrument will be exposed during flight operation. While most of the present paper is concerned with measurements made with a tagged gamma-ray beam, one section is included on an investigation of the

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TABLEI Materialsandthicknessesofthe platesin thespark-chamber. Plate number

Material and thickness

1 2 to 6 to 10 to 17 to

300/tm A1 300/~mA1+ 400/~m W 300#m Al+200/zm W 300/~mAI+ 100/~m W 300 #m A1

5 9 16 21

Total thickness

Radiation length 0.003 × 1 0.127 × 4 0.065×4 0.034 × 7 0.003 × 5 1.02

effects of exposure to beams of energetic charged particles.

2. Description of the instrument The design philosophy of the instrument has been described elsewhere3'5). The layout is shown schematically in fig. 1. The instrument consists of four subsystems as described below: - T h e spark-chamber (SC) consists of a stack of 20 pairs of orthogonal grids of 192 parallel wires, each pair defining a 3 m m gap. The spacing between the wires of a grid is 1.25 mm. The gaps are interleaved with metal plates 240 m m square and consecutive gaps are 13 m m apart. In order to achieve a high conversion efficiency for high-energy photons and to keep a good directional information for lowenergy photons a non-uniform material distribution, having thicker tungsten plates at the top of the chamber, was chosen. The materials and thicknesses of the plates are given in table 1. The stack is mounted inside a vessel which is filled with 2 bar neon and an admixture of 0.7% of ethane. - T h e triggering telescope consists of two plastic scintillation counters (B1, B2) and a Cherenkov counter (C). It is designed to minimise the scattering and absorption of particles traversing it. In particular, each of its three elements is constructed so that the photomultipliers are outside of the field of view. Counter B1 is 6 m m thick and is divided into two rectangular segments of area 110 × 220 mm 2, each viewed through an adiabatic strip light guide by a photomultiplier tube. The plexiglass Cherenkov counter C is 3 0 m m thick and is divided into 4 segments of area l l 0 x l l 0 m m 2, each viewed through a hollow lightguide by a photomultiplier tube. Its upper surface is painted black in order to avoid triggering by charged particles coming from the bottom. The circular counter B2, 10 m m thick and 250mm diameter, is divided into four quadrants,

ACCELERATOR

EVALUATION

OF A SPARK-CHAMBER

each viewed by one photomultiplier tube via a flat plexiglas lightguide. This telescope triggers the spark chamber when a charged particle has passed the BI, C, B2 detectors. The threshold of the B1 counter may be selected to correspond to the passage of one or two particles. The outputs of the B1 and B2 counters are pulse-height analysed in order to estimate the energy lost by the particles traversing them. - The anticoincidence counter (A) is constructed of scintillation plastic 10 mm thick and it is placed around the spark chamber and the upper elements of the telescope for rejection of triggers produced by charged particles. The dome-shaped counter is viewed by 9 photomultiplier tubes, evenly spaced around its lower edge. - Below the telescope is the energy calorimeter, the purpose of which is to measure in coincidence with the triggering of the spark chamber the residual energy of the secondary electrons and photons. A caesium-iodide scintillation counter was selected, after comparative tests with lead-glass and lead/ plastic-scintillator sandwich counters, as having the best energy resolution attainable within the available weight. This counter (E) has a thickness of 4.7 radiation lengths and is viewed by 4 photomultipliel s. In order to provide additional information on highenergy events its bottom is covered by a cup-shaped plastic scintillator (D) also viewed by 4 photomultipllers. The outputs of counters D and E are pulseheight analysed, but the energies at which the D counter plays a role in the energy measurement lie outside the range of energies investigated in the tests.

259

EXPERIMENT

Spark chamber patterns were classified as defined in table 2, events of class 0 being rejected from the data set. For accepted events the class and the coordinates of all cores belonging to assigned tracks were added to the event data. Further automatic analysis was then performed by an IBM 360-50 system. Pulse-height distributions could be obtained as a function of event class, start point of tracks, triggering condition of counters A, B1, C and B2.

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3. Data analysis system The data generated by the instrument were recorded on punched paper tape and included: 1) coordinates of cores set in the spark chamber, 2) pulse-height information of counters B l, B2, E and D, 3) information on which elements of B1, C and B2 were triggered and whether counter A was triggered. The computer analysis was separated into an interactive phase and an automatic phase. The interactive analysis was performed on a Hewlett-Packard HP 2116 system, coupled to a Tektronix 611 crt display using a specially constructed keyboard 6) for semiautomatic track assignment and classification of events. An example of a display of a 95 MeV gamma-ray event before and after this editing process is shown in fig. 2.

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260

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et al.

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TABLE 2 Definition of the classification of the spark-chamber patterns.

Event class

Pattern description

spurious cores only unrelated tracks all events starting in top gap or side wall m o r e t h a n one track, b u t only one identifiable single " t h i n " track single " t h i c k " track two tracks with c o m m o n origin m o r e t h a n two tracks identified, o f which at least two have c o m m o n origin

An intuitive method for the determination of arrival direction of y-rays was chosen. The directions of individual tracks at the point of creation were defined by a least-squares straight-line fit through the first four available coordinates in each of the two orthogonal projections. (Note: if a track had more than 1 assigned core per gap, the arithmetic mean was used.) F o r single tracks (class 3, 4 and 5 events), the derived gamma-ray direction was the vector sum of the two projections. For class 6 and 7 events, the vector direction of each of the electron tracks forming the pair was calculated. The reconstituted arrival direction of the photon was defined as the weighted average of the electron directions, the chosen weight factors being the inverse of the r.m.s, deviations of the respective coordinates from the individual tracks. The calculated difference between the reconstituted angle and the nominal gamma-ray beam direction was used to study the directional resolution capability of the instrument. For class 6 events the angle between the two electron directions was also computed. In order that electrons which were likely to escape

the energy calorimeter could be identified, the directions of tracks leaving the spark chamber were calculated from the last 4 available coordinates in each projection.

4. The tagged gamma-ray beam For the calibration of the instrument a beam was set up at the 450 MeV electron synchrotron of Bonn University, as shown schematically in fig. 3. Collimated g a m m a rays produced in a tungsten target within the primary electron beam are incident on a copper target T 1 in which they produce electron-positron pairs. The

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Fig. 4. Variation with energy o f triggering efficiency (curve A) and o f the overall efficiency after s c a n n i n g (curve B: pairs and single tracks, curve C: pairs a n d multiples only).

ACCELERATOR

E V A L U A T I O N OF A S P A R K - C H A M B E R

magnet M t deflects the positrons and counters C~, C 2 and C3 select a monoenergetic beam of energy E 1 which is also passively collimated by a lead collimator K of diameter 30 mm. After passing through the copper t a r g e t T2, of thickness 0.07 radiation length, the positrons are again momentum analysed by a magnet M 2 and those having a residual energy E2 are selected by counters C4 and C5. In the energy region of interest the dominant process for energy loss in T 2 is blemsstrahlung and the probability of double bremsstrahlung in such a thin target is small so that any energy loss may be attributed to the radiation of a single photon of energy E~ = E z - E 2 . Photons radiated close to the beam direction pass through an aperture in the magnet to reach the instrument. This aperture is completely covered by a counter C 6 which vetoes any events accompanied by charged particles. The triggering telescope logic of the instrument was gated by the coincidence (C 1 "C2 "C3 "C4 "C5 "C6 "P), where P represents the synchrotron spill pulse. The fraction of gamma rays defined by this logic that actually reached the instrument was measured by means of a total absorption calorimeter and found to vary between 30% at E~ = 25 MeV and 65% at E~ = 300 MeV. The profile of the gamma-ray beam was determined from the coordinates of the points of conversion in the spark chamber. The beam width (fwhm) was about 50 ram. The spread in energy of the tagging system (due to the finite dimensions of counters C~ and C 5 and to fluctuations in the energy-loss process) was measured to be ± 15 MeV (independent of E~) which is small

261

EXPERIMENT

compared withi the energy resolution of the instrument under investigation (see section 7). i

!

5. Detection efficiency 5. l. TRIGGERING AND OVERALL EFFICIENCIES

The triggering efficiency, defined as the ratio between the number of itelescope triggers and the total number of incident photons, is plotted in fig. 4 as a function of gamma-ray energy for axially incident photons. Curve A shows the efficiency for /~ .B1 .C .B2 coincidences. Fig. 4 also shows the overall efficiencies for ~, .B1 .C .B2 coincidences after the scanning and class assignment processes (see Section 3). Curve B gives the ratio of the sum of the events in classes 3, 4, 5, 6 and 7 to the total number of incident photons, while curves C refers only to classes 6 and 7. The reduction in overall efficiency introduced by scanning and :classification is apparent. The relative decrease in the overall efficiencies towards higher energies arises: from the increased complexity of the pictures the human scanner has to deal with so that the probability for an event to be rejected as unclassifiable is greater. Table 3 shows the efficiencies for gamma-rays of 45, 95 and 195 MeV incident at various off-axis and inclined directions. These directions are shown in fig. 1. The coordinate system of table 3 refers to the position of the iintersection of the gamma-ray trajectory with the first spark-chamber gap (the x and y axes being parallel to the grid wires) and to the angle 0 between this trajectory and the z axis (optical axis).

TABLE 3 Efficiencies measured for various positions and directions o f incidence. Position and direction

N u m b e r of incident photons

See x y 0 fig. l ( m m ) (mm)

45 MeV

95 195 MeV MeV

I

0

0

0°

23000 11000

5600

2

80

0

0°

31000 13000

6700

3

80

80

0°

5600

2300

1300

4

30

0

15 °

4600

2000

950

5

94

0

20 °

27000 16000

4900

6

117

0

30 °

11000

2900

4800

BI" B2" C" A. triggering efficiency 45 MeV

95 MeV

195 MeV

0.062 4-0.002 0.046 4-0.001 0.039 4-0.003 0.045 4-0.003 0.033 4-0.001 0.016 4-0.001

0.158 4-0.004 0.134 4-0.003 0.102 4-0.007 0.I01 4-0.007 0.079 4-0.002 0.032 4-0.003

0.298 4-0.007 0.252 4-0.006 0.190 4-0.012 0.209 4-0.015 0.131 4-0.005 0.057 4-0.004

Classes 3, 4, 5, 6, 7 45 MeV

95 MeV

0.040 0.109 4-0.001 4-0.003 0.026 0.079 4-0.001 ~0.002 0.012 0.052 4-0.001 d:O.O05 0.0241 0.051 4-0.002 4-0.005 0.018 10.048 4-0.001 ~0.002 0.009 ' 0.018 4-0.001 4-0.002

Classes 6 and 7 195 MeV

45 MeV

95 MeV

195 MeV

0.205 4-0.006 0.133 4-0.004 0.075 4-0.008 0.125 4-0.011 0.073 4-0.004 0.031 4-0.003

0.015 4-0.001 0.011 4-0.001 0.004 4-0.001 0.009 4-0.001 0.009 4-0.001 0.004 4-0.001

0.057 4-0.002 0.038 4-0.002 0.025 4-0.003 0.024 4-0.003 0.030 4-0.001 0.009 4-0.001

0.128 -t-0.005 0.078 4-0.003 0.039 4-0.006 0.090 4-0.010 0.053 4-0.003 0.021 4-0.003

262

G.F.

BIGNAMI

Only directions parallel to the x z plane were investigated. The effective area for the detection of gamma-rays from a point source located at an angle 0 from the optical axis may be derived by integrating the efficiency for the angle 0 over the whole area of the instrument. In order to perform this integration the efficiency was represented by an analytic function which fitted the available data points and which can be interpreted in terms of a geometrical model in which the parameters are the boundary conditions on direction and position of incidence of gamma rays outside which the efficiency is effectively zero. The effective area for axial incidence was found to be 300 ~/o(E) cm 2, where r/0(E ) is the appropriate curve of fig. 4, while the variation with 0 has a fwhm of about 30 °.

~ produced electrons to have the proper directions and sufficient range to penetrate the triggering telescope. These factors impose conflicting requirements on the selection of the optimum total thickness of the sparkchamber. The available data provide a quantitative means to improve this aspect of the design, since the data analysis procedure permits a breakdown of the events according to the plate in which the photon conversion has taken place, i.e. the one above the gap in which the track is seen to start. The distribution obtained differs from the theoretical gamma-ray absorption curve because of the third effect mentioned above. Fig. 5 displays the probability of axially-incident events of classes 3, 4, 5, 6 and 7 and two energies converting at or beyond the gap specified by the upper abcissa and triggering the telescope with the A .B1 .B2 coincidence requirement. A correction has been applied for the fraction of gamma rays already converted above this gap, computed according to Rossi7). This presentation shows the relative efficiency of the instrument as a function of total spark-chamber thickness (lower abscissa). The data have been normalised to the efficiency of the actual spark chamber (1.02 radiation length thickness). This result clearly indicates that at low energies the fraction of events lost by absorption or scattering of the electrons exceeds the fractional increase of converted

5.2. DISTRIBUTIONOF EVENTSTARTPOSITIONS The probability for gamma rays to convert in a given plate of the spark-chamber and to trigger the instrument depends on: 1) The thickness of material above that plate, which determines the probability of absorption of the incident photons. 2) The thickness of the plate itself, which determines the conversion probability. 3) The thickness and distribution of material below the plate, which determines the probability for the SPARK CHAMBER 161'514131211 10 9 8 7 6

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efficiency for gamma-ray

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06 Q8 THICKNESS (RADtATION LENGTH)

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thickness,

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ACCELERATOR TABLE

EVALUATION

OF A S P A R K - C H A M B E R

TABLE

4

Variation o f the a n g u l a r resolution p a r a m e t e r cr~ with energy. Event

EXPERIMENT

263

5

Variation o f the a n g u l a r resolution p a r a m e t e r crw with incidence angle 0.

aw (degrees)

classes

45 MeV

95 M e V

195 M e V

295 MeV

Incidence

3, 4, 5 6

9.4±0.5 9.3±0.5

7.34-0.3 7.9±0.3

5.3±0.3 5.7±0.3

4.0±0.4 4.5±0.3

gamma rays if the thickness is increased beyond about 0.6 radiation length. The indicated excess above 1.0 represents the possible increase in efficiency achievable by reducing the spark-chamber thickness.

6. Angular resolution The computer programme as described in section 3 gives the angle ¢ between the reconstituted direction of incidence of the photon and the nominal beam direction. The integral distribution of this angle has been constructed for all runs but with some limitations on the photon population given by requirements on event class, plate of conversion and, for pair events, on the angle co between the two tracks of the pair. An

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trw (degrees) 45 M e V

95 MeV

195 MeV

0 15 20

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angular resolution parameter a o was defined such that 68% of the angles are less than a~. The variation with energy of the angular resolution parameter aq, for different sets of event classes is presented in table 4, for events initiated by axially incident photons. The variation of ~q, with the photon incident angle, 0, for all event classes and different energies is presented in table 5. The parameter aq, is weakly affected by the deviation of the incidence directions of individual photons from the nominal beam direction; the average deviation has been computed for each axial run and was found to be less than (0.9-t-0.1) degree for 68% of the events, independent of energy. The dispersion of individual gamma-ray directions about this mean deviation can be seen from the beam width (section 4) to be about 0.5 ° at the position of the detector (2.55 m from target T2). It is thus evident that this deviation cannot contribute significantly to the measured value of a~. Samples of data leading to an improved angular resolution can be obtained by selecting, for instance, pair events with a chosen upper limit on the opening angle co. Such a selection has been applied to balloonflight data by Niel et alfl). The effect on the present data is illustrated for co _< 10 ° in fig. 6, which also shows the associated reduction in efficiency. It is premature to compare in detail the values of ao with the angular resolutions claimed by other authors. At the time of the measurements the spark-chamber performance, as reflected in the gap efficiency and the track width, could not be properly adjusted. In addition the chosen analysis method may have introduced some degradation. Therefore the measured angular resolution is larger than the values obtainable with the same configuration. On the other hand it should be noted that theoretical estimates of the angular resolution will generally result in underestimates if the actual spark-chamber performance is not properly accounted for.

264

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B I G N A M I et al.

7. Energy measurement The energy calorimeter measures that part of the energy of the incident photon that is carried by secondary electrons and photons entering the CsI crystal. Between the point of conversion of the incident photon and the energy calorimeter the secondary electrons lose energy by ionisation and radiation. Some of this energy is retained by the system and may be estimated from the responses of the spark chamber and telescope counters, but energy carried by scattered electrons and photons may be lost from the system. A major aim of the tests was to investigate the influence of these effects on the energy resolution of the experiments as a whole. First the energy loss in various parts of the instrument was measured directly, using mono-energetic electrons, while successively integrating the different units, starting with the energy calorimeter alone. The following conclusions were reached: 1) The ionisation loss follows directly from the thickness of matter traversed. 2) Radiation loss, the dominant energy loss mecha-

nism in the spark chamber, has little effect on the energy measurement. As expected, most of the secondary photons dissipate their energy in the calorimeter. The E-counter pulse height distribution for all classes of axially incident gamma rays of 195 MeV which converted in gaps 10 to 16 of the spark-chamber is shown as an example in fig. 7. This distribution, and those for other incident energies, were used to derive the variation with incident gamma-ray energy of the peak channel number of the pulse height distributions, shown in fig. 8. A detailed account of the measured energy at primary energies of 95 and 195 MeV for various types of events is given in fig. 9. The observed peak pulse height is displayed as a function of the point of conversion in the spark-chamber for all accepted events. Fig. 10 shows the same variation for class 6 events of 195 MeV divided into four categories according to the number of tracks seen to leave the bottom of the spark-chamber and the number that, when extrapolated, intersect the upper surface of the calorimeter. The energy resolution of the complete instrument was obtained using the individual pulse height distributions leading to the data shown in figs. 9 and 10, together with similar data for events of other classes

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50 1OO CHANNEL NUMBER

200

Fig. 7. Energy calorimeter pulse height distribution for axially incident 195 MeV g a m m a rays converting in gaps 10 to 16 of the spark chamber.

O O

I 1OO

I 200 ENERGY ( M e V )

I 300

Fig. 8. Energy calorimeter response as a function of gamma-ray energy.

ACCELERATOR EVALUATION

OF A S P A R K - C H A M B E R EXPERIMENT

and energies. Each distribution was normalised to a fixed point of creation (gaps 10-16) and to category A of fig. 10 and the distributions were added using the frequency of occurrence of each type of event as the weight factor. Fig. 11 shows the overall energy resolution (fwhm) as a function of gamma-ray energy for events with class (4, 5, 6), corrected for category and start position, and for all accepted events, corrected for start position only. Because of low-energy background contamination and poorer statistics it was not possible to apply this method to the 45 MeV measurements. At 25 MeV the beam resolution was too poor to perform any energy measurements. In principle, the correction according to the different categories should have been achieved, without extrapolation, by comparing the numbers of particles traversing the B1 and B2 counters, This was not possible because of a deterioration in gain in one of the B2 counter quadrants during the tests. The analysis technique used for the spark chamber data was not suitable to assess the amount of energy information which may be gained from the sparkchamber picture.

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266

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of the energy measurement. Data taken at inclined positions of the instrument with respect to the beam have indicated, as expected, an increased fraction of such events. Nevertheless, using the pictorial information of the spark-chamber and the pulse height information of the B1 and B2 counters, a sample of events with a reliable energy measurement can always be defined.

8. Background rejection The ability of the instrument to select incident gamma-rays from a flux of charged particles depends mainly on the following functions: 1) The anticoincidence veto for charged particles within the field of view of the telescope. 2) The two-particle threshold option of the B1 counter which favours the photon-inititated pairproduction process. 3) The directionality of the Cherenkov counter response which gives a low efficiency for upwardmoving particles. 4) The pictorial information of the spark-chamber. The first three of these must limit the background triggering rate to a level at which the dead-time correction is small. The last serves to identify gamma rays from the residual background. The overall response of the experiment to a chargedparticle background depends very much on the way the particles interact in the satellite material as a whole and especially in the payload. The isotropic flux of cosmicray particles present in space may not be easily simulated in the laboratory, but worst cases can be investigated using charged-particle accelerator beams. Since the inefficiency of the anticoincidence counter for minimum-ionising particles within the field of view of the triggering telescope has been found in a separate accelerator investigation 9) to be ~ 10 -5, it has been estimated that the largest contribution to spurious

triggering will come from secondary particles generated by photon interactions in the concentrated mass of the Energy Calorimeter. In order to obtain as realistic a measureas possible of the expected counting rate due to such processes the Scientific Model was exposed to beams of charged particles from the proton synchrotron 'Saturne' at the Centre d'Etudes Nucl~aires de Saclay. A beam, approximately 6 cm in diameter, of protons of momentum 1450 MeV/c was used to bombard the Energy Calorimeter in what were deemed to be the directions most likely to favour interactions in which scattered or secondary particles might trigger the experiment. These directions are indicated in fig. 12. The experiment triggering mode was the simple B1 .B2 coincidence (except in direction 4 for which it was BI -B2 .C) and events recorded in coincidence with a beam pulse were displayed on the computer crt to indicate the counter

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Fig. 12. Directions of incidence used for investigation of background triggering.

TABLE 6 Relative triggering frequencies for the various coincidence modes, as a function of the direction of incidence.

Direction (see fig. 12)

1 2 3 4 5

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2.0 x 1.4x 2.1 × ~,1 1.4x

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0.1 0.03 0.07

ACCELERATOR

EVALUATION

OF A S P A R K - C H A M B E R

pulse heights and the triggering of counters not included in the coincidence requirements. Table 6 shows, for the given direction of incidence, the relative triggering frequencies appropriate to the various coincidence modes and the frequency of recording tracks in the spark chamber. Corrections have been made for experiment dead-time and for impurity of the beam, protons being identified by a time-of-flight method. The most important factor in the rejection of this type of background is seen to be the reverse rejection capability of counter C. However in the case of direction 5 about one third of the B1 .B2 coincidences also give a pulse in C due to the passage of a particle through the photomultiplier tube. This effect was investigated at Saclay by bombarding a photomultiplier with a beam (about 1 cm diameter) of pions of momentum 500 MeV/c, incident from a number of typical directions. The magnitude of the effect was found to be dependent on the position of incidence on the tube, being most pronounced for particles which pass through the window in front of the photocathode. It is not possible to give a firm statement on the probability that events satisfying the full BI -C .B2 .,~ coincidence requirement will produce tracks in the spark-chamber since only two such events (both without tracks) were recorded for the first four directions. However, some indication of this possibility may be derived from the last column of table 6 which gives the fraction of B1 .B2 events showing tracks. This must be an upper limit since most of the particles producing the tracks were seen to continue to trigger the anticoincidence counter. 9. Conclusions

Because of limitations on time and the range of energiesavailable from the beam it was not possible to perform a complete calibration of the instrument. Nevertheless the measurements have given useful information of what can be achieved with this type of instrumentation and software. From the measurements the following conclusions can be drawn: 1) The overall efficiency for detection of gamma-rays is improved by reducing the total spark-chamber thickness to approximately 0.5 radiation length. 2) The determination of the matter distribution in the spark chamber leading to the best compromise between efficiency and angular resolution, especially in the energy range 25-100 MeV, needs a refined measurement and an improved analysis

EXPERIMENT

267

procedure for the direction determination (see, for example ref. 10). 3) The quality of the spark-chamber picture, determined by the gap efficiency and the track width has a major effect on the overall detection efficiency and the achievable angular resolution. 4) The obtainable energy resolution depends on how well one can estimate the number of particles (and their energy) which do not enter the energy calorimeter. In addition the energy loss in the spark-chamber and the triggering telescope must be taken into account. The resolution of the pulse height measurement of the telescope counters (BI and B2) should be better than 50% fwhm for this purpose. 5) The largest contribution to the triggers caused by background comes from charged particles penetrating the photomultipliers of the Cherenkov counter. This effect can be reduced sufficiently by coating the sidewalls of the tubes with a black paint and by covering the tube windows with thin sheets of plastic scintillator used in anticoincidence. 6) The pulse height distributions in the telescope counters were markedly different for gamma-ray events and for background events. Since the trigger rate caused by background is expected to be of the same order as that for gamma rays a positive identification of gamma rays is required. For this purpose the typical gamma-ray pattern in the spark chamber is supplemented by a pulse-height measurement of the B1, B2 and C counters. 7) Since the trigger rate in orbit due to background cannot be predicted accurately a variety of coincidence requirements must be selectable by telecommand, in order to assure the best compromise between gamma-ray detection efficiency and deadtime. A new model of the instrument, incorporating detailed design changes following the above conclusions will be calibrated in a gamma-ray beam with energies between 20 MeV and 5 GeV and will be tested in proton beams of 0.5 to 5 GeV. The results of these tests will provide the required information to determine the optimum matter distribution in the spark-chamber and should prove the rejection capability of the instrument against background. The gamma-ray measurements will be supplemented by Monte Carlo calculations to facilitate interpolation and possible extrapolation beyond the limits of obtainable experiment data11). Meanwhile, the data obtained during the tests will prove valuable during the development and testing

268

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B I G N A M I et al.

of software for the future reduction and analysis of flight data. As far as is possible future analysis will be carried out automatically be means of a suitable pattern recognition program (see, for example ref. 12). The analysis of the present measurements has shown that it is not practical to analyse the expected large quantity of data entirely by visual scanning. The measurements described in this paper have shown not only the value of a tagged gamma-ray beam for calibration of this type of experiment but also the advantages to be gained from carrying out such measurements as early as possible in the development of the equipment. The authors wish to express their gratitude to Mr P. Coufleau, the COS-B payload manager, and to the COS-B project division of ESTEC for their continued support and assistance. Prof. G. Boella, Dr H. A. Mayer-Hasselwander, Dr B. G. Taylor and Mr P. Keirle are thanked for their leadership during the design and preparation of the experiment hardware. The provision of the tagged gamma-ray beam in Bonn was made possible by the kind permission of Prof. W. Paul and with the close cooperation of the synchrotron group led by Dr G. von Holtey and Dr H. Genzel. The facilities at Saturne were made available by Prof. J. Labeyrie. The assistance of Mr P. Coffaro, Mr P. de Korte and Mr M. Goiisse during the accelerator measurements deserves special mention. The software for the IBM and Hewlett-Packard computers was provided by Mr C. M. Cooper and Mr K. H. Schenkl

respectively. Finally the authors would like to acknowledge many stimulating discussions with Prof. K. Pinkau, Dr J. J. Burger, and Prof. L. Scarsi. References I) C. E. Fichtel, I. A. U. Symp. no. 41, New techniques in space astronomy (eds. F. Labuhn and R. Liist; Reidel, Dordrecht, 1971) p. 14. 2) W.L. Kraushaar, Proc. ESRO Colloq. Gamma-ray astrophysics, ESRO SP-58 (ESRO, Neuilly-sur-Seine, 1970) p. 15. 3) B . N . Swanenburg, Proc. ESRO Colloq. Gamma-ray astrophysics, ESRO SP-58 (ESRO, Neuilly-sur-Seine, 1970) p. 69. 4) G. Altmann, Proc. ESRO Colloq. Gamma-ray astrophysics, ESRO SP-58 (ESRO, Neuilly-sur-Seine, 1970) p. 87. 5) H . C . van de Hulst, A. Scheepmaker, B . N . Swanenburg, H . A . Mayer-Hasselwander, E. Pfeffermann, K. Pinkau, H. Rothermel, H. Schneider, W. Voges, J. Labeyrie, P. Keirle, J. Paul, G. Bellomo, G. Bignami, G. Boella, L. Scarsi, G . W . Hutchinson, A.J. Pearce, D. Ramsden, R. D. Wills and P. J. Wright, I. A. U. Syrup. no. 41, New techniques in space astronomy (eds. F. Labuhn and R. Lfist; Reidel, Dordrecht, 1971) p. 37. 6) K. H. Schenkl, MPI-PAE/Extraterr. 42 (Max-Planck lnstitut fiir Physik und Astrophysik, Mtinchen, 1970). 7) B. Rossi, High energy particles (Prentice Hall, New York, 1952) p. 81. 8) M. Niel, G. Vedrenne and R. Bouigue, Astrophys. J. 171 (1972) p. 529. 9) M. Gorisse, Memoire C N A M (Conservatoire National des Arts et M6tiers, Paris, 1971). 10) K. Pinkau, Nucl. Instr. and Meths. 104 (1972) 517. 11) R. D. Wills, EPS Conf. The impact of computers on physics (Geneva, 1972) to be published in Computer Phys. Commun. 12) G . F . Bignami, G. Cioni, A. Della Ventura, P. Mussio, M. J. L. Turner and U. Volonte, EPS Conf. The impact o f computers on physics (Geneva, 1972) to be published in Computer Phys. Commun.