Adv. Space Res. Vol.3, No.4, pp.197—201, 1983 0273—1177/83 $0.00 + .50Printed in Great Britain. All rights reserved. Copyright ©COSPAR

TECHNIQUES FOR FINE GAMMA-RAYBURST SPECTROSCOPY

C. Barat

Centred’Etude SpatialedesRayonnements,CNRS/UPS,B.P. 4346,31029Toulouse,Cedex,France

ABSTRACT

A better understanding of the origin of gamma—ray bursts requires a significant improvementin present detector sensitivity, particularly for fine line spectroscopy in the 5—200 keVenergy range. This paper presents a critical analysis of some detectors which may be usedto obtain high energy resolution measurements of photon spectra from cosmic gamma—ray burstsources.

INTRODUCTION

The physics of gamma—ray bursts remains poorly understood, in spite of the large number ofevents detected to date (almost 200). However, the observation of lines near 60 keV and 420keV in the spectra of some bursts is the most striking discovery of the past three years [I]it has allowed an estimate of the magnetic field and the gravitational potential at the pho-ton source, which is probably located at the surface of a 1 to 1.5 M~neutron star.

The number of features observed in different events, both around 60 key and 420 keV, makesthe existence of these lines plausible. However, some lines are detected at the limit of sen-sitivity of current instruments. Moreover, the interpretation of the low energy part of hardspectra is ambiguous, due to the Compton contribution to the detector response. For thesereasons we have studied a variety of detectors with greater sensitivity, particularly thosehaving better energy resolution for the 5—200 key range, which contains the major part ofthe total energy of most gamma—ray bursts.

DETECTION OF SPECTRAL LINES

The detection of a line at an energy E becomes possible when the count rate from that linediffers significantly from the count rate expected in the absence of a line. In most cases,the source flux is less than the detector background, which is thus the maim component in thecontinuum. If the background obeys Poisson statistics, the minimum detectable flux at the 3sigma confidence level is given by

S(E) 3(B(E) .R(E) .V. t)~2 (1)

A (E) . t

where B(E) is the detector background in counts cm 3s 1MeV l, R(E) is the resolution (FWHM,MeV), V is the detector volume in cm3, t is the time resolution in seconds, and A(E) is theeffective area in cm2. The product B(E)R(E)Vt cannot be less than 1 count, so that the mini-mum value of S(E) is 3/A(E)t. Formula (1) shows that the line sensitivity of gamma—ray burstdetectors can be increased by improving their energy resolution, using large detection areasand higher stopping power.

Up to now most gamma—ray bursts have been detected using solid detectors, mostly NaI scintil—lators and germanium crystals cooled to low temperatures (T<l3O~K). For detectors of the samearea, the use of materials with higher stopping power has several advantages : smaller volume,lower background, better resistance to radiation damage and larger energy range where thephotoelectric effect is dominant. This last aspect plays an important role in the interpreta-tion of absorption lines below 100 keV, especially for hard spectra (kT > 500 keV), due tothe Compton contribution. For example, figure 1 shows a count rate spectrum and the corres-ponding photon spectrum for the 1978 November 19 event, detected by the SIGNS experiments onVemera 11 and 12. A strong absorption appears between 50 and 85 key in the count rate spec-trum ; this feature is significant at the 5—6 sigma confidence level with respect to thecontinuum obtained from all other energy channels. However, the Compton contribution repre-sents 85 percent of the observed count rate at this energy.

197

198 C. Barat

SIGNE EXPERIMENT — 19 NOV 78 GAMMA-RAY BURSTI I I

SPECTRUM N’ 3 SPECTRUM N’3dT~O.25 ~T.O25

2 __~__ 2

- ± ± V-Il .. V-li

± _V-12 -2- - ~1o

w IiiQ_ S 0..U) LI)

Z2

0

2 2

I I15/-__j_ I

10 2 ~ to2 2 5 2 5 ~ 1 2 5 102 2 5 2 5

ENERGY (keV) ENERGY(keV)Figure 1 Count spectrum and photon spectrum of the 1978 November 19 gamma—rayburst showing a strong absorption between 50 and 85 keV.

Taking into account the propagation of the errors from the high energy to the low energy partof the spectrum, the flux near 60 keV could be anywhere between 0 and 3.2 10—2 photonscm2s1kew1. For the last value, the statistical significance of the absorption line is only1—1.5 sigma with respect to the continuum [3] . Thus the large Compton contribution degradesthe significance of the spectral feature. To avoid this sort of problem, one solution consistsof using detectors with a larger atomic number than that of Nal (say, HgI

2), or with betterenergy resolution (say, Ge), if the line is narrow. The use of a Xe (Z54) gas scintillationproportional counter (GSPC) in which the photons interact by photoabsorption is another solu-tion.

The noise of a semiconductor detector can be represented by a normal distribution whosevariance results from fluctuations in the number of collected charges (VARF), in the probabi-lity of charge collection (VARC) and in the electronic noise (VABE). The resolution of thedetector is related to these three independent components through the relation

R(FWHM) = 2.355 (VARy + VARC+ VARE)~’

2 (2)

The variance resulting from the fluctuation in the number of collected charges may be writtenas

VAR~=FcE (3)

where t is the energy needed to produce an electron—hole pair, and F is the Fano factor. Theintrinsic resolution of the detector is deduced from expressions (2) and (3), assuming thatVARc and VARF are zero ; at 60 keV, it is 310 eV (c = 2.9 eV, F = 0.1, and T = 77°K) for Geand 510 keV ~e = 4.15 eV, F = 0.19, and I = 300°K) for Hg1

2. The expression for VARC hasbeen given by Henck [4]; this variance is negligible for intrinsic germanium, because thedistance between electrodes is much smaller than the product ur~. (where l~ is the mobility,-r the lifetime, and ~ the electric field) which represents the distance traveled by thecharge carriers before recombination or trapping. However this is not true for HgI2, becausethe product ~i-r~.for holes is of the order of several tins [5] . The expression for VARE hasbeen calculated by Radeka [6] ; it shows that the detector resolution is proportional to thedetector area and the temperature of the FET at the preamplifier input. The resolution of theGSPC is given by the following expression

R(FWHM) = 2.355 (F c E + k 52/A)l/2 (4)

where k is a constant and ~ is the average amplitude of the light pulse [7] . These two compo-nents are due to the fluctuation in the number of primary electrons created by the photoabsorp—tion of X and gamma—ray photons, and to the fluctuation in the number of UV photons createdby the de—excitation of the gas, respectively. With F = 0.17 and e = 21.9 eV for Xe [8] , theintrinsic resolution of the detector is 1.11 key at E 60 keV.

SEMICONDUCTORDETECTORS

The interaction of a photon in a semiconductor liberates electron—hole pairs which are collec—l~d under the influence of an electric field. The detection of these charges implies a

Techniques for Fine Gamma—RayBurst Spectroscopy 199

depleted zone which exists at 300°K for HgI~ and CdTe, or only at low temperature(T<l30°K) for Ge.

Mercuric iodide is probably the most sensitive detector available today, due to its efficien-cy [~], which is greater than 60 5 between 5 and 100 keV (Figure 2), its intrinsic energy re—so1utio~ which is equal to 510 eV at 60 keV for a Fano factor of 0.19 [10] , its large photo-electric cross section, and its small volume. Moreover, it may be assumed that HgI

2 has agood resistance to radiation damage, because of its small thickness and its large atomic num-ber, which causes the crystalline structure to be more elastic. However, the growth of thecrystal and the fabrication techniques do not seem to have been perfected yet, consideringthe large number of impurities and defects which cause polarization phenomena to occur andgive a small value of the product ~ir for holes [11] . CdTe is another material used at roomtemperature [9] . However, its efficiency (Z=48 for Cd and Z = 52 for Te) and resolution aresmaller than those of Hg12[9] due to a forbidden gap which is less wide (1.5 eV, comparedto 2.1 eV for HgI2) and to a larger value of e (4.43 eV as opposed to 4.15 eV for Hg12). Onthe other hand, the product ~i-r is larger for CdTe [s~

r—~ -‘—*..~~ I Figure 2 : Efficiency and

I n~~crondead layer Hg 12 resolution of mercuric

\ iodide as a function of> - .._Iheoretical cube (500 mIcrons I0 \ energy.

0.1 — • Slapa e1 al. l~00microns) -

~ Schoroger •5 01.1500 microns)

A£

w - 0~)

A’ -4 - ,.

o 0.56 Cm

2 ~ ~AU-

I - 07 Cm2 0 I cm2 + ~roger 01 01 1

7 ~.hbo!~ i E0E10.19ENERGY (MeVI

Cooled silicon and germanium crystals (77°K) are the most currently used detectors for fineX and gamma—ray spectroscopy. Germanium gives the best energy resolution [12] which followsthe empirical formula R(FWHM) = 1.86 EO’

22(MeV) (Figure 3). This behavior changes very littlebetween 77°Kand 130°K [13] . Silicon becomes less suitable around 60 keV because its Comptoncross section is greater than its photoelectric cross section at higher energies. Also, itsenergy resolution is less than that of germanium (e 3.6 eV, compared to 2.9 eV for Ge).

1 , •—~:.~•~ n-type

~5rnrn Be ~Inyow HPGe Figure 3 : Efficiency and>_ coaxiaL resolution of Germanium

- detector - as a function of energy.

~0.o1 - - ID —

4 0 Raudor~ et olw I

A Llocer

— Ill400etIcal l~rv,

1t(Fr0.I0l _4~.-

- ,_~i0_ii~o:_:_~ii:,ENERGY LM0V)Germanium comes in coaxial or planar form ; n type HPGe coaxial detectors (active impuritydensity < 3 x 1010 cm

3) have an emergy threshold comparable to that of planar detectors, butremain efficient beyond 1 MeV. In addition, they may be thermal cycled between300°Kand 77°K,which is not possible with Ge(Li) ; the latter detector must be maintained at liquid nitrogentemperaturesto avoid compensation of the lithium, n—type HPGe has two important advantagesover p—type. First, it has a lower energy threshold (5 keV instead of 40 keV) due to thewindow thickness, which is 0.3 pm for n—type and about 300 pm for p—type.

200 C. Barat

The best periphery contact should be made by the boron ion implantation technique rather thenby metallic evaporation, which is degraded when the detector temperature cycles from 77°K to300°K. Second, it resists radiation damage better ; a resolution of 2 keV (FWHM) at 1.33 MeVwill degrade, when subjected to a fast neutron fluence of around 1010 neutrons cm

2, to 3 keVfor n—type and 70 keV for p—type, for a 4.5 cm diameter by 4.5 long HPGe coaxial detector [14]

The differences are even greater for larger detectors. Fast neutrons create defects in Germa-nium, which act to trap holes ; but in n—type material electron collection processes dominatethe current pulse and this explains its good resistance to radiation damage.

GAS SCINTILLATION PROPORTIONALCOUNTERS

In a GSPC, X and gamma—rays interact in the gas via the photoabsorption effect, giving riseto primary electrons(drift region); under an electric field, these electrons may gain enoughenergy between collisions to excite the medium (excitation region). The excited atoms formdiatonic molecules which de—excite by the emission of UV photons in the range 1500—1950 X[15}The light is collected with a phoromultiplier through a suprasil window, with a vacuum photo—diode, or with a photoionization proportional counter [16] . Contrary to the case of a propor-tional counter, the electric field in the excitation region is kept below the limit at whichsecondary electrons are produced in order to obtain a better energy resolution.

Noble gases are the best scintillators, especially Xenon, due to its low value of t (21.9 eV)and its high stopping power (Z=54) [8] . However, the GSPC needs a getter to work, so that ahigh gas purity can be maintained for long periods. The elimination of impurities, whichabsorb CV, means that the GSPCmust be constructed using high vacuum techniques.

The almost isotropic distribution of gamma—ray bursts makes it necessary to use a GSPC witha large (~2 ii sr) field of view. Studies carried out by the Columbia Astrophysics Laborato-

ry have shown that a conical GSPCwith a 45° half—angle works well, despite a slight degra-dation in the resolution above 40° [17] . The use of a hemispherical geometry should give evenbetter resolution performance, while a 10 atmosphere filling should increase the efficiencynear 100 keV and reduce the fluorescent escape radiation for energies above the K edge(34.56 keV).

The good sensitivity of the GSPC is due to its excellent energy resolution, (which is morethan 2 times better than that of a proportional counter) to its large efficiency at a pres-sure of 10 atmospheres (Figure 4), and to its large area (> 300 cm2). Pulse shape discrimina-tion may be used to further improve the sensitivity.

DI, I 5~ndow ulm SPC

~ooi - 10 cm ecOIh drifted region \\\\ - energy.

a — Theorel~cul1111111 I F~0.17)

a Ku ~1at.

I I ii I I III I I it i I 1Ifl7

0.0? 0.1 I IDEN ERGY 1Mev)

SENSITIVITY

Two possible satellite experiments are currently being studied at the Centre d’Etude Spatialedes Rayonnements which should allow fine spectroscopy of gamma—ray bursts by 1985. One woulduse a 6.5 cm diameter by 6.5 cm thick n—type HPGe detector and a 7.5 cm diameter by 7.5 cmthick Nal scintillator for high energies. The Germanium detector will be passively cooled tol3O°K using a technique developed by the Centre National d’Etudes Spatiales for Meteosat.

A Xenon GSPC could be used in the second experiment for low energy measurements (E<300 keV). 2It would have a field of view of ~ 2 ii sr, and would operate near 10 atmospheres, with 300 cmarea. This study is being carried out in collaboration with CERNand the University of Orsay,France [18]

Techniques for Fine Gamma—RayBurst Spectroscopy 201

The minimum detectable line flux at the 3 sigma confidence level is plotted vs. energy inFigure 5. The background has been estimated from the measurements obtained on Apollo 15

(near the spacecraft), ISEE—3 and SIGNE experiments ; determination of the solid detectorefficiencies has been computed only taking into account the full energy peak while the GSPC

sensitivity does not include the decrease of the detector background obtained by rise

time discrimination.

- Figure 5 Minimum detectable line

trltegrotion tIme I fluxes at the 3 sigma level confidencefor n—type HPGe, Nal and GSPCdetectors.

—.. The data points show the line intensi—— a Tee~ardon and CIlne . ties observed by Teegarden and Cline

~ ~ 01 al T [2] ; the cyclotron line intensitieso from Mazets et al. [1] are indicated byill the shaded area.

CoaxiaL~l0 - HPGe

‘~.

LU10 ~ ~ _‘,~‘~0a60° -

GsPc

to.2 It I I It I

0.01 0.1 1ENERGY 1MeV)

The results show that a NaI scintillator has the best sensitivity at high energies, despitethe fact that its energy resolution is only 6 1 at 1 MeV. It is also clear that the use of aGSPC between 5 and 300 key would result in a considerable progress for the understanding ofabsorption processes at lower energies.

REFERENCES

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490 (1969)18 H. Nguyen Ngoc, Nucl. Inst. and Meth., 154, 597 (1978)

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