A Ge + BaF2 composite γ-ray spectrometer

Nuclear Instruments and Methods in Physics Research A316 (1992) 306-310 North-Holland

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Sechon A

A Ge + BaF 2 composite -f-ray spectrometer A. Krasznahorkay ], J. Bacelar, A. Balanda 2 and A. Buda Kern.fysisch Versneller Instimut, 9747,4,4 Groningen, Netherlands Received 13 January 1992

The design of a new v-ray spectrometer for detection of high energy photons in the 10-20 MeV region with high resolution and efficiency is presented. Tests with a prototype of the Ge + BaF2 composite ,/-ray spectrometer are discussed. The measured energy resolution and efficiency of the system are in agreem,.nt with the results of Monte Carlo calculations. This type of spectrometer can have better than 0.5% energy resolution and more than 10% efficiency at 15 MeV.

!. Introduction The detection of high energy v-rays following the decay of giant resonances to specific final states demands a detector system with high efficiency and good energy resolution in the 10-20 MeV energy region. The most widely used detector which fulfills these criteria is a large (usually 240 mm in diameter and 240-360 mm long) NaI(TI) scintillator together with an anticoincidence plastic scintillator shield. Depending on the energy threshold of the shield these spectrometers can have 2 5 - 4 0 % detection efficiency for 22 MeV v-rays. A high degree of uniformity of the crystal is essential to achieve high energy resolution. S~ndorfi and Collins [1], for example reported 2.2% FWHM at 22 MeV. At the Kernfysisch Versneller Instituut (KVI) a similar spectrometer [2] has 2.0% F W H M at 22 MeV. Recently Hakansson et al. [3] obtained 1.6% resolution at 22 MeV which is the best reported value for this type of detector up till now. This type of spectrometer is extensively used for the investigation of the ~/-decay of giant dipole resonances (GDR) [4]. In special applications however, like in the investigation of the ~/-decay of the G D R to separate final states [5,6] the resolution of such a detector is a limiting factor. Recently Bruce et al. used a new method [7] for measuring high energy gamma-rays, employing Ge + BGO anti-Compton detector systems designed for low energy gamma-ray spectroscopy. In this measurement

I On leave from the Institute of Nuclear Research of the Hungarian Academy of Sciences, P.O. Box 51, H-4001 Debrecen, Hungary. 2 Permanent address: Institute of Physics, Jagellonian University, Cracow, 30-059 Krak6w, Reymonta 4, Poland.

the energy of the ~/-quanta was reconstructed by summing (off-line) the energies deposited in coincidence in the Ge detector and in its BGO suppression shield. This hybrid Ge + B G O system gave more than 30% efficiency and a F W H M of 225 keV energy resolution at 6.13 MeV. Since most of the v-ray energy is measured in the Ge detector this system shows excellent gain stability. The same detection principle was applied recently by Yoshida et al. [8] who used a cluster of 7 hexagonal BaF 2 crystals by irradiating only the central crystal to improve the response of the BaF z array. One should mention, however, that the energy resolution of such a system is worse compared to either of the above mentioned spectrometers. In the present paper we discuss the performance of a hybrid Ge + BaF 2 spectrometer built at KVI, Groningen, in order to make use of the advantages of both systems of the latter type. The use of BaF 2 instead of BGO scintillator material together with a central Ge detector improves both the energy and time resolution of such a spectrometer. The measured energy resolution and efficiency of the system at 6.13 and 15.1 MeV v-ray energy was compared to the calculated response using a Monte Carlo technique through the GEANT3 [9] code.

2. The Ge + BaF 2 spectrometer The working principle of the spectrometer is based on the fact that above an energy of about 8 MeV the main mode of interaction of gamma-rays in the Ge detector is pair production. For a 10-20 MeV ~/-radiation the range of the electrons and positrons created is typically less than the size of the Ge crystal so their full

0168-9002/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

A. Krasznahorkay et al. / Ge + BaF2 composite "y-ray spectrometer

energy could in principle be detected. The radiation which usually escapes from the crystal is the bremsstrahlung emitted during the slowing down process of the particles, and the two annihilation quanta following positron capture. This radiation can be detected with high efficiency in a scintillator shield surrounding the Ge crystal. The energy deposited in the shield is therefore only a small part of the total energy of the original photon. Nevertheless, since the resolution of the Ge is much better than that of the scintillator, the effective resolution of the spectrometer is still mainly determined by the intrinsic resolution of the shield. By replacing the BGO scintillator shield by a BaF 2 scintillator the energy resolution of the system is improved by about a factor of 2. The gain stability of the spectrometer depends largely on the Ge detector and is therefore extremely good when compared to detectors based on photomuitiplier as pointed out in ref. [6]. Another advantage of using a BaF 2 scintillator instead of BGO is its much better time resolution. The rediscovery of BaF 2 scintillators started after observing a relative intense fast component superimposed on the known slow decay of the light pulse. Laval et al. [10] observed a decay time constant of 600 ps for the fast and 620 ns for the slow component. In coincidence experiments one can replace the Ge timing signal by the BaF 2 one and in this way the timing of the Ge + BaF 2 system can be much better (sub ns) than the timing of a big Nal (typically = 3 ns). This is important in coincidence experiments a n d / o r in reactions where time of flight discrimination of ~/s and neutrons is required. According to the recent developments of BaF 2 crystals (see for instance, the TAPS, two arm photon spectrometer detector array [11]) as good as 10.9% energy resolution was achieved for the ~37Cs 662 keV line using 25 cm long and 5.9 cm inner diameter hexagonal crystals. To do test measurements for the proposed Ge + BaF 2 system, six of these TAPS crystals were used as a shield for a 20% Ge detector. The schematic layout of the detector system is shown in fig. 1.

307

--F

J

a

6cm

F2

Fig. 1. Schematic layout of the Ge + BaF 2 spectrometer.

Exp. 1000 800

Monte-Carlo !

I

Ge

!

600 -.J 400 uJ Z Z 6 MeV) and detected in the BaF 2 usually exceed the upper discriminator level. The spectrometer permits active discrimination against cosmic rays too. If a cosmic shower interacts with the Ge crystal leaving about 20 MeV energy in it, it should also interact with at least two of the six BaF 2 crystals, leaving more than 20 MeV in each. With an upper limit for the energy detected by the shield of for instance 2 MeV, the cosmic ray events can be effectively rejected. For further improvement of the spectrometer the effect of using larger Ge detector and (or) BaF 2 shields was also simulated by the Monte Carlo calculations. We can summarize the results as follows: a) By increasing the diameter of the Ge detector only the solid angle of the spectrometer can be increased, but the efficiency remains almost the same. b) The full energy efficiency increases almost linearly with the length of the Ge crystal. This would be a promising way to improve the spectrometer. c) Increasing the thickness of the shield by a factor of 2 increases the full energy efficiency from 10% to 12%, although the height of the first escape peak can be decreased by about a factor of 3. d) A scintillator plug of 5 cm placed behind the Ge detector increases the full energy peak efficiency from 10% to 11%.

5. Conclusions The results of our measurements and Monte Carlo calculations indicate that the Ge + BaF 2 combined detector can have more than 10% detection efficiency and better than 0.5% energy resolution at E v = 15

Acknowledgements We wish to thank our colleagues at KVI, Groningen and Gesellschaft f'ur Schwerionenforschung Darmstadt

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A. Krasznahorkay et a L / Ge + BaF2 composite 7-ray spectrometer

mbH, D a r m s i a d t for making thc B a F 2 detectors availablc to us. This work was performed as a part of the research program of the Stichting voor F u n d a m e n t e e l O n d e r zoek der Materie ( F O M ) with financial support from the N e d e r l a n d s e Organisatie voor Wetenschappelijk O n d e r z o e k (NWO).

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