High-purity germanium Gamma-Ray Spectrometer with stirling cycle cryocooler

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Adv. Space Res. Vol. 30, No. 8, pp. 1927-1931, 2002 © 2002 COSPAR. Publishedby ElsevierScience Ltd. All rights reserved Printed in Great Britain 0273-1177/02 $22.00 + 0.00 PII: S0273-1177(02)00490-8

HIGH-PURITY GERMANIUM GAMMA-RAY SPECTROMETER WITH STIRLING CYCLE CRYOCOOLER M. N. Kobayashi 1, N. Hasebe 1, T. Miyachi 1, T. Doke 1, J. Kikuchi 1, H. Okada I, A. Oka 1, O. Okudaira I, H.Souri I, N.Yamashita I, E. Shibamura2, T. Kashiwagi 3, T. Takashima4, K. Narasaki 5, K. Tsurumi 5, K. Mori 6, C. d'Uston 7, S. Maurice 8, M. Grande9, and R.C. Reedy 1°

i Adv. Res. Inst. for Science and Engineering, Waseda Univ., 3-4-10kubo, Shinjuku-ku, Tokyo, 169-8555 Japan 2 College of Health Science, Saitama Prefectural Univ., Saitama, Japan Faculty of Engineering, Kanagawa Univ., Yakohama, Japan 4 Department of Physics, Nagoya Univ., Chikusa-ku, Nagoya, Japan 5 Niihama Works, Sumitomo Heavy Industries Ltd., Ehime, Japan 6 Clear Pulse Ltd., 6-25-17 Chuo, Ohta-ku, Tokyo, 143-0024 Japan 7 CESR, CNRS/UPS, Colonel Roche, B.P 4346, France 8 Observatoire Midi-Pyrenee, Toulouse, France 9 RAL, Dedicot, Oxfordshire OXllOQX, UK 1oLos Alamos Nat. Lab., Los Alamos, New Mexico, 87545, USA ABSTRACT The Japanese lunar polar orbiter SELENE carries a gamma-ray spectrometer which uses a high-purity Ge detector cooled to 80-90 K by a Stifling mechanical cooler. The Gamma-Ray Spectrometer (GRS) consists of a large volume n-type Ge detector (252 cc) as the main detector and bismuth-germanate (BGO) and plastic scintillators as an active shielding. The engineering model still maintains excellent energy resolution even after severe vibration testing. The Gamma-Ray Spectrometer will globally map of the Moon for the major elements ofO, Mg, AI, Si, Ti, Fe, etc. and natural radioisotopes of K, Th and U with a high precision. The energy resolution of the GRS is such that it would identify prompt gamma-ray line from hydrogen and the location and the amount of ice, if it exists at the polar regions. © 2002 COSPAR. Published by Elsevier Science Ltd, All rights reserved.

INTRODUCTION The SELENE project (Selenological Engineering Explorer) is a Japanese lunar polar orbiter scheduled to be launched in 2004. It is a three-axis-stabilized spacecraft with a nominal life of -1 year at an altitude of 100 km. The primary objectives of the mission are to carry out selenological and engineering studies of the Moon, as well as the environment of the Moon and its neighbor. A surface of a planetary body with thin or no atmosphere emits gamma-ray lines produced by the nuclear interaction of planetary material with cosmic ray particles, whose intensity can be observed by orbital remote sensing (Feldman et al., 1991). The average concentration of various elements in the upper layers of a few 10 cm can be measured by the Gamma-Ray Spectrometer (GRS) remote sensing. The SELENE GRS using Ge-detector is expected to offer the first global high precision mapping of lunar surface (Hasebe et al., 1999), which will promote a better understanding of the origin and evolution of the Moon and the future utilization of the lunar resources. The present status of the development of the engineering model (EM) of the GRS for the SELENE mission is reported in this paper. INSTRUMENT The Gamma-Ray Spectrometer GRS consists of Gamma-Ray Detector GRD, Compressor Driver Unit CDU and a common electronics unit GPE and charged particle spectrometer CPS (Hasebe et al., 1999).

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A. Detector Configuration In previous missions for lunar exploration, NaI(TI) and BGO were used as the gamma-ray spectrometer. In SELENE, however, a Ge detector is to be used at first time for global mapping of chemical composition of lunar surface because of its excellent energy resolution. The detector configuration of GRD is schematically shown in Figure 1. The GRD consists of Ge/BGO/Plastic detectors, vacuum container, Stirling refrigerator, preamplifiers, high voltage supplies and radiators. A use of high purity n-type Ge crystal is important because of its high resistance against proton/neutron induced radiation damage (Koenen et al., 1995). The Ge detector has a volume of 252cc and is hermetically encapsulated in a vacuum-tight A1 canister (provided by Eurisys Measures). A high vacuum of the order of 108 torr is maintained in the canister in a temperature region from -80 K to -390 K. The Ge-canister is placed in a high vacuum chamber which is contacted to the cooler head through a copper de-coupler. In order to increase the sensitivity of GRS, it is essential to decrease background gamma-rays. The major background components are cosmic ray particles entering the detectors, produced particles due to primary and secondary cosmic ray interactions with materials of spacecraft, and scattered gamma-rays produced in planetary surface and detector itself. The SELENE GRS employs BGO and plastic scintillators as an active shield. The Ge detector is surrounded by the horseshoe-shaped BGO detector (see Figure 1). The thickness of BGO in the spacecraft (S/C)-side is so thick that the background from the S/C can be greatly reduced, while no BGO shield is placed to the lunar-side, which allows the lunar gamma-rays to easily enter the Ge detector. A massive shield of BGO greatly decreases the background gamma-rays produced in the spacecraft and materials surrounding the Ge detector. The BGO shield also reduces the Compton background by anticoincidence counting between Ge and BGO detector. A plastic scintillator is located in the window of GRS toward the Moon. It decreases the background caused by albedo particles. The combination of Ge detector with BGO and plastic shields in the GRS on SELENE greatly improves the sensitivity and the energy resolution. B. Thermal and Vibration Control The Ge detector must be cooled below 110 K to measure gamma-rays. The Ge crystal in GRS is cooled to 80-90 K by a Stirling mechanical cooler developed by Sumitomo Heavy Industries (SHI). The cooler consists of a compressor module, a displacer and a drive electronics. The cooling capacity is 1.0 W at 80 K and the total power consumption is 55 W. The cooling unit and its AC driving unit weigh 8.4 kg in total. The cooler uses dual opposed pistons and a free displacer in the cold head to greatly reduce a vibration levels in the compressor. The cooler has been successfully operating over 18000 hrs at SHI. The cold finger is connected to the canister through a flexible thermal link with high thermal conductance, which is made of 40 copper foils with which a high damping is achieved. Two Glass Fiber Resin Plastics (GFRP) supports for the Ge canister are attached to the vacuum

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Fig. 1 Schematic drawing of the Gamma-Ray Spectrometer onboard SELENE

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chamber to minimize the thermal leakage and to withstand the vibration load at launch. Conductive thermal flux from these supports and radiative flux between the surface coupling are estimated to be -490 mW and -510 mW or less, respectively. An additional ring-shaped GFRP support, touching at four points of Ge canister, is placed in order to reduce acceleration load. When the canister is cooled, this support is separated from the cold head due to shrinkage and prevents thermal leakage. We have made twice random vibration tests for the engineering model of GILD with full vibration level (QT : 10 Grms). The performance (BGO and Ge-detectors) of GRD has not changed at all after severe tests of random and sine vibrations. It takes about 24 hrs to cool the Ge crystal from room temperature to 90 K.

C. Electronics and Digital Processing The first cold stage of the charge sensitive preamplifier mounted on the back of the canister is cooled down to 130-160 K. Signals from Ge are fed to a shaping-amplifier. The pulses are analyzed by a 8000-channel pulse height analyzer for gamma rays ranging from 0.1 MeV to 10 MeV. The GPE includes two CPU boards. One is the SI-CPU board which controls data taking and management. The other is the SI-OBC that processes telemetry, format data, analyzes commands from ground telemetry center and so on. After data are accumulated for 20 sec in a histogram in memory and then time indexed, they are compressed by the SI-OBC and sent to the Data Handling Unit of the SELENE satellite system through the 1553B bus line.

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Fig. 2 Expected energy spectra of gamma-rays from the lunar surface by a Ge and NaI

PERFORMANCE The SELENE GRS is approximately twice as sensitive as the Lunar Prospector (Lawrence et al., 1997) and four times as sensitive as the APOLLO GRS (Harrington et al., 1974) because of the excellent energy resolution of the Ge detector. Indeed, it is 20 times better than that of scintillators. It determines the concentration of various elements in the lunar surface with better precision than studies carried out so far. The performance of the EM expected for the GRD is shown in Figure 2. It shows a comparison between gamma ray energy spectra measured by a Ge and NaI(TI) detector. In the calculation, Reedy's results are used as the emission of gamma-rays from the lunar surface (Reedy, 1978). The counting interval for each spectrum is 84 hrs. A clear difference between the observed peaks is seen from the figure. Therefore, it is anticipated that SELENE will provide the first precise global maps for the elements; O, Mg, A1, Si, Ti, Fe, K, Th, U, and so on, which is the major scientific goal of the GRS. The existence of water ice on the Moon is controversial (Watson et al., 1961; Arnold et al., 1979).

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The lunar water ice is quite interesting, not only from a scientific point of view, but also for lunar utilization in the future. Recently, Lunar Prospector data reported a possibility of water presence on the lunar surface (Feldman et al., 1998). It is therefore important to know detailed information on the location, depth and concentration of water ice at lunar poles. We have made a computer simulation of energy spectra expected by the SELENE GRS with an energy resolution of 3 keV in fwhm at 1.33 MeV. Here we assumed that the lunar water was in form of ice and homogeneously distributed in the uppermost several 10 cm of lunar regolith. Gamma-ray spectra measured at the polar region for 40 hours were shown in Figure 3, from which GRS is possible to identify 2.223 MeV photons from hydrogen via neutron capture processes, 2.210 MeV photons from A1 and 2.235 MeV photons from Si via neutron inelastic scattering. Both NaI(T1) and BGO are not able to separate the prompt capture line apart of 12-13 keV from the strong lines. The SELENE GRS can measure water ice with 0.1% mass fraction when water ice is homogeneously distributed in the top layer of regolith deep to 30 g/cm 2. If water ice was deposited between 30- 60 g/cm 2, SELENE GRS would not be able to detect it unless the water ice has a mass fraction of 0.2 % or higher. In summary, GRS, consisting of a large Ge detector, cooled by a Stifling refrigerator, and actively shielded by BGO and plastic scintillators, will be a payload on SELENE and can measure gamma-rays emitted from the lunar surface with precise spectral resolution. The compositional mapping by the GRS includes the identification and delineation of basalt in the maria, to determine the composition of ancient mare found in the highlands using Fe and Ti data, and to search for anomalous areas with unusual elemental compositions. Such mapping will also determine the distribution and the amount of water ice at the polar regions. These maps would be valuable, not only for a better understanding of the origin of the Moon and the evolution of the lunar crust, but also for resource-utilizing applications in future exploration.

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ACKNOWLEDGEMENT We would like to thank all of the SELENE team, especially people in ISAS and NASDA. REFERENCES Arnold J. R. (1979): Ice in the Lunar Polar Regions. J. Gephys. Res., 84, 5659-5668, 1979. Feldman W.C., et al., Introduction to Planetary Remote Sensing Gamma Ray Spectrometer: in Remote Geochemical Analysis: Elemental and Mineralogical Composition, ed. by Pieters & Englerty, Cambridge Univ. Press, 167-198, 1991. Feldman W. C., et al., Fluxes of Fast and Epithermal Neutrons from Lunar Prospector; Evidence for

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Water Ice at the Lunar Poles. Science, 281, 1496-1500, 1998. Harrington T.M., et al., The Apollo Gamma Ray Spectrometer, Nucl. Instr. and Methods, 118, 401-411, 1974. Hasebe N., et al., Gamma-Ray a~d Alpha-Ray Spectrometer Experiment on SELENE Mission, Proc. Lunar Planet. Sci. Conf. 30u', 1171-1172, 1999; N. Hasebe et al., Gamma-Ray Spectrometer for Japanese Lunar Polar Orbiter, Adv. Space Res., 23, 1837-1840, 1999. Koenen M., et al., Radiation Damage in Large Volume n- and p-Type High Purity Germanium Detectors Irradiated by 1.5 GeV Protons, IEEE Trans. on Nucl. Science, NS-42, 653-659, 1995. Lawrence et al., (1998): Global Elemental Maps of the Moon; The Lunar Prospector Gamma-Ray Spectrometer: Science, 281, 1484-1489, 1998. Reedy R.C., Planetary gamma-ray Spectroscopy, Proc. Lunar Planet. Sci. Conf. 9~h, 2961-2984, 1978. Watson K., Murray B C. and Brown H., The Behavior of Volatiles on the Lunar Surfaces, J. Geophys. Res., 66, 3033-3045, 1961.