Elsevier Science 1 Journal logo A novel design of the MeV gamma-ray imaging detector with Micro-TPC R.Orito *,H.Kubo,K.Miuchi,T.Nagayoshi,A.Takada,T.Tanimori,M.Ueno Department of Physics,Graduate School of Science,Kyoto University,Sakyo-ku,Kyoto 606-8502,Japan Elsevier use only: Received date here; revised date here; accepted date here Abstract A new type of MeV gamma-ray imaging detector in the energy band from 0.2 to 30 MeV, based on a gaseous time projection chamber with Micro Pixel Chamber (Micro-TPC), is proposed. This detector consists of a gas chamber and a scintillation camera. The Micro Pixel Chamber (µ-pic) has pixel type anodes and cathodes with 400 µm pitch, which enables it to measure the fine tracks of electrons or positrons in Compton scattering or pair creation. The scintillation camera measures the positions and energies of Compton-scattered gamma-rays, electrons and positrons by pair creation. Using information from both the gas chamber and scintillation camera, the incident gamma-rays can be reconstructed event by event with high angular resolution. Simulation of the detector performance and the status of the prototype are reported. 2001 Elsevier Science. All rights reserved Keywords: MeV gamma-rays;imaging;compton;tpc 1. Introduction The most classical MeV gamma-ray imaging detector is a phoswich detector with a lead collimator, such as OSSE[1]. The design is simple but the field of view is narrow because of the collimator. COMPTEL[2], another type of the MeV gamma-ray detector based on the double Compton method doesn t need a collimator. In this detector, the * Corresponding author.e-mail:orito@cr.scphys.kyoto-u.ac.jp incident gamma-ray can be reconstructed only on the direction cone, since the direction of the recoil electron by Compton scattering can t be measured. The MeV gamma-ray detectors based on the multiple Compton scatterings method[3] with layers of semiconductor detectors are recently studied. In these detectors, it is also difficult to measure the direction of low energy recoil electron because of the large angle deflection by multiple scattering in the solid detector. Since multiple scattering is smaller in the gas phase than that in the solid, the angular resolution
2 Elsevier Science of the initial direction of the recoil electron in gaseous detectors is better than solid detectors. Thus, we propose a MeV gamma-ray imaging detector based on the gaseous Micro-TPC[4] enclosed by a scintillation camera as shown in Fig. 1. The Micro- TPC consists of the µ-pic[5][6],which has pixel type electrodes arranged perpendicular to each other with 400 µm spacing. It has long-term stability and higher gain than the present MSGCs[7]. In the Micro-TPC, the gaseous target and the microelectrode readout allow us to get the three-dimensional direction of the low energy recoil electron at the point which Compton scattering occurs. Combined with information from the scintillation camera enclosing the Micro-TPC for detecting the position and energy of the scattered gamma-ray, event by event reconstruction of the incident gamma-ray direction is realized. The event by event reconstruction gives a high efficiency of background rejection without a large anti-coincidence counter of the inorganic scintillator. A large field of view is expected because no widespread collimator is needed. Thus, application of this detector in astrophysics, medical science and radioactive pollution monitoring is expected. 2. Principle of gamma-ray imaging Fig.1 shows a schematic view of the detector and the principle of gamma-ray imaging. The Micro-TPC is filled with noble gases, and a position sensitive scintillation camera encloses half the volume of the Micro-TPC. If Compton scattering occurs in the gas, both the three-dimensional track and the energy of the recoil electron are measured by the Micro-TPC, while the position and energy of the scattered gamma-ray are measured by the scintillation camera. The depth of the electron track in the Micro-TPC is measured with the drift time of electrons in the gas, from a trigger of the scintillation camera. The start and end points of the track are distinguished by the difference of the energy deposit of the electron. Thus, the original direction of the incident gamma-ray can be reconstructed event by event on a small segment of the standard Compton event circle (Fig. 1). Fig. 2 shows the definition of the axes used in this paper. ϕ is defined as the angle between the scattered and incident gamma-ray axes, which can be found from the kinetic energy E e of the recoil electron and the energy of the scattered gamma-ray E sg as follows. cos ϕ=1-m e c 2 (1/E sg - 1/E ig ), E ig =E sg +E e (1) δ is the angle between projected vectors of the recoil electron and the incident gamma-ray on the normal plane to the scattered gamma-ray. The axes of the zenith and azimuth angles are fixed to the detector. If the scattered gamma-ray doesn t lose all its energy in the scintillation camera, the incident gamma-ray direction is not reconstructed correctly. But we can reject such an event from the Fig. 1 Concept of the MeV gamma-ray detector with Micro-TPC and scintillation camera Fig. 2 Definition of axes
Elsevier Science 3 inconsistency of two angles, the α angle from Eq.(2) and measured α angle between the recoil electron and the scattered gamma-ray (hereafter a-cut). We can also reject the background gamma-rays with a high efficiency by the same way. cosα = Ε e (Ε sg - mc 2 ) / (E sg 1/2 (E e 2 +2E e mc 2 )) (2) In the pair creation process, the three-dimensional tracks of both positron and electron are measured in the Micro-TPC, and the energies of the electron and positron passing through the Micro-TPC volume are measured by the enclosing scintillation camera. Thus the incident gamma-ray is reconstructed from simple summation of the electron and positron momentum vectors. Fig. 3 Detection efficiency as a function of the energy of the incident gamma-ray from zenith 3. Monte Carlo simulation The detector performance was simulated by Geant 4.4.1[8]. The volume of the Micro-TPC is supposed to be 30 cubic cm and filled with xenon gas at 1.5 atm. The scintillator which encloses the Micro-TPC to the half height is 2.5 cm thick CsI(Tl) with 5mm pitch segments which are optically separated by reflectors. The energy resolution of the Micro-TPC is supposed to be 30% at 5.9keV[6].The depth of the interaction point in the scintillator is always fixed at the center of the scintillator. The energy resolution of the scintillation camera is supposed to be 7% at 662 kev. 3.1. Compton-scattering process(0.2-2mev) In the Compton scattering process, the detection efficiency as a function of the energy of the incident gamma-ray from the zenith is shown in Fig. 3. The detection efficiency of the MeV gamma-rays is calculated by requiring the following four constraints. Compton scattering occurs only once in the gas. The recoil electron stops in the gas and deposits all its energy. The scattered gamma-ray doesn t absorb by the photoelectric effect in the gas. The scattered gamma-ray loses all its energy in the scintillator. The efficiency is lower in the higher energy region, because of the higher probabilities that the scattered Fig. 4 Effective area as a function of the zenith angle of the incident gamma-ray at 1MeV. The azimuth angle is 0 Fig. 5 Angular resolution as a function of the incident gamma-ray energy. The solid and dashed lines represent the ϕ and δ angles, respectively gamma-ray passes through the scintillator and that the electron passes through the gas volume. If scintillators are thick enough to detect all scattered gamma-rays, and the measurement of the electrons which don t stop in the gas is possible, the efficiency is 10 times higher at 2 MeV. The effective area at 1MeV as a function of the zenith angle is shown in Fig. 4,which indicates a wide field of view, 2 str (FWHM). The angular resolution of ϕ and of δ as a function of the incident gamma-ray energy is shown in Fig. 5. The direction of the recoiled electron is determined at a position 1mm away from the Compton scattering point in the gas. The result is after a-cut that a < 10 degrees at 1 MeV. Now we don t use any other cuts, such as rejecting an electron
4 Elsevier Science with low energy, which leads to higher δ resolution but lower efficiency. If the measurement of the electrons which don t stop in the gas is possible, the δ resolution is twice as good at 2MeV because of a small multiple scattering of high energy electrons. 3.2. Pair creation process (1-30MeV) In the pair creation process, the detection efficiency and the angular resolution are shown in Figs. 6,7. We suppose that no medium exists between the gas and the scintillators. Fig. 8 Prototype of the MeV gamma-ray imaging detector 5.Summary Fig. 6 Detection efficiency as a function of the incident gamma-ray energy in the pair creation process A new type of MeV gamma-ray imaging detector based on the gaseous Micro-TPC and enclosing scintillation camera is proposed. In this detector, incident gamma-rays can be reconstructed event by event with high angular resolution, because of the fine tracking of electrons and positrons by the Micro- TPC. Monte Carlo simulation of the detector performance and the status of the prototype have been reported. Fig. 7 Angular resolution of ϕ as a function of the incident gamma-ray energy in the pair creation process 4.Prototype development Fig. 8 shows a prototype of the MeV gamma-ray imaging detector for the Compton scattering process. This prototype is intended to confirm the feasibility of this imaging method. The prototype Micro-TPC has a 10cm square detection area and 8.5cm drift length. The position sensitive scintillation camera is an Anger camera[9]. A NaI(Tl) scintillator of 4 4 1 size is read by 25 single anode PMTs of 3/4 diameters. When the Micro-TPC is triggered in a few µ-sec drift time after the scintillation camera triggered, pulse heights of the signals from 25 PMTs and encoded position of the electron track are collected. Now we begin to take data with radioisotopes. 6.Acknowlegement This work is supported by a Grant-in-Aid in Scientific Research of the Japan Ministry of Education, Culture, Science, Sports and Technology, and Ground Research Announcement for Space Utilization promoted by Japan Science Forum. References [1] W.N.Johnson et al,apjs 86(1993)693 [2] V.Schönfelder et al,apjs 86(1993)657 [3] T.Kamae et al, Nucl.Instr.and Meth.A 260(1987)254 [4] H.Kubo et al, these proceedings [5] A.Ochi et al,nucl.instr.and Meth.A 471(2001)264 [6] T.Nagayoshi et al, these proceedings [7] T.Tanimori et al,nucl.instr.and Meth.A 381(1996)280 [8] Geant4, http://geant4.web.cern.ch/geant4 [9] H.O.Anger,Rev.Sci.Instr.29(1958) 27
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