Simulation study of the 511 keV annihilation line observation with the Soft Gamma-ray Detector onboard ASTRO-H Yuto Ichinohe ([email protected]), H. Odaka, T. Sato, S. Takeda, S. Watanabe, M. Kokubun, T. Takahashi, H. Tajima, T. Tanaka, Y. Uchiyama, K. Nakazawa 1. Introduction International Workshop on Positrons in Astrophysics March 20-23, 2012 - Mürren, Switzerland SGD performance at 511 keV The 511 keV annihilation line is the one of the key targets of the Soft Gamma-ray Detector (SGD) onboard ASTRO-H. The soft gamma-ray band, including the 511 keV annihilation line, is less explored than the hard X-ray or GeV gamma-ray bands, due to 1) high in-orbit background and 2) low detection efficiency. To overcome these observational difficulties, the SGD adopts a new concept of “narrow fieldof-view (FOV) Compton camera”. In order to study the SGD performance at 511 keV, we conducted detailed Monte-Carlo simulations with full implementation of the detector mass model and event analysis, considering realistic activation background estimation based on the radiation environment at the low Earth orbit. Angular Resolution Counts 7000 Energy Resolution 6000 9000 8000 7000 6000 5000 4000 Counts Counts -- Energy Resolution: 4.1 keV -- Angular Resolution: ~5.0 deg -- Effective Area: 2.7 cm2 -- Background: 2-4x10-5 counts/sec/keV 5000 9000 8000 5.0º (FWHM) 4000 7000 4.1 keV (FWHM) 6000 5000 3000 4000 3000 In the simulations, we assumed following parameters: -- Obserbation time: 1Msec -- Energy cut range: 511.0 ± 4.1 keV -- Theta cut range: -5.0 - 5.0 deg 3000 2000 2000 2000 1000 0 500 505 510 515 1000 0 0 100 200 300 520 Energy (keV) 400 1000 500 Energy (keV) 0 -4 -2 0 2 4 ARM (deg) → H. Odaka’s talk on Friday 2. How to reduce the backgrounds? There are many background / noise origins -- Proton, CXB, etc... coming from outside of the line of sight toward the target -- Photon escaped from the detector after Compton Scattered by the detector -- Gamma-rays or electrons emitted by the radioisotopes generated inside the detectors themselves due to radioactivation by trapped protons (the most dominant background origin for the 511 keV observation) Simulated Background Spectrum (SAA vs. non-SAA) Counts/sec/keV Soon after the satellite passes the SAA (South Atlantic Anomaly), the detector material is radioactivated, resulting in high background level at 511 keV. In this simulation, in order to achieve higher sensitivity, we selected the events 30000 sec after passage of the SAA. SAA pass non-SAA pass 10-4 10 -4 10-5 10 -5 300 350 400 450 500 550 600 650 700 480 Energy (keV) 490 500 510 520 530 540 Energy (keV) (c) NASA The SGD achieves high sensitivity by the powerful background rejection Compton Camera Compton Reconstruction Incident Photon Incident direction cut FOV: 10ºx10º Hit Data (E1, r1), (E2, r2), ... Photon coming from inside of the detector (Activation) NO Compton Reconstruction → Compton Cone CdTe (E1 , r1 ) Si CdTe 10-3 10 10-4 10-4 ③ Event Background 2 me c 1 E2 10-5 1 E1 + E2 ① Before anti-coincidence 10-6 3 hit 2 Hit 3 Hit 4 Hit 4 > Hit 56% 2 hit Si Si CdTe CdTe CdTe When the incident photon energy is high (> 400 keV), the ratio of the number of 3 or more hit events to the number of 2 hit events becomes larger. Currently, Si-SiCdTe (SSC) events and Si-CdTe-CdTe (SCC) events are included in the analysis. CdTe Effective Area at 511 keV The Si hit is regarded as the first Compton scattering. 2 possibilities Si-CdTe1-CdTe2 and Si-CdTe2-CdTe1 are compared in the point of view which is more physically consistent. By reconstructing each last two CdTe-CdTe events regarding the first Si hit as the source, we can calculate the Δθ (ARM). We choose the possibility of which Δθ is smaller than the other. 0.030cm2 0.592cm2 0.520cm2 Total: 2.7 cm2 0.494cm2 1.052cm2 480 490 500 510 520 530 540 Energy (keV) 4. SGD Sensitivity for the 511 keV Line The sensitivity of the SGD for the 511 keV annihilation line reaches to the 5-7x10-5 ph/ sec/cm2 with exposure of 1 Msec (depends on the systematic error of the background). 511 keV candidates 3 or more hit Compton Events -6 One β+ decay positron annihilates into two 511 keV lines. Background rejection efficiency of the active shields for such events is better than that for fluorescence lines or Gamma decay lines. Background is suppressed by 1/20 with Compton Reconstruction Source Si-CdTe-CdTe event reconstruction 10 103 Energy (keV) 102 511 keV Flux (10-5 ph/cm2/sec) GC, bulge 100 GC, Disk 47 LMXB ~1 BH Binary 40 (Cyg X-1) Nova 10 511 511 keV keV Line Line Sensitivity Sensitivity (5σ) (5σ) 7.0E-05 2.0E-04 511 keV Flux (ph/sec/cm2) 511 keVPhoton Photon Flux (ph/sec/cm2) 29% 5% -5 ③ After initial direction cut Number of hits per event at 511keV 11% 10 ② After anti-coincidence By using the information on deposited energy and interacted positions, we can limit the incident direction of the photon that is consistent with the Compton Kinematics (Compton Cone). If this direction is not in the BGO FOV, this event will be rejected as background. 3. To Improve the Detection Efficiency Inter-CC -3 507 keV (121Te) YES NO When SGD detects an event including some BGO hits, this event is rejected as background. 511 keV (β+ decay) BGO cos = 1 BGO 10-2 (E2 , r2 ) β+ Decay Annihilation Outside of the BGO FOV ? 1SGD ARMcut 5deg systematic error of the background 1.8E-04 6.0E-05 systematic error of the background 1.6E-04 5.0E-05 1.4E-04 1.5% 3% 1.5% 5% 3.0% 5.0% 1.2E-04 4.0E-05 1.0E-04 8.0E-05 3.0E-05 6.0E-05 2.0E-05 4.0E-05 2.0E-05 1.0E-05 2E+06 3E+06 4E+06 5E+06 6E+06 7E+06 8E+06 9E+06 1E+07 0E+001E+06 0E+00 1E+06 2E+06 3E+06 4E+06 5E+06 6E+06 7E+06 8E+06 9E+06 1E+07 Exposure Time (sec) Exposure Time (sec) non-SAA pass Energy cut range 511.0 ± 4.1 keV (based on 1 Msec simulation data) 100 Case: Galactic Center Counts/keV Escaping photon 10-2 Photon coming from the outside of the BGO FOV YES Hits at BGO? ② Si Compton Cone ① Proton, CXB, etc... Counts/sec/keV BGO Active Shield Anti-Coincidence Simulated Background Spectrum (non-SAA pass) Counts/sec/keV Narrow FOV 90 Observed Spectrum Signal Background 80 70 2Hit Si-Si 2Hit Si-CdTeBottom 2Hit Si-CdTeSide 3Hit SSC 3Hit SCC Effective Area: 2.7 cm2 (Total) Effective area is highly improved by using the 3 hit Compton Events Galactic Center Bulge Region Radius: 3 deg Flux: 1x10-3 ph/sec/cm2 Line width: 2.4 keV (FWHM) Exposure: 1 Msec 60 50 40 30 20 24σ Detection 10 0 480 490 500 510 520 530 540 Energy (keV) ・The Background rejection technique for the SGD reduces the activation background count by 1/100. 5. Conclusions ・The SGD will enable precise measurement of 511 keV line flux from the GC bulge. ・The SGD 1Msec sensitivity for the 511 keV annihilation line will be ~5x10-5 ph/cm2/sec.
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