Simulations of Advanced Compton Telescopes in a Space Radiation Environment

Transcription

1 Simulations of Advanced Compton Telescopes in a Space Radiation Environment Andreas Zoglauer, C.B. Wunderer, S.E. Boggs, UC Berkeley Space Sciences Laboratory G. Weidenspointner CESR, France

2 The Advanced Compton Telescope Enable high sensitivity -ray spectroscopy and imaging from 0.2 to 10 MeV Life Cycles of Matter Supernovae & nucleosynthesis Supernova remnants & interstellar medium Neutron stars, pulsars, novae Black Holes Creation & evolution Lepton vs. hadron jets Deeply buried sources Fundamental Physics & Cosmology Gamma-ray bursts & first stars History of star formation MeV dark matter 100 sensitivity improvement for spectroscopy, imaging & polarization ( MeV) Advanced 3-D positioning -ray spectrometers, 25% sky field-of-view LEO equatorial orbit, zenith-pointing survey mode (baseline mission), 80%/orbit 2/15

3 Principle of a Compton telescope Photons interact multiple times in active detector.the interaction sequence can be determined from redundant information (scatter angles). The origin of a single nottracked event can be restricted to the so called event circle. The photon originated at the point of overlap. 3/15

4 Baseline ACT instrument D1: 27 layers 2-mm thick Si 10x10 cm2, 64x64 strips 3888 det., 248,832 chns -30 C, Stirling cycle cooler D2: 4 layers, 16-mm thick Ge 9.2x9.2 cm2, 90x90 strips 576 det., 103,680 chns 80 K, Turbo-Brayton cooler BGO: 4-cm thick shield ACD: plastic scintillator ACT mass model on GLAST bus. Realistic mass model based on ISAL and IMDC (NASA/GSFC) instrument and mission engineering studies! More details about ACT: Boggs et al /15

5 Simulation & Analysis Package End-to-end simulation package for space-borne -ray telescopes (source and background). Background predictions verified on WIND/TGRS, INTEGRAL/SPI, RHESSI (MGGPOD, Weidenspointner et al. 2005). The packages includes: Comprehensive space environment model (ACTenvir, R.M. Kippen) Flexible instrument model (ACTmodel by R.M. Kippen & MEGAlib s Geomega by A. Zoglauer) Almost complete physics model (MGGPOD based on Geant3, written and maintained by Georg Weidenspointner) Compton imaging & analysis tools (MEGAlib, A. Zoglauer) ACTtools: MGGPOD: MEGAlib: 5/15

6 MGGPOD What is MGGPOD? Monte-Carlo suite consisting of the Fortran tools MGEANT (Geant3), GCALOR, PROMPT, ORIHET & Decay Designed for background simulation of gamma-ray telescopes Packaged, written, and maintained by Georg Weidenspointner (CESR, France), with neutron cross section updates by Elena Novikova (NRL) and contributions by Mike Harris (prompt deexcitation) Advantages: Verified, mostly working & faster than Geant4 Disadvantages: Base libraries (Geant3, GCalor) no longer supported Unstable and undebuggable (ZEBRA data structures) Not all required physics processes, cross sections, etc. included Develop replacement based on Geant4? 6/15

7 MGGPOD verification Weidenspointner et al Good agreement between measurement and simulation for TGRS, Integral and RHESSI (simulations ca. x2 to low). 7/15

8 ACT background simulations Equatorial low earth orbit after 2 years in orbit: Between 0.5 and 3.5 MeV activation background dominates 8/15

9 847 kev broad line from SN Ia Albedo Neutrons (activation) 2% Cosmic diffuse gamma-rays 50% Albedo Neutrons (prompt) 2% Albedo Photons 6% Trapped Protons (activation) 12% Cosmic Protons (activation) 28% Activation (protons & neutrons) still second largest background component Correct simulation critical for performance prediction 9/15

10 Layout of a space background simulation program Detector geometry, characteristics and trigger conditions Inspired by MGGPOD Radiation environment & history Geant4 based simulation program Level 1: Handle initial particles and secondaries as well as prompt (within coincidence window) deexcitation, decay, etc. and record all longer-lived radioactive isotopes Determine radioactive build-up according to irradiation history event list Level 2: Handle radioactive decay including all secondaries High level data analysis tools 10/15

11 Simulation requirements I Photon interactions (5 kev 1 TeV) Polarized Compton scattering (including subsequent Compton scatters) with Doppler broadening? Polarized gamma conversion (at least down to a few MeV) including conversion on electrons? Rayleigh scattering (taking care of polarization)? Photo effect Photo nuclear reaction (e.g. Giant Dipole Resonance) Electron interactions (5 kev 1 TeV) Energy loss via ionization (must work for thin media!) Molière scattering (must work for thin media!) Bremsstrahlung Delta rays Møller scattering Positron interactions (5 kev 1 TeV) see electrons Bahaba scattering instead of Møller scattering 11/15

12 Simulation requirements II Proton interactions (1 MeV up to 1 TeV) Ionization and scattering Bremstrahlung Spallation Capture Interaction cross sections for ALL isotopes? Correct generation of radioactive isotopes? Generate and track secondaries (, e+-, p, n, etc.) Alpha particles interactions See protons (ex capture) Ion interactions (up to Fe)? See Alpha 12/15

13 Simulation requirements III Neutron interactions (thermal 1 TeV) Elastic scattering Inelastic scattering Interaction cross-sections for all isotopes? Handle all excitation and all channels of deexcitations? Neutron capture Interaction cross-sections for all isotopes Generation of correct radioactive isotopes with the correct amount? Handle all channels of deexcitations and decays? 13/15

14 Simulation requirements IV Spallation, proton capture, inelastic neutron scattering, and neutron capture: Correctly handle meta-stable isotopes, excitation, deexcitation, radioactive decay, daughter nuclids, etc. for each possible isotope Distinguish between PROMPT (within detectors coincidence window) and DELAYED de-excitation and radioactive decay Handle all generated secondaries Keep record of all generated unstable isotopes Generated radioactive elements: Determine build-up of radioactive elements over mission life Handle radioactive decay (again take care of prompt and delayed) including tracking of all secondaries Correct handling of all possible decay channels! 14/15

15 Features not only of advantage for ACT but all current and future low-to-medium energy gamma-ray instruments: INTEGRAL Suzaku NCT (balloon-borne mini-act) GRI (potential European Gamma-ray lens imager) NeXT (next Japanese gamma-ray telescope) EXIST (possible successor of SWIFT) 15/15

16 Backup slides start 16/15

17 The radiation environment LEO Expected environment in an equatorial low-earth orbit (525 km) which avoids SAA 17/15

18 The gamma-ray imager GRI Gamma-ray lens space craft Focal plane detector space craft 18/15

19 Principle of a gamma-ray lens imager Laue diffraction of gamma rays within the crystal s volume under Bragg condition Compton scatter detector 19/15

20 Simulation requirements V Interface between SPENVIS and Geant4 Direct radiation environment input from SPENVIS to Geant4 as a function of: Orbit height Orbit inclination Timed SAA passages 20/15