Modeling of proton-induced radioactivation background in hard X-ray telescopes: Geant4-based simulation and its demonstration by Hitomi’s measurement in a low Earth orbit

Abstract

Hard X-ray astronomical observatories in orbit suffer from a significant amount of background due to radioactivation induced by cosmic-ray protons and/or geomagnetically trapped protons. Within the framework of a full Monte Carlo simulation, we present modeling of in-orbit instrumental background which is dominated by radioactivation. To reduce the computation time required by straightforward simulations of delayed emissions from activated isotopes, we insert a semi-analytical calculation that converts production probabilities of radioactive isotopes by interaction of the primary protons into decay rates at measurement time of all secondary isotopes. Therefore, our simulation method is separated into three steps: (1) simulation of isotope production, (2) semi-analytical conversion to decay rates, and (3) simulation of decays of the isotopes at measurement time. This method is verified by a simple setup that has a CdTe semiconductor detector, and shows a 100-fold improvement in efficiency over the straightforward simulation. To demonstrate its experimental performance, the simulation framework was tested against data measured with a CdTe sensor in the Hard Preprint submitted to Nuclear Instruments and Methods in Physics Research Section A X-ray Imager onboard the Hitomi X-ray Astronomy Satellite, which was put into a low Earth orbit with an altitude of 570 km and an inclination of 31 • , and thus experienced a large amount of irradiation from geomagnetically trapped protons during its passages through the South Atlantic Anomaly. The simulation is able to treat full histories of the proton irradiation and multiple measurement windows. The simulation results agree very well with the measured data, showing that the measured background is well described by the combination of proton-induced radioactivation of the CdTe detector itself and thick Bi 4 Ge 3 O 12 scintillator shields, leakage of cosmic X-ray background and albedo gamma-ray radiation, and emissions from naturally contaminated isotopes in the detector system.

Origin of the in-orbit instrumental background of the Hard X-ray Imager onboard Hitomi

“…The background of irradiated CdTe-DSD was thoroughly investigated with Monte Carlo simulations in our previous work. 4 In that paper, we successfully reproduced the observed background of CdTe-DSD with the simulations, and revealed that it is dominated by the protoninduced radioactivation of the CdTe-DSD itself.…”

“…We used a Monte Carlo simulation framework COMPTONSOFT,11 identical to that used in a previous work on the radioactivation background of the CdTe-DSD. 4 In the COMPTONSOFT, interactions between incident particles and detector materials are simulated by the software based on the GEANT4 toolkit library. [12][13][14] In addition, detailed detector responses due to the charge carrier transport inside semiconductor detectors are also implemented.…”

Origin of the in-orbit instrumental background of the Hard X-ray Imager onboard Hitomi

“…The background of irradiated CdTe-DSD was thoroughly investigated with Monte Carlo simulations in our previous work. 7 In that paper, we successfully reproduced the observed background of CdTe-DSD with the simulations, and revealed that it is dominated by the proton-induced radioactivation of the CdTe-DSD itself.…”

Reconstruction of multiple Compton scattering events in MeV gamma-rayCompton telescopes using a physics-based probabilistic model

“…In order to test the reconstruction algorithm developed in this work, we apply it to a simulation data set generated by ComptonSoft, a software package of Geant4-based simulation and data analysis for Compton telescopes [26,27,28]. The algorithm has a great advantage in the event reconstruction of Compton telescopes with a large sensitive volume.…”