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

Hirokazu Odaka*, Makoto Asai, Kouichi Hagino, Tatsumi Koi, Greg Madejski, Tsunefumi Mizuno, Masanori Ohno, Shinya Saito, Tamotsu Sato, Dennis H. Wright, Teruaki Enoto, Yasushi Fukazawa, Katsuhiro Hayashi, Jun Kataoka, Junichiro Katsuta, Madoka Kawaharada, Shogo B. Kobayashi, Motohide Kokubun, Philippe Laurent, Francois LebrunOlivier Limousin, Daniel Maier, Kazuo Makishima, Taketo Mimura, Katsuma Miyake, Kunishiro Mori, Hiroaki Murakami, Takeshi Nakamori, Toshio Nakano, Kazuhiro Nakazawa, Hirofumi Noda, Masayuki Ohta, Masanobu Ozaki, Goro Sato, Rie Sato, Hiroyasu Tajima, Hiromitsu Takahashi, Tadayuki Takahashi, Shin'ichiro Takeda, Takaaki Tanaka, Yasuyuki Tanaka, Yukikatsu Terada, Hideki Uchiyama, Yasunobu Uchiyama, Shin Watanabe, Kazutaka Yamaoka, Tetsuya Yasuda, Yoichi Yatsu, Takayuki Yuasa, Andreas Zoglauer

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    14 Citations (Scopus)


    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 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 Bi4Ge3O12 scintillator shields, leakage of cosmic X-ray background and albedo gamma-ray radiation, and emissions from naturally contaminated isotopes in the detector system.

    Original languageEnglish
    Pages (from-to)92-105
    Number of pages14
    JournalNuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
    Publication statusPublished - 2018 May 21


    • In-orbit background
    • Monte-Carlo simulation
    • Radioactivation
    • X-ray astronomy

    ASJC Scopus subject areas

    • Nuclear and High Energy Physics
    • Instrumentation


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