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

Kavli Affiliate: Tadayuki Takahashi

| First 5 Authors: Kouichi Hagino, Hirokazu Odaka, Goro Sato, Tamotsu Sato, Hiromasa Suzuki

| Summary:

Understanding and reducing the in-orbit instrumental backgrounds are
essential to achieving high sensitivity in hard X-ray astronomical
observations. The observational data of the Hard X-ray Imager (HXI) on board
the Hitomi satellite provides useful information on the background components,
owing to its multi-layer configuration with different atomic numbers: the HXI
consists of a stack of four layers of Si (Z = 14) detectors and one layer of
CdTe (Z = 48, 52) detector surrounded by well-type BGO (Bi4Ge3O12) active
shields. Based on the observational data, the backgrounds of top Si layer, the
three underlying Si layers, and the CdTe layer are inferred to be dominated by
different components, namely, low-energy electrons, albedo neutrons, and
proton-induced radioactivation, respectively. Monte Carlo simulations of the
in-orbit background of the HXI reproduce the observed background spectrum of
each layer well, thereby verifying the above hypothesis quantitatively. In
addition, we suggest the inclusion of an electron shield to reduce the

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