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

TL;DR

This paper investigates the origins of the in-orbit instrumental background of Hitomi's Hard X-ray Imager, identifying different background components for each detector layer and validating these with Monte Carlo simulations to improve future observations.

Contribution

It provides a detailed analysis of background sources in the HXI and validates the findings through simulations, offering insights for background reduction strategies.

Findings

Different background components dominate each detector layer.

Monte Carlo simulations accurately reproduce observed spectra.

An electron shield is suggested to reduce background.

Abstract

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 background.