Abstract
Understanding and reducing 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) onboard the Hitomi satellite provide useful information on the background components due to its multilayer configuration with different atomic numbers: the HXI consists of a stack of four layers of Si (Z = 14) detectors and one layer of cadmium telluride (CdTe) (Z = 48, 52) detector surrounded by well-type Bi4Ge3O12 active shields. Based on the observational data, the backgrounds of the 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 quantitatively verifying the above hypothesis. In addition, we suggest the inclusion of an electron shield to reduce the background.
Highlights
The hard x-ray imager (HXI) was one of the four observational instruments onboard the Hitomi satellite, which was launched on February 17, 2016.1,2 Combined with the hard x-ray telescopes,[3] the Hard X-ray Imager (HXI) was designed to perform imaging spectroscopy in the 5- to 80-keV band
Since the background spectra of layers 1 to 3 (Fig. 2) are similar to each other, hereafter, we separate the detectors of the HXI into three groups: the top layer of the double-sided Si strip detector (DSSD), the remaining three layers of the DSSD, and the cadmium telluride (CdTe)-DSD layer
4.1 Incident Path of Low-Energy Electrons In Secs. 2 and 3, we showed that the low-energy electrons with energies of ∼100 keV are the major background component of the top-layer DSSD
Summary
The hard x-ray imager (HXI) was one of the four observational instruments onboard the Hitomi satellite, which was launched on February 17, 2016.1,2 Combined with the hard x-ray telescopes,[3] the HXI was designed to perform imaging spectroscopy in the 5- to 80-keV band. The Si detectors detect lowerenergy x-rays with less radioactivation background than high-Z detectors, while CdTe detectors detect higher-energy x-rays that cannot be absorbed by Si. The Si detectors detect lowerenergy x-rays with less radioactivation background than high-Z detectors, while CdTe detectors detect higher-energy x-rays that cannot be absorbed by Si This stacked configuration of the main imager is effective in optimizing the in-orbit instrumental background and in achieving high sensitivity in the hard x-ray band. The multilayer configuration of low-Z (Si) and high-Z (CdTe) sensors is very useful, in achieving high sensitivity, and in decomposing the background components since interactions between the detector materials and cosmic-ray particles/photons have different. The multilayer configuration of the HXI would provide us with fruitful knowledge on the origins of the in-orbit instrumental backgrounds, which is essential to achieve high sensitivity in hard x-ray astronomical observations.
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