Abstract
Hitomi (ASTRO-H) was the sixth Japanese x-ray satellite that carried instruments with exquisite energy resolution of <7 eV and broad energy coverage of 0.3 to 600 keV. The Soft Gamma-ray Detector (SGD) was the Hitomi instrument that observed the highest energy band (60 to 600 keV). The SGD design achieves a low background level by combining active shields and Compton cameras where Compton kinematics is utilized to reject backgrounds coming from outside of the field of view. A compact and highly efficient Compton camera is realized using a combination of silicon and cadmium telluride semiconductor sensors with a good energy resolution. Compton kinematics also carries information for gamma-ray polarization, making the SGD an excellent polarimeter. Following several years of development, the satellite was successfully launched on February 17, 2016. After proper functionality of the SGD components were verified, the nominal observation mode was initiated on March 24, 2016. The SGD observed the Crab Nebula for approximately two hours before the spacecraft ceased to function on March 26, 2016. We present concepts of the SGD design followed by detailed description of the instrument and its performance measured on ground and in orbit.
Highlights
IntroductionHitomi was the sixth Japanese x-ray astronomy satellite[1,2,3,4,5,6] developed to investigate a variety of astrophysical problems, including the formation mechanisms and dynamics of galaxy clusters, formation and evolution of supermassive black holes, and to Journal of Astronomical Telescopes, Instruments, and Systems 021411-1
Hitomi was the sixth Japanese x-ray astronomy satellite[1,2,3,4,5,6] developed to investigate a variety of astrophysical problems, including the formation mechanisms and dynamics of galaxy clusters, formation and evolution of supermassive black holes, and to Journal of Astronomical Telescopes, Instruments, and Systems 021411-1Apr–Jun 2018 Vol 4(2)Tajima et al.: Design and performance of Soft Gamma-ray Detector onboard. . .uncover the behavior of matter under extreme conditions, such as high density, strong magnetic field, and strong gravity in the vicinity of neutron stars and black holes
To fulfill the above objectives, the Hitomi satellite carried the following instruments: a very high-energy-resolution soft x-ray spectrometer (SXS) covering the 0.3- to 12-keV band consisting of thin-foil x-ray optics [Soft X-ray Telescope (SXT)]7 and a microcalorimeter array (SXS);[8] a soft x-ray imaging spectrometer sensitive in the 0.4- to 12-keV band, consisting of an SXT focusing x-rays onto CCD sensors (Soft x-ray Imager);[9] a hard x-ray imaging spectrometer, sensitive in the 3- to 80-keV band, consisting of multilayer-coated, focusing hard x-ray mirrors (Hard X-ray Telescope)[10] and silicon (Si) and cadmium telluride (CdTe) cross-strip sensors [Hard X-ray Imager (HXI)];11–20 and a soft gamma-ray spectrometer covering the 60- to 600-keV band, utilizing multilayer semiconductor Compton cameras with an active shield [Soft Gamma-ray Detector (SGD)]
Summary
Hitomi was the sixth Japanese x-ray astronomy satellite[1,2,3,4,5,6] developed to investigate a variety of astrophysical problems, including the formation mechanisms and dynamics of galaxy clusters, formation and evolution of supermassive black holes, and to Journal of Astronomical Telescopes, Instruments, and Systems 021411-1. The sensitivity of the HXD GSO was limited by gamma-ray backgrounds arising from the contamination of signal from celestial sources, and by background resulting from radiation by radioisotopes in the GSO scintillators and the activation of the detector materials mainly due to the particle environment of the South Atlantic Anomaly (SAA). The design of the SGD attempted to reduce those internal backgrounds by employing direction-sensitive gamma-ray detectors made of semiconductor sensors. Directional sensitivity can further suppress backgrounds by rejecting events that do not originate from within the field of view (FOV)
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