The Si/CdTe semiconductor Compton camera of the ASTRO-H Soft Gamma-ray Detector (SGD)

Abstract

a b s t r a c tThe Soft Gamma-ray Detector (SGD) is one of the instrument payloads onboard ASTRO-H, and will cover a wide energy band (60-600 keV) at a background level 10 times better than instruments currently in orbit. The SGD achieves low background by combining a Compton camera scheme with a narrow field-of-view active shield. The Compton camera in the SGD is realized as a hybrid semiconductor detector system which consists of silicon and cadmium telluride (CdTe) sensors. The design of the SGD Compton camera has been finalized and the final prototype, which has the same configuration as the flight model, has been fabricated for performance evaluation. The Compton camera has overall dimensions of 12 cm  12 cm  12 cm, consisting of 32 layers of Si pixel sensors and 8 layers of CdTe pixel sensors surrounded by 2 layers of CdTe pixel sensors. The detection efficiency of the Compton camera reaches about 15% and 3% for 100 keV and 511 keV gamma rays, respectively. The pixel pitch of the Si and CdTe sensors is 3.2 mm, and the signals from all 13,312 pixels are processed by 208 ASICs developed for the SGD. Good energy resolution is afforded by semiconductor sensors and low noise ASICs, and the obtained energy resolutions with the prototype Si and CdTe pixel sensors are 1.0-2.0 keV (FWHM) at 60 keV and 1.6-2.5 keV (FWHM) at 122 keV, respectively. This results in good background rejection capability due to better constraints on Compton kinematics. Compton camera energy resolutions achieved with the final prototype are 6.3 keV (FWHM) at 356 keV and 10.5 keV (FWHM) at 662 keV, which satisfy the instrument requirements for the SGD Compton camera (better than 2%). Moreover, a low intrinsic background has been confirmed by the background measurement with the final prototype.

Design and performance of Soft Gamma-ray Detector onboard the Hitomi (ASTRO-H) satellite

“…Taking advantage of 15 years of development, [11][12][13]31,[36][37][38][39][40][41][42] the SGD Compton camera is optimized to detect gamma rays up to 600 keV. A Compton camera consists of a stack of 32 layers of 0.6-mm-thick Si pixel sensors 43 followed by 8 layers of 0.75-mm-thick CdTe pixel sensors, 44,45 surrounded by 2 layers of 0.75-mm-thick CdTe pixel sensors on the four sides as shown in Fig.…”

“…A detailed description of the Compton camera data acquisition is given in Ref. 41. The BGO data acquisition operates independently of the Compton camera.…”

“…The energy resolution in orbit is slightly better than the performance observed on ground due to lower operating temperature in orbit, and no degradation was observed from the performance obtained in the component level tests. 23,24,41 …”

A compact imaging system with a CdTe double-sided strip detector for non-destructive analysis using negative muonic X-rays

“…The CdTe-DSD has dimensions of 3.2 × 3.2 cm 2 and a thickness of 750 µm, providing a detection efficiency of 90% at 60 keV. Signals from each strip are amplified and converted to digital pulse height in dedicated analog Application Specific Integrated Circuits (ASICs) [9]. The CdTe-DSD covers an energy range of 5-200 keV with an energy resolution of 1.6 keV (FWHM) at 60 keV.…”

ASTRO-H CdTe detectors proton irradiation at PIF

“…On the other hand, SGD [3] is an optic-free imaging spectrometer system, working in Compton mode. SGD is extremely sensitive up to 600 keV thanks to the combination of the narrow field-of view of its active shield made of BGO, and the use of Compton kinematics to reject background events.…”