Expected radiation damage of reverse-type APDs for the Astro-H mission

Abstract

Scheduled for launch in 2014, Astro-H is the sixth Japanese X-ray astronomy satellite mission. More than 60 silicon avalanche photodiodes (Si-APDs; hereafter APDs) will be used to read out BGO scintillators, which are implemented to generate a veto signal to reduce background contamination for the hard X-ray imager (HXI) and a soft gamma-ray detector (SGD). To date, however, APDs have rarely been used in space experiments. Moreover, strict environmental tests are necessary to guarantee APD performance for missions expected to extend beyond five years. The radiation hardness of APDs, as for most semiconductors, is particularly crucial, since radiation in the space environment is severe. In this paper, we present the results of radiation tests conducted on reverse-type APDs (provided by Hamamatsu Photonics) irradiated by gamma rays ( 60 Co) and 150 MeV protons. We show that, even under the same 100 Gy dose, high energy protons can cause displacement (bulk) damage in the depletion region and possibly change the activation energy, whereas gamma-ray irradiation is less prone to cause damage, because ionization damage dominates only the surface region. We also present quantitative guidance on how to estimate APD noise deterioration over a range of temperatures and radiation doses. As a practical example, we discuss the expected degradation of the BGO energy threshold for the generation of veto signals, following several years of Astro-H operation in Low Earth Orbit (LEO), and directly compare it to experimental results obtained using a small BGO crystal.

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

“…[52][53][54][55] The BGO crystal is dense (7.1 g∕cm 3 ), has a high stopping power (i.e., short radiation length of 1.1 cm), high optical transparency, and ability to form a large crystal although its light output is lower than NaI or CsI.…”

Development of high performance Avalanche Photodiodes and dedicated analog systems for HXI/SGD detectors onboard the Astro-H mission

“…Strictly speaking, however, this is too optimistic because the APD suffers bulk damage mainly caused by high energy protons as well as surface damage demonstrated by 60 Co irradiation. In fact, Kataoka et al [16] reported that significant increase in bulk current, which is the multiplied component of the APD dark current. Assuming that about 5 percent of the total dose will be attribute to high energy protons, they concluded that the energy threshold of the BGO readout for Astro-H may increase by a factor of 1.3-1.6 after the expected five-year life of the Astro-H mission, whereby its impact on background rejection power must be carefully considered before the launch.…”

“…The dark current is composed of surface current and bulk current (with only the latter being multiplied in proportion to the avalanche gain) [16]. At the in-orbit temperature the bulk current should be dominant, while at room temperature it is not clear which component is responsible for the net dark current.…”

“…Since the APD has relatively large capacitance as a photodetector, the capacitive noise is dominant at the in-orbit temperature [16]. There, we developed a dedicated CSA specialized in noise performance for a detector with large capacitance.…”

Development of the hard x-ray monitor onboard WF-MAXI

“…1 (left). We employ existing technologies developed for ASTRO-H [5] and micro-satellite TSUBAME [6] to quickly realize the WF-MAXI mission.…”