Proton Radiation Tests of Single Photon Avalanche Diode for Space Applications

We study the possibility to use Single Photon Avalanche Diodes (SPAD) as photodetectors for applications in space missions. SPADs are silicon devices operated above the junction breakdown voltage (with the typical overvoltage of 5V), for which a single photon interacting in the active region is sufficient to trigger a self-sustainable avalanche discharge. SPADs can thus be used for the detection of very low light levels with a fast time response around 50ps FWHM for single photon detection, without spectroscopic capabilities. Large area SPAD (500 µm in diameter) have been designed and fabricated at the CNR-IMM facility, with an intrinsic noise lower than 10kHz at -15°C, before irradiation. We performed bulk damage radiation tests with protons in order to evaluate their radiation hardness properties and their suitability for application in a Low Earth Orbit (LEO) space mission. With this aim SPAD devices have been irradiated up to 3×109 (28.5 MeV) p/cm2. The test performed show that large area SPAD are very sensitive to proton doses as low as 1×108 p/cm2 with a significant increase in dark counts rate as well as in the manifestation of the "random telegraph signal" effect. Lower protons doses in the range 1-5×107 p/cm2 result in a lower increase of DCR suggesting that applicability in low-inclination LEO, particularly useful for gamma-ray astrophysics, is still feasible.