Scintillating fibers readout by single photon avalanche diodes (SPAD) for space applications

We present the design and performances of a radiation detector based on plastic scintillating fibers with doubleside readout by means of large-area Single Photon Avalanche Diodes (SPAD). This can be the basic step toward the realization of a large-area, cost-effective position sensitive detector to be employed in future space gammaray observatories. 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 selfsustainable 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 ?m in diameter) have been designed and fabricated at the CNR-IMM facility, with an intrinsic noise lower than 10kHz at -15oC, and are optically coupled to both ends of 3-meter long scintillating fibers, with the same diameter. Double-side readout is required to operate the devices in coincidence (10ns coincidence window), in order to reduce the rate of false detections to the level of 1Hz. The detectors have been tested with minimum ionizing particles at CERN PS demonstrating a detection efficiency larger than 90% and a moderate position resolution along the fiber due to the difference in time of arrival between the two photodetectors. Radiation hardness tests on SPADs have also been carried out, showing that large-area SPADs can be safely employed in low-inclination low Earth orbits.