Neutron hardness of photodiodes for use in passive rubidium frequency standards

Experiments are reported to evaluate the neutron hardness of several different types of planar silicon photodiodes. The diodes tested had approximately 1 cm² of active area, and were maintained at 100°C during neutron irradiation and testing. They were exposed to neutron fluences ranging from 7×10¹¹ to 1.4×10 ¹³ n/cm². For each of the diodes, the DC short-circuit current, the AC short-circuit current, and the small-signal AC shunt resistance were measured as a function of neutron fluence. Of the various types of diodes tested, the n/p types exhibited better performance that the p /n types normally used in rubidium frequency standards (RFSs). Detailed experimental results are presented and compared with previously published data, where possible. The tradeoffs inherent in using the different types of diodes for RFS operation are also discussed