High resolution measurements of space radiation effects on silicon surface-barrier detectors

Two high-resolution charged particle spectrometers employing silicon surface-barrier detectors have been operated in an 800 km altitude polar orbit for a 26-month period between October 1971 and December 1973-Based on existing trapped radiation models and the measured solar flare environment during this period the detectors were exposed to a proton fluence of 2.8 &time; 10¹³ protons/cm² at energies > 400 keV and an electron fluence of 4.5 &time; 10¹³ electrons/cm² at energies > 50 keV. High-resolution pulse height spectra of the alpha particles from low-intensity Americium-241 in-flight calibration sources show that the resolutions of the detectors did not degrade more than 5 percent and that the gains of the system did not vary more than 1.2 percent as a result of this exposure. Two aspects of detector operation are considered responsible for this performance. First, the incident detectors were operated with the rear, aluminum contact exposed to the radiation environment instead of the front, gold contact. The more intense lower energy radiation is therefore stopped in the rear ohmic contact region instead of in the front surface-barrier junction region and the overall damage effects are significantly reduced. Second, a detector bias technique was employed that automatically maintained a constant depletion potential across the detector independent of variations in leakage current due to radiation damage.