Radiation Damage to Solar Cells on Relay I and Relay II

The short circuit currents from 1 ohm-cm silicon N/P, silicon P/N, and gallium arsenide solar cells, bearing shields up to 60 mils thick, and carried aboard the Relay I and Relay II satellites were monitored. The currents from all cells decreased because of damage by the trapped particles in the Van Allen radiation belts, The currents from unshielded cells fell to about half initial values in one day. The heavily shielded silicon cells finally degraded at the rate of about 17% per decade of time. Heavily shielded N/P silicon cells lasted about 10 times as long as similar P/N cells. Tentatively, gallium arsenide cells with 3 and 12 mil glass shields appear inferior to silicon N/P cells with 60 mil fused silica shields, while gallium arsenide cells with 30 and 60 mil silica shields appear superior, judging by comparing times to fall to given percentages of initial short circuit current values, and ignoring absolute efficiencies. The results from Relay I and Relay II, where comparable are in good agreement. The severe stepwise damage experienced by the unshielded cells is in good agreement with values calculated from the expected fluxes of protons of 0.5 Mev and greater. The calculated proton damage to the heavily shielded silicon cells is somewhat greater than that observed in orbit. Electron damage, after being discounted for belt decay, is considered to be a contributing but minor factor.