UHF Losses in Sapphire and Ruby at Low Temperatures

Losses in Al2 O3 rods, placed along the axis of a superconducting helical resonator, were observed at frequencies from 130 MHz to 450 MHz and at sample temperatures up to 20 K. Losses at 2 K corresponding to ¿"/¿\´ of 0.8 x 10-6 were found in the two different grades of sapphire tested. Losses associated with Cr+3 impurities were larger, corresponding to X"/X\´ of 6 x 10-6 for pink ruby and 2 x 10-5 for red ruby at 130 MHz and 3 K. The ruby losses decreased with increasing temperature and increasing frequency. Magnetic coupling within the Cr3+ spin system seems to account for most of the ruby losses.