Acoustic loss and frequency stability studies of gamma and proton irradiated alpha quartz crystal resonators

Piezoelectric alpha quartz crystal resonators are widely used as precision frequency sources in satellite communication and navigation systems. Their apparent susceptibility to space radiation appears as transient frequency shifts. For satellites in low-earth orbits with altitudes ranging from 500-1500 km, the quartz resonators accumulate radiation at rates less than 0.1 rad/min and doses not in excess of 10 rad/orbit. The objectives of this work are to examine the radiation induced effects in alpha quartz crystal resonators and to distinguish the various acoustic losses responsible for the frequency susceptibility over these dose ranges. Simulation of low-earth orbit proton radiation was accomplished with protons from the Harvard University Cyclotron using a novel proton beam modulator, which was designed to emulate a 10-120 MeV proton spectrum for the radiation susceptibility and acoustic loss studies on AT quartz resonators. Quartz resonators having aluminum defect center concentrations between 0.01 and 19 ppm experienced proton induced frequency shifts not correlated to their aluminum impurity content. It was also found that AT quartz resonators of the electrodeless BVA design experienced the smallest frequency shifts. Experiments conducted with 1.25 MeV gamma rays from a cobalt 60 source demonstrated identical frequency shifts in quartz, indicating that the energy losses of gamma rays and protons in quartz over the examined dose and energy ranges were similar. Acoustic loss measurements conducted over the 0.3-70 K range revealed that the phonon-phonon and two-level energy excitation peaks near 20 and 5 K, respectively, were not affected by proton or cobalt 60 radiation. However, the loss peak due to sodium relaxation near 50 K decreased for accumulated doses up to 1000 rad. The two-level energy losses in quartz showed a similarity to losses previously observed in vitreous silica. The relation between acoustic losses and radiation susceptibility is discussed in terms of the interaction mechanisms of protons and gamma rays with the defect centers, phonon gas, and quartz resonator electrode-interface.