Sweeping quartz crystals

Sweeping is a purification process that removes certain impurities and thereby improves the radiation sensitivity of quartz crystals. The major steps of a typical sweeping process consist of applying electrodes to the Z-surfaces of a lumbered quartz bar; applying voltage to the electrodes to produce an electric field of approximately 1 kV/cm; heating the bar slowly either in air, vacuum, or hydrogen-containing or inert atmospheres to about 500°C; monitoring the current through the bar; and, when the current decays to some near-constant value, cooling the bar slowly to room temperature and removing the voltage. Alkali-metal impurities, such as Li⁺ and Na⁺, are charge compensators for trivalent aluminium impurities in quartz. When sweeping is performed in air or H₂ the alkalis diffuse to the cathode and are replaced by the indiffusion of hydrogen at the anode. The alkali-metal impurities are subsequently removed by lapping the Z-surfaces. The sweeping process also modifies dislocations, reducing the tendency for formation of etch channels. Recent advances have involved the use of impurity characterizations such as electron spin resonance, acoustic loss measurements, and infrared absorption spectroscopy