Radiation induced frequency offsets and acoustic loss in AT-cut quartz crystals

Abstract

Ionizing radiation modifies point defects already present in quartz. For example, the A1-Li center present in as-grown high quality cultured quartz is converted into a mixture of Al-OH and Al-hole centers. While neither the A1-Li nor the A1-OH center have acoustic l o s s peaks, peaks at 23K, lOOK and 135K are unswept quartz, the ratio of stable Al-hole centers to associated with the presence of the Al-center. In A1-OH centers is determined by the availability of hydrogen trapped in the growth defect sites. Since these peaks occur at temperatures below the temperature of a crystal oscillator their production causes a negative frequency offset. The magnitude of the radiation-induced offset due to the production of the Al-hole center should match the sum of the heights of these three loss peaks. We have measured the acoustic loss and frequency versus temperature spectra as a function of radiation dose of a number of matching Naswept, unswept. and H-swept AT-cut quartz crystals. As expected, large positive frequency offsets at both the upper and lower turnovers were observed for the Naswept crystals. However, the magnitude of the offset was consistently much less than the change in height of the strong 53K (5MHz) A1-Na loss peak. This smaller offset was caused by a change in "apparent" angle of the crystal that comes about when the A1-Na center is replaced the A1-OH or Al-hole center. King first noted that AT-cut crystals with large A1-Na concentrations behave as if they were cut at a smaller angle than natural or swept crystals. Both the unswept crystals and the H-swept crystals usually showed negative frequency offsets at the turnover temperatures. The offsets of the swept crystals were usually smaller than those of the unswept samples. Only part of the could be explained by summing the heights of the l o s s observed fractional frequency offset at the turnovers peak changes. An additional radiation induced offset was observed at temperatures below 15 K where the the offset does not appear to be related to any acoustic frequency versus temperature curves flatten out. This l o s s peaks; instead, it is caused by shifts in the elastic moduli that take place when a defect is modified. Once this low temperature offset is subtracted from the offset measured at the turnover the by the changes in the acoustic loss peaks. remaining frequency shift matches the value predicted JNTRODUCTION quartz oscillators must be considered in a number of The effects of ionizing radiation on precision aerospace applications. Both transient and steadyof the crystal have been observed following the state frequency offsets and series resistance changes exposure to radiation [ l -1 0 1 . King and Koehler [l]have reviewed the radiation response of quartz with respect to frequency control applications. Norton, Cloeren, and Suter; Suter and Maurer; and Suter have recently reported low-dose frequency offset studies [11,12,13]. Several mechanisms for low-dose effects have been discussed by Flanagan, Leadon, and Shannon [14]. Palkuti and Truong ( ...

Mechanical properties of MEMS materials: reliability investigations by mechanical- and HRXRD-characterization related to environmental testing

“…Aluminum impurities are recognized to be the main reason for radiation-sensitivity of α-Quartz [9], [59], [60]. The aluminum can take the place of silicon in the crystal lattice.…”

“…For instance, in ref. [59] several quartz samples with 0.2 to 70 ppm Al were swept in presence of sodium, lithium and hydrogen and the effect on the radiation sensitivity to 60 Co gamma-rays was investigated. The measurements were made on AT-cut resonators at the upper turnover point (at about 26°C) where the slope of the frequency versus temperature is zero.…”

Sweeping quartz crystals

“…Rutherford backscattering (RBS) [7]; secondary ion mass spectroscopy(S1MS) [8]; acousticloss measurements [9]; and infrared (IR) absorption spectroscopy [lo-141.…”