Introduction to Quartz Frequency Standards - Radiation Effects

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Ionizing radiation changes a crystal oscillator's frequency primarily because of changes the radiation produces in the crystal unit [33,34]. Under certain conditions, the radiation will also produce an increase in the crystal unit's equivalent series resistance. The resistance increase can be large enough to stop the oscillation when the oscillator is not radiation hardened.

Figure 31 shows a crystal oscillator's idealized frequency response to a pulse of ionizing radiation. The response consists of two parts. Initially, there is a transient frequency change that is due primarily to the thermal-transient effect caused by the sudden deposition of energy into the crystal unit. This effect is a manifestation of the dynamic f vs. T effect discussed earlier. The transient effect is absent in SC-cut resonators made of high purity quartz.

Figure 31. Crystal oscillator's response to a pulse of ionizing radiation: f₀ = original preirradiation frequency, Df_ss = steady-state frequency offset (0.2 hours to 24 hours after exposure), f_t = instantaneous frequency at time t.

In the second part of the response, after steady state is reached, there is a permanent frequency offset that is a function of the radiation dose and the nature of the crystal unit. The frequency change versus dose is nonlinear, the change per rad being much larger at low doses than at large doses. At doses above 1 kilorad (SiO₂), the rate of frequency change with dose is quartz-impurity-defect dependent. For example, at a 1 megarad dose, the frequency change can be as large as 10 ppm when the crystal unit is made from natural quartz; it is typically 1 to a few ppm when the crystal is made from cultured quartz, and it can be as small as 0.02 ppm when the crystal is made from swept cultured quartz.

The impurity defect of major concern in quartz is the substitutional Al³⁺ defect with its associated interstitial charge compensator, which can be an H⁺, Li⁺, or Na⁺ ion, or a hole. This defect substitutes for a Si⁴⁺ in the quartz lattice. Radiation can result in a change in the position of weakly bound compensators, which changes the elastic constants of quartz and thereby leads to a frequency change. The movement of ions also results in a decrease in the crystal's Q, i.e., in an increase in the crystal's equivalent series resistance, especially upon exposure to a pulse of ionizing radiation. If the oscillator's gain margin is insufficient, the increased resistance can stop the oscillation for periods lasting many seconds. A high level pulse of ionizing radiation will produce photocurrents in the circuit which result in a momentary cessation of oscillation, independent of the type of quartz used in the resonator. In oscillators using properly designed oscillator circuitry and resonators made of swept quartz, the oscillator recovers within 15 ps after exposure [35,36].

Sweeping is a high-temperature, electric-field-driven, solid-state purification process in which the weakly bound alkali compensators are diffused out of the lattice and replaced by more tightly bound H+ ions and holes [37,38]. In the typical sweeping process, conductive electrodes are applied to the Z surfaces of a quartz bar, the bar is heated to about 500°C, and a voltage is applied so as to produce an electric field of about 1 kilovolt per centimeter along the Z direction. After the current through the bar decays (due to the diffusion of impurities) to some constant value, the bar is cooled slowly, the voltage is removed' and then the electrodes are removed. Crystal units made from swept quartz exhibit neither the radiation-induced a degradation nor the large radiation-induced frequency shifts. Swept quartz (or low aluminum content quartz) should be used in oscillators which are expected to be exposed to ionizing radiation.

At low doses (e.g., at a few rads) the frequency change per rad can be as high as 10^-9 per rad [39]. The low-dose effect is not well understood. It is not impurity dependent, and it saturates at about 300 rads. At very high doses (i.e., at >> 1 Mrad), the impurity-dependent frequency shifts also saturate because, since the number of defects in the crystal are finite, the effects of the radiation interacting with the defects are also finite.

When a fast neutron hurtles into a crystal lattice and collides with an atom, it is scattered like a billiard ball. A single such neutron can produce numerous vacancies, interstitials, and broken interatomic bonds. The effect of this ''displacement damage'' on oscillator frequency is dependent primarily upon the neutron fluence. The frequency of oscillation increases nearly linearly with neutron fluence at rates of: 8 x 10^-21 neutrons per square centimeter (n/cm²) at a fluence range of 10¹¹ to 10¹² n/cm², 5 x 10^-21/n/cm² at 10¹² to 10¹³ n/cm², and 0.7 x 10^-21/n/cm² at 10¹⁷ to 10¹⁸ n/cm².

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