ESR modulation-spectrum study of x-ray irradiated samples of fused quartz: Detection of three types of E' centers

It was demonstrated for the first time that the modulation-spectrum (MS) signal from x-ray irradiated fused quartz could be separated into three groups of lines by adjusting the first-modulation frequency and its phase angle. The average linewidths of the three groups were found to be approximately 1.5, 10.2, and 26.0 kHz, respectively. From the measurements of their dependence on microwave power and radiation dose, it was concluded that these three groups arose from three different types of E' centers, called types 1, 2, and 3. Moreover, the MS method was shown to allow one to observe the ESR absorption arising from each type of these centers under the application of the second modulation with a frequency selected for each absorption. Estimating the dipolar broadening in these ESR curves and assuming a cubic distribution of radicals, the average separation of radicals was found to be approximately 52 angstrom, 46 angstrom, and 42 angstrom for types 1, 2, and 3, respectively. The relative concentrations of the three types were found to be approximately 1:2:1. Based on these measurements, it was proposed that the radicals known to be E' centers exist in three types of species differing in radical separation