No abstract

A comprehensive study of x-ray stimulated luminescence has been carried out on four types of high-purity, amorphous silica (a-SiO2). Both high OH and low OH as well as oxygen-excess and oxygen-deficient materials were studied. The room-temperature, visible x-radio luminescence (XRL) was measured continuously as a function of x-ray dose from zero to 400 Mrad volume average dose. In addition to the XRL measurements, electron paramagnetic resonance (EPR) was used to determine the concentrations of the two key radiation-induced defects, the E′ center and the nonbridging oxygen hole center (NBOHC). The XRL spectra were deconvolved into four Gaussian components with centers at 1.9, 2.2, 2.6, and 2.75 eV. The same centers and widths could be used to describe the spectra in all four types of a-SiO2, only the intensities varied. The 2.6 and 2.75 eV lines are strongly dose dependent, rising from near zero intensity at zero dose in all four materials. These two lines are strongly correlated with each other; they have essentially the same dependence on dose and sample type. This correlation suggests that these two lines are due to the same radiation-induced defect, or to closely related defects. The dose dependence and sample-to-sample variation of these two lines bear some similarities to the E′ concentrations. In contrast to the 2.6 and 2.75 eV lines, the 1.9 eV line has a high intensity at the lowest doses measurable. A simple phenomenological model is proposed to describe the 1.9 eV XRL line. This model involves two populations of defects; one population is present at zero dose and is assumed to be dose independent, while the second population is dose dependent. Evidence is presented that the dose-dependent defect is the NBOHC. The XRL due to the dose-independent population may be associated with a transient response to the x rays, or to a metastable defect; this population may not be observable in post-irradiation experiments such as EPR and conventional photoluminescence. Similar to the 1.9 eV line, the 2.2 eV line also has relatively high intensity at the lowest measurable x-ray dose. The behavior of this line is in general agreement with the self-trapped exciton model.

An electron spin resonance (ESR) signal comprised of three resolved lines of equal 19.3 Gauss separation (3×19.3 G), but unequal amplitude, is observed in x-irradiated amorphous silicon dioxide. The radical appears exclusively in silica samples which also exhibit the methyl radical, a familiar indicator of trace carbon and hydrogen contamination. The 3×19.3 G signal is observed to grow most rapidly versus irradiation dose when methyl radical concentration is near maximum. This evidence suggests that the ESR signal is due to a radiolytic, organic radical which evolves after the methyl radical and, like the methyl radical, is trapped and stabilized in the amorphous silica network. Experimental methods of radical generation are presented, followed by discussion of models for the chemical structure of the 3×19.3 G radical.

A three-line electron-spin-resonance ͑ESR͒ spectrum has been observed following x irradiation in three of four high purity amorphous silica samples independently manufactured via the flame hydrolysis method ͑type-III silica͒. This spectrum, with a line separation of approximately 18 G, was previously attributed to an unpaired spin undergoing a hyperfine interaction with nitrogen. Optimization of ESR parameters and irradiation procedures have been employed in the present work to obtain spectra of unprecedented signal quality. A direct correlation of the three-line ESR signal with the presence or absence of two carbon radicals, HCO˙and CH 3˙, in the four different silica samples is observed. Studies of defect concentration vs x-ray dose at room temperature show that the three-line defect forms concurrently with the decay of HCO˙, and prior to the appearance of CH 3˙. Absolute spin counts are consistent with the evolution of all three defects from a single trace impurity. It is proposed that the three-line spectrum results from a hyperfine splitting due to the nuclear spins of two equivalent 1 H ͑Iϭ1/2͒, not 14 N ͑Iϭ1͒. Simulation of the experimental line shape gives excellent agreement with the two hydrogen model. The magnitude of the hyperfine splitting of this extrinsic silica defect is nearly the same as that due to the two equivalent hydrogens of the˙CH 2 OH radical in methanol. Thus the three-line defect is identified as a ϪCH 2˙r adical, not a nitrogen defect as had been previously supposed.