Radiation damage studies on non-metals utilizing measurements made during irradiation

Abstract To investigate radiation effects in non-metals, particularly radiation-induced color center formation, two facilities were constructed at Brookhaven National Laboratory for making optical absorption, luminescence, and other measurements during irradiation. In one facility the radiation is provided by a 60Co gamma-ray source. In the second facility samples are irradiated with 0.5–3.0 MeV electrons from an accelerator. Both facilities have large “walk-in” irradiation chambers. Optical measurements are made with a 13-m-long optical relay system that functions as a double-beam spectrophotometer, light-collecting system, etc. All equipment sensitive to radiation is located outside the irradiation chamber. A large variety of measurements can be made, e.g. simultaneous optical absorption and (radio)luminescence measurements can be made from approximately 210–1000 nm on samples before, during, and after irradiation. Usually separate 250 point absorption spectra and luminescence spectra are recorded simultaneously as often as every 2 min. However, more detailed spectra can be recorded conveniently. At a fixed wavelength measurements have been made every 1.4 ms; much faster measurements are possible. Samples can be measured during irradiation at temperatures between liquid helium temperatures and 900 °C. Measurements have been made on a large variety of crystals and glasses including alkali halides, quartz, fused silica, optical component glasses and minerals—especially natural rock salt. Numerous examples are given in the text. These examples, and other measurements, are the basis for a number of conclusions: the radiation-induced absorption in non-metals measured during irradiation differs from that measured after the irradiation is terminated, with only one or two possible exceptions. In most samples the absorption decays after irradiation. In certain samples some absorption bands decay and others increase. A few bands increase immediately after irradiation and then decrease. In a few cases, e.g. synthetic NaCl at room temperature, the resumption of radiation after an interruption initiates a complex sequence of F-center absorption band changes. Curves of absorption band intensity vs irradiation time measured during irradiation at a constant dose rate can usually be described precisely by simple functions, e.g. one, two, or three saturating exponential components plus a linear component. Similarly, some, but not all, changes occurring after irradiation can be described precisely by decaying exponential components and increasing saturating exponential components. Many samples emit copious radioluminescence during irradiation. Often, e.g. in quartz, the luminescence is strongly temperature dependent. Also, the radiation-induced absorption is usually strongly temperature dependent; most often the dependence on temperature is complex. In synthetic NaCl crystals and natural rock salt, irradiations in the temperature range 100–250°C produce F, V and other centers at low doses but, in addition, Na metal colloid bands at higher doses. Below 150°C the radiation-induced color center and colloid particle bands in natural and synthetic rock salt change little after irradiation. However, at higher temperatures the decay occurring after irradiation increases with increasing irradiation temperature. Based on measurements made during irradiation at the high dose rate of 120 Mrad h−1, the colloid growth rate is low at 120°C, increases to a maximum at 150–170°C, and decreases with increasing temperature to a low rate at 250–300°C. If the colloid growth temperature dependence were determined from measurements made a few hours after irradiation an entirely different, and misleading, temperature dependence would appear to occur. This example, as well as numerous other examples of the type sketched above, illustrates a general conclusion resulting from these studies. Namely, in almost all non-metals, to determine the levels of radiation damage present during irradiation and, more importantly, to determine the kinetics of radiation-induced processes, it is necessary to make measurements during irradiation.