The effects of high energy radiation on the luminescence properties of solids are surveyed. Of the four effects considered—radiophotoluminescence, radiophotostimulation, radiothermoluminescence, and the ``killing'' of luminescence by x-rays or gamma-rays—the first effect is shown to have advantages in principle over the others as a basis for dosimetry.
The absorption, excitation, emission, sensitivity, energy dependence, and stability characteristics of a radiophotoluminescent Ag-activated phosphate glass are described. It is shown that dosage measurements can be made with this material, employing a simple fluorophotometer, from 10 roentgens to a few thousand roentgens of gamma-rays. By proper shielding, such a dosimeter element can be made reasonably independent of energy. The dosage indication obtained is quite stable with time and is not seriously affected by exposure of the glass to visible or ultraviolet light or to temperatures in the range of −70° to +100°C. Because of its sensitivity range and its stability over a wide range of ambient conditions, the dosimeter appears to be particularly suited for monitoring personnel that may be exposed to rather high doses of gamma-rays, such as may be encountered in A-bomb explosions.
The color centers and free radicals produced in irradiated solid alcohols, ketones, ethers, and other compounds have been examined experimentally by means of optical absorption and EPR spectroscopy to obtain information regarding the products of ionization and the associated electronic processes. At liquid nitrogen temperature these products of electron or x-ray bombardment are stable; however, the color centers are photosensitive and can be bleached both optically and thermally. The alcohols also show an increase in absorption near the characteristic uv bands. Removing the visible color centers increases the EPR hfs, but bleaching in the uv band markedly reduces or changes the hfs. In methanol and ethanol, free radicals are formed with good efficiency requiring about 18 and 12 ev per radical, respectively. Saturation concentrations of 2×1019 and 9×1019 spins per cc were produced in acetone and methanol, respectively. The hfs of ethanol and methanol indicate an alkyl hydrogen is missing from the radicals produced by irradiating these materials. No sign of the atomic hydrogen doublet was observed at liquid nitrogen temperature. Support for the α—β hypothesis was found in the hfs displayed by the irradiated alcohols, paraffins, ketones, and ethers.
The conductivity induced in polyethylene and Teflon by bombardment with x-rays from a 2-Mev Van de Graaff and gamma rays from Co60 has been investigated as a function of time, temperature, geometry, exposure rate, and applied electric field. Within the range of the variables studied, the observed photocurrents were directly proportional to the exposure rate and the applied electric field. The photocurrent could be divided into three components, the current due to the action of monodirectional photons, the increase in this current due to the influence of the electric field, and a component of less than 10% made up of all other possible charge carriers. During irradiation, the conductivity increased by a factor of about 103. Between 78°K and 273°K the photocurrent was nearly independent of temperature. In general, the conductivity of Teflon was greater than that of polyethylene.
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