Philip H. Yuster

Some Optical Properties of Potassium Iodide‐Thallium Phosphors

The absorption spectra of potassium iodide‐thallium phosphors have been studied at both liquid nitrogen and liquid helium temperatures. Eight absorption bands have been observed which are due to the presence of thallium. The maximum absorption coefficients of four of these bands appear to vary linearly, while the other four appear to vary with the square of the thallium concentration. The origin of these bands is discussed in terms of an extension of the model proposed by Seitz.

Paramagnetic Resonance and Optical Absorption of Trapped Holes and Electrons in Irradiated KCl: Ag

The electron spin resonance and optical absorption spectra of crystals of KCl containing silver impurity ions were examined after the crystals had been X -irradiated at 77 °K. It is shown that the silver impurity ions act as traps for both electrons and positive holes. A description is given of the annealing behaviour of the crystals after low-temperature irradiation.

A New Type of Short Wavelength Absorption Band in Alkali Halides Containing Color Centers

Photo‐chemically and additively colored potassium iodide crystals exhibit absorption bands in the ultraviolet which do not have the characteristic properties of V‐bands. An interpretation of the origin of these bands is proposed.

V‐Centers in Alkali Halides

V‐centers are defined as stabilized positive holes. The peak wave‐lengths of three V‐bands of KCl, KBr, and NaCl are listed. Most of the paper deals with KCl, but the results are typical for KBr and NaCl as well. Experimental data are given concerning the conditions under which the various V‐bands are obtained and their unequal stability with regard to bleaching by light and temperature. Crystals irradiated at various temperatures and observed at room temperature show superpositions of the V2‐ and V3‐bands with relative intensities depending greatly on the temperature at which the crystals were irradiated and on the nature of the primary radiation. The V1‐band can be observed only at temperatures below −100°C. These data are compared with those published by other investigators, and some remarks are made concerning the possible mechanisms which are responsible for the various phenomena.

Stability of Color Centers in Alkali Halides

Color centers have been formed in alkali halide crystals by exposing them to x‐rays, electron bombardment, and pile irradiation, and their stability with respect to temperature and light bleaching has been investigated. In general such crystals contain centers of non‐uniform stability. The lower the temperature at which the color centers have been produced, the more unstable they are on the average. In the absorption spectra taken at low temperature of crystals irradiated at low temperature new bands appear on the short wave‐length side of the F‐band which are never observed in crystals irradiated at room temperature and which, on warming the crystal to room temperature, vanish together with a part of the F‐centers. It is suggested that the instability of some of the color centers formed at low temperature is connected with the presence of these new absorption bands.

Photon‐Induced Diffusion in Thin Films and Single Crystals of Potassium Iodide

Thin films of potassium iodide have been prepared at both room and liquid nitrogen temperatures. Absorption spectra are presented which show that films deposited at liquid nitrogen temperature are quite disordered, but after warming to room temperature are very similar to those films deposited at room temperature. Evidence is given which shows that in thin films and also in single crystals of potassium iodide diffusion is promoted by the absorption of light. It is proposed that diffusion is caused by point thermal spikes at the absorbing centers.

Crystal Technique for Measuring Cyclotron Beam Energies

A lithium fluoride crystal when irradiated with deuterons from a cyclotron develops a yellow color; if the deuterons are stopped within the crystal, there is a sharp demarcation in color between the irradiated and unirradiated portions. It has been found that the measurement of the depth of penetration of the color into lithium fluoride can be used to determine accurately (±0.1 Mev at 20 Mev, ±0.2 Mev at 10 Mev) and rapidly the energy of deuterons in a cyclotron beam. A single exposure of a crystal to the deuteron beam allows one to determine not only the energy of the deuterons but also the angular orientation of the beam trajectories.