Alkali halide crystals become colored when irradiated with x-rays. Theoretical considerations indicate that the coloration is due to absorption centers which consist of electrons trapped at negative ion vacancies. Precision measurements of the density of uncolored crystals, however, indicate that it is very unlikely that there is a sufficient number of vacancies present in well annealed crystals to account for the number of color centers, which may be determined optically, unless vacancies are formed as a result of the irradiation.Potassium chloride crystals were irradiated with x-rays. Their change in density was measured by the "crystal suspension method." It was found that the crystals changed in density, and that the number of vacancies calculated from the decrease in density was in reasonable agreement with the number of color centers determined by optical measurement.

An anisotropic electron spin resonance spectrum was observed in three natural Type Ib diamonds. The diamonds which exhibit the spectrum also show the spectrum from substitutional nitrogen donors previously observed by others. The spectrum consists of three anisotropic groups of lines. The spectrum is interpreted in terms of a model in which each defect center is characterized by a spin Hamiltonian of the form H=βS·g·H+A·I·S, where S = ½ and I = 1, and in which the tensors g and A each have one component g1 and A1 oriented in a 〈110〉 direction. The spin-one nucleus is believed to be nitrogen. The principal components of g and A have the values g1 = 2.0031±0.0003, g2 = 2.0019±0.0003, g3 = 2.0025±0.0003, A1 = 5.303±0.005×10−4 cm−1, A2 = 7.164±0.005×10−4 cm−1, and A3 = 5.293±0.005×10−4 cm−1. The components g2 and A2 make angles Ψ = 45.2°±0.3° and α = 22.4°±0.1°, respectively, with 〈110〉 directions. The theoretical angular dependencies of the spectra when the magnetic field H is rotated in {100}, {110}, and {111} crystalline planes were calculated on the basis of the above model and are in excellent agreement with the experimentally observed angular behaviors.

Electron spin resonance (ESR) was studied in five semiconducting diamonds in the temperature range 108°-370oK and at 4.2°K. The g factor is 2.0030±0.OOO3, and the linewidth varies from 0.3 to 8 Oe at room temperature. The number of spins contributing to the ESR absorption varies between 10 1 3-10 14 and agrees with the number of uncompensated acceptors in the case of two diamonds on which Hall and resistivity measurements were made. The number of spins varies as liT in the temperature range 108°-370oK. The spin-lattice relaxation time was estimated to be of the order of 10-8 sec at 300 o K. The results indicate that the spin center is associated with the acceptors in the p-type diamonds. The effect of heat treatment on the ESR absorption (g=2.0029) observed in crushed nonconducting diamond was measured. The intensity and linewidth (2.9 Oe) for the crushed diamond was reduced on heating at 6OO°Cj further heating at 1100°C did not change the ESR line after the initial decrease.

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Studies of the rectification between a metal point and £-type semiconducting diamond show that the formation of the potential barrier is essentially independent of the work function of the metal. The rectifying barrier apparently is formed by the establishment of equilibrium between charges in surface and interior states as proposed by Bardeen for the case of silicon. The semiconducting diamonds are photoconducting in the ultraviolet and visible regions with the maxima occurring at 224, 228, 640, and 890 m/x. Generally, diamonds have not been observed to be photoconducting in the visible region; however, it has been observed that in some cases an enhancement of conductivity induced by ultraviolet radiation results upon simultaneous irradiation with red light. There is agreement between the spectral response of photoconductivity and photovoltages developed at metal contacts with the exception that photovoltages developed near 440 m/i were not obtained in photoconductivity.