Absorption Spectrum of γ-Irradiated Lead Silicate Glass
IN an earlier communication1 it was stated that the absorption spectrum of γ-irradiated lead silicate glass showed three absorption bands at 1.7 eV., 2.3 eV. and 3.3 eV. respectively. The separation of the spectral curve into these bands was obtained by assuming that the peak at 3.3 eV. was uninfluenced by the presence of any other bands. The points in the vicinity of this peak were used to estimate the first Gaussian and the other bands were obtained by successive subtraction.
Gamma Irradiation of Lead Silicate Glass
GLASSES exposed to gamma radiation darken due to the trapping of electrons in defects. Much work has been done by Seitz and others on colour centres produced in alkali halides by irradiation. Glass has also been studied by many workers, and it is well known that the intensity of the colour produced in glass fades in the dark and that this process is accelerated by increase in temperature or by exposing the glass to light. The present work was undertaken to investigate the mechanism involved in the darkening of glass, and the recovery in the dark with time, in an attempt to obtain better understanding of the structure of glass. A lead silicate glass of weight percentage composition PbO–80, SiO2–20 was chosen because of the simplicity of its chemical composition (approximately PbO.SiO2). Such glass might be expected to provide a simple absorption spectrum after irradiation. Rectangular blocks of glass 1 cm. × 1 cm. × 3 cm. cut from a single block of glass had their 1 cm. × 3 cm. faces ground and polished to be flat and parallel.
The thermal conductivity of diamond at low temperatures
The thermal conductivity of a type I diamond has been measured between 2° K and room temperature. Above 100° K the temperature variation of conductivity accords with Peierls’s theory of Umklapp processes. To account for the results at the lowest temperatures it is necessary to assume that some specular reflexion of phonons occurs at the crystal boundaries. Casimir’s theoretical treatment of boundary scattering has been extended to take into account the finite length of specimens and the smoothness of the crystal walls, and the experimental results show that at 3° K the proportion of specular reflexions is as great as 40%. Between 50 and 100° K the conductivity is considerably less than that calculated for an ideal diamond, and some conclusions are drawn as to the imperfections which cause this discrepancy.