It is well known that coloured crystals can be stored at room temperature, in dark, without observing any appreciable loss of colouration for a period of several years /l/. On the other hand, colouration of microcrystalline powders of alkali halides is found to decay very fast, when the coloured powders are stored in dark a t mom temperature. The decay of colour centres is through some non-radiative procesa. This non-radiative decay process is drastically modified in the powderls of the mixed and doped alkali halide /2 to 4/. It is possible to store the mixed and doped alkali halide powders for several days. Now attempts are being made to study the role of alkaline earth i m p r i t i e s (Ca2+, Sr2+, Ba2+) in stabilizing the colouration of the microcrystalline powders. The stability of colour centres in alkaline earth doped alkali halide powders having a maximum number of 3 centres is found to be less compared to as-irradiated powder. The systematic results of these investigations are reported in this paper with the view'of obtaining an additional information about the interaction between colour centres and dislocations.The single crystals of pure and alkaline earth doped (500 ppm) KCl were grown from melt by using the Kyropoulos technique. Crystals 80 obtained were crushed to micrystalline powders (80 to 100 mesh) and then exposed to p r a y s from a 6oCo source for a dose of 1.15X10 rad. The isothermal decay of colouration in microcrystalline powders was studied at 300 K by measurillg the diffuse reflectance a8 a function of time on a Carl Zeise Spectmphotometer VSU-2P, with a 45/0 reflectance attachment. Thermoluminescence glow curves were detected by RCA 931A photomultiplier and recorded on a millivolt recorder.The details of the T L assembly are described previously /5/. A linear heating rate of 50 K/min was employed using a Powertech temperature controller programmer. 5 1) Nagpur 440010, India.
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