Graduate Thesis Or Dissertation
 

The effect of impurities on the gamma-ray coloration of potassium chloride crystals

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  • The incorporation of europium and dysprosium in potassium chloride crystals was investigated. The crystals were prepared by the Kyropoulos method from melts containing the rare-earth chlorides. The concentration of these rare-earth ions in the crystals was determined by neutron-activation analysis. The distribution coefficient of europium and dysprosium between the molten and solid potassium chloride was found to be . 21 and 1.3 x 10⁻⁴, respectively. The molar absorptivity of europium in the KC1 crystals at two wavelengths, 241 and 325 mμ, was found to be 2880 and 2750 liter mole⁻¹ cm⁻¹, respectively. The crystals doped with dysprosium did not exhibit any absorption bands. The effect of europium, dysprosium, praseodymium, calcium, and lead on the gamma-ray coloration of KC1 crystals was studied. It was found that these five cations had the same qualitative effect on the coloration curve of pure KC1, although the magnitude of the effect depended on the specific impurity. At low impurity concentrations the total first-stage coloration was not significantly changed, however, the rate of second-stage coloration was suppressed. As the impurity concentration was increased further, the coloration during the first stage increased and the second-stage coloration rate began to increase after reaching a minimum. The minimum in the rate of the second-stage coloration occurred at impurity concentrations below 1 x 10¹⁶ ions per cc. After the minimum in the second-stage rate of coloration had been attained, the ratio of the total first-stage coloration to the second-stage rate of coloration was a constant which was independent of the impurity concentration. Crystals of potassium chloride were grown from aqueous solutions employing both convection and evaporation methods. The size and the perfection of the crystals were the same for both solution-grown methods. The dislocation density of the solution-grown crystals was a factor of three less than crystals grown from the melt. The solution-grown crystals exhibited lower first-stage coloration than the melt-grown crystals, however, in the second stage the rate of coloration was larger for the solution-grown crystals. The second stage, which was linear for the melt-grown crystals, was generally not linear for the solution-grown crystals. The gamma-ray coloration of several nominally pure KCl crystals obtained from several different sources was investigated. From this study it was suggested that the colorability can be employed as a criterion of purity; the pure crystals will have a small first-stage coloration and a large second-stage rate of coloration. The ratio of the total first-stage coloration to the rate of second-stage coloration will become smaller as the purity increases. However, there are differences between the purity order established by the above criterion and conductivity data.
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