Formation of Color Centers in KCl:Tl by Gamma Irradiation at 21 °C

The formation of color centers in KCl, both "pure" and with thallium impurity, by ${\mathrm{Co}}^{60}\ensuremath{\gamma}$ irradiation at 21 \ifmmode^\circ\else\textdegree\fi{}C has been carefully studied. The samples were grown from zone-refined material which had been chemically purified. The pure crystals did not contain detectable amounts of thallium, while the doped crystals were adjusted to contain from 4 to 145 ppm of Tl impurity. Presumably, ${\mathrm{Tl}}^{+}$ enters the KCl lattice substitutionally and, except possibly at the highest Tl concentrations, is incorporated as an isolated ion. As the irradiation dose increases, the ${\mathrm{Tl}}^{+}$ $A$, $B$, and $C$ bands decrease; however, the $F$ band, a broad "$V$-region" absorption, and, except at lowest doses, the $M$ band all concomitantly increase. The $V$-region absorption is a function of the Tl content. Below \ensuremath{\sim} ${10}^{6}$ rad the $F$-center formation rate increases and then decreases as the Tl content increases. Above ${10}^{6}$ rad the level of the $F$-center absorption decreases as the ${\mathrm{Tl}}^{+}$ content increases. Also, the $M$-center concentration is proportional to the square of the corrected $F$-center concentration, and the $M$- to-$F$-center ratio increases as the Tl content increases. These and other observations indicate that, with few exceptions, if any, the effects on the coloring curves produced by substituting monovalent ${\mathrm{Tl}}^{+}$ ions in the KCl lattice are very similar to those produced by substituting divalent impurities such as ${\mathrm{Ca}}^{2+}$ ions. Consequently, it must be concluded that the role of monovalent and divalent metallic impurities in the coloring process is similar and is not related to the initial valence state of the impurity. In particular, it would appear that those positive-ion vacancies that are almost certainly introduced when divalent ions are substituted in the KCl lattice, to provide charge neutrality, are not essential to the impurity-related coloring mechanisms.