On the State of Activator in CsI(Na) Crystals Grown Under Forced Mixing of the Melt

Abstract

Effect of Nal concentration (C) on variations of some Csl(Na) crystal characteristlcs has been studied. The characteristics considered include: the optical absorption coefficient in the activator band; emission intensities of excitons localized at Na+ ions (420 nm) and in the regular lattice (840 nm) in the radioluminescence (RL) at 80 K; yields of stationary RL (241Am, 60 kev) (trnJ, y-scintillation 11s76s, 662 kev) (lr) and cr-scintillation (241Am, 5.9 meV) (Zo) for Csl(-lG) crystals grown under the forced melt mixing. The number of activator emission centers in those crystals has been shown to increase linearly as C rises up to about 2.2'l}-z mol.%. The -Lo value attains a maximum at C = 2.3.10-2 mol.% Nal while those of. Lpy and. L", at C = 910-3 moluh. The Nal solubility in Csl has been concluded to be at least 2.'2.10-2 mol.oh and thus to exceed substantially the literature data (from 8.10-3 mol.% to 1.10-2 mol.%). Therefore, the maximum tro value is due to a sufficient number of emission centers at the specified excitation density. The fact that the C value corresponding to the maximum .L" of Csl(Na) is lower than for Csl(Tl) crystals (about 2.5..1O-2 mol.% of Tll) is assumed io be caused by a larger size of the distorted lattice volume in the neighborhood of the light Na+ cation where the probability of the hole localization is increased.