Segregation of (134Cs)+ impurity during growth of polycrystalline ice

Abstract

The distribution of radioactive impurities in the liquid and solid phases was measured during the growth of ice under a condition of one dimensional heat flow. The concentration of impurities along the specimen was proportional to the intensity of gamma rays detected by an Nal (Tl) photomultiplier and single-channel analyzer. A large impurity accumulation took place ahead of the crystal growing vertically upwards and the thickness of this impurity cloud is of the order of a few mm corresponding to the hydrodynamic instability region, between 0 and 4 °C, of maximum water density. Freezing downward does not show a detectable peak due to the strong convection where the diffusion coefficient is about 2500 times larger than for the diffusion of impurity in the case of freezing upward. Accumulation of impurity is a strong function of the V rate of freezing and of the initial impurity concentration C0. Impurity entrapment in ice is given in terms of the partition coefficients, Kef and Ki, which depend on V and C0. A morphological substructure of the solid-liquid interface is presumably changed during freezing runs thus causing a complicated dependence of Kef, Ki, and the coefficients of the impurity cloud size on the V rate and concentration C0.