Inhomogeneous strain induced by fast neutron irradiation in NaKSO4 crystals

Abstract

A characteristic first-order phase transition of a solidsolid transformation in NaKSO4 crystals was observed using thermal analysis, infrared and X-ray diffraction techniques [1]. Previous studies of thermal properties of NaKSO4 crystals showed an anomaly in the temperature dependence of specific heat (Cp), linear thermal expansion coefficient, ~j, and heat transition, AQ, at 453 K [2]. These observations were confirmed by the results of the electrical conductivity in the same temperature range [3].The electrical conductivity measurements in the temperature range 300 to 500 K of NaKSO 4 crystals irradiated by fast neutrons of 14.5MeV energy, showed a pronounced change in the phase-transition temperature [4].In this letter the effect of neutron irradiation on the thermal properties of NaKSO4 crystals, in the same temperature range, was studied and the effect of the induced inhomogeneous strain was noted.Stoichiometric ratios of NazSO 4 and KzSO 4 were added to produce NaKSO4 crystals which were purified five times. Large single crystals were grown by means of slow dynamical isothermal evaporation methods [5].Chemical analysis, X-ray diffraction and optical inspection were used for purity control and to ensure the quality of the crystal.A Heraeus TMA 500 dilatometer was used for the thermomechanical analysis measurements. The sample temperature was monitored by means of a Ni-Cr-Ni thermocouple fitted in a sample holder of standard design which was made of quartz. The linear expansion coefficient was calculated on the assumption that the quartz expansion coefficient has an insignificant effect on the measured values, the heating rate used was 2 K min-~. The samples were shaped from a large single crystal of NaKSO4 in the form of rectangular rods whose dimensions were 2 x 2 x 20mm 3. The longer part of the sample was oriented along the crystallographic direction (0 0 1).The specific heat under constant pressure, Cp, was determined using a differential scanning calorimeter (DSC) technique, where a Heraeus DSC cell was connected to the Heraeus DTA 500 thermal analyser. Measurements were performed by applying the base line methods [6]. Lidded pans, made of aluminium, were used to eliminate the sloping of the base line. A Pt-100 thermocouple was used as the temperature sensor while a heating rate of 2 K min-1 was applied.The samples were irradiated by different neutron doses at room temperature (298 K) using a compact 0261 8028/87 $03.00 + .12