!Qc/. Tracks, Vol. 9, No. 2, pp. 139- 142, 1984 Printed in Great Britain. 0191-278X/84$3.00+ .oO Pergamon Press Ltd. SHORT COMMUNICATION zyxwvutsrqponmlkjihgfedcbaZYXWVU ISOTHERMAL DECAY STUDIES OF CaS : Bi PHOSPHOR USING UV EXCITATION K. MAHESH, D. R. VIJ, N. SINGH, N. LAL and K. K. NAGPAUL Department of Physics, Kurukshetra University, Kurukshetra-132119, India (Received 27 May 1983; in revised form 4 January 1984) Abstract-The isothermal decay records of two TL peaks (peak I at 130°C and peak II at 280°C) of CaS : Bi phosphor samples (prepared with different Bi-dopant concentrations) have been analysed to determine the kinetic order parameter (A4) after irradiating them with a given UV dose. The value of M for both the peaks is found to be unity at a low dopant concentration of 0.012% (by weight of CaS). Further increase in the dopant concentration to 0.45% changes the Mvalue from 1 fo 2 for peak II, while for peak I, it remains unchanged. These measurements of kinetics and activation energy suggest that the Bi-impurity introduces two sets of trapping states whose density - profile shifts towards deeper states as the dopant concentration increases. 1. INTRODUCIION CALCIUM sulphide, though an early TL phosphor, has not received much attention on account of instability of composition and lack of reproducibility of data. One such problem, namely the hydrolization of the prepared phosphor, has been overcome by Vij and Mathur (1969) by washing the freshly prepared samples with distilled water. In this process the flux component (Na,SO,), which is converted into Na,S during heating in the reducing atmosphere, is removed, as it is soluble in water. Furthermore, there is a strong possibility for the revival of interest in CaS : Bi phosphors as these have been shown recently by Marwaha er al. (1979) to be good UV dosimeters. There is a suitable dosimetric’peak at about 280°C in this phosphor. As the impurity states are known to influence the performance of a phosphor significantly, we decided to investigate the characteristic features of the impurity states in CaS : Bi phosphors under UV excitation. In the present work we have determined the activation energies (E) and the kinetic order parameter (M) for the two glow peaks of the phosphor from the glow curves and isothermal decay records of the peaks (using the method given in Kathuria and Sunta, 1979) respectively with different Bi-impurity concentrations. 2. PHOSPHOR PREPARATION AND EXPERIMENTAL FEA WRES The phosphors have been prepared by reducing the charge, which consists of the CaS host taken in the form of CaS04. 2H,O (5 g) and the flux-Na>SO, (1.5 g), to which carbon in excess amount (I.8 g) is also added. These materials, which are all reagent grade from Merck, India, are properly mixed in a mortar. The Bi-activator impurity in the form of Bi(NOI), 5H,O solution is now added to it in such a way that the relative proportion between the solution and CaSO,. 2H,O gives net Bi-concentrations of 0.012%, 0.24%, 0.36% and 0.48% by weight of CaS. To ensure uniform mixing of the activator, a few drops of alcohol are also added. The whole charge in the graphite crucible is then heated at 900°C in a muffle furnace. In order to avoid the formation of luminescent CaO, the reducing atmosphere in the muffle furnace is maintained by keeping the carbon in a separate crucible. After the first firing for an hour, the charge is taken out, allowed to cool, and then thoroughly crushed so as to ensure proper mixing of the dopant. The crushed charge is given a second firing for half an hour at 900°C. The charge, after cooling, is crushed and then washed in boiling distilled water to remove any Na,S. The samples are placed in a holder of the TL 139