Nucl. Tracks, Vol. 10, Nos. 1/2, pp. 101-105, 1985 0191-278X/85 $3.00 + .00 Printed in Great Britain Pergamon Press Ltd. PHOTO-TRANSFER THERMOLUMINESCENCE AND THERMAL QUENCHING IN CaSO4:Dy A. S. PRADHAN*and R. C. BHATr Division of Radiological Protection, Bhabha Atomic Research Centre, Bombay--400 085, India (Received 12 March 1984; in revised form 14 September 1984) Abaract--Like many other thermoluminescence materials, CaSO4:Dy, irradiated to doses above 102 Gy, annealed partially and exposed to UV, also exhibits strong photo-transfer thermoluminescence (FITL). The aim of the present study was to understand the mechanism of the photo-transfer process. In CaSO4:Dy, the UV irradiation does not appear to produce free charge carriers from the filled deep traps for the transfer. The PTFL and the reduction in residual thermoluminescence suggest that the transfer originates not only from the residual thermoluminescence traps but also from other radiation induced non-luminescent centres. 1. INTRODUCTION (reduction in luminescence efficiency with increas- SOME thermoluminescence dosimetry (TLD) phos- ing the temperature) of the high temperature glow phors, when subjected to a UV exposure after a peaks (Kathuria and Sunta, 1981). In order to high level gamma irradiation followed by a partial accurately assess this thermal quenching factor in high temperature glow peaks of CaSOg:Dy by the annealing (which only erases dosimetry glow peaks ~ method, we have, in the present work, but retains high temperature glow peaks), exhibit studied the FITL, over a wide range of doses. regeneration of the dosimetric glow peaks and a reduction in the intensity of the unerased high temperature glow peaks (residual thermolurnin- 2. MATERIALS AND METHODS escence, RTL). The regeneration of thermolumin- escence (TL) glow peaks is understood to be due to The CaSO4:Dy powder used in our study was the transfer of charge carders from filled deep traps taken from one of the phosphor batches used in our (related to high temperature glow peaks) to national personnel monitoring programme. The shallower empty traps and is known as photo- TLD reader, the annealing oven, the 6°Co gamma transfer thermoluminescence (FTTL). Photo- ray source, and the UV lamp used by us have been transfer studies in CaSO4:Dy (Pradhan, Bhatt and described elsewhere (Pradhan et al., 1980). The Supe, 1984) have shown that the UV exposure does UV lamp emits radiation mostly of 254 nm not produce free charge carders from filled deep wavelength. For photo-transfer studies, the phos- traps for the transfer; the PTTL for a sample phor grains after gamma-ray irradiation and partial irradiated to a gamma ray dose of 5 x 104 Gy and annealing (300°C-lh), were spread out (forming a annealed partially 300°C for 1 h) is about two times monolayer) in an aluminium planchette of diameter the reduction in RTL intensity (ARTL); and during 3.0 cm and thickness 0.4 ram. For all UV exposures the photo-transfer process there is no significant the sample to lamp distance was 7.5 cm. retrapping in high temperature glow peaks. The PTFL in LiF has been observed to be higher by an 3. RESULTS AND DISCUSSION order of magnitude as compared to the reduction in corresponding RTL and the excess in PTTL has Figure 1 shows the growth in P'ITL glow peak been attributed to a high thermal quenching (210°C) with UV exposure and the decay in the * Present address: Dr A. S. Pradhan, Universit~itsldinikum, Hufelandstrasse 55, D-4300 Essen-1, W. Germany. 101