Photo-Transferred Thermoluminescence

Abstract

This chapter deals with transfer of TL intensity from one glow peak to another under exposure to light. The transferred intensity is called photo-transferred TL (PTTL). This phenomenon is observed almost in all thermoluminescent materials, usually showing a transfer from higher temperature glow peak to those of lower temperatures. In this Chapter transfer is shown from a glow peak to lower as well as to higher temperature glow peaks. This chapter also describes the effect of sample temperature on the transfer efficiency for incident light. This phenomenon is used to find the energy levels of the donor trap. Using the monochromatic incident light for photo-transfer, optical trap depth is found for the donor trap. Studies describing the dependence of transfer efficiency on sample temperature have possibly not been given in earlier books of this field. These studies are used to propose a model for photo-transfer in mineral CaF₂. Application of PTTL in cumulative dose measurement and retrospective dosimetry is described briefly. Finally the topic of photo-bleaching is taken up. It is an important subject in sediment dating. It is shown that in a multi-peak glow curve photo-bleaching and photo-transfer run together. On prolonged light exposure, all glow peaks, donors as well as accepters attain a low and nearly stable level. At this stage most of the traps are empty and correspondingly the population of the recombination centres also is negligibly small. As a result the available option for the photo-excited electrons from any of the traps is simply to get retrapped into any of the vacant traps. In other words a to and fro transfer between the traps continues almost indefinitely. This leads to a nearly unbleachable TL signal which is used as the zero level in the TL dating of geological sediments.