Optically stimulated luminescence of Al2O3

Anion-deficient aluminum oxide doped with carbon (Al2O3:C) is not only an extremely sensitive thermoluminescence (TL) material, but is well suited to optically stimulated luminescence OSL applications due to a high cross-section for interaction of light with radiation-induced trapped charge. Several different OSL readout protocols have been suggested, including pulsed OSL (POSL), and "delayed" OSL (DOSL). This paper examines the properties of Al2O3:C for application using these two readout protocols. The POSL technique utilizes the prompt luminescence that results from the direct recombination of released charge carriers at luminescence sites (F-centers in Al2O3:C). Following a pulse of stimulation light using a laser, the POSL signal is observed to decay with a temperature-independent lifetime of similar to 35-36 ms. The DOSL signal, on the other hand, utilizes the temperature-dependent signal resulting from the capture of released charge carriers by shallow traps. The decay Of the luminescence component after the stimulating pulse has a lifetime of several hundred ms, depending upon temperature. The dependence of the DOSL signal on readout temperature can be explained in terms of the involvement of the shallow traps in the process. However, the intensity (not the lifetime) of the POSL signal is also slightly temperature dependent. It is conjectured that this may be caused by a thermally assisted optical detrapping process involving localized excited states. Different forms of Al2O3:C are examined. By modifying both the concentration and energy distribution of the shallow traps material optimized for DOSL applications can be engineered. In contrast, the best material for POSL is grown with no shallow traps. The integrated light output in a typical POSL measurement is approximately a factor of 7-8 greater than that of DOSL, even for a DOSL-quality sample. (C) 1998 Elsevier Science Ltd. All rights reserved.