TABLE OF CONTENTS Optical bleaching indicates the effect of light, of a specific wavelength, on irradiated TL samples, in the sense that charge carrier stimulation of a particular defect center can be achieved via absorption of optical energy, resulting then in a photodepopulation of the center. The charge carriers released may recombine with opposite sign carriers, emitting light during the illumination (bleaching light), or may be retrapped in other trapping centers. Observing then the changes occurring in the glow-curve resulting after the optical stimulation, relationships between thermoluminescence traps and optically activated centers can be obtained.
The term beaching is taken from the vocabulary of color centers; a crystal is colored by high dose of ionizing radiation and a subsequent illumination produces the color fading, i.e., the sample is bleached.
The effect of light on an irradiated thermoluminescent sample consists of a reduction of the TL signal, depending on the light intensity, its wavelength and duration of exposure.
For practical applications (personel, environmental and clinical dosimetry), the sensitivity to the light of different TL materials can be avoided by wrapping the dosimeters in light-tight envelopes. If this procedure is not applied, fading correction factors have to be determined carrying out experiments in dark and light conditions.
The oven used for annealing should be able to keep predetermined temperature oscillations within well specified margins. However, it must be noted that the reproducibility of the annealing procedure, concerning both heating up and cooling down processes, is much more important...
