Sources of error in phosphate glass dosimetry

Recent silver-activated glass dosimeters are superior to photographic film and thermoluminescent dosimeters for many reasons described briefly, especially in high-level accident dosimetry, because fast dose evaluation and the possibility of remeasuring doubtful values are necessary. A main disadvantage of glass dosimeters is possible changes in the fluorimetric or in the photometric dose reading between exposure and evaluation. These changes have to be considered if exact dose readings are desired, UV irradiation can induce or destroy radiation-induced fluorescence and colour centres in the glass, depending on the quantum energy of the UV light. More important, in practice, is the increase of the measured RPL immediately after exposure, superimposed later by the fading process. Increase and decrease rates depend strongly on the storage temperature and the composition of the glass. For instance, the increase after exposure decreases with increasing si!ver concentration in a glass with constant basic composition, and fading occurs earlier. At constant silver concentration there is also an effect of different glass base compositions. There are no observable differences in the kinetics of X-ray, $\gamma$-ray and thermal neutron effects in the glass. In the $10^{-4}$ to $^{3}$ r $\gamma$ dose range (fluorimetric dose reading) the kinetic behaviour of the RPL centres determines this effect, while in the $10^{3}$ to $10^{7}$ r dose range the somewhat different stability of the absorption centres is important if absorption measurements are taken (the fading depends on the wavelength of the photometric measurement and is lowest at the 320 nm absorption band), If fluorimetric measurements are also taken, the superimposing effects determine the dose reading because of the UV absorption in the glass at the fluorimetric: reading. A brief theoretical interpretation of the kinetics in irradiated glass is given by means of a simplified band model of the physical processes involved. A general expression, for the possible radiation effect alterations is given, After a high-dose exposure the glass can be regenerated by a special heat treatmem. Too extended heating or too high temperatures may increase the predose, In practice the possible inaccuracies in the dose measurement can be overcome by: (a) A short heat treatment of the glass stabilization or "development" of the radiation effect); or (b) The use of the correction curves given in the paper. The UV-induced RPL behaves somewhat differently