The effect of high gamma-ray doses on the track-registration properties of natural mica

The effect of gamma-ray irradiation of natural mica is given in terms of the track-registration property. Specimens of muscovite mica were first exposed to various gamma-ray doses from a 60Co source ranging from 1.9 x 103 to 1.6 x 104 MRad, and then to fission fragments. The results show that the average etched width of fission-fragment tracks decreases with increasing gamma-ray dose. Shallow pits were observed in etched specimens when the gamma-ray dose exceeded 5 x 103 MRad. The number of shallow etch pits per unit area increases with increasing gamma-ray dose and depends on the local lattice structure in the specimen as well. Numerous shallow etch pits caused by the gamma-ray irradiation of the order of 104 MRad interfered with the observation of fission tracks in the specimen. Other specimens were exposed to gamma-ray doses up to 1.8 x 104 MRad after having been heated for 100 min at various temperatures up to 6000C. No shallow etch pits were observed on the surface of the specimen annealed for 100 min at 6000C before the gamma-ray irradiation. Pre-annealing extends the 'safety limits' of the gamma background below which muscovite mica can be used to observe fission tracks without any gamma-ray interference. (orig.)