Radiation damage in muscovite mica by gamma and neutron irradiation

Samples of natural muscovite mica have been exposed to high doses of gamma rays from a 60Co source. Numerous shallow etch pits are observed in chemically etched mica exposed to gamma doses above 2 or 3 x 103 Mrad(2 or 3 x 107 Gy). Similar etch pits can be observed in the samples exposed to neutrons in an experimental hole of a reactor. Muscovite used loses its mechanical strength and turns silvery white when heated at a temperature of 730 deg C for 1 hr, whereas the sample exposed to gamma doses above 7 x 103 Mrad shows the same behavior at about 840 deg C. The resistance to thermal decomposition is increased with increasing gamma dose in the range from 3 x 103 to 7 x 103 Mrad. The samples exposed to fast neutron fluences up to 9 x 1016 n/cm2 show no significant change of the resistance to thermal decomposition. However, that of the samples which are previously exposed to a gamma dose of about one thousand Mrad is increased with increasing fast neutron fluences ranged from 5.4 x 1015 to 2.2 x 1016 n/cm2. A simple mechanism is suggested to understand the etch pit formation and the change of the resistance to thermal decomposition by the gamma and neutron irradiation. (author)