Elution behaviour of alpha-recoil atoms into etchant and ovservation of their tracks on the mica surface☆

Abstract Muscovite samples, which were irradiated with alpha-recoil atoms emitted from a thinly electrodeposited 232 U-source in a vacuum chamber of about 10 −2 Torr, were subjected to a chemical etching treatment with a hydrofluoric acid solution to develop alpha-recoil tracks. The transferred alpha-activities of 224 Ra and 212 Po, supported by 212 Pb, on the mica surface were repeatedly measured after every etching treatment. The results showed that the 224 Ra could be rapidly eluted out at earlier etching stages, in contrast to appreciably delayed elution of 212 Po. These findings, along with annealing experiments on mica, imply that the recoil range of 224 Ra, originated from the parent 228 Th by a single decay process, is shorter than the total recoil range of 212 Po, which can penetrate partially into inner mica layers through its preceding multiple alpha-decay processes after injection of its precursors into the mica. Scanning electron and phase-contrast microscopic observation of the etched mica surfaces indicated an apparent dependence of the recoil-track etch pit size on the number of succesive alpha-decays.