Radioactivity of neutron-irradiated materials used for fission track dating—improvement on dating procedures to eliminate radiation exposure by using a diallyl phthalate detector

Radioactivity of neutron-irradiated materials used for fission track (FT) dating was evaluated. We measured effective dose rates and specific activities resulting from neutron irradiation for commonly dated minerals (zircon, apatite and sphene), a muscovite detector, a diallyl phthalate (DAP) resin detector and a NIST-SRM612 standard glass. All of the materials except DAP had much higher specific activities after irradiation, and their activities persist over a long period because they contain several isotopes with long half-lives such as Sc-46 and Ta-182. In contrast, no radioisotopes were detected in DAP, except for a small amount of Br-82, whose activity was found to be far lower than the aforementioned radioisotopes as it decreased to background level after several days. By adopting the track counting procedure in which the fossil tracks in the dated minerals are counted prior to the irradiation of the samples and by using DAP detectors instead of muscovite, radiation exposure from the irradiated samples may be avoided. The procedure proposed here should reduce the effective dose to a microscopist during FT counts to less than 80μSv per year.