High specific activity of organically bound tritium in pine needles and search for its cause in the environment

In order to measure low level tritium in biological materials, an improved method was established. This method was applied to a number of pine needles. Tritium in pine needles is divided into two categories, i.e., tritium in tissue free water (TFWT) and organically bound tritium (OBT). The former is easily collected by freeze drying, but the latter requires combustion of dried samples. The combustion water is collected in a trap immersed in dryice-alcohol. The water thus collected is used for scintillation counting after distillation. We measured tritium concentrations in pine needles collected from 21 locations in all over Japan which are considered to be far enough removed from the influences of nuclear facilities. In addition to them, the same procedures were applied to pine needles collected in the vicinity of nuclear facilities. Tritium concentrations in pine needles of ordinary locations are higher in organic matter than in tissue free water with the ratio, ca. 1.5. The ratios are quite variable for the pine needles in the vicinity of nuclear installations suggesting that this method is useful for the environmental monitoring. The cause for high specific activity of organically bound tritium is not fully confirmed. However, we consider that this excess OBT may be attributed to direct assimilation of HT that presents in the atmosphere with extremely high specific activity. (author)