Takuya Abe

Relationship between variations of 7Be, 210Pb and 212Pb concentrations and sub-regional atmospheric transport: Simultaneous observation at distant locations

In order to investigate the applicability of (212)Pb as a tracer for atmospheric transport in the sub-regional scale (few hundred kilometers in horizontal direction and up to approximately 1km by height), we measured the air concentrations of the short-lived radionuclide (212)Pb along with the long-lived (7)Be and (210)Pb near the ground surface. For this purpose, simultaneous observations were continued for several days at three locations: a reference point representative for standard land surface atmosphere conditions, a second location at an altitude 650 m near the reference point, and on a solitary island approximately 180 km from the reference point. Measurements of radioactivity in aerosol particle samples collected at intervals of 2-3h with a high-volume air sampler were performed by extremely low background gamma-ray spectrometry with the use of Ge detectors located at the Ogoya Underground Laboratory. Concentration of (7)Be or (210)Pb and their variation patterns was found to be similar among the three points during the whole observation period except for moment of the passage of a cold front. The results indicate that distributions of concentrations of the long-lived nuclides were uniform in this range. On the other hand, concentration levels and the variation patterns of the short-lived (212)Pb differed greatly from one location to another, reflecting differences in geographical location and altitude of the observation points. Additionally, there were certain indications that observed concentration of (212)Pb contained two components: an autogenous component from sources nearby and a heterogenous one from faraway sources carried by atmospheric horizontal transport. Results of this study provide experimental proof that (212)Pb can be used as a tracer of sub-regional atmospheric transport.

Benchmarking of activation reaction distribution in an intermediate energy neutron field

Neutron-induced reaction rate depth profiles inside concrete shield irradiated by intermediate energy neutron were calculated using a Monte-Carlo code and compared with an experiment. An irradiation field of intermediate neutron produced in the forward direction from a thick (stopping length) target bombarded by 400 MeV nucleon(-1) carbon ions was arranged at the heavy ion medical accelerator in Chiba. Ordinary concrete shield of 90 cm thickness was installed 50 cm downstream the iron target. Activation detectors of aluminum, gold and gold covered with cadmium were inserted at various depths. Irradiated samples were extracted after exposure and gamma-ray spectrometry was performed for each sample. Comparison of experimental and calculated shows good agreement for both low- and high-energy neutron-induced reaction except for (27)Al(n,X)(24)Na reaction at the surface.