AMS analysis of Cl-36 induced in concrete of accelerator facilities

摘要

Accelerator mass spectrometry (AMS) was applied to the analysis of Cl-36 induced in concrete samples obtained from accelerator facilities. In order to use a small amount of concrete sample and to separate chlorine as pure as possible, an improvement of separation process was developped. Chlorine was extracted from 1-5g of concrete into 0.01 M nitric acid in a pressurized decomposition vessel. After determining chlorine using ion chromatography, a certain amount of NaCl solution was added to obtain sufficient amounts of AcyCl precipitate and to dilute to a suitable isotope ratio of Cl-36 to Cl-35 (Cl-36/Cl-35) for the AMS (10(-12) - 10(-10)). A careful purification procedure was applied to reduce S-36 interference in AMS. Good reproducibility and small error throughout the chemical process for sample preparation was attained. Depth profiles of Cl-36/Cl-35 in concrete of a medium-energy cyclotron were measured by the developed method and compared with the results of gamma-emitters induced by thermal neutrons. Since it was confirmed that Cl-36 was produced by thermal neutron capture of Cl-35, the thermal neutron fluence irradiated during accelerator operation could be obtained usin Cl-36/Cl-35. In order to estimate the neutron fluences, the Cl-36/Cl-35 measurement by AMS is more useful than radioactivity measurements of other isotopes such as gamma-emitters because AMS directly provides the isotope ratio and the half-life of Cl-36 is very long.