2020 年 19 巻 1 号 p. 11-14
The regulation for isotopes which emit radiation with extremely long half-lives such as Calsium-48 (t1/2＝1.9×1019 year) or Bismuth-209 (t1/2＝2.01×1019 year) was examined. At permission usage facilities pertaining to unsealed radioisotopes, all the radiation-emitting isotopes have to be controlled as radioisotopes regardless of their quantities or half-lives, even if radioactivity of these isotopes rarely exceeds the lower bound quantities (as with “exemption levels”). This is because the lower bound quantities are evaluated in terms of integrated radioactivity of all the unsealed radiation-emitting isotopes placed in a factory or place of business. In most of such cases, these isotopes can be excluded from the subject of radiation control by considering the regulation on the concentration limit for the unsealed radiation-emitting isotope even at the permission usage facilities pertaining to unsealed radioisotopes.
