陽子線治療に伴う放射化空気の放射能濃度測定における検出器の較正

概要

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While patients are irradiated by a proton beam for the cancer therapy, high energy protons unavoidably traverse the air of the treatment room. This makes the air getting radioactive due to nuclear reactions such as the p-alpha reaction in nitrogen nuclei. Thus, the measurement of the radioactive concentration of the air is important from the view point of radiation protection. The response of a gas monitor depends not only on the energy spectrum of emitted beta-rays but also on the wall material as well as the geometry of its ionization chamber. Consequently, it is necessary to calibrate the detector. Since samples having some standard conentration of radioactivity are not available for those nuclei of short half lives, such as carbon-11, nitrogen-13, etc., an EGS-4 simulation code was utilized to evaluate the detector response. The result shows the detector used in this facility (Tritium Room Monitor MGR-120X, Aloka) is about ten times less sensitive to beta-rays of tritium than to those of carbon-11 and nitrogen-13. With this consequence the radioactive air contamination of the treatment room is estimated to be 0.6 Bq/cc at a maximum, where the energy and the fluence rate of the proton beam are 160 MeV and 6.4 x 106cm-2 sec-1, respectively.