PURPOSE:To make a sensitivity and a sensitivity characteristic freely variable by controlling a switch valve in such a way that it becomes a component ratio of an ionization gas sealed in an ionization chamber main body, according to the sensitivity or sensitivity characteristic required. CONSTITUTION:Ionization gases of different compositions are preliminarily prepared in multiple gas cylinders 3-1 to 3-n, and the composition ratio as well as gas pressure of the ionization gase correspondent to various sensitivity characteristics are stored in the data base of a computer 8, while the composition ratio as well as the gas pressure of the ionization gases correspondent to the required sensitivity as well as the sensitivity characteristic are read, and an electromagnetic valve 4 of the gas cylinder 3 in which the correspondent component is sealed, is opened, an a command for controlling the gas pressure according to the read information is given to the electromagnetic valve 3, so as to seal the gas of each composition in an ionization chamber main body 1. When the sensitivity as well as the sensitivity characteristic of the sealed ionization gas is to be verified, a check source is used, and radiations of different energies are irradiated, and a radiation measurement value is analyzed, and a calibration value is calculated, so as to flow the correspondent gas from the gas cylinder 3 to the ionization chamber main body 1.
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