Absolute calibration of microfission chamber in JT-60U

We have conducted calibrations of a microfission chamber, which was installed between the vacuum vessel and the toroidal field coils, by both a Cf-252 neutron source and real deuterium plasmas in JT-60U. The detector employs both pulse counting and Campbell (mean square voltage) modes in the electronics to cover a wide dynamic range of the neutron source strength. The pulse counting and Campbell modes were calibrated by Cf-252 and deuterium plasmas, respectively. Point efficiencies, counts per neutron from a point at a single angle, were measured for 27 locations of the neutron point source in toroidal scan. The efficiencies were influenced by the various components such as the vacuum vessel, port, and graphite tiles. The point efficiencies can be integrated and averaged with angle to provide toroidal line efficiencies. The line efficiencies of the microfission chamber and the nearest neutron monitor of the U-235 fission chamber were 5.38x10(-9) and 1.77x10(-8), respectively. Then the calibration for the Campbell mode was also performed by using a real deuterium plasma. The detection efficiency in the Campbell mode was about three-tenths of that of the nearest neutron monitor, which is consistent with the calibration result obtained by using a Cf-252 neutron source for pulse counting mode.