Absolute calibration of the NSTX neutron monitor system

Abstract

NSTX has a complement of six neutron detectors consisting of two fission chambers, one NE-451 ZnS scintillator and 3 plastic BC-400 scintillators. The primary purpose of the fission chambers is to provide an absolute calibration of the neutron rate, while the scintillator detectors monitor fast excursion in the neutron yield due for instance, to fast MHD events. Within the last 10 years NSTX has performed 4 separate calibrations of the neutron monitoring system. Initially, a californium neutron source on a long tether was introduced into the vessel through 10 different top ports on NSTX corresponding to 10 of the 12 bays of the vessel. Each of the ports was at the same major radius just slightly larger than the nominal plasma major radius and a point-wise ring source was simulated to perform the calibration. Recently, the fission chambers were relocated to accommodate new equipment and the detector system required recalibration. This most recent calibration employed a commercial G-gauge model train and three different diameter circular tracks to transport the 252CF neutron source around the midplane of the NSTX vessel. In each of the calibrations, the fission chambers were operated in the pulse counting mode. During plasma operations one of the detectors transitions to the pulse saturated or current mode of operation. To complete the calibration, a series of low performance He discharges heated with one deuterium neutral beam were performed. Neutrons in this case come predominantly from beam-beam reactions. These low-yield discharges produced neutron levels that allowed the FC2 to remain in pulse counting mode while FC1 transitioned into the current mode. This allows a cross calibration to be performed between FC2 in pulse counting mode and FC1 in the current mode. The current mode value is then transferred to each of the four scintillating detectors. The results of the four insitu calibrations are presented.