Absolute Calibration of the NSTX Neutron Monitor System
Authors: L. Roquemore, D.S. Darrow, and S.S. Medley
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 252 CF
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.
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Submitted to: 38th International Conference on Plasma Science & 24th Symposium on Fusion Engineering/ICOPS 2011 SOFE, Chicago, IL, June 26-30, 2011
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