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Report No.

Measurements of power profile in TRACY supercritical experiment by detecting epithermal neutrons

Nakajima, Ken; Yanagisawa, Hiroshi; Miyoshi, Yoshinori

We have tried to measure the power profile in the TRACY supercritical experiment with high accuracy by detecting epithermal neutrons. In order to measure the epithermal neutrons, a cadmium covered $$^{235}$$U fission chamber was used, and polyethylene, a neutron moderator, was set inside the cadmium to enhance the neutron detection efficiency. In addition, a lead shield was used to reduce the noise current due to gamma-rays. The measured results were compared with the ones using a thermal neutron detector, and it was found that the time delay effect in the thermal neutron detection, which was caused by the flight time of neutrons to reach the detector, distorted the power profile and reduced its peak value. The reduction ratio of peak power was about 4% for the relatively slow power change with the inserted reactivity of 1.5$, but it became over than 40% for the rapid power change with the reactivity of about 3$.



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