Metal Discs as Very Low Thermal Neutron Flux Monitors in Reactor Environment

摘要

Metal discs of pure materials like Au, W, Ta, Dy, Sm, In, Re and Mn can be used as monitors of very low thermal neutron fluxes. In order to reach low detection limits, the discs were activated for about 3 half-lives and measured with ultra low-level gamma-ray spectrometry for another 3 half-lives in an underground laboratory located at a depth of 225 m. The minimum detectable activity for these discs is in the range 0.5 mBq to 50 mBq, which was found through calibration to correspond to thermal neutron fluxes in the range 10~(-4) to 10~(-2) cm~(-2) s~(-1). The discs were calibrated in the well defined neutron flux of the 1-m diameter large cavity facility located in the thermal column of the BR1 reactor at SCK*CEN. Thermal and ~(235)U fission spectrum neutron standard fields were used in the calibration procedure. The objectives of this technique, the calibration results as well as the first monitoring of thermal neutron fluxes at work places outside the BR1-reactor are presented.