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The B%W Owners Group (BWOG) Cavity Dosimetry Benchmark Experiment included, in addition to various types of neutron dosimeters, lithium-7 fluoride (LiF) optical absorbance gamma dosimeters placed at eight cavity locations. The purpose of these dosimeters was to indirectly measure the gamma field in the vicinity of the neutron dosimeters. These measured data were used in a semi-empirical procedure to characterize the gamma-ray dose rate distribution in the power reactor cavity.
The LiF chip dosimeters were irradiated for one cycle in the high-level mixed neutron-gamma field that existed in the power reactor cavity. The 380.3-day cycle length was sufficient to ensure absorbed doses in the tenths of megagrays. After being removed from the reactor, the LiF chips were shipped to the National Institute of Standards and Technology (NIST) for measurement of the optical absorbance, annealing, and re-calibration in a known gamma ray field. The neutron flux spectrum at each LiF chip location was calculated using a semi-empirical methodology that reconciles measured neutron dosimeter data to calculated neutron spectra. Appropriate energy-dependent normalization factors were developed and applied to the a priori flux calculated by the DOT4.3 discrete ordinates transport code. The neutron flux spectra were then used to determine the total neutron dose absorbed in each LiF chip. Finally, the calculated neutron dose was subtracted from the measured (total) dose, yielding the integrated gamma-ray dose. Calculated-to-measured (C/M) gamma dose ratios were then established for each LiF chip and are reported herein.
gamma field, reactor cavity, lithium fluoride high fluence rate gamma detector
Principal Engineer, B%W Nuclear Technologies, Lynchburg, Virginia
Research Scientist, National Institute of Standards and Technology, Gaithersburg, Maryland