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Calibration of a long counter for fast neutrons with energies from 2 to 14 MeV
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To determine if a Hansen and McKibben type shielded long counter has a flat response from 2 MeV to 14 Mev detector efficiency was experimentally measured using a PuBe source. Calculations using the Monte Carlo program, MCNP, were performed to determine the efficiency of the detector for both PuBe and 14 MeV neutrons. The detector used a boron triflouride proportional counter as its counting device. Measurements were made using two 1 0 curie PuBe neutron sources (combined source strength of 4.02E+07 neutrons/second) to determine the detectors efficiency at the mean energy of the source, 4.3 MeV. The detector was found to have an efficiency of 0.85 counts-centimeter2/neutron at a source to detector distance of 1 meter. This compares favorably with previous measurements with long counters of similar configuration. The determination of the counter's effective center indicated that the effective center for the counter is 8.7 + 0.1 centimeters behind the front face of the detector. Attempts to use foil activation to determine the flux at the counter proved unsuccessful as the source strength was insufficient to activate the foils sufficiently. The Monte Carlo code, MCNP, was used to model a 150 KeV neutron generator source for 14 MeV neutrons, and the combined 1 0 Ci PuBe source experiment in order to determine the (n, a) reaction rate in the BF3 detector of the long counter, thereby simulating the long counter's response. For the PuBe source the efficiency of the long counter was computed to be 0.54 counts-centimeter2/particle at a source to detector distance of 60 centimeters. This is slightly less than the 0.6 counts-centimeter2/neutron achieved experimentally with the actual long counter, for a 20 Ci PuBe source prior to applying the correction for the detector's effective center, at the same distance. The MCNP model also was used to determined a long counter efficiency of 0.39 counts-centimeter2/particle for 14.74 MeV neutrons at a source to detector distance of 60 centimeters. This suggests that the long counter response is not flat over the 2 to 14 MeV energy range; however, tests indicated that the long counters efficiency on depends on the BF3 tube position in the long counter and that a flatter response over the energy range of interest may be obtained by adjusting the position of the BF3 tube.
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Orr, Michael Lee (1993). Calibration of a long counter for fast neutrons with energies from 2 to 14 MeV. Master's thesis, Texas A&M University. Available electronically from
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