The response matrix of a Bonner sphere spectrometer was calculated by use of the MCNP code. As thermal neutron counter, the spectrometerhas a 3.2 cm-diameter 3He-filled proportional counter which is located at the center of a set of polyethylene spheres. The response wascalculated for 0, 3, 5, 6, 8, 10, 12, and 16 inches-diameter polyethylene spheres for neutrons whose energy goes from 10?9 to 20 MeV.The response matrix was compared with a set of responses measured with several monoenergetic neutron sources. In this comparison thecalculated matrix agrees with the experimental results. The matrix was also compared with the response matrix calculated for the PTB Cspectrometer. Even though that calculation was carried out using a detailed model to describe the proportional counter; both matrices doagree, but small differences are observed in the bare case because of the difference in the model used during calculations. Other differencesare in some spheres for 14.8 and 20 MeV neutrons, probably due to the differences in the cross sections used during both calculations.
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