Comparison of Response Function Calculations for Multispheres

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Abstract
An intercomparison of computer calculations to determine the response function of a multisphere neutron spectrometry system has been undertaken by EURADOS-CENDOS Working Committee IV. For this purpose, a standard multisphere spectrometer was adopted. This consisted of four polyethylene spheres, radii 38.1, 63.5, 101.6 and 152.4 mm with a central 3He detector, diameter 32 mm, having an atomic density of 4.25 x 1019 atoms.cm-3 (pressure, 172 kPa). The calculations were made for two polyethylene densities (0.92 and 0.95 g.cm-3). The Monte Carlo computer codes MCNP and BOKU (developed at PTB specifically for these calculations) and the discrete ordinates code ANISN were used to calculate the response of the detector for a variety of monoenergetic incident neutrons. Preliminary results for the 38.1 and 63.5 mm radii spheres are presented to compare these codes and their application at different laboratories. Discrepancies are discussed. In addition, further calculations were undertaken to compare the effects of various factors on the response functions, including: the effect of using cross sections without taking into account the binding of the carbon and hydrogen in the polyethylene; the effect of the steel walls of the 3He detector; and the effect of the stem of the counter.

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