High Energy Neutron Spectral Unfolding Using Activation Foils

L.S. Walker1; R.H. Olsher1; J. Oostens2; and M. James1 (1Los Alamos National Laboratory; 2Campbellsville University, Kentucky)

This experiment is a proof of principal experiment to verify the technique for unfolding the high energy component of a neutron spectrum proposed in Scott Walker's Masters Degree thesis. The neutron spectrum is measured by neutron time of flight (TOF) techniques and compared to the spectrum estimated with activation foils. Seven, 1 cm x 2.54 cm diameter activation foils (Indium, Terbium, Holmium, Tantalum, Iridium, Gold and Bismuth) were exposed to a high energy neutron beam at the Weapons Neutron Research Center (WNR). After irradiation, a hyper-pure germanium detector was used to measure the gamma spectra emitted by each of the individual foils. Up to 14 different nuclei are created in the individual activation foils via (n,) or (n, xn) reactions (depending on the foil). The resulting nuclei measurements were then corrected for decay and detector efficiency to calculate total nuclide activity. Response functions for each of the (n, xn) activation reactions were previously calculated using physics models (CEM and Bertini) in the MCNPX computer code. The particular geometric model used to calculate the response functions incorporated a mono energetic, parallel, planar neutron disc source that irradiated the foil stack perpendicular to the cylindrical axis. The low energy component of the spectrum (calculated from (n,nx) reactions in the activation foils) was unfolded using standard Bonner sphere type unfolding codes while the high energy component spectrum unfolding was attempted using available codes and matrix algebra. The spectrum estimated from each activation foil is analyzed individually and then, normalized into an average spectrum. TOF and activation foil estimated spectra are then compared. Note: Because of the nature of TOF measurement techniques, the low energy component of the spectrum cannot be unfolded.

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