NTA film as a personnel neutron monitor in criticality accidents

Abstract

Kodak nuclear track emulsions (known as NTA films) used by personnel for routine fast neutron monitoring are not useful in criticality accidents because of the high gamma background and the large proton track population expected in the emulsion. However the bromine content of the emulsion gets activated to 82Br on exposure to thermal and intermediate energy neutrons. 82Br is a beta-gamma emitter decaying with a half-life of 35.9 h. The induced activity in the cadmium-covered film is used to evaluate the intermediate energy neutron dose. Further, the NTA emulsion base is made of cellulose acetate. The carbon recoils produced in the base give rise to damage tracks. The damage track density is a measure of fast neutron dose. The induced bromine activity as well as the response of cellulose acetate in producing damage tracks, vary according to the leakage neutron spectral shape. To overcome these energy dependence problems, the average neutron activation cross section values for 81Br as well as the cellulose acetate response were determined for about twenty spectra from various critical assemblies. The method provides an independent estimate of neutron dose component along with the criticality badges that are available at present. As most threshold detector systems consist of gold, copper and indium foils as well as sulphur pellets, the σ Au / σ Cu and σ S / σ In ratios for these spectra are also correlated with the bromine cross section and the etch track response.