Development of neutron spectrum unfolding method for advanced nuclear emulsion

Abstract

In order to realize neutron spectroscopy in high-intensity gamma ray fields such as those present in spent nuclear fuel storage facilities using an advanced nuclear emulsion based on a non-sensitized OPERA film with AgBr grain sizes of 60 nm, a new neutron spectrum unfolding method has been developed. The response functions were evaluated in the neutron energy range between 0.1 and 4.5 MeV by Monte Carlo calculations. To realize a highly reliable unfolding method that does not require an initial guess spectrum, an algorithm was formulized based on the maximum entropy principle and the maximum likelihood method, and the new unfolding code MEALU was developed. The code performance was checked by analyses of mock-up data. As an example, the neutron spectrum from fresh fuel of the experimental fast reactor Joyo was calculated and track lengths were simulated using the calculated response function and the estimated spectrum. Comparison between the original neutron spectrum and the unfolded one confirmed the effectiveness of the proposed method.