Development of neutron spectrum unfolding method for advanced nuclear emulsion

Maeda, Shigetaka   ; Ito, Chikara   ; Ishihara, Kohei*; Takagi, Keisuke*; Minato, Haruna*; Sakai, Yosuke*; Kawarabayashi, Jun*; Tomita, Hideki*; Iguchi, Tetsuo*

In order to realize neutron spectroscopy in high-intensity ray fields, an advanced nuclear emulsion based on a non-sensitized OPERA film with AgBr grain sizes of 60 nm has been developed together with a neutron spectrum unfolding method. 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. Through analyses of mock-up data, its performance was checked. The neutron spectrum from fresh fuel from the experimental fast reactor Joyo was estimated and track lengths were simulated using the calculated response function and the estimated spectrum. Comparison between the assumed neutron spectrum and the unfolded one confirmed the effectiveness of the proposed technique.