Evaluation of Nal(Tl) Scintillator as a Fast Neutron Monitor

The fast neutron induced reaction ¹²⁷1(n,n') followed by the absorption of the 57 keV gamma ray of the deexcitation of the ¹²⁷1 first excited state allows a conventional Nal(Tl) detector to be utilized as a high efficiency, fast neutron detector. The interaction length for the emission of the 57 keV gamma ray produced by the passage of neutrons of energy 2.8 MeV and 14.1 MeV through Nal(Tl) has been measured. A† 2.8 MeV, λ = 0.0079 ± 0.0009 cm^-1; at 14.1 MeV, λ = 0.0050 ± .0005 cm^-1.