Nanosecond timing in prompt γ ray analysis of bulk media using fast neutrons

Abstract

High resolution time and energy spectra of γ rays emitted after inelastic scattering of 14.6 MeV neutrons in a bulk sample of concrete have been observed. The time spectrum suggests that about 90% of γ rays of interest for the prompt γ ray analysis are emitted from the 12 cm thick surface layer in less than 10 ns after the neutrons hit the sample. The decay of the observed γ ray intensity was found to be approximately exponential, yielding the effective fast neutron lifetime of 2.1 ns. The signal-to-background ratio in the high resolution energy spectrum can thus be substantially increased by applying nanosecond timing techniques. In this way we identified all major (abundance ≳ 1%) elements in the sample.