Transmission-based detection of nuclides with nuclear resonance fluorescence using a quasimonoenergetic photon source

Abstract

We provide a detailed experimental validation of the concept of transmission-based isotope detection. The dominant background processes in this class of systems were measured by studying the detection of U238 with a quasimonochromatic (ΔE∕E∼3%) photon beam. A notch develops in the spectrum transmitted through our test objects due to the preferential attenuation of photons with an energy that resonantly excites a bound nuclear state in U238 near 2 MeV. The notch was measured downstream of our test objects by means of resonant photon scattering from a secondary U238 target. The dominant backgrounds measured in the notch detector due to radioactive decay and elastic scattering of the transmitted beam are presented. Processes that refill the notch with off-resonance photons will obscure the signal and result in a higher probability of false negatives. A measurement of the refill process produced a null result, and we report an upper limit on the magnitude of the notch fill factor.