Non-destructive assay (NDA) of 239Pu in spent nuclear fuel is possible using the integral resonance transmission (IRT) method. IRT measures the integral nuclear resonance fluorescence scattering on a mono-isotopic witness target following resonance absorption in nuclear fuel. Lowering the temperature of the witness target could substantially improve measurement sensitivity by narrowing the resonance width due to Doppler broadening. Calculations studying this effect will be presented.
