Spectra observed following cargo irradiation

We present calculations of photon spectra observed following irradiation of bare HEU, HEU embedded in steel and wood cargos, and steel and wood alone. These spectra might be useful starting points for statistical detection efforts aimed at determining whether fissile material is present in a cargo. Detailed comparisons between calculations and experiments are presented and overall quite good (small χ²) agreement is found. We do not present a complete solution to the problem of determining whether a given spectrum contains contributions from post-fission photons. However, it is shown that a brute-force fitting of observed spectra in terms of a few calculated "basis" spectra gives meaningful predictions about the presence of ²³⁵U in cargo. Though this may not be the most powerful method, it does give well defined confidence limits and seems to have strong predictive power.