CAN ULTRA-HIGH ENERGY RESOLUTION GAMMA-RAY SPECTROMETERS DIRECTLY DETECT FISSILE ISOTOPES IN SPENT NUCLEAR FUEL?

Superconducting ultra-high energy resolution Gamma-ray spectrometers offer an order of magnitude improvement in energy resolution over conventional high-purity germanium detectors. This not only reduces line overlap in non-destructive isotope analysis (NDA) and thus improves the measurement accuracy; ultra-high energy resolution also increases the peak-to-background ratio and therefore improves the detectability of weak lines on a high spectral background. We have performed Monte-Carlo simulations of our superconducting spectrometer to model its low-energy response function and understand the dominant contributions to the spectral background. We observe the different contributions from Compton scattering in the detector itself, its shielding and its thermal heat sink, which qualitatively agree with the measured response function. We discuss the results in the context of plutonium assay in the presence of a high spectral background, and outline the design of an instrument to directly detect radiative Pu signatures in spent nuclear fuel.