Ultra-high resolution gamma-ray spectrometer development for nuclear attribution and non-proliferation applications

Cryogenic Gamma-ray spectrometers based on superconducting thermistors provide more than an order of magnitude improvement in energy resolution over conventional high-purity germanium detectors. They are based on measuring the temperature increase upon Gamma-ray absorption with a sensor operated at the transition between its superconducting and normal state. We are developing Gamma-ray calorimeters using Mo/Cu multilayer sensors with an attached Sn absorber for increased absorption efficiency ("UltraSpec"). We have also developed two-stage adiabatic demagnetization refrigerators for user-friendly detector operation at the required temperatures of similar to 0.1 K. The spectrometer has achieved an energy resolution between 50 and 90 eV FWHM for photon energies up to 100 keV, and can be operated up to 0.4 MeV with reduced resolution. We present an update on spectrometer performance and sensitivity, and discuss the relevance of this technology for Gamma-ray analysis in nuclear attribution and nuclear nonproliferation applications.