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Future nuclear arms reduction efforts will require technologies to verify that warheads slated for dismantlement are authentic without revealing any sensitive weapons design information to international inspectors. Despite several decades of research, no technology has met these seemingly paradoxical requirements simultaneously. The physical cryptographic measurement protocol recently developed at MIT presents one possible solution, exploiting nuclear resonance fluorescence (NRF) interactions to make isotope- and geometry-sensitive measurements of candidate nuclear warheads without relying on electronics or software for encryption. To protect sensitive information, the NRF signal from the warhead is convolved with that of an 'encryption foil' that contains key warhead isotopes in amounts unknown to the inspector. The convolved spectrum from a candidate warhead is then statistically compared against that from an authenticated template warhead to determine whether the candidate itself is authentic.
In this talk, I will discuss my contributions to the MIT NRF measurement protocol, which proceeded in four main stages. First, I carried out proof-of-concept Monte Carlo simulations of the NRF measurement for (proxy) 'genuine' and 'hoax' warheads using the Geant4+G4NRF codes, and showed that the measurement is capable of detecting both isotopic and geometric hoaxes in realistic measurement times and facilities. Next, I improved the physics accuracy and performance of the G4NRF code, developed a high-accuracy semi-analytical model for the expected NRF count rates in a warhead verification measurement, and subsequently benchmarked the improved version of G4NRF against the semi-analytical model to within 1–3%. I then used Geant4+G4NRF to design proof-of-concept NRF warhead verification experiments, which were recently conducted at the Massachusetts Institute of Technology. Using high-purity germanium (HPGe) detectors, I measured the NRF spectra generated from the interrogation of proxy 'genuine' and 'hoax' objects by a 2.52 MeV endpoint bremsstrahlung beam. The observed differences in U-238 NRF intensities near 2.2 MeV demonstrate experimentally that the physical cryptographic protocol can distinguish between proxy genuine and hoax objects with high confidence in realistic measurement times. Finally, I conducted an analysis of the absolute U-238 NRF count rates detected in order to experimentally validate the G4NRF Geant4 code to within ~20%, further enabling the design and analysis of possible future NRF experiments.