Superheated Droplet Detector Response to the Source System for Zero-Knowledge Verification

Verifying the authenticity of nuclear warheads requires gaining confidence as to whether a presented object is a nuclear weapon, without revealing sensitive information about either its design or its material composition. A Zero-Knowledge Protocol (ZKP) differential radiography technique, using pre-loadable superheated-emulsion (bubble) detectors, has previously been tested for this purpose. We are both extending this technique for higher spatial resolution, and also developing the ZKP technique to verify fissile material in presented objects, using the new “EXCALIBUR” neutron source available at the Princeton Plasma Physics Laboratory. First, to test higher-resolution differential radiography with 14 MeV neutrons from EXCALIBUR, we placed test-tube-shaped bubble detectors transverse to the direction of the neutron flux and also to a jump in neutron opacity in the test object. We found a corresponding jump in the bubble density along the axis of the detector, indicating an achievable spatial resolution of a few mm. Secondly, for fissile material detection, we plan to use bubble detectors with an energy threshold at 1 MeV to detect fission neutrons driven by sub-MeV neutrons, as can be provided by the EXCALIBUR neutron source in its moderated mode. A key requirement is that the neutron spectrum emitted from EXCALIBUR should result in a dramatically reduced bubble count rate compared with the 14 MeV spectrum, with the result that bubbles measured in the presence of fissile material should dominantly come from fission events, not from the small fraction of higher-energy neutrons emitted by EXCALIBUR. The required drop in bubble count rate was measured, roughly consistent with MCNP calculations taking into account both the neutron moderation in EXCALIBUR and the energy response of the detectors provided by Yale University. Justification This work is needed to develop the Zero-Knowledge Protocol approach to warhead verification towards both higher spatial resolution and also sensitivity to fissile vs. fissionable isotopes. Significance The radiography experiment with transverse detectors demonstrates improved spatial resolution of superheated droplet detectors and the measurements in different source configuration confirms the basic capability required for isotope sensitivity. This work supported by Defense Nuclear Nonproliferation R&D.