Zero Knowledge Warhead Verification: System Requirements and Detector Technologies

We have previously proposed a “Zero-Knowledge” approach to nuclear warhead verification that avoids the need for an electronic information barrier, since sensitive information is never stored electronically. The basic concept is to use a Zero-Knowledge Protocol to make differential transmission radiographs and neutron emission measurements, comparing templates with objects presented for verification. An array of non-electronic neutron detectors are preloaded by the host with counts that complement those that will be accumulated during measurement. Here we focus on the system requirements for this approach, and particularly on the detector technology. We find that at least two detector technologies should be able to meet these requirements: superheated drop (“bubble”) detectors and neutron activation imaging. Bubble detectors will require a high density of small droplets to achieve the required total counts. An appropriate chemical formulation and good temperature control will be required for controlled energy selectivity. Magnetic resonance imaging or optical tomography may be used to count the dense array of bubbles produced. For transmission neutron activation imaging with 14 MeV neutrons, the 90 Zr(n,2n) 89 Zr reaction has adequate efficiency and a convenient threshold of 12.1 MeV. It has a half-life of 3.27 days and emits a 909 keV ? . We have detected the presence of 95 Zr from the 96 Zr(n,2n) 95 Zr reaction, which will require a particular preloading procedure to preserve zero knowledge. We have begun to analyze the requirements for emission measurements. The 115 In(n,n’) 115m In reaction is well suited for detection of emitted neutrons from spontaneous and actively-driven fission. It has a half-life of 4.5 hours and emits a 336 keV ? . A ~250 keV neutron source would be attractive for discriminating between fissile and fissionable material in driven emission measurements. Such a source could be provided using the 7 Li(p,n) 7 Be reaction, which has a threshold at 1.88 MeV and a resonance near 2.25 MeV.