Detection and Interdiction of Shielded and Unshielded Special Nuclear Material Using Tensioned Metastable Fluid Detectors
The study of scientific and engineering underpinnings underlying the deployment of tensioned metastable fluid detectors (TMFDs) for active neutron and photon based interrogation of cargo containers formed the primary basis for this dissertation. The improvement of TMFDs for passive interrogation applications for cargo scanning and handheld dosimetry was also addressed. Active interrogation applications were primarily explored, with three specific techniques. These techniques are: threshold energy neutron analysis (TENA), interrogation of cargo for photo-fission (ICPF), and differential die away analysis (DDAA). The TENA technique uses a time continuous source of 2.45MeV neutrons interrogating potentially fissile special nuclear material (SNM) bearing cargo. These interrogation neutrons cause fissions in cargo containing SNM. A threshold enabled detector can then be used to detect only those neutrons that have energies higher than the interrogating beam, therefore unequivocably indicating the presence of fissile SNM. A 40cm3 centrifugally tensioned metastable fluid detector (CTMFD) was configured and shown able to detect > 2.45MeV neutrons at efficiencies of up to 4% while rejecting up to 10,000× more source neutrons than fission neutrons. It was found that using a panel of 15 (16cm3 sensitive volume) sealed CTMFDs for TENA based active interrogation with d + d generator neutrons, rejection ratios of ∼10,000× and higher are attainable with 14MeV neutron background subtraction. In addition, a panel of 15 CTMFDs was configured and able to detect ∼50g of ∼10% average enriched uranium dioxide within 5min. In a direct comparison, the industry standard 100cm3 NE-213 liquid scintillation detector, exhibited detection efficiencies up to only ∼0.1%, and with rejection ratios of less than 12×. The ICPF approach was also studied using a 6MV endpoint x-ray source. Photon based active interrogation was conducted using CTMFDs and conventional detectors. This method proved impossible to utilize with unshielded conventional detectors because of the large background field of photons causing false positives and detector saturation. However, this study verified that the CTMFD architecture was readily able to detect 630g of natural uranium with appropriate calibration while also rejecting a background source of ∼100, 000 n/s photoneutrons from x-ray interaction with deuterium in concrete. The differential die away analysis (DDAA) technique was investigated through simulation means using the MCNP-PoliMi framework. This simulation indicated the presence of fission neutrons in the time band between 10−5 s and 10−3s after interrogation shutoff at levels possibly detectable. Hardware was created to synchronize the timing between acoustically tensioned metastable fluid detector (ATMFD) systems and a d + t neutron generator. Successful rejection of 14.1MeV neutrons was demonstrated. It appears that, with higher efficiency ATMFDs, sizeable fission signals may then be detected from cargos, although as of yet, there is no experimental evidence to that effect.
Taleyarkhan, Purdue University.
Nuclear engineering|Nuclear physics
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