Characterization of Fissionable Material using a Time-Correlated Pulse-Height Technique for Liquid Scintillators.

Abstract

In recent years, the threat of an extremist group obtaining and using a nuclear weapon has moved to the forefront of nuclear security concerns. Fortunately, the production of special nuclear materials (SNM) requires a level of infrastructure that is not available to a non-state entity; an extremist group would only be able to obtain existing weapons or the SNM needed to build one. To prevent any loss or diversion of existing SNM, new material accountability and safeguards technologies are needed to replace currently-deployed systems and expand their capabilities. With the aim of combatting these threats, this work portrays an evolution of time-correlation-based characterization systems, starting with benchmarking a commercially-available system and concluding with the initial development of the novel, time-correlated pulse-height (TCPH) characterization technique. Past work has demonstrated that cross-correlation measurements with liquid scintillator detectors can use fast-timing information to characterize source material; TCPH is an expansion of these cross-correlation measurements that incorporates pulse-height information. Using this additional information, it is possible to estimate the source multiplication, which is a key piece of information that can be used to identify SNM from benign sources. To facilitate the development of the TCPH technique, a program called MPPost was developed to simulate a detector response based on the particle transport performed with MCNPX-PoliMi. The capabilities of MPPost were expanded to provide a detailed detector response for a wide variety of common detectors and several analysis techniques. The feasibility of the TCPH technique was demonstrated with measurements of a 252Cf source. To validate MCNPX-PoliMi/MPPost, results of this measurement were compared to simulation. The simulated results showed excellent agreement, within 2% of the measured data. Measurements of low-multiplying materials were made at the Joint Research Center in Ispra, Italy. These measurements were used to further validate the capabilities of the MCNPX-PoliMi/MPPost simulation package for complex, multiplying sources. Lastly, simulations of reflected plutonium spheres, with multiplications ranging from 4 to 18, were performed. Using the TCPH response, each case could be identified and distinguished from a non-multiplying source, successfully demonstrating that TCPH can characterize the multiplication of an unknown source.