Jennifer L. Dolan

Neutron and gamma-ray cross-correlation measurements of MOX fuel using liquid scintillators

The detection of special nuclear material (SNM) is a very difficult problem: typically, the signals are weak thus difficult to distinguish from the background and in many practical applications additional shielding can be expected. In order to detect SNM more accurately, new methods to isolate the source signal are needed. Using cross-correlation functions it may be possible to identify the time-correlated particles from fission events above the background. To examine this feasibility, measurements were performed on 1-kg MOX fuel samples, a 252Cf source, and an AmBe source. Using pulse shape discrimination the particle types were isolated and the individual correlation contributions were determined. By comparing the (n, n) peak of the correlation curves a distinct difference is observed between a spontaneous fission source and an (α, n) source.

Nuclear nonproliferation measurements performed on mixed-oxide fuel pins at the Idaho National Laboratory

New results are presented on the passive measurement of mixed-oxide fuel pin assemblies. A fast and robust methodology for the characterization of mixed-oxide fuel pins is discussed. Four EJ-309 liquid scintillation detectors coupled with an accurate pulse timing and digital, offline, optimized pulse-shape discrimination method were used. Measurement analysis including pulse-height distributions and time-dependent cross-correlation functions were performed. The use of Monte Carlo particle transport codes, specifically MCNP-PoliMi, is discussed in conjunction with the anticipated measurements.

Validation of MCNPX-PoliMi fission models

We present new results on the measurement of correlated, outgoing neutrons from spontaneous fission events in a Cf-252 source. 16 EJ-309 liquid scintillation detectors are used to measure neutron-neutron correlations for various detector angles. Anisotropy in neutron emission is observed. The results are compared to MCNPX-PoliMi simulations and good agreement is observed.

The estimation of neutron energy spectra of nuclear materials by passive measurements for nuclear nonproliferation applications

The development of technologies to accurately characterize nuclear materials is becoming an area of significant importance in the United States and internationally. The development of new measurement systems with improved characterization capabilities is a top priority. Specifically, the study of neutron detection via passive measurement yields many opportunities. In this research, passive measurements were performed on mixed-oxide samples (MOX) and several isotopic neutron sources at the Joint Research Centre in Ispra, Italy in June of 2010. These measurements are used to test the capabilities of a neutron detection system and analysis algorithms developed at the University of Michigan. The measurements are also utilized to evaluate the use of liquid scintillation detectors for the determination of neutron energy distributions emitted from MOX samples and from various isotopic neutron sources. This measurement campaign provided the opportunity to identify and evaluate new candidates for the replacement of current detector technologies.

Plutonium metal spontaneous fission neutron cross-correlation measurements

A plutonium metal sample was measured by a fast-neutron multiplicity counter for characterization of spontaneous fission neutron anisotropy and for verification of MCNPX-PoliMi calculations. Accurate neutron angular distribution models are important to properly simulating fast neutron coincidence measurements for nuclear nonproliferation and safeguards applications. A majority of prompt neutrons are emitted from fully accelerated fission fragments; those neutrons carry momentum from the fission fragments, and thus an anisotropic neutron angular distribution is observed in the laboratory reference frame. The fast-neutron multiplicity counter was used with pulse shape discrimination techniques to produce neutron-neutron cross-correlation time distributions from spontaneous fission in a lead-shielded 0.84 g 240Pueff metal sample. Due to neutron anisotropy, the number of observed neutron cross-correlations varied as a function of angle between a detector pair and fission source. Fewer neutron correlations were observed at detector angles near 90 degrees, relative to higher and lower detector angles. Both the neutron coincident detections as a function of time difference and detector pair angle are compared with MCNPX-PoliMi calculations and show good agreement.