Marc L. Ruch

Active neutron and gamma-ray imaging of highly enriched uranium for treaty verification

The detection and characterization of highly enriched uranium (HEU) presents a large challenge in the non-proliferation field. HEU has a low neutron emission rate and most gamma rays are low energy and easily shielded. To address this challenge, an instrument known as the dual-particle imager (DPI) was used with a portable deuterium-tritium (DT) neutron generator to detect neutrons and gamma rays from induced fission in HEU. We evaluated system response using a 13.7-kg HEU sphere in several configurations with no moderation, high-density polyethylene (HDPE) moderation, and tungsten moderation. A hollow tungsten sphere was interrogated to evaluate the response to a possible hoax item. First, localization capabilities were demonstrated by reconstructing neutron and gamma-ray images. Once localized, additional properties such as fast neutron energy spectra and time-dependent neutron count rates were attributed to the items. For the interrogated configurations containing HEU, the reconstructed neutron spectra resembled Watt spectra, which gave confidence that the interrogated items were undergoing induced fission. The time-dependent neutron count rate was also compared for each configuration and shown to be dependent on the neutron multiplication of the item. This result showed that the DPI is a viable tool for localizing and confirming fissile mass and multiplication.

Comparison between silicon photomultipliers and photomultiplier tubes for pulse shape discrimination with stilbene

A stilbene crystal was coupled to a silicon photomultiplier (SiPM) to assess the performance of the detectors pulse shape discrimination (PSD) between fast neutrons and gamma rays. Pulses were digitized from a measurement of Cf-252 and digital charge comparison was used to perform PSD. The stilbene crystal was then coupled to a photomultiplier tube (PMT) and the measurement was repeated. The PSD performance when using the SiPM was compared to that of the system using the PMT. Both systems demonstrate efficient ability to discriminate between neutrons and gamma rays. While PMTs have long been the standard technology for light readout, SiPMs show similar capabilities while being less expensive, significantly more compact in size, significantly less sensitive to magnetic fields, and having lower power requirements. Potential drawbacks of SiPMs include elevated levels of noise and nonlinearity at high energies.

Author Correction: Active neutron and gamma-ray imaging of highly enriched uranium for treaty verification

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Position sensitivity within a bar of stilbene coupled to silicon photomultipliers

A 6-mm by 6-mm by 50-mm bar of stilbene was coupled on both ends to silicon photomultipliers (SiPMs) to assess the detectors position sensitivity to interactions throughout the bar. A Na-22 gamma ray source was collimated with a pair of lead bricks to produce a source beam that was used to irradiate five positions along the length of the bar. A logarithmic relationship between the ratio of the pulse heights obtained from the two SiPMs and the position of the collimated source was established. The standard deviation of the distribution of ratios from each measurement was propagated through the functional form to determine position resolution. The position resolution along the length of the bar was determined to have an average value of 4.9 mm.