Scott J. Thompson

Background radiation measurements at high power research reactors

Abstract Research reactors host a wide range of activities that make use of the intense neutron fluxes generated at these facilities. Recent interest in performing measurements with relatively low event rates, e.g. reactor antineutrino detection, at these facilities necessitates a detailed understanding of background radiation fields. Both reactor-correlated and naturally occurring background sources are potentially important, even at levels well below those of importance for typical activities. Here we describe a comprehensive series of background assessments at three high-power research reactors, including γ-ray, neutron, and muon measurements. For each facility we describe the characteristics and identify the sources of the background fields encountered. The general understanding gained of background production mechanisms and their relationship to facility features will prove valuable for the planning of any sensitive measurement conducted therein.

Parametric evaluation of active neutron interrogation for the detection of shielded highly-enriched uranium in the field

Parametric studies using numerical simulations are being performed to assess the performance capabilities and limits of active neutron interrogation for detecting shielded highly enriched uranium (HEU). Varying the shield material, HEU mass, HEU depth inside the shield, and interrogating neutron source energy, the simulations account for both neutron and photon emission signatures from the HEU with resolution in both energy and time. The results are processed to represent different irradiation timing schemes and several different classes of radiation detectors, and evaluated using a statistical approach considering signal intensity over background. This paper describes the details of the modeling campaign and some preliminary results, weighing the strengths of alternative measurement approaches for the different irradiation scenarios.

Optimization of a fast neutron scintillator for real-time pulse shape discrimination in the transient reactor test facility (TREAT) hodoscope

We present a multi-channel, fast-neutron/photon detection system that uses ZnS(Ag) scintillator detectors. The system employs field-programmable gate arrays to pulse-shape analysis for real-time all-digital neutron/photon differentiation, producing particle-dependent pulse height and temporal distributions while allowing count rates in excess of 1,000,000 events per second per channel. The system size is scalable in blocks of 16 channels.

Estimation of the performance of multiple active neutron interrogation signatures for detecting shielded HEU

A comprehensive modeling study has been carried out to evaluate the utility of multiple active neutron interrogation signatures for detecting shielded highly enriched uranium (HEU). The modeling effort focused on varying HEU masses from 1 kg to 20 kg; varying types of shields including cement, wood, polyethylene, aluminum, and steel; varying depths of the HEU in the shields, and varying engineered shields immediately surrounding the HEU including steel, lead, and cadmium. Neutron and gamma-ray signatures were the focus of the study and false negative detection probabilities versus measurement time were used as a performance metric. To facilitate comparisons among different approaches an automated method was developed to generate receiver operating characteristic (ROC) curves for different sets of model variables for multiple background count rate conditions. This paper summarizes results or the analysis, including laboratory benchmark comparisons between simulations and experiments.

Observation of natural background radiation during the Great American Eclipse

Observations of photon and neutron background radiation were made in Rigby, Idaho, during the Great American Eclipse on August 21, 2017. Photon measurements were made using a mechanically-cooled, high-purity germanium gamma-ray spectrometer, segmenting the data into four energy bands of < 1 MeV, 1-2 MeV, 2-3 MeV, and 3-7 MeV. Neutron measurements were made using 3He proportional counter arrays embedded in polyethylene, either bare or wrapped with Cd or B filters. All data was analyzed in 900-s intervals starting one day before the eclipse and extending to one day after the eclipse. More detailed analyses were made in 90-s intervals for the photon data and 110-s intervals for the neutron data. Meteorological data was simultaneously recorded in 60-s intervals, recording solar radiance, temperature, air pressure, relative humidity, and dew point. For the observations described here, no statistically-significant (> 3σ) variations in signal count rates were observed in either the photon or neutron data. This level corresponds to the lack of observed photon variations exceeding 2.1%, 12.2%, 21.6%, or 43.2% of mean values in the four photon energy groups, respectively; it corresponds to a lack of observed neutron variations exceeding 25.3%, 25.6%, or 16.1% of mean values in the three neutron detector arrays, respectively.

Post-blast radiological dispersal device source term estimation

Research is underway to develop instruments and methods to determine the activity of radionuclides present in the fallout debris from the detonation of a radiological dispersal device (RDD). Handheld instruments, including commonly used health physics survey instruments, have been incorporated into a portable telemetry kit containing a global positioning system receiver, WiFi and radio communications, and a small microcomputer to facilitate data processing, logging, and transmission. An operator carries the system and walks through the RDD post-blast environment, real-time radiological data is logged, stored locally, and transmitted to a base station outside of the RDD hot zone. A map of the distributed radiological dispersal is generated, subdividing the world into 1-m2 squares. Separate measurements of ground activity taken at a finite number of discrete locations is used to cross-correlate the survey data, transforming the health physics data (e.g., mrem hr−1) to surface activity (Bq m−2). The map data, smoothed using standard Kriging approaches, is then analyzed by summing each discrete square area, producing an estimate for the total dispersed ground activity. The instrumentation and method have been field tested multiple times at Idaho National Laboratory during field exercises using short-lived radionuclides detonated in small-scale experiments.

Testing The High-Energy Prompt Neutron Signature At Low Beam Energies

Prompt fission neutrons continue to be examined as a signature for detecting the presence of fissionable material. This technique exploits the neutron energy limitations inherent with photonuclear emissions from non‐fissionable material, allowing prompt fission neutrons to be identified and engaged for detecting nuclear material. Prompt neutron signal measurements were acquired with bremsstrahlung endpoint energies of 6 MeV for 18 targets comprised of both fissionable and non‐fissionable material; delayed neutron measurements were also collected as a reference. The 238U target was also shielded with increasing thicknesses of lead or borated polyethylene to compare the resulting detection rates of the prompt and delayed fission neutron signals.