Scott M. Watson

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.

Comparison of BCF-10, BCF-12, and BCF-20 scintillating fibers for use in a 1-dimensional linear sensor

One-dimensional fiber-bundle arrays may prove useful in a number of radiation sensing applications where radiation detection over large areas is needed. Tests have been performed to evaluate the light generation and transmission characteristics of IS-meter long, 10-fiber bundles of BCF-10, BCF-12, and BCF-20 scintillating fibers (Saint Gobain) exposed to collimated gamma-ray sources. The test set-up used one R9800 (Hamamatsu) photomultiplier tube (PMT) at each end, with a high-speed waveform digitizer to collect data. Time constraints were imposed on the waveform data to perform time-of-flight analysis of the events in the fiber bundles, eliminating spurious noise pulses in the high gain PMTs and also allowing 1-dimensional localization of interactions along the lengths of the fiber bundles. This paper will present the results of these measurements including the attenuation coefficients of the three fiber types and the timing resolution (position uncertainty) possible for each fiber bundle when using the R9800 PMTs.

Detection of Special Nuclear Material by means of promptly emitted radiation following photonuclear stimulation

Techniques have been developed to exploit abundant prompt emissions from photonuclear reactions for the identification of special nuclear material (SNM). These enhancements are designed to reduce inspections times and delivered dose in systems which have, historically, relied solely on delayed emissions. Experimental evidence is presented for prompt neutron time-of-flight measurements, neutron/photon correlations in multiple detectors, and novel detector development, specifically LaBr3 scintillators with new gating and buffering circuits to identify prompt gamma signatures. Significant and specific signatures indicative of the presence of SNM can be distinguished for the prompt neutron time-of-flight experiment and the neutron/photon correlations in multiple detectors.

Fast digitization and discrimination of prompt neutron and photon signals using a novel silicon carbide detector

Current requirements of some Homeland Security active interrogation projects for the detection of Special Nuclear Material (SNM) necessitate the development of faster inspection and acquisition capabilities. In order to do so, fast detectors which can operate during and shortly after intense interrogation radiation flashes are being developed. Novel silicon carbide (SiC) semiconductor Schottky diodes have been utilized as robust neutron and photon detectors in both pulsed photon and pulsed neutron fields and are being integrated into active inspection environments to allow exploitation of both prompt and delayed emissions. These detectors have demonstrated the capability of detecting both photon and neutron events during intense photon flashes typical of an active inspection environment. Beyond the inherent insensitivity of SiC to gamma radiation, fast digitization and processing has demonstrated that pulse shape discrimination (PSD) in combination with amplitude discrimination can further suppress unwanted gamma signals and extract fast neutron signatures. Usable neutron signals have been extracted from mixed radiation fields where the background has exceeded the signals of interest by >1000:1.

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.

Nuclear resonance fluorescence using different photon sources

Nuclear resonance fluorescence (NRF) is a photon-based active interrogation approach that provides isotope-specific signatures that can be used to detect and characterize samples. As NRF systems are designed to address specific applications, an obvious first question to address is the type of photon source to be employed for the application. Our collaboration has conducted a series of NRF measurements using different photon sources to begin to examine this issue. The measurements were designed to be as similar as possible to facilitate a straight-forward comparison of the different sources. Measurements were conducted with a high-duty factor electron accelerator using bremsstrahlung photons, with a pulsed linear accelerator using bremsstrahlung photons, and with a narrow bandwidth photon source using Compton backscattered photons. We present our observations on the advantages and disadvantages of each photon source type. Issues such as signal rate, the signal-to-noise ratio, and absorbed dose are discussed.

Photon dosimetry using plastic scintillators in pulsed radiation fields

Simulations and experiments have been carried out to explore using a plastic scintillator as a dosimetry probe in the vicinity of a pulsed bremsstrahlung source in the range 4 to 20 MeV. Taking advantage of the tissue-equivalent properties of this detector in conjunction with the use of a fast digital signal processor near real-time dosimetry was shown to be possible. The importance of accounting for a broad energy electron beam in bremsstrahlung production, and photon scattering and build-up, in correctly interpreting dosimetry results at long stand-off distances is highlighted by comparing real world experiments with ideal geometry simulations. Close agreement was found between absorbed energy calculations based upon spectroscopic techniques and calculations based upon signal integration, showing a ratio between 10 MeV absorbed dose to 12 MeV absorbed dose of 0.58 at a distance of 91.4 m from the accelerator. This is compared with an idealized model simulation with a monoenergetic electron beam and without scattering, where the ratio was 0.46.

Actively-induced, Prompt Radiation Utilization in Nonproliferation Applications

The Pulsed Photonuclear Assessment (PPA) technique, which has demonstrated the ability to detect shielded nuclear material, is currently based on utilizing delayed neutrons and photons between accelerator pulses. While most active interrogation systems have focused on delayed neutron and gamma-ray signatures, the current requirements of various Homeland Security issues necessitate bringing faster detection and acquisition capabilities to field inspection applications. This push for decreased interrogation times, increased sensitivity and mitigation of false positives requires that detection systems take advantage of all available information. Collaborative research between Idaho National Lab (INL), Idaho State Universitys Idaho Accelerator Center (IAC), Los Alamos National Laboratory (LANL), and Oak Ridge National Laboratory (ORNL), has focused on exploiting actively-induced, prompt radiation signatures from nuclear material within a pulsed photonuclear environment. To date, these prompt emissions have not been effectively exploited due to difficulties in detection and signal processing inherent in the prompt regime as well as an overall poor understanding of the magnitude and yields of these emissions. Exploitation of prompt radiation (defined as during an accelerator pulse/(photo)fission event and/or immediately after (les 1 mus)) has the potential to dramatically reduce interrogation times since the prompt neutron yields are more than two orders of magnitude greater than delayed emissions. Successful exploitation of prompt emissions is critical for the development of an improved robust, high-throughput, low target dose inspection system for detection of shielded nuclear materials.

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.