Brandon W. Blackburn

Pulsed D-D Neutron Generator Measurements of HEU Oxide Fuel Pins

Pulsed neutron interrogation measurements have been performed on highly enriched uranium (HEU) oxide fuel pins and depleted uranium (DU) metal using a D‐D neutron generator (2×106u2009neutrons‐s−1) and moderated 3He tubes at the Idaho National Laboratory Power Burst Facility. These measurements demonstrate the ability to distinguish HEU from DU by coincidence counting using a pulsed source. The amount of HEU measured was 8 kg in a sealed 55‐gallon drum compared to 31 kg of DU. Neutron events were counted during and after the pulse with the Nuclear Materials Identification System (NMIS) and used to calculate the neutron coincidence time distributions. Passive measurements were also performed for comparison with the pulsed measurements. This paper presents the neutron coincidence time distribution and Feynman variance results from the measurements.

Development of an Intense Pulsed Characteristic γ-Ray Source for Active Interrogation of Special Nuclear Material

An intense source of characteristic gamma-rays is developed as a potential probe to identify special nuclear material. A pinch-reflex ion diode is operated on the Gamble II pulsed power generator to produce proton beams with 270-kA peak current and 2.0-MV peak voltage. These beams bombard a PTFE (Teflon) target to produce characteristic gamma-rays by the 19F(p,alphagamma)16O reaction with energies of 6.13, 6.92, and 7.12 MeV and with an intensity of 3.1x1011 gamma-rays into 4pi in a single 50-ns duration pulse. Simple ballistic transport is used to transport the proton beam one meter so that the gamma-ray signal is separated in time and space from the diode bremsstrahlung pulse.

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.

Electronics and signal processing for prompt radiation

Active interrogation with pulsed bremsstrahlung beams can saturate detectors and produce high count rates of overlapping pulses in the prompt region (<1 mus) after the interrogating pulse. We describe a method to eliminate saturation by modifying the photomultiplier voltage divider and by gating intermediate pairs of dynodes and the anode. To process the high count rate of overlapping pulses, we convert the output current pulses to charge-pulse steps that can be digitally processed more easily and rapidly in real time. We discuss the application to LaBr3, liquid, and plastic scintillators and present some preliminary data.

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.

Radiation Fields in the Vicinity of Compact Accelerator Neutron Generators

Intense pulsed radiation fields emitted from sealed tube neutron generators provide a challenge for modern health physics survey instrumentation. The spectral sensitivity of these survey instruments requires calibration under realistic field conditions while the pulsed emission characteristics of neutron generators can vary from conditions of steady-state operation. As a general guide for assessing radiological conditions around neutron generators, experiments and modeling simulations have been performed to assess radiation fields near DD and DT neutron generators. The presence of other materials and material configurations can also have important effects on the radiation dose fields around compact accelerator neutron generators.

Optimizing Inspection Parameters for Long Stand‐Off Detection of SNM

Detection of special nuclear material (SNM) at extended ranges (>100 m) through the utilization of high energy (>20 MeV) bremsstrahlung photons requires optimizing the structure and interrelation of irradiation (beam-on) and detection (counting) periods. Conventional inspection schemes at lower energies and smaller distances primarily operate by pulsing an accelerator at frequencies of 0.1-1 kHz while collecting emitted radiation from the target under inspection for the few milliseconds in between pulses. Simulation and experimental results for long stand-off scenarios with source photons >20 MeV, however, indicate that two primary phenomena--(1) induced photoneutrons in proximity to the accelerator and (2) beam induced activation of air and soil--preclude the use of conventional inspection schemes. By considering the time structure and magnitude of the beam-induced photon and neutron backgrounds, signals of interest from the target, and natural backgrounds, inspection schemes have been developed to maximize signal to noise ratios (SNR). Analysis of the data indicates that the highest SNR values are found with short (2-5 s) irradiations followed by a 1-2 s period of collecting emitted neutron and photon signatures.