David L. Chichester

Using electronic neutron generators in active interrogation to detect shielded fissionable material

Experiments have been performed at Idaho National Laboratory to study methodology and instrumentation for performing neutron active interrogation die-away analyses for the purpose of detecting shielded fissionable material. Here we report initial work using a portable DT electronic neutron generator with a 3He neutron detector to detect shielded fissionable material including enriched uranium and reactor grade plutonium. Measurements have been taken of bare material as well as of material hidden within a large plywood cube. Results from this work have demonstrated the efficacy of the die-away neutron measurement technique for quickly detecting the presence of special nuclear material hidden within plywood shields by analyzing the time dependent neutron signals in-between neutron generator pulses. Using a DT electronic neutron generator operating at 300 Hz with a yield of approximately 0.36 times 108 neutrons per second, 2.2 kg of enriched uranium hidden within a 61 cm times 61 cm times 71 cm volume of plywood was positively detected with a measurement signal 2-sigma above the passive background within 1 second. Similarly, for a 500 second measurement period a lower detection limit approaching the gram level could be expected with the same simple set-up.

Using Electronic Neutron Generators in Active Interrogation to Detect Shielded Fissionable Material

Experiments have been performed at Idaho National Laboratory to study methodology and instrumentation for performing neutron active interrogation die-away analyses for the purpose of detecting shielded fissionable material. Here we report initial work using a portable DT electronic neutron generator with a He-3 fast neutron detector to detect shielded fissionable material including enriched uranium and reactor grade plutonium. Measurements have been taken of bare material as well as of material hidden within a large plywood cube. Results from this work have demonstrated the efficacy of the die-away neutron measurement technique for quickly detecting the presence of special nuclear material hidden within plywood shields by analyzing the time dependent neutron signals in-between neutron generator pulses. Using a DT electronic neutron generator operating at 300 Hz with a yield of approximately 0.36 x 108 neutrons per second, 2.2 kg of enriched uranium hidden within a 0.60 m x 0.60 m x 0.70 m volume of plywood was positively detected with a measurement signal 2-sigma above the passive background within 1 second. Similarly, for a 500 second measurement period a lower detection limit of approaching the gram level could be expected with the same simple set-up.

An integrated field emission array for ion desorption

Field emission arrays that are used for ion desorption must be capable of operating at high applied voltages. The large electric fields can lead to dielectric breakdown or electron emission from the gate, both of which may result in catastrophic failure. Methods were developed to fabricate tip arrays with integrated gate electrodes, separated from the substrate with sufficient dielectric to sustain high voltages. To suppress gate electron emission, processes were developed to fabricate geometries that favor high fields at the tip while minimizing the field at the gate.

Dose profile modeling of Idaho National Laboratory's active neutron interrogation laboratory

A new laboratory has been commissioned at Idaho National Laboratory for performing active neutron interrogation research and development. The facility is designed to provide radiation shielding for deuterium-tritium (DT) fusion (14.1 MeV) neutron generators (2 x 10(8) n/s), deuterium-deuterium (DD) fusion (2.5 MeV) neutron generators (1 x 10(7) n/s), and (252)Cf spontaneous fission neutron sources (6.96 x 10(7) n/s, 30 microg). Shielding at the laboratory is comprised of modular concrete shield blocks 0.76 m thick with tongue-in-groove features to prevent radiation streaming, arranged into one small and one large test vault. The larger vault is designed to allow operation of the DT generator and has walls 3.8m tall, an entrance maze, and a fully integrated electrical interlock system; the smaller test vault is designed for (252)Cf and DD neutron sources and has walls 1.9 m tall and a simple entrance maze. Both analytical calculations and numerical simulations were used in the design process for the building to assess the performance of the shielding walls and to ensure external dose rates are within required facility limits. Dose rate contour plots have been generated for the facility to visualize the effectiveness of the shield walls and entrance mazes and to illustrate the spatial profile of the radiation dose field above the facility and the effects of skyshine around the vaults.

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 (2×106 neutrons‐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.

The MARVEL assembly for neutron multiplication

A new multiplying test assembly is under development at Idaho National Laboratory to support research, validation, evaluation, and learning. The item is comprised of three stacked, highly-enriched uranium (HEU) cylinders, each 11.4 cm in diameter and having a combined height of up to 11.7 cm. The combined mass of all three cylinders is 20.3 kg of HEU. Calculations for the bare configuration of the assembly indicate a multiplication level of >3.5 (k(eff)=0.72). Reflected configurations of the assembly, using either polyethylene or tungsten, are possible and have the capability of raising the assemblys multiplication level to greater than 10. This paper describes simulations performed to assess the assemblys multiplication level under different conditions and describes the resources available at INL to support the use of these materials. We also describe some preliminary calculations and test activities using the assembly to study neutron multiplication.

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.

Fast-neutron spectrometry using a 3He ionization chamber and digital pulse shape analysis

Digital pulse shape analysis (dPSA) has been used with a Cuttler-Shalev type (3)He ionization chamber to measure the fast-neutron spectra of a deuterium-deuterium electronic neutron generator, a bare (252)Cf spontaneous fission neutron source, and of the transmitted fast neutron spectra of a (252)Cf source attenuated by water, graphite, liquid nitrogen, and magnesium. Rise-time dPSA has been employed using the common approach for analyzing n +(3)He→(1)H+(3)H ionization events and improved to account for wall-effect and pile-up events, increasing the fidelity of these measurements. Simulations have been performed of the different experimental arrangements and compared with the measurements, demonstrating general agreement between the dPSA-processed fast-neutron spectra and predictions. The fast-neutron resonance features of the attenuation cross sections of the attenuating materials are clearly visible within the resolution limits of the electronics used for the measurements, and the potential applications of high-resolution fast-neutron spectrometry for nuclear nonproliferation and safeguards measurements are discussed.

Addressing different active neutron interrogation signatures from fissionable material

In a continuing effort to examine portable methods for implementing active neutron interrogation for detecting shielded fissionable material research is underway to investigate the utility of analyzing multiple time-correlated signatures. Time correlation refers here to the existence of unique characteristics of the fission interrogation signature related to the start and end of an irradiation, as well as signatures present in between individual pulses of an irradiating source. Traditional measurement approaches in this area have typically worked to detect die-away neutrons after the end of each pulse, neutrons in between pulses related to the decay of neutron emitting fission products, or neutrons or gamma rays related to the decay of neutron emitting fission products after the end of an irradiation exposure. In this paper we discus the potential weaknesses of assessing only one signature versus multiple signatures and make the assertion that multiple complimentary and orthogonal measurements should be used to bolster the performance of active interrogation systems, helping to minimize susceptibility to the weaknesses of individual signatures on their own. Recognizing that the problem of detection is a problem of low count rates, we are exploring methods to integrate commonly used signatures with rarely used signatures to improve detection capabilities for these measurements. In this paper we will discuss initial activity in this area with this approach together with observations of some of the strengths and weaknesses of using these different signatures.

Measurement Of The Neutron Spectrum Of A DD Electronic Neutron Generator

A Cuttler‐Shalev (C‐S) 3He proportional counter has been used to measure the energy spectrum of neutrons from a portable deuterium‐deuterium electronic neutron generator. To improve the analysis of results from the C‐S detector digital pulse shape analysis techniques have been used to eliminate neutron recoil artifacts in the recorded data. Data was collected using a 8‐GHz, 10‐bit waveform digitizer with its full scale corresponding to approximately 6‐MeV neutrons. The measurements were made with the detector axis perpendicular to the direction of ions in the ENG in a plane 0.5‐m to the side of the ENG, measuring neutrons emitted at an angle from 87.3° to 92.7° with respect to the path of ions in the ENG. The system demonstrated an energy resolution of approximately 0.040 MeV for the thermal peak and approximately 0.13 MeV at the DD neutron energy. In order to achieve the ultimate resolution capable with this type of detector it is clear that a higher‐precision digitizer will be needed.