N. S. Bowden

Observation of neutrons with a Gadolinium doped water Cherenkov detector

Abstract Spontaneous and induced fission in Special Nuclear Material (SNM) such as 235 U and 239 Pu results in the emission of neutrons and high energy gamma-rays. The multiplicities of and time correlations between these particles are both powerful indicators of the presence of fissile material. Detectors sensitive to these signatures are consequently useful for nuclear material monitoring, search, and characterization. In this article, we demonstrate sensitivity to both high energy gamma-rays and neutrons with a water Cherenkov-based detector. Electrons in the detector medium, scattered by gamma-ray interactions, are detected by their Cherenkov light emission. Sensitivity to neutrons is enhanced by the addition of a Gadolinium compound to the water in low concentrations. Cherenkov light is similarly produced by an 8xa0MeV gamma-ray cascade following neutron capture on the Gadolinium. The large solid angle coverage and high intrinsic efficiency of this detection approach can provide robust and low cost neutron and gamma-ray detection with a single device.

Investigation of large LGB detectors for antineutrino detection

Abstract A detector material or configuration that can provide an unambiguous indication of neutron capture can substantially reduce random coincidence backgrounds in antineutrino detection and capture-gated neutron spectrometry applications. Here we investigate the performance of such a material, a composite of plastic scintillator and Li 6 nat 6 Gd ( BO 3 10 ) 3 : Ce (LGB) crystal shards of ≈ 1 xa0mm dimension and comprising 1% of the detector by mass. While it is found that the optical propagation properties of this material as currently fabricated are only marginally acceptable for antineutrino detection, its neutron capture identification ability is encouraging.

Comparison of Lithium Gadolinium Borate Crystal Grains in Scintillating and Nonscintillating Plastic Matrices

We present a method for detecting neutrons using scintillating lithium gadolinium borate crystal grains in a plastic matrix while maintaining high gamma rejection. We have procured two cylindrical detectors, 5 × 5, containing 1% crystal by mass and with the crystal grains having a typical dimension of 1 mm. One detector was made with scintillating plastic, and one with nonscintillating plastic. Pulse shape analysis was used to reject gamma ray backgrounds. The scintillating detector was measured to have an intrinsic fast fission neutron efficiency of 0.4% and a gamma sensitivity <; 4.93 × 10-9, while the nonscintillating detector had a neutron efficiency of 0.6 or 0.7%, depending on analysis integration limits, with gamma sensitivity <; 4.93 × 10-9 and (3.25 ±2.84) × 10-7, respectively. We determine that increasing the neutron detection efficiency by a factor of 5-6 will make the detector competitive with moderated 3He tubes, and we discuss several simple and straightforward methods for obtaining or surpassing such an improvement. We end with a discussion of possible applications, both for the scintillating-plastic and nonscintillating-plastic detectors.

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.

A note on neutron capture correlation signals, backgrounds, and efficiencies

Abstract A wide variety of detection applications exploit the timing correlations that result from the slowing and eventual capture of neutrons. These include capture-gated neutron spectrometry, multiple neutron counting for fissile material detection and identification, and antineutrino detection. There are several distinct processes that result in correlated signals in these applications. Depending on the application, one class of correlated events can be a background that is difficult to distinguish from the class that is of interest. Furthermore, the correlation timing distribution depends on the neutron capture agent and detector geometry. Here, we explain the important characteristics of the neutron capture timing distribution, making reference to simulations and data from a number of detectors currently in use or under development. We point out several features that may assist in background discrimination, and that must be carefully accounted for if accurate detection efficiencies are to be quoted.

Advances towards readily deployable antineutrino detectors for reactor monitoring and safeguards

The large flux of neutrinos that leaves a nuclear reactor carries information about two quantities of interest for safeguards: the reactor power and fissile inventory. Our SNL/LLNL collaboration has demonstrated that antineutrino-based nuclear reactor monitoring is feasible using a relatively small cubic scale detector made of Gadolinium loaded liquid scintillator at tens of meters standoff from a commercial Pressurized Water Reactor, deployed in an underground gallery that lies directly under the containment.

Advances toward a transportable antineutrino detector system for reactor monitoring and safeguards

Nuclear reactors have served as the neutrino source for many fundamental physics experiments. The techniques developed by these experiments make it possible to use these very weakly interacting particles for a practical purpose. The large flux of antineutrinos that leaves a reactor carries information about two quantities of interest for safeguards: the reactor power and fissile inventory. Our SNL/LLNL collaboration has demonstrated that such antineutrino based monitoring is feasible using a relatively small cubic meter scale liquid scintillator detector at tens of meters standoff from a commercial Pressurized Water Reactor (PWR). With little or no burden on the plant operator we have been able to remotely and automatically monitor the reactor operational status (on/off), power level, and fuel burnup. The initial detector was deployed in an underground gallery that lies directly under the containment dome of an operating PWR. The gallery is 25 meters from the reactor core center, is rarely accessed by plant personnel, and provides a muon-screening effect of some 20–30 meters of water equivalent earth and concrete overburden. Unfortunately, many reactor facilities do not contain an equivalent underground location. We have therefore attempted to construct a complete detector system which would be capable of operating in an aboveground location and could be transported to a reactor facility with relative ease. A standard 6-meter shipping container was used as our transportable laboratory — containing active and passive shielding components, the antineutrino detector and all electronics, as well as climate control systems. This aboveground system was deployed and tested at the San Onofre Nuclear Generating Station (SONGS) in southern California in 2010 and early 2011. We will first present an overview of the initial demonstrations of our belowground detector. Then we will describe the aboveground system and the technological developments of the two antineutrino detectors that were deployed. Finally, some preliminary results of our aboveground test will be shown.

Integrated readout of organic scintillator and ZnS:Ag/ 6 LiF for segmented antineutrino detectors

Antineutrino detection using inverse beta decay conversion has demonstrated the capability to measure nuclear reactor power and fissile material content for nuclear safeguards. Current efforts focus on aboveground deployment scenarios, for which highly efficient capture and identification of neutrons is needed to measure the anticipated antineutrino event rates in an elevated background environment. In this submission, we report on initial characterization of a new scintillation-based segmented design that uses layers of ZnS:Ag/6LiF and an integrated readout technique to capture and identify neutrons created in the inverse beta decay reaction. Laboratory studies with multiple organic scintillator and ZnS:Ag/6LiF configurations reliably identify 6Li neutron captures in 60 cm-long segments using pulse shape discrimination.

Performance of a MICROMEGAS-based TPC in a high-energy neutron beam

Abstract The MICROMEGAS (MICRO-MEsh GAseous Structure) charge amplification structure has found wide use in many detection applications, especially as a gain stage for the charge readout of Time Projection Chambers (TPCs). Here we report on the behavior of a MICROMEGAS TPC when operated in a high-energy (up to 800 MeV ) neutron beam. It is found that neutron-induced reactions can cause discharges in some drift gas mixtures that are stable in the absence of the neutron beam. The discharges result from recoil ions close to the MICROMEGAS that deposit high specific ionization density and have a limited diffusion time. For a binary drift gas, increasing the percentage of the molecular component (quench gas) relative to the noble component and operating at lower pressures generally improves stability.

Neutron detection with water Cerenkov based detectors

Legitimate cross border trade involves the transport of an enormous number of cargo containers. Especially following the September 11 attacks, it has become an international priority to verify that these containers are not transporting Special Nuclear Material (SNM) without impeding legitimate trade. Fission events from SNM produce a number of neutrons and MeV-scale gammas correlated in time. The observation of consistent time correlations between neutrons and gammas emitted from a cargo container could, therefore, constitute a robust signature for SNM, since this time coincident signature stands out strongly against the higher rate of uncorrelated gamma-ray backgrounds from the local environment. We are developing a cost effective way to build very large neutron detectors for this purpose. We have recently completed the construction of two new water Cherenkov detectors, a 250 liter prototype and a new 4 ton detector. We present both the results from our prototype detector and an update on the newly commissioned large detector. We will also present pictures from the construction and outline our future detector development plans.