Spontaneous fission in Special Nuclear Material (SNM) such as plutonium and highly enriched uranium (HEU) results in the emission of neutrons with energies in the MeV range (hereafter 'fast neutrons'). These fast neutrons are largely unaffected by the few centimeters of intervening high-Z material that would suffice for attenuating most emitted gamma rays, while tens of centimeters of hydrogenous materials are required to achieve substantial attenuation of neutron fluxes from SNM. Neutron detectors are therefore an important complement to gamma-ray detectors in SNM search and monitoring applications. The rate at which SNM emits fast neutrons varies from about 2 per kilogram per second for typical HEU to some 60,000 per kilogram per second for metallic weapons grade plutonium. These rates can be compared with typical sea-level (cosmogenic) neutron backgrounds of roughly 5 per second per square meter per steradian in the relevant energy range [1]. The fact that the backgrounds are largely isotropic makes directional neutron detection especially attractive for SNM detection. The ability to detect, localize, and ultimately identify fast neutron sources at standoff will ultimately be limited by this background rate. Fast neutrons are particularly well suited to standoff detection and localization of SNM or other fast neutrons sources. Fast neutrons have attenuation lengths of about 60 meters in air, and retain considerable information about their source direction even after one or two scatters. Knowledge of the incoming direction of a fast neutron, from SNM or otherwise, has the potential to significantly improve signal to background in a variety of applications, since the background arriving from any one direction is a small fraction of the total background. Imaging or directional information therefore allows for source detection at a larger standoff distance or with shorter dwell times compared to nondirectional detectors, provided high detection efficiency can be maintained. Directional detection of neutrons has been previously considered for applications such as controlled fusion neutron imaging [2], nuclear fuel safety research [3], imaging of solar neutrons and SNM [4], and in nuclear science [5]. The use of scintillating crystals and fibers has been proposed for directional neutron detection [6]. Recently, a neutron scatter camera has been designed, constructed, and tested for imaging of fast neutrons, characteristic for SNM material fission [7]. The neutron scatter camera relies on the measurement of the proton recoil angle and proton energy by time of flight between two segmented solid-state detectors. A single-measurement result from the neutron scatter camera is a ring containing the possible incident neutron direction. Here we describe the development and commissioning of a directional neutron detection system based on a time projection chamber (TPC) detector. The TPC, which has been widely used in particle and nuclear physics research for several decades, provides a convenient means of measuring the full 3D trajectory, specific ionization (i.e particle type) and energy of charged particles. For this application, we observe recoil protons produced by fast neutron scatters on protons in hydrogen or methane gas. Gas pressures of a few ATM provide reasonable neutron interaction/scattering rates.

New algorithms for data analysis were coupled with new techniques for event-by-event data handling. These were tested using data collected with an AmBe fast neutron source and compared to simulated data. Using the measured fast neutron background and estimates of the measurement uncertainties from stationary data runs, simulations involving relative motion between source and detector show promising results for this technology.

Since the dawn of the nuclear weapons era, political, military, and scientific leaders around the world have been working to contain the proliferation of Special Nuclear Material and explosively fissile material. This paper describes the construction of a prototype, directional, fast neutron detector, modeled after the Dark Matter Time Projection Chamber. Fast neutrons are emitted by a host of interesting sources, including medical isotopes, research sources, cosmic neutron spallation, and the most interesting, SNM and WGP. This detector was built for remote operations using a computer terminal for the detection of Special Nuclear Material (SNM) and Weapons Grade Plutonium (WGP). Further, this paper discusses the baseline and characterization testing using a low intensity Americium-Beryllium neutron source, compared with probability-based rate calculations and Geant4 simulation rates. The detector, in its current configuration agrees well with the expected rates, showing a 95% track reconstruction efficiency, computed from both the probability-based rate calculation and the Geant4 simulation rates. Scaling this to an appropriate size, this detector provides an entirely unique and new piece of information in the world of radiation detection: direction to fast neutron source. The primary concept of employment focuses on building statistics through large total cross section, increased scan time, or proximity of scan source. With these three variables balanced for a specific operational environment, this detector technology is able to pinpoint fast neutron sources in a wide variety of scenarios.