A new coded aperture thermal neutron imager system has been developed at Brookhaven National Laboratory. The cameras use a new type of position-sensitive 3He-filled ionization chamber, in which an anode plane is composed of an array of pads with independent acquisition channels. The charge is collected on each of the individual 5x5 mm2 anode pads, (48x48 in total, corresponding to 24x24 cm2 sensitive area) and read out by application specific integrated circuits (ASICs). The new design has several advantages for coded-aperture imaging applications in the field, compared to the previous generation of wire-grid based neutron detectors. Among these are its rugged design, lighter weight and use of non-flammable stopping gas. The pad-based readout occurs in parallel circuits, making it capable of high count rates, and also suitable to perform data analysis and imaging on an event-by-event basis. The spatial resolution of the detector can be better than the pixel size by using a charge sharing algorithm. In this paper we will report on the development and performance of the new pad-based neutron camera, describe a charge sharing algorithm to achieve sub-pixel spatial resolution and present the first stereoscopic coded aperture images of thermalized neutron sources using the new coded aperture thermal neutron imager system.
We are developing a high data gamma/neutron spectrometer suitable for active interrogation of special nuclear materials
(SNM) activated by a single burst from an intense source. We have tested the system at Naval Research Laboratory's
(NRL) Mercury pulsed-power facility at distances approaching 10 meters from a depleted uranium (DU) target. We
have found that the gamma-ray field in the target room "disappears" 10 milliseconds after the x-ray flash, and that
gamma ray spectroscopy will then be dominated by isomeric states/beta decay of fission products. When a polyethylene
moderator is added to the DU target, a time-dependent signature of the DU is produced by thermalized neutrons. We
observe this signature in gamma-spectra measured consecutively in the 0.1-1.0 ms time range. These spectra contain the
Compton edge line (2.2 MeV) from capture in hydrogen, and a continuous high energy gamma-spectrum from capture or
fission in minority constituents of the DU.
We have investigated neutron spectrometry using fast gamma-ray detectors (BaF2) in coincidence with a gamma/neutron
detector (plastic scintillator). Neutron spectra of spontaneous fission sources are determined by time-of-flight between
correlated gamma and neutron detections using the associated particle (AP) technique. When the source is within a ~1
meter zone of detector separation, the correlated neutron detection efficiency is high because of the multiplicity of
gamma-rays (10 gammas/fission in 252Cf) and neutrons (3.6 neutrons/fission). Cosmic-ray produced neutron detection
efficiency is quite low in an AP measurement using a <50ns coincidence window because time-of-flight of most events
is long from its creation within the 120 meters e-folding neutron range of air. We found that the AP signal to
background ratio was dominated by uncorrelated coincidences and propose a triple coincidence system (1 neutron and 2
gamma-ray detectors) to improve performance. The gamma/gamma-ray coincidence time distribution is related to the
target's production history where fast neutron multiplication may be a dominant physical process. MCNPX calculations
suggest that the gamma-ray time history of Depleted Uranium (DU) and Highly Enriched Uranium (HEU) provide
separable signatures because fast neutron multiplication is much higher in HEU.
We have constructed a fast-neutron double-scatter spectrometer that efficiently measures the neutron spectrum and direction of a spontaneous fission source. The device consists of two planes of organic scintillators, each having an area of 125 cm2, efficiently coupled to photomultipliers. The four scintillators in the front plane are 2 cm thick, giving almost 25% probability of detecting an incident fission-spectrum neutron at 2 MeV by proton recoil and subsequent ionization. The back plane contains four 5-cm-thick scintillators which give a 40% probability of detecting a scattered fast neutron. A recordable double-scatter event occurs when a neutron is detected in both a front plane detector and a back plane detector within an interval of 500 nanoseconds. Each double-scatter event is analyzed to determine the energy deposited in the front plane, the time of flight between detectors, and the energy deposited in the back plane. The scattering angle of each incident neutron is calculated from the ratio of the energy deposited in the first detector to the kinetic energy of the scattered neutron.
We have measured the neutron spectra of cosmic-rays and a spontaneous fission emitting source (Cf-252) using a neutron double scatter spectrometer. The energy range of measurements was 0.1-10 MeV where the spectrometer efficiency is determined to be up to 8.7%, depending on the separation between detection planes. Our cosmic-ray neutron spectrum measurement is in good agreement with the sea-level data reported by Goldhagen and his co-workers. In the energy range 0.1-1.0 MeV, the cosmic-ray and Cf-252 spectra are different and separable. This difference is expected from the applicable models that describe the phenomena, 'equilibrium slowing down' (cosmic-rays) and 'Maxwellian kinetic temperature' emission (spontaneous fission). We show that >80% of Cf-252 neutrons and <25% of cosmic-ray related neutrons are emitted in this energy range of measurement, and conclude that neutron spectroscopy provides effective ways to distinguish a fission source from the cosmic-ray background.
Fast neutrons can be detected with relatively high efficiency, >15%, using two planes of hydrogenous scintillator detectors where a scatter in the first plane creates a start pulse and scatter in the second plane is separated by time-offlight. Indeed, the neutron spectrum of the source can be determined as the sum of energy deposited by pulse height in the first added to the energy of the second found by time-of-flight to the second detector. Gamma rays can also create a double scatter by Compton interaction in the first with detection in the second, but these events occur in a single time
window because the scattered photons all travel at the speed of light. Thus, gamma ray events can be separated from neutrons by the time-of-flight differences. We have studied this detection system with a Cf-252 source using Bicron 501A organic scintillators and report on the ability to efficiently detect fast neutrons with high neutron/gamma detection ratios.
We have further studied cosmic-ray neutron background detection response that is the dominant background in long range detection. We have found that most of the neutrons are excluded from the time-of-flight window because they are either too high in energy, >10MeV, or too low, < 10 keV. Moreover, if the detection planes are position-sensitive, the angular direction of the source can be determined by the ratio of the energy of scattered protons in the first detector relative to the position and energy of the scattered neutron detected in the second. This ability to locate the source in
theta is useful, but more importantly increases the signal to noise relative to cosmic-ray produced neutrons that are relatively isotropic. This technique may be used in large arrays to detect neutrons at ranges up to 0.5 kilometer.
Thermal neutrons passing through air have scattering lengths of about 20 meters. At further distances, the majority of neutrons emanating from a moderated source will scatter multiple times in the air before being detected, and will not retain information about the location of the source, except that their density will fall off somewhat faster than 1/r2. However, there remains a significant fraction of the neutrons that will travel 20 meters or more without scattering and can be used to create an image of the source. A few years ago, a proof-of-principle "camera" was demonstrated that could produce images of a scene containing sources of thermalized neutrons and could locate a source comparable in strength with an improvised nuclear device at ranges over 60 meters. The instrument makes use of a coded aperture with a uniformly redundant array of openings, analogous to those used in x-ray and gamma cameras. The detector is a position-sensitive He-3 proportional chamber, originally used for neutron diffraction. A neutron camera has many features in common with those designed for non-focusable photons, as well as some important differences. Potential applications include detecting nuclear smuggling, locating non-metallic land mines, assaying nuclear waste, and surveying for health physics purposes.
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