Leon Forman

Calibration and testing of a large-area fast-neutron directional detector

We have developed a new directional fast-neutron detector based on double proton recoil in two separated planes of plastic scintillators with continuous position-sensitive readout in one of two dimensions. This method allows the energy spectrum of the neutrons to be measured by a combination of peak amplitude in the first plane and time of flight to the second plane. The planes are made up of 100-cm long, 10-cm high paddles with photomultipliers at both ends, so that the location of an event along the paddle can be estimated from the time delay between the optical pulses detected at the two ends. The direction of the scattered neutron can be estimated from the locations of two time-correlated events in the two planes, and the energy lost in the first scattering event can be estimated from the pulse amplitude in the first plane. The direction of the incident neutron can then be determined to lie on a cone whose angle is determined by the kinematic equations. The superposition of many such cones generates an image that indicates the presence of a localized source. Setting upper and lower limits on time of flight and energy allows discrimination between gamma rays, muons and neutrons. Monte Carlo simulations were performed to determine factors affecting the expected angular resolution and efficiency. These models show that this design has a lower energy limit for useful directional events at about 250 keV, because lower energy neutrons are likely to scatter more than once in the first plane.

An 8-element neutron double-scatter directional detector

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.

Demonstration of a directional fast neutron detector

A detector array consisting of 8 photomultiplier tubes equipped with liquid organic scintillators and arranged in two planes has been demonstrated to be capable of detecting both neutrons and gamma rays and distinguishing them by time-of-flight. Moreover, the neutron double-scatter events can be analyzed to determine both the energy spectrum and the probable direction to the source. This information can help to improve the capability to distinguish a localized man-made isotopic source of neutrons from natural background events generated ubiquitously by cosmic rays. The system was constructed using mostly commercial off-the-shelf components, and the analysis is performed with some customized software algorithms

Design of a Large-Area Fast Neutron Directional Detector

A large-area fast-neutron double-scatter directional detector and spectrometer is being constructed using 1-meter-long plastic scintillator paddles with photomultiplier tubes at both ends. The scintillators detect fast neutrons by proton recoil and also gamma rays by Compton scattering. The paddles are arranged in two parallel planes so that neutrons can be distinguished from muons and gamma rays by time of flight between the planes. The signal pulses are digitized with a time resolution of one gigasample per second. The location of an event along each paddle can be determined from the relative amplitudes or timing of the signals at the ends. The angle of deflection of a neutron in the first plane can be estimated from the energy deposited by the recoil proton, combined with the scattered neutron time-of-flight energy. Each scattering angle can be back-projected as a cone, and many intersecting cones define the incident neutron direction from a distant point source. Moreover, the total energy of each neutron can be obtained, allowing some regions of a fission source spectrum to be distinguished from background generated by cosmic rays. Monte Carlo calculations have been compared with measurements.

Radiation imaging of dry-storage casks for nuclear fuel

We report the results of a measurement campaign conducted on six dry-storage, spent-nuclear-fuel storage casks at the Idaho National Laboratory. A gamma-ray imager, a thermal-neutron imager and a Ge-spectrometer were used to collect data. The campaign was conducted to examine the feasibility of using cask radiation signatures as unique identifiers for individual casks as part of a safeguards regime. The results clearly show different morphologies for the various cask types although the signatures are deemed insufficient to uniquely identify individual casks of the same type. Based on results with the Ge-spectrometer and differences between thermal neutron images and neutron-dose meters, this result is attributed to the limitations of the extant imagers used, rather than of the basic concept.

Thermal neutron imaging in an active interrogation environment

We have developed a thermal‐neutron coded‐aperture imager that reveals the locations of hydrogenous materials from which thermal neutrons are being emitted. This imaging detector can be combined with an accelerator to form an active interrogation system in which fast neutrons are produced in a heavy metal target by means of excitation by high energy photons. The photo‐induced neutrons can be either prompt or delayed, depending on whether neutron‐emitting fission products are generated. Provided that there are hydrogenous materials close to the target, some of the photo‐induced neutrons slow down and emerge from the surface at thermal energies. These neutrons can be used to create images that show the location and shape of the thermalizing materials. Analysis of the temporal response of the neutron flux provides information about delayed neutrons from induced fission if there are fissionable materials in the target. The combination of imaging and time‐of‐flight discrimination helps to improve the signal‐to...

Directional stand-off detection of fast neutrons and gammas using angular scattering distributions

We have investigated the response of a Double Scatter Neutron Spectrometer (DSNS) for sources at long distances (>200 meters). We find that an alternative method for analyzing double scatter data avoids some uncertainties introduced by amplitude measurements in plastic scintillators. Time of flight is used to discriminate between gamma and neutron events, and the kinematic distributions of scattering angles are assumed to apply. Non-relativistic neutrons are most likely to scatter at 45°, while gammas with energies greater than 2 MeV are most likely to be forward scattered. The distribution of scattering angles of fission neutrons arriving from a distant point source generates a 45° cone, which can be back-projected to give the source direction. At the same time, the distribution of Compton-scattered gammas has a maximum in the forward direction, and can be made narrower by selecting events that deposit minimal energy in the first scattering event. We have further determined that the shape of spontaneous fission neutron spectra at ranges >110 m is still significantly different from the cosmic ray background.

Directional detection of fission-spectrum neutrons

Conventional neutron detectors consisting of 3 He tubes surrounded by a plastic moderator can be quite efficient in detecting fission spectrum neutrons, but do not indicate the direction of the incident radiation. We have developed a new directional detector based on double proton recoil in two separated planes of plastic scintillators. This method allows the spectrum of the neutrons to be measured by a combination of peak amplitude in the first plane and time of flight to the second plane. It also allows the determination of the angle of scattering in the first plane. If the planes are position-sensitive detectors, then the direction of the scattered neutron is known, and the direction of the incident neutron can be determined to lie on a cone of a fixed angle. The superposition of many such cones generates an image that indicates the presence of a localized source. Typical background neutron fluences from the interaction of cosmic rays with the atmosphere are low and fairly uniformly distributed in angle. Directional detection helps to locate a manmade source in the presence of natural background. Monte Carlo simulations are compared with experimental results.

Stand-off detection of special nuclear materials using neutron imaging methods

Neutrons can travel considerable distances through the air, and can be used for stand-off detection of special nuclear materials. Plutonium emits neutrons by spontaneous fission, while uranium can be induced to fission by active interrogation with either energetic photons or neutrons. Traditional neutron detectors consisting of 3He tubes embedded in polyethylene moderator do not record the directions of incident neutrons. Their efficiency is usually less than 10% for fission spectrum neutrons. We have developed two neutron imaging methods, one for fast neutrons and one for thermal neutrons. The fast neutron directional detector is based on double proton recoil in two layers of plastic scintillators consisting of arrays of paddles with photomultipliers at both ends. This arrangement provides relatively smooth spatial uniformity and allows neutrons to be distinguished from background gammas and muons using time of flight. The thermal neutron imager is a coded aperture camera based on a 3He wire chamber. Neutrons that are thermalized by materials close to the source have a mean free path in air of about 20 meters, and can be imaged at distances up to 60 meters. In this case, no moderation takes place at the detector, and the quantum efficiency of the 3He wire chamber is about 60%. Background neutrons generated by cosmic rays arrive from all directions at a low rate, whereas a manmade source is likely to produce a bright spot in an image. Various methods of image enhancement can be applied, such as auto filtering, but the practical limitation stems from the requirement that the real source must stand out against artifacts in the image.

Active nuclear material detection and imaging

An experimental evaluation has been conducted to assess the operational performance of a coded-aperture, thermal neutron imaging system and its detection and imaging capability for shielded nuclear material in pulsed photonuclear environments. This evaluation used an imaging system developed by Brookhaven National Laboratory. The active photonuclear environment was produced by an operationally-flexible, Idaho National Laboratory (INL) pulsed electron accelerator. The neutron environments were monitored using INL photonuclear neutron detectors. Results include experimental images, operational imaging system assessments and recommendations that would enhance nuclear material detection and imaging performance.