Cynthia Salwen

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.

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.

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.

A new pad-based neutron detector for stereo coded aperture thermal neutron imaging

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.

CODED APERTURE THERMAL NEUTRON CAMERA WITH ASIC-BASED PAD READOUT

A new generation of coded aperture neutron imagers is being developed at Brookhaven National Laboratory. The detector of the camera is a position sensitive thermal neutron chamber. The new device is a 3He-filled ionization chamber, which uses only anode and cathode planes. The anode is composed of an array of individual pads. The charge is collected on each of the individual 5 × 5 mm2 anode pads, (48 × 48 in total corresponding to a 24 × 24 cm2 sensitive area) and read out by application specific integrated circuits. The new design has several advantages for the field of coded aperture applications compared to the previous generation of wire-grid based neutron detectors. Among these are the rugged design, lighter weight and use of non-flammable stopping gas. The pad-based readout is event by event, thus capable of high count rates, and can 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 charge sharing between adjacent pads. In this paper, we will report on the development and performance of the new, prototype pad-based neutron camera and present the first pad-based coded aperture images of thermalized neutron sources.

A new pad-based neutron detector for stereo coded-aperture thermal neutron imaging

A new generation of coded aperture neutron imagers is being developed at Brookhaven National Laboratory. The detector of the camera is a position-sensitive thermal neutron chamber. The new device is a 3He-filled ionization chamber, which uses only anode and cathode planes. The anode is composed of an array of individual pads. The charge is collected on each of the individual 5×5 mm2 anode pads, (48×48 in total, corresponding to 24×24 cm2 sensitive area) and read out by application specific integrated circuits. The new design has several advantages for the coded-aperture 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 is event by event, thus capable for high count rates, and also 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 charge sharing between adjacent pads. In this paper we report on the development and performance of the new, prototype pad-based neutron camera, and present the first stereoscopic coded aperture images of thermalized neutron sources.