Peter Marleau

Results With the Neutron Scatter Camera

We describe the design, calibration, and measurements made with the neutron scatter camera. Neutron scatter camera design allows for the determination of the direction and energy of incident neutrons by measuring the position, recoil energy, and time-of-flight (TOF) between elastic scatters in two liquid scintillator cells. The detector response and sensitive energy range (0.5-10 MeV) has been determined by detailed calibrations using a 252Cf neutron source over its field of view (FOV). We present results from several recent deployments. In a laboratory study we detected a 252Cf neutron source at a stand off distance of 30 m. A hidden neutron source was detected inside a large ocean tanker. We measured the integral flux density, differential energy distribution and angular distribution of cosmic neutron background in the fission energy range 0.5-10 MeV at Alameda, CA (sea level), Livermore, CA (174 m), Albuquerque, NM (1615 m) and Fenton Hill, NM (2630 m). The neutron backgrounds are relatively low, and non-isotropic. The camera has been ruggedized, deployed to various locations and has performed various measurements successfully. Our results show fast neutron imaging could be a useful tool for the detection of special nuclear material (SNM).

Results with the neutron scatter camera

We present results from recent deployments with the neutron scatter camera. We successfully detected and pinpointed a hidden 252Cf neutron source in a large ocean tanker at Alameda, CA. In a lab study we detected a 252Cf neutron source at a stand off distance of about 100 ft. We measured the integral flux, differential flux and angular distribution of cosmic neutron background in the fission energy range 0.5–10MeV at Alameda, CA (sea level), Livermore, CA (570 ft), Albuquerque, NM (5300 ft) and Fenton Hill, NM (8630 ft). The neutron backgrounds are relatively low, uniform and well understood. We recently increased the camera effective area 3x. The camera has been successfully ruggedized, deployed to various locations and has performed various measurements. Our results are encouraging and suggest fast neutron imaging could be a useful tool for the detection of special nuclear material (SNM).

Development of a Neutron Scatter Camera for Fission Neutrons

Special nuclear material (SNM) emits high energy radiation during active and passive interrogation. This radiation can be imaged thus allowing visualization of shielded and/or smuggled SNM. Although gamma-ray imaging is appropriate for many cases, neutrons are much more penetrating through hi-Z materials, and are thus preferred in certain scenarios (e.g. weapons grade Pu or HEU smuggled inside a lead pig several inches thick). Techniques for thermal neutron imaging have already been developed, but these approaches only image the moderating material, not the true SNM source. Traditional neutron detectors such as He3 tubes and scintillators simply count neutrons. We are developing an instrument that will directly image the fast fission neutrons from an SNM source using a neutron scatter camera. This technique has been shown to be 10 times more sensitive for solar neutrons over traditional neutron counting techniques. In addition to being a 4pi neutron imager, this instrument will also be an excellent neutron spectrometer, and will be able to differentiate between different types of neutron sources (e.g. fission, gamma-n, cosmic ray, and dd or dt fusion). Our instrument will be able to pinpoint the source location. We will present results from a prototype detector and discuss key parameters that determine detector performance.

Advances in imaging fission neutrons with a neutron scatter camera

Special nuclear material (SNM) emits high energy radiation during active and passive interrogation. This radiation can be imaged thus allowing visualization of shielded and/or smuggled SNM. Lower backgrounds and higher penetration through hi-Z materials make neutrons the preferred detectable in many scenarios. We have developed a neutron scatter camera that directly images fast fission neutrons from SNM sources while simultaneously measuring energy spectra. We have made many significant advances in the design and implementation of such instruments leading to an over 30 fold improvement in sensitivity. We will present results from our detector including analysis techniques that we have developed for neutron imaging and particle discrimination techniques. We will discuss camera calibration and performance under realistic threat detection scenarios, and future prospects in this field.

Measurement of the Fast Neutron Energy Spectrum of an

We have measured the neutron energy spectrum of an 241Am-Be(α,n) source between 1.5 MeV and 9 MeV using a neutron scatter camera. The apparatus consists of two segmented planes each with 16 liquid scintillator cells (Eljen EJ-309), for a total of 32 elements; the neutron energy spectrum is measured using double elastic scatter events. After unfolding resolution effects using a maximum likelihood technique, the measurement is compared to reference Am-Be spectra. Further, we discuss the ability of the neutron scatter camera to distinguish between an Am-Be source and a spontaneous fission source.

^{241}{\rm Am\!-\!Be}

Because of their penetrating power, energetic neutrons and gamma rays (∼1 MeV) offer the best possibility of detecting highly shielded or distant special nuclear material (SNM). Of these, fast neutrons offer the greatest advantage due to their very low and well understood natural background. We are investigating a new approach to fast-neutron imaging — a coded aperture neutron imaging system (CANIS). Coded aperture neutron imaging should offer a highly efficient solution for improved detection speed, range, and sensitivity. We have demonstrated fast neutron and gamma ray imaging with several different configurations of coded masks patterns and detectors including an “active” mask that is composed of neutron detectors. Here we describe our prototype detector and present some initial results from laboratory tests and demonstrations.

Source Using a Neutron Scatter Camera

Because of their penetrating power, energetic neutrons and gamma rays (>~1 MeV) offer the best possibility of detecting highly shielded or distant special nuclear material (SNM). Of these, fast neutrons offer the greatest advantage due to their very low and well understood natural background. We are investigating a wholly new approach to fast-neutron imaging-an active coded-aperture system that uses a coded mask composed of neutron detectors. The only previously demonstrated method for long-range fast neutron imaging is double-scatter imaging. Active coded-aperture neutron imaging should offer a highly efficient alternative for improved detection speed, range, and sensitivity. We will describe our detector including design considerations and present initial results from a lab prototype.

Results from the coded aperture neutron imaging system

Passive detection of special nuclear material (SNM) at long range or under heavy shielding can only be directly achieved by observing the penetrating neutral particles that it emits: gamma rays and neutrons in the MeV energy range. The ultimate SNM standoff detector system would have sensitivity to both gamma and neutron radiation, a large area and high efficiency to capture as many signal particles as possible, and good discrimination against background particles via directional and energy information. We are exploring the use of time-modulated collimators that may lead to practical gamma-neutron imaging detector systems that are highly efficient with the potential to exhibit simultaneously high angular and energy resolution. We will present results from a large standoff SNM detection demonstration using a prototype high sensitivity time encoded modulation imager.

Active coded aperture neutron imaging

A Bayesian approach is proposed for pulse shape discrimination of photons and neutrons in liquid organic scinitillators. Instead of drawing a decision boundary, each pulse is assigned a photon or neutron confidence probability. In addition, this allows for photon and neutron classification on an event-by-event basis. The sum of those confidence probabilities is used to estimate the number of photon and neutron instances in the data. An iterative scheme, similar to an expectation-maximization algorithm for Gaussian mixtures, is used to infer the ratio of photons-to-neutrons in each measurement. Therefore, the probability space adapts to data with varying photon-to-neutron ratios. A time-correlated measurement of Am–Be and separate measurements of 137Cs, 60Co and 232Th photon sources were used to construct libraries of neutrons and photons. These libraries were then used to produce synthetic data sets with varying ratios of photons-to-neutrons. Probability weighted method that we implemented was found to maintain neutron acceptance rate of up to 90% up to photon-to-neutron ratio of 2000, and performed 9% better than the decision boundary approach. Furthermore, the iterative approach appropriately changed the probability space with an increasing number of photons which kept the neutron population estimate from unrealistically increasing.

Time encoded fast neutron/gamma imager for large standoff SNM detection

Organic scintillators are widely used for fast neutron detection and spectroscopy. Several effects complicate the interpretation of results from detectors based upon these materials. First, fast neutrons will often leave a detector before depositing all of their energy within it. Second, fast neutrons will typically scatter several times within a detector, and there is a non-proportional relationship between the energy of, and the scintillation light produced by, each individual scatter; therefore, there is not a deterministic relationship between the scintillation light observed and the neutron energy deposited. Here we demonstrate a hardware technique for reducing both of these effects. Use of a segmented detector allows for the event-by-event correction of the light yield non-proportionality and for the preferential selection of events with near-complete energy deposition, since these will typically have high segment multiplicities.