Nicholas Mascarenhas

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).

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}

This report summarizes the results of a one-year, feasibility-scale LDRD project that was conducted with the goal of developing new plastic scintillators capable of pulse shape discrimination (PSD) for neutron detection. Copolymers composed of matrix materials such as poly(methyl methacrylate) (PMMA) and blocks containing trans-stilbene (tSB) as the scintillator component were prepared and tested for gamma/neutron response. Block copolymer synthesis utilizing tSBMA proved unsuccessful so random copolymers containing up to 30% tSB were prepared. These copolymers were found to function as scintillators upon exposure to gamma radiation; however, they did not exhibit PSD when exposed to a neutron source. This project, while falling short of its ultimate goal, demonstrated the possible utility of single-component, undoped plastics as scintillators for applications that do not require PSD.

Source Using a Neutron Scatter Camera

When searching for SNM simply designing a better detector to optimize the signal S from the source is not enough. It is important to know the background B to maximize S/N, where N is the noise in B. Cosmic rays are a dominant source of neutron background. It is therefore important to know their flux, angular and energy distribution. Over the last 50 years work has been done to study cosmic ray neutrons and their variation. The full hemispherical neutron flux is usually quoted at a certain altitude (e.g. Altitude = 0 meters above sea level, pressure = 1033 g/cm2) and geomagnetic rigidity (e.g. GMR = 1.2GV). Neutron fluxes at other locations are scaled from the sea level data using a well determined prescription. However, there is a lack in knowledge of the angular dependence of the neutron flux at sea level. The angular dependence is important for two reasons; first many detectors have an efficiency that changes with the direction of the incident neutron. Second none of the measurements to date have determined how the flux changes with angle, their data must be modeled to estimate the full hemispherical flux. In this paper we present the cosmic neutron background flux measured by a neutron scatter camera in the energy range 0.2-10MeV. Our measurements are in agreement with the best fit to past data. We present for the 1st time the neutron zenith angle dependence at fission energies which is observed to be a function of the form cos2.7thetas.

Final LDRD Report: Advanced Plastic Scintillators for Neutron Detection

Active neutron interrogation is an effective technique used to locate fissionable material. This paper discusses a portable system that utilizes a AmBe neutron source. The AmBe source consists of an americium alpha source and a beryllium target that can be switched into alignment to turn the source on and out of alignment to turn the source off. This offers a battery operated backpack portable source. The detector system that has been fabricated for use with this source is a fifteen tube 3He neutron detector. The results of initial experiments with the detector and MCNP calculations are discussed.

A measurement of the flux, angular distribution and energy spectra of cosmic ray induced neutrons at fission energies

Several improvements were made to the NSC over the course of this project. The liquidscintillator-cell configuration was changed from nine cells in each plane to 16 cells in each plane (2” deep, 5” diameter cells in the front plane, 5” deep, 5” diameter cells in the rear plane). The cells were mounted in a new shock-proof frame that also provided motorized adjustment of the spacing between the two planes. To simplify transporting the system, the liquid scintillator material itself was changed from EJ-301 to the less-hazardous EJ-309 (higher flashpoint, more benign chemical content). Dual-mode imaging capabilities were implemented in software, enabling simultaneous Compton-camera gamma imaging in addition to the neutron imaging. Data acquisition was converted to an all-digital system using a newly available VME digitizer system, leading to both enhanced data analysis capabilities, and to a much more portable configuration (with a large separate electronics rack replaced by a single VME crate attached to the scatter-camera frame, as shown in Fig. 1). Maximum-Likelihood Expectation-Maximization (MLEM) methods were added to our image reconstruction toolkit.

A portable active interrogation system using a switchable AmBe neutron source

We have successfully ruggedized and deployed a field portable fast-neutron imager. Detection efficiency has been increased by > 5x more over the one fielded last year. Imaging has been demonstrated though 30 cm thick concrete and at a stand-off distance of 75 m in air. We have also imaged a pulsed D-D neutron generator and recorded its energy spectrum. These demonstrations show that our neutron scatter camera could be a useful tool not only in the lab but as a field detector for hidden SNM.

A High-Sensitivity Fast Neutron Imager

A Monte Carlo model has been developed for the Sandia Neutron Scatter Camera. We have used this model to optimize cell geometry and detector efficiency. The simulations have been used to improve and understand detector response and calibrations.