Mark D Gerling

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}

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.

Source Using a Neutron Scatter Camera

We describe a very compact (0.9 m high, 0.4 m diameter, 40 kg) battery operable neutron scatter camera designed for field deployment. Unlike most other systems, the configuration of the sixteen liquid-scintillator detection cells are arranged to provide omnidirectional (4π) imaging with sensitivity comparable to a conventional two-plane system. Although designed primarily to operate as a neutron scatter camera for localizing energetic neutron sources, it also functions as a Compton camera for localizing gamma sources. In addition to describing the radionuclide source localization capabilities of this system, we demonstrate how it provides neutron spectra that can distinguish plutonium metal from plutonium oxide sources, in addition to the easier task of distinguishing AmBe from fission sources.

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

The authors have investigated the use of fast neutrons—primarily the fast neutron energy spectrum—as a signature for uranium hexafluoride (UF6) nuclear accountancy measurements. Detailed modeling of UF6 storage cylinders and a proposed neutron detection system indicates that the measured neutron energy spectrum is indeed a function of uranium enrichment. Field measurements at the Department of Energy’s Paducah Gaseous Diffusion Plant with a detection system similar to the modeled system provided an opportunity to collect signatures from several storage cylinders containing UF6 with a range of enrichments. Subsequent analysis lends credibility to the modeling results, indicating that enrichment over the range measured (0.72% to 4.95% uranium-235) can be extracted from the measured neutron energy spectrum. These results were scaled to estimate the tradeoff in measurement system size and counting time to achieve a relative enrichment measurement uncertainty of 5%.

A compact neutron scatter camera for field deployment

We present advances with a 32 element scalable, segmented dual mode imager. Scaling up the number of cells results in a 1.4 increase in efficiency over a system we deployed last year. Variable plane separation has been incorporated which further improves the efficiency of the detector. By using 20 cm diameter cells we demonstrate that we could increase sensitivity by a factor of 6. We further demonstrate gamma ray imaging in from Compton scattering. This feature allows for powerful dual mode imaging. Selected results are presented that demonstrate these new capabilities.

Using fast neutron signatures for improved UF6 cylinder enrichment measurements.

The neutron scatter camera was originally developed for a range of SNM detection applications. We are now exploring the feasibility of applications in treaty verification and warhead monitoring using experimentation, maximum likelihood estimation method (MLEM), detector optimization, and MCNP-PoliMi simulations.

Results with a 32-element dual mode imager

Our previous conference report on this instrument emphasized its use for fast-neutron imaging spectroscopy. We describe here its additional measurement capabilities, namely active interrogation, time-correlated pulse-height multiplication measurements, and gamma imaging.

Applying the neutron scatter camera to treaty verification and warhead monitoring

Our research group has been developing a fast neutron imaging platform to enhance the capabilities of emergency responders in the localization and characterization of special nuclear material. This mobile imager of neutrons for emergency responders (MINER) is a compact neutron scatter camera optimized to provide omni-directional (4-Pi) imaging with only a ~twofold decrease in sensitivity compared to our much larger neutron scatter cameras. The system performance is tuned for fission energy neutron imaging and spectroscopy, and it also can function as a Compton camera for gamma imaging. Results will be presented relating to detector response as well as several measurement campaigns at external facilities.

Additional capabilities of a compact neutron scatter camera: Active interrogation, time-correlated pulse-height multiplication measurements, and gamma imaging

We are investigating two promising new families of materials derived from proven, low-cost, well-understood, versatile scintillator hosts CsI and NaI. These are modified by incorporating Li ions to achieve the desired spectroscopic properties, producing high quality, combined n/γ sensors. Specifically we report on the synthesis and characterization of Li3Cs2I5 (LCI) and LixNa1-xI (LNI), both of which demonstrate pulse shape discrimination (PSD) and pulse height discrimination (PHD) for effective suppression of gamma background from neutron signals. In the case of LCI, the primary decay time for thermal neutron interactions is faster than for gamma interactions, and is on the order of 250 ns for neutrons and 500 ns for gamma rays. LNI is opposite to LCI in this respect, which shows slower, 210 ns, decay for neutron interactions and relatively faster, 180 ns, decay for gammas. The measured light yield for LCI is ~40,000 to 55,000 photons/neutron, which corresponds to an electron equivalent energy of 2.1 to 2.8 MeV. Whereas LNI demonstrates a much brighter yield of over 100,000 photons/neutron and electron equivalent energy per neutron interaction of over 4.5 MeV, very close to the 6Li(n,α) Q value of 4.7 MeV. Relatively narrow emission bands with a peak at 450 nm for LCI:Eu and at 420 nm for LNI:Tl make these sensors well matched to the quantum efficiency of conventional photodetectors such as PMTs. Our data show that significant red shift in emission can be achieved by doping LCI with Tl and LNI with Eu, making these materials well suited for use with such sensors as solid state photomultipliers (SSPMs). In addition to their excellent scintillation properties, the use of widely available, proven host materials, and a possible vapor deposition method for their synthesis, are promising features of this development. This combination allows mass production of large-area, high-performance neutron sensors in a uniquely time efficient manner.

MINER - a mobile imager of neutrons for emergency responders

Various active interrogation techniques are under investigation with the goal of increasing sensitivity to the presence of special nuclear material (SNM) in an unknown target. The Intense Pulsed Active Detection approach uses a single very short but intense pulse of high-energy photons to interrogate a target. Interrogation with short pulses opens the possibility of accessing the strong prompt neutron signal from induced fission in the target. In the current experimental campaign, a bremsstrahlung spectrum of hard x-rays - endpoint energies between 8 MeV and 16 MeV - is provided by the HERMES-III pulsed power facility at Sandia National Laboratories. We fielded a combination of neutron-sensitive organic scintillator detectors at the HERMES campaign, including liquid, plastic, and crystalline scintillators at ~45 m from the target. We have investigated the sensitivity of these organic scintillators to the neutron and gamma signatures at three timescales of interest in the active interrogation context: the prompt signal from photofission at O(μs), the “delayed prompt” signal from thermalized neutrons inducing fission in fissile material at O(ms), and the delayed gammas and beta-delayed neutrons from fission daughters at O(s).