We compare the pulse-shape-discrimination performance of a large Stilbene detector, manufactured at the Lawrence Livermore National Laboratory, to a similarly sized liquid scintillator, made by Eljen Technologies, Inc. (type EJ309), under various high-count-rate scenarios, such as can be expected during Active Interrogation of objects using an electric neutron generator for detecting the presence of Special Nuclear Materials and contraband. Pulse Shape Discrimination (PSD) is performed with an extension of previously described wavelet and event fitting algorithms, which are also used to reject pileup events and to correct the energy calibration for event signals clipped during digitization. We find that strong PSD and pileup rejection cuts are required to function in this high-count-rate environment. Whereas the Stilbene detector has intrinsically better pulse-shape discrimination performance, the pulses of events from the liquid scintillator are somewhat narrower, reducing pileup. We use an event fitting cut that provides simultaneous PSD identification and pileup rejection. When applying the same cut to data taken with both detectors, we find that Stilbene has higher event use efficiency by about a factor of two and retains a better gamma rejection ratio compared to EJ309.

The detection of special nuclear materials (SNM) in commercial cargoes is a major objective in the field of nuclear security. In this work we investigate the use of two-neutron time-correlations from photo-fission using the Prompt Neutrons from Photofission (PNPF) detectors in Passport Systems Inc.’s (PSI) Shielded Nuclear Alarm Resolution (SNAR) platform for the purpose of detecting ∼5 kg quantities of fissionable materials in seconds. The goal of this effort was to extend the secondary scan mode of this system to differentiate fissile materials, such as highly enriched uranium, from fissionable materials, such as low enriched and depleted uranium (LEU and DU). Experiments were performed using a variety of material samples, and data were analyzed using the variance-over-mean technique referred to as Y2F or Feynman-α. Results were compared to computational models to improve our ability to predict system performance for distinguishing fissile materials. Simulations were then combined with empirical formulas to generate receiver operating characteristics (ROC) curves for a variety of shielding scenarios. We show that a 10 second screening with a 200 μA 9 MeV X-ray beam is sufficient to differentiate kilogram quantities of HEU from DU in various shielding scenarios in a standard cargo container.

Prompt γ-ray spectra were measured for the spontaneous fission of 240,242Pu and the neutron-induced fission of 239,241Pu with incident neutron energies ranging from thermal to about 100 keV. Measurements were made using the Detector for Advanced Neutron Capture Experiments (DANCE) array in coincidence with the detection of fission fragments using a parallel-plate avalanche counter. The unfolded prompt fission γ-ray energy spectra can be reproduced reasonably well by Monte Carlo Hauser–Feshbach statistical model for the neutron-induced fission channel but not for the spontaneous fission channel. However, this entrance-channel dependence of the prompt fission γ-ray emission can be described qualitatively by the model due to the very different fission-fragment mass distributions and a lower average fragment spin for spontaneous fission. The description of measurements and the discussion of results under the framework of a Monte Carlo Hauser–Feshbach statistical approach are presented.

A consistent set of high-precision measurements have been performed to study the energy dependence of the fission product yields of U-235, U-238, and Pu-239 using monoenergetic neutrons between 0.5 and 14.8 MeV. The results confirm the progression towards symmetric fission at higher incident neutron energy, i.e., 14.8 MeV. However, at lower energies (E-n < similar to 3 MeV) the experimental data reveal a peculiar energy dependence of some of the fission-product yield from neutron-induced fission of Pu-239: a positive slope up to about 4-5 MeV which then turns negative as the incident neutron energy increases. This latter finding at low-energy is in conflict with present theoretical predictions.

A program has been initiated to measure the energy dependence of selected high-yield fission products used in the analysis of nuclear test data. We present out initial work of neutron activation using a dual-fission chamber with quasi-monoenergetic neutrons and gamma-counting method. Quasi-monoenergetic neutrons of energies from 0.5 to 15MeV using the TUNL 10MV FM tandem to provide high-precision and self-consistent measurements of fission product yields (FPY). The final FPY results will be coupled with theoretical analysis to provide a more fundamental understanding of the fission process. To accomplish this goal, we have developed and tested a set of dual-fission ionization chambers to provide an accurate determination of the number of fissions occurring in a thick target located in the middle plane of the chamber assembly. Details of the fission chamber and its performance are presented along with neutron beam production and characterization. Also presented are studies on the background issues associated with room-return and off-energy neutron production. We show that the off-energy neutron contribution can be significant, but correctable, while room-return neutron background levels contribute less than <1% to the fission signal.

The Precision Reactor Oscillation and Spectrum Experiment, PROSPECT, is designed to make a precise measurement of the antineutrino spectrum from a highly-enriched uranium reactor and probe eV-scale sterile neutrinos by searching for neutrino oscillations over meter-long distances. PROSPECT is conceived as a 2-phase experiment utilizing segmented $^6$Li-doped liquid scintillator detectors for both efficient detection of reactor antineutrinos through the inverse beta decay reaction and excellent background discrimination. PROSPECT Phase I consists of a movable 3-ton antineutrino detector at distances of 7 - 12 m from the reactor core. It will probe the best-fit point of the $\nu_e$ disappearance experiments at 4$\sigma$ in 1 year and the favored region of the sterile neutrino parameter space at $>$3$\sigma$ in 3 years. With a second antineutrino detector at 15 - 19 m from the reactor, Phase II of PROSPECT can probe the entire allowed parameter space below 10 eV$^{2}$ at 5$\sigma$ in 3 additional years. The measurement of the reactor antineutrino spectrum and the search for short-baseline oscillations with PROSPECT will test the origin of the spectral deviations observed in recent $\theta_{13}$ experiments, search for sterile neutrinos, and conclusively address the hypothesis of sterile neutrinos as an explanation of the reactor anomaly.

Fission Product Yields (FPY) have historically been one of the most observable features of the fission process. They are known to have strong variations that are dependent on the fissioning species, the excitation energy, and the angular momentum of the compound system. However, consistent and systematic studies of the variation of these FPY with energy have proved challenging. This is caused primarily by the nature of the experiments that have traditionally relied on radiochemical procedures to isolate specific fission products. Although radiochemical procedures exist that can isolate all products, each element presents specific challenges and introduces varying degrees of systematic errors that can make inter-comparison of FPY uncertain. Although of high importance in fields such as nuclear forensics and Stockpile Stewardship, accurate information about the energy dependence of neutron induced FPY are sparse, due primarily to the lack of suitable monoenergetic neutron sources. There is a clear need for improved data, and to address this issue, a collaboration was formed between Los Alamos National Laboratory (LANL), Lawrence Livermore National Laboratory (LLNL) and the Triangle Universities Nuclear Laboratory (TUNL) to measure the energy dependence of FPY for 235U, 238U and 239Pu. The measurements have been performed at TUNL, using a 10 MV Tandem Van de Graaff accelerator to produce monoenergetic neutrons at energies between 0.6 MeV to 14.8 MeV through a variety of reactions. The measurements have utilized a dual-fission chamber, with thin (10-100 μg/cm2) reference foils of similar material to a thick (100-400 mg) activation target held in the center between the chambers. This method allows for the accurate determination of the number of fissions that occurred in the thick target without requiring knowledge of the fission cross section or neutron fluence on target. Following activation, the thick target was removed from the dual-fission chamber and gamma-ray counted using shielded HPGe detectors for a period of 1-2 months to determine the yield of various fission products. To the extent possible all irradiation and counting procedures were kept the same to minimize sources of systematic errors. FPY have been determined at incident neutron energies of 0.6, 1.4, 2.4, 3.5, 4.6, 5.5, 8.9 and 14.8 MeV.

Determining the amount, chemical form, physical shape, and multiplication of the fissile material in an unknown assembly is a challenging problem that is of great importance in a number of national security contexts. The discrimination of plutonium in metal versus oxide form in special nuclear materials is crucial and yet remains a challenge for current methods. It has previously been shown that the alpha-ratio for un-moderated plutonium oxides can be rapidly determined by comparing the fast neutron spectrum measured with a liquid scintillator with pure fission and (alpha, n) neutron spectra measured with the same liquid scintillator. In this paper we show that the chemical form can be obtained for moderated nuclear materials using fast neutron detection systems. We describe a new method to distinguish moderated plutonium metal from oxide using liquid scintillators, neutron spectroscopy and neutron correlations.

A meter-long, 23-liter EJ-309 liquid scintillator detector has been constructed to study the light collection and pulse-shape discrimination performance of elongated scintillator cells for the PROSPECT reactor antineutrino experiment. The magnitude and uniformity of light collection and neutron-gamma discrimination power in the energy range of antineutrino inverse beta decay products have been studied using gamma and spontaneous fission calibration sources deployed along the cell axis. We also study neutron-gamma discrimination and light collection abilities for differing PMT and reflector configurations. Key design features for optimizing MeV-scale response and background rejection capabilities are identified.

The primary objective of this project was to better understand the time-correlations between the muons and neutrons produced as a result of high energy primary cosmic ray particles hitting the atmosphere, and investigate whether these time correlations might be useful in connection with the detection of special nuclear materials. During the course of this project we did observe weak correlations between secondary cosmic ray muons and cosmic ray induced fast neutrons. We also observed strong correlations between tertiary neutrons produced in a Pb pile by secondary cosmic rays and minimum ionizing particles produced in association with the tertiary neutrons.

This report describes the NA-22 supported cosmic ray experimental and analysis activities carried out at LLNL since the last report, dated October 1, 2013. In particular we report on an analysis of the origin of the plastic scintillator signals resembling the signals produced by minimum ionizing particles (MIPs). Our most notable result is that when measured in coincidence with a liquid scintillator neutron signal the MIP-like signals in the plastic scintillators are mainly due to high energy tertiary neutrons.

Using dual-fission chambers each loaded with a thick (200-400-mg/cm(2)) actinide target of U-235,U-238 or Pu-239 and two thin (similar to 10-100-mu g/cm(2)) reference foils of the same actinide, the cumulative yields of fission products ranging from Sr-92 to Nd-147 have been measured at E-n = 8.9MeV. The H-2(d,n) He-3 reaction provided the quasimonoenergetic neutron beam. The experimental setup and methods used to determine the fission product yield (FPY) are described, and results for typically eight high-yield fission products are presented. Our FPYs for U-235(n, f), U-238(n, f), and Pu-239(n, f) at 8.9 MeV are compared with the existing data below 8 MeV from Glendenin et al. [Phys. Rev. C 24, 2600 (1981)], Nagy et al. [Phys. Rev. C 17, 163 (1978)], Gindler et al. [Phys. Rev. C 27, 2058 (1983)], and those of Mac Innes et al. [Nucl. Data Sheets 112, 3135 (2011)] and Laurec et al. [Nucl. Data Sheets 111, 2965 (2010)] at 14.5 and 14.7 MeV, respectively. This comparison indicates a negative slope for the energy dependence of most fission product yields obtained from 235U and 239Pu, whereas for 238U the slope issue remains unsettled.

We describe a method for simultaneously determining the {alpha}-ratio and k{sub eff} for fissile materials using fast neutrons. Our method is a generalization of the Hage-Cifarrelli method for determining k{sub eff} for fissile assemblies which utilizes the shape of the fast neutron spectrum. In this talk we illustrate the method using Monte Carlo simulations of the fast neutrons generated in PuO{sub 2} to calculate the fast neutron spectrum and Feynman correlations.

For many years at LLNL, we have been developing time-correlated neutron detection techniques and algorithms for applications such as Arms Control, Threat Detection and Nuclear Material Assay. Many of our techniques have been developed specifically for the relatively low efficiency (a few percent) inherent in man-portable systems. Historically, thermal neutron detectors (mainly 3 He ) were used, taking advantage of the high thermal neutron interaction cross-sections, but more recently we have been investigating the use of fast neutron detection with liquid scintillators, inorganic crystals, and in the near future, pulse-shape discriminating plastics that respond over 1000 times faster (nanoseconds versus tens of microseconds) than thermal neutron detectors. Fast neutron detection offers considerable advantages, since the inherent nanosecond production timescales of fission and neutron-induced fission are preserved and measured instead of being lost in the thermalization of thermal neutron detectors. We are now applying fast neutron technology to the safeguards regime in the form of high efficiency counters. Faster detector response times and sensitivity to neutron momentum show promise in measuring, differentiating, and assaying samples that have modest to very high count rates, as well as mixed neutron sources (e.g., Pu oxide or Mixed Cm and Pu ). Here we report on measured results with our existing liquid scintillator array and promote the design of a nuclear material assay system that incorporates fast neutron detection, including the surprising result that fast liquid scintillator becomes competitive and even surpasses the precision of 3 He counters measuring correlated pairs in modest (kg) samples of plutonium.

Our group at Livermore Lab specializes in nuclear materials measurements and assessments. We have built detectors that we have taken from breadboard to fieldable. We have even commercialized some detectors for rugged non-expert use, such as the Fission Meter - a thermal neutron multiplicity counter and the Detective - a mechanically-cooled germanium spectrometer. We are currently developing instrumentation and analysis algorithms that rely on fast neutron and gamma scintillator detectors. Our systems merge data from dozens of detectors at rates of a million events per second. Fast neutron and gamma arrival times and energy can be used to assay fissioning materials and estimate moderation and shielding, as well as make 3-D images. The underlying hardware and software to operate these detection systems represent a jump in complexity from the previous systems and are similar to subsystems of high energy and particle physics. We will describe the challenges to convert these scintillator detection systems from expert-user laboratory instruments to non-expert field use, including automating pulse-analysis, data-cuts, data-calibration, data-compression, and especially real-time analysis and assessment.

To resolve long-standing differences between LANL and LLNL regarding the correct fission basis for analysis of nuclear test data [M.B. Chadwick et al., Nucl. Data Sheets 111, 2891 (2010); H. Selby et al., Nucl. Data Sheets 111, 2891 (2010)], a collaboration between TUNL/LANL/LLNL has been established to perform high-precision measurements of neutron induced fission product yields. The main goal is to make a definitive statement about the energy dependence of the fission yields to an accuracy better than 2-3% between 1 and 15 MeV, where experimental data are very scarce. At TUNL, we have completed the design, fabrication and testing of three dual-fission chambers dedicated to 235U, 238U, and 239Pu. The dual-fission chambers were used to make measurements of the fission product activity relative to the total fission rate, as well as for high-precision absolute fission yield measurements. The activation method was employed, utilizing the mono-energetic neutron beams available at TUNL. Neutrons of 4.6, 9.0, and 14.5 MeV were produced via the 2H(d,n)3He reaction, and for neutrons at 14.8 MeV, the 3H(d,n)4He reaction was used. After activation, the induced γ-ray activity of the fission products was measured for two months using high-resolution HPGe detectors in a low-background environment. Results for the yield of seven fission fragments of 235U, 238U, and 239Pu and a comparison to available data at other energies are reported. For the first time results are available for neutron energies between 2 and 14 MeV.

One of the aims of the Stockpile Stewardship Program is a reduction of the uncertainties on fission data used for analyzing nuclear test data [1,2]. Fission products such as Nd-147 are convenient for determining fission yields because of their relatively high yield per fission (about 2%) and long half-life (10.98 days). A scientific program for measuring fission product yields from U-235, U-238 and Pu-239 targets as a function of bombarding neutron energy (0.1 to 15 MeV) is currently underway using monoenergetic neutron beams produced at the 10 MV Tandem Accelerator at TUNL. Dual-fission chambers are used to determine the rate of fission in targets during activation. Activated targets are counted in highly shielded HPGe detectors over a period of several weeks to identify decaying fission products. To date, data have been collected at neutron bombarding energies 4.6, 9.0, 14.5 and 14.8 MeV. Experimental methods and data reduction techniques are discussed, and some preliminary results are presented.