We have developed a Compact Fast Neutron Spectrometer (CFNS) for passive assay of special nuclear material (SNM) through the observation of fast neutrons. The CFNS consists of eight organic glass scintillators (OGS) coupled to silicon photomultipliers and a waveform digitizer, which are integrated within a human-portable box. The CFNS determines the neutron energy profile by spectrum unfolding using the Maximum-Likelihood Expectation Maximization method. The detector acquisition system was optimized to have a dynamic range of up to 10 MeV neutron energy. Bulk special nuclear material (SNM) measurements from the National Criticality Experiments Research Center were analyzed for SNM validation/examination. The results show that the CFNS can be used to distinguish between fission and (α, n) neutron emitters, regardless of intervening material type (Cu and polyethylene) and thickness, by taking the ratio of neutron counts at different regions in the unfolded energy spectrum. Additionally, by fitting an exponential curve to the unfolded energy spectrum of PuO2 and Pu neutron emitters, the CFNS showed the ability of distinguishing between pure Pu oxide, pure Pu metal and mixed oxide-metal configurations.

Distributed and networked mobile sensor platforms using unmanned aerial and/or ground vehicles to survey areas of interest offer a safer and more efficient method for radiological contamination mapping; however, most applications rely on uniformly sweeping of the area in a raster-type motion without utilizing the information available in a dynamic sense. We have developed a fully autonomous optimal motion planning procedure for networks with two or more mobile sensors. The procedure utilizes well-established concepts of Gaussian processes in combination with control laws based on centroidal Voronoi tessellations to achieve optimal next-iteration sensor movements. A new method of informing optimal motion planning is proposed, whereby the absolute difference between the prior and current full-map prediction, referred to as the prediction-difference map, is used as the spatial density function within each Voronoi cell, providing immediate and iterative feedback for dynamic use of available information. The Gaussian process regression model used to estimate the contamination in unvisited locations also provides prediction uncertainties, and can be used as a quantitative metric to assess the confidence in the calculated contamination map; these estimates and prediction uncertainties are unavailable for standard uniform survey routines as they can only produce maps in the vicinity of observed locations. We present through simulation the achievable performance gains from using this new method by directly comparing to a uniform survey method. Results show that using the prediction-difference maps to inform motion planning procedures offers a faster rate of producing an accurate and convergent map relative to a uniform survey route.

and iodine have been identified as two important classes of compounds related to nuclear weapons proliferation issues.

Rossi-alpha measurements of fissionable assemblies are used to estimate the prompt neutron decay constant alpha. Reactivity can be inferred from alpha if the values of the neutron generation time and effective delayed neutron fraction are assumed. If multiple measurements are performed on an assembly near delayed critical, one can determine alpha at delayed critical and directly infer reactivity (without needing to assume values for the neutron generation time or effective delayed neutron fraction). Previous works have demonstrated that two-exponential fits for Rossi-alpha measurements of reflected assemblies have better fit metrics than those of one-exponential fits; however, the two-exponential probability density function that is needed to obtain alpha from the fit parameters has not been derived. This paper derives the two-exponential fit based on a two-region point kinetics model for Rossi-alpha measurements of reflected assemblies, a generalization of the current, one-region model (one-exponential fit). The new model is validated for shielded assemblies, a special case of reflected assemblies where the reflector-to-core leakage is negligibly small. The validation is performed using shielded, fissionable assemblies (highly enriched uranium with k(eff) approximate to 0.95 and weapons-grade plutonium with k(eff) > 0.77). The results show that the two-exponential model can (1) predict the constant alpha within two standard deviations, and (2) deconvolve alpha and the time a neutron spends in the reflector region, neither of which is possible with the one-exponential model.

Determining the reactivity of systems containing fissile material is essential for verifying safety, and possession and enrichment levels. The goal of this experiment was to characterize a subcritical nuclear fuel assembly irradiated by a pulsed neutron generator through differential die-away analysis. Similar characterizations are commonly performed with thermal neutron detectors, but these detectors are bulky and cumbersome, the materials are not widely available, and timing can be clouded by the required thermalization time. In this work, an organic scintillator capable of detecting both neutrons and photons was used to measure the die-away profiles of both particles. These time profiles were then used to estimate the reactivity of the assembly. The calculated reactivity estimates compare favorably to simulated estimates, and therefore provide a useful tool for fissile material characterization.

The purpose of this technical note is to consolidate the notations used for describing parameters that pertain to neutron multiplicity mathematics relevant to various applications including nonproliferation, international safeguards, and criticality safety among others. The nomenclatures used in these techniques vary widely depending on the origin of the work and their applications. We aim to consolidate many of the previously used notations in a single document to enhance past, present, and future technical exchanges pertaining to neutron multiplicity. This will help avoid confusion in future publications and will facilitate wider application-independent advancements and utility of peer-reviewed findings. A brief introduction and history of neutron multiplicity counting is presented, followed by a summary of commonly used techniques in a variety of different applications. In each section, we present the notations used in previous publications for the reader's reference.

The efficient and accurate detection of neutrons is essential in many instruments developed for nuclear nonproliferation and safeguards applications. Recent advances in silicon photomultiplier (SiPM) technology and new organic scintillator materials have made it possible to develop new capabilities for neutron detection that have the potential to improve the current systems used in the field. Areas of application include multiplicity counters for fast neutrons emitted by spontaneous and induce fission in the actinides, and imaging systems used for detecting, locating, and characterizing fission sources. Our group has shown the ability of SiPMs to be used as a replacement for traditional photomultiplier tubes to achieve better timing while retaining pulse shape discrimination capabilities (i.e., the ability to distinguish neutron-induced pulses from gamma ray-induced pulses) for stilbene detectors. In this work, we show the application of arrays of organic fast neutron detectors for multiplicity, imaging, and other applications.

Fission neutron anisotropy, due to the kinematics of the fission process, has been studied for non-multiplying sources and highly multiplying subcritical plutonium metal assemblies (i.e. relatively long fission chains). The studies on highly multiplying assemblies show that the observed neutron–neutron angular distribution appear isotropic, while the studies on non-multiplying sources show that the neutron–neutronangular distribution appear anisotropic. No measured data exists, however, that investigates the dependence of neutron anisotropy on multiplication for low-multiplying assemblies. We have experimentally characterized the dependence of fission neutron anisotropy on multiplication for low-multiplying plutonium metal assemblies. Here, an array of 16 organic scintillators was used to measure plutonium metal assemblies (95% 239Pu, by mass) exhibiting a leakage multiplication of 1.0722(3) to 1.6006(4). Full neutron–neutron angular distributions were measured, and the fission neutron anisotropy was quantified with the ratio of neutron–neutron coincidences observed at 180°and 90°. The results show that the neutron–neutron angular distribution becomes more isotropic as the multiplication increases. Additionally, energy–angle correlations were also characterized showing that the angular distributions are more anisotropic when observing neutrons of higher energy.

The neutron light-output response from quasi-monoenergetic neutrons was measured for a empty set 5.08 x 5.08 cm trans-stilbene and a empty set5.08 x 5.08 cm small-molecule organic glass scintillator. Quasi-monoenergetic neutrons were isolated from a time-of-flight measurement of a Cf-252 spontaneous fission source with a flight distance of 200 cm. Two different methods of waveform analysis were implemented, where the neutron light output response proportional to the detected pulse height distribution (PHD) and pulse integral distribution (PID) were extracted for both types of scintillators. The light output response proportional to the pulse integral was determined by integrating the digitized waveform with an integration window length of 150 ns, which contained > 90% and > 95% of the scintillation light for an averaged neutron and photon waveform above 0.5 MeVee. The extracted light-output data were fitted with a semi-empirical function based on the Birks' formula. The results show that the small-molecule organic glass scintillator produced more light than the trans-stilbene scintillator for a range of neutron energies of 0.79 +/- 0.04 MeV to 3.65 +/- 0.38 MeV. The fitted semi-empirical function was used in MCNPX-PoliMi with MPPost to simulate the detector response from an independent measurement of a Cf-252 spontaneous fission source to test the fidelity of the extracted light output response functions. The simulated and measured total neutron count rate agreed to within < +/- 3 % and < +/- 1% for the trans-stilbene and small-molecule organic glass scintillators. Due to the different light-output response of the two scintillators, the intrinsic neutron detection efficiency was calculated for equal observable ranges in light output and neutron-equivalent energy units. The intrinsic neutron detection efficiency in the observable light-output range of 0.06 MeVee to 2.4 MeVee was calculated to be 28.66 +/- 1.43 % and 34.66 +/- 1.73 % for pulse height and pulse integral analysis, respectively. For the same observable light-output range, the intrinsic efficiency of small-molecule organic glass was calculated to be 32.54 +/- 1.63 % and 37.39 +/- 1.87 % for pulse height and pulse integral analysis, respectively. When observing equal light-output ranges, the small-molecule organic glass was 11.92 +/- 6.23% and 7.88 +/- 7.35% more efficient than the trans-stilbene using pulse height and pulse integral analysis, respectively. When observing equal neutron-equivalent energy ranges, the trans"-stilbene scintillator was 8.4 +/- 6.46% and 11.95 +/- 6.23% more efficient than the small-molecule organic glass for pulse height and pulse integral analysis, respectively.

Accurate and timely characterization of physical properties pertinent to plutonium bearing materials is important for fulfilling nuclear nonproliferation and safeguards goals. Physical properties include the fissile mass, leakage multiplication, and the α-ratio, defined as the ratio of (α,n) neutrons to spontaneous fission neutrons. Traditionally, these properties can be inferred by relating the measured neutron multiplicity count rates to the well-established point kinetics moments equations; the current state-of-the-art utilizes 3He-based detection systems. Organic scintillators have been used extensively to study and measure characteristic signatures in the neutron angular and energy distributions. Previous work has proposed techniques that independently leverage the energy and angle sensitivity of organic scintillators to estimate the α-ratio of plutonium bearing material; however, it is expected that the energy and angular distributions are correlated to one another due to the underlying physics of fission and (α,n) neutron emissions. This work presents experimental results that characterize neutron-neutron angular distribution and subsequently the neutron-neutron energy-angle correlations for plutonium samples of similar mass and multiplication, but varying α-ratio due to the type of low-Z impurity. Full neutron-neutron angular distributions are presented using a low-energy detection threshold of 0.10, 0.15, and 0.20 MeVee (0.73, 0.96, 1.16 MeV neutron-equivalent energy). Neutron anisotropy was quantified by taking the ratio of neutron-neutron coincidences at 180°to those at 90°, where a value of unity indicates a purely isotropic source. The results show that the observed neutron-neutron correlations transition away from fission-induced signal to the cross-talk signal associated with single (α,n) neutrons with increasing α-ratio. Energy-angle correlations are characterized by calculating the neutron anisotropy at various detection thresholds and show positive correlation between the observed anisotropy and the energy of the neutrons.

We developed a fast-neutron multiplicity counter (FNMC) based on stilbene and EJ-309 organic scintillators. The system can detect and discriminate correlated photon and neutron multiplets emitted by fission reactions. We used the system to estimate the fissile mass of uranium oxide samples in active interrogation mode at the Zero Power Physics Reactor of Idaho National Laboratory (INL). Two sets of certified reference material (CRM) samples were characterized. The U-235 enrichment of the first set is constant at 93.2 wt%, and the UO2 mass ranges between 0.5 and 4 kg. The second set includes samples of increasing enrichment (from 20 wt% to 97 wt%) and constant UO2 mass of 230 g. We used two AmLi sources to induce fission reactions in the samples. Despite the intense gamma-ray background of the UO2 and interrogating sources, the system could measure induced fission neutrons emerging from the interrogated samples without additional shielding surrounding the sample and only relying on pulse shape discrimination to classify neutron and gamma-ray pulses. The overall neutron count rate and time-correlated counts are well correlated with the sample fissile mass. We also proved that CRM samples can be used to build a calibration curve to assay the U-235 mass of unknown samples of different mass, geometry and enrichment, with an average bias error of 8%, for U-235 mass higher than 390 g.

Traditionally, capture-based neutron multiplicity counters (i.e. He-3 based systems) are used for non-destructive assay of special nuclear material. Using capture-based detectors for non-destructive assay requires intervening moderating material that inhibits the ability to observe characteristic energy and angular signatures. Therefore, these systems rely on observing only the emitted neutron multiplicity to infer physical properties such as fissile mass, leakage multiplication, and contribution from non-fission neutrons (alpha-ratio). We have developed a fast neutron multiplicity counter using 24 - 5.08 cm x 5.08 cm stilbene detectors coupled to ETL 9214B photomultiplier tubes, and demonstrate that the system is sensitive to the emitted neutron energy and angular distributions, in addition to the multiplicity distribution. The system was used for passive assay of Pu-metal and Pu-oxide samples, and the detected neutron multiplicity, energy, and angular distributions are used to demonstrate that new correlated signatures exist related to the physical properties of the item. We also demonstrate the neutron cross-talk effects can be considered in order to yield more accurate estimates of the aforementioned physical properties. The final paper will present correlated signatures in energy, angle, and multiplicity that can potentially provide new quantities for characterizing special nuclear material when traditional techniques become unreliable.

We developed and tested a compact neutron detector, based on a cylindrical stilbene crystal, directly read out by a silicon photomultiplier (SiPM). We experimentally studied the effect of light guides to improve the light collection efficiency when coupling small SiPMs to large-area crystals. The use of the light guide for a crystal-SiPM area ratio of 14x increased the overall detection efficiency approximately 5%, compared to a direct coupling. This modest efficiency improvement hardly justifies the additional design complexity due to adding a light guide to the assembly. Based on a simple readout scheme, a pair of detectors was used to measure correlated emissions from laboratory fission sources (i.e. Cf-252). An amplified SiPM readout board is being developed. Detectors equipped with this readout device will be used in a multiplicity counter assembly to inspect special nuclear materials.

This paper compares simulated prompt, time-correlated, neutrons emitted by plutonium metal plates and detected by organic scintillators to experimental data, measured at the Zero Power Physics Reactor of Idaho National Laboratory. These experimental results are compared to simulations with the MCNPX and MCNPX-PoliMi Monte Carlo codes. The number and energy of the neutrons emitted by each fission was simulated using a standard bounded integer approach and two more advanced models.

Neutron multiplicity counting (NMC) techniques are widely used for nuclear materials accountability and international safeguards applications to quantitatively evaluate characteristic properties pertaining to fissile material. Mathematical models for NMC moments have been previously derived for systems that use capture-based detectors; however, these models are not applicable when scatter-based detectors are used because of "neutron cross talk." Neutron cross talk occurs when a single neutron scatters and deposits energy above threshold into multiple detectors causing spurious increase in multiplicity counts; this, in turn, has caused fissile mass to be overestimated when not treated. In this paper, we propose new mathematical models derived from point kinetics to correct for neutron cross-talk effects up to any arbitrary order N, where N denotes the maximum number of counts a single neutron can cause. The new models were used to estimate the fissile mass of plutonium metal and oxide samples with effective Pu-240 mass ranging from 2.5 to 250 g. The adequacy of the models was confirmed using simulations of a conceptual scatter-based neutron multiplicity counter (e.g., organic scintillators) using MCNPX v2.7e with the PoliMi fission event generating extension. The fissile mass estimates with no correction for neutron cross-talk events yielded an average relative deviation from the true Pu-240(eff) mass of 55.94% and 84.56% for metal and oxide samples, respectively. When neutron cross-talk events of order N = 2 are included in the model, the fissile mass estimates yielded an average relative deviation of 11.89% for metal and 13.21% for oxide samples. Accounting for neutron cross-talk events of order N = 3 resulted in fissile mass estimates with an average relative deviation of 9.58% and 10.51% for metal and oxide samples, respectively. These mass estimates were compared to a reference case (i.e., no neutron crosstalk effects) that yielded an average relative deviation of 6.81% and 4.77% for metal and oxide samples, respectively. The discrepancy between the estimates from the proposed model and the reference case is attributed to the assumed value of N, which sets a finite upper bound on the order of cross-talk events the model treats (i.e., the model for N = 3 assumes that a neutron will never cause more than three counts).

Timely detection of HEU is one of the greatest challenges in homeland security applications, especially if any amount of shielding material is present. The specific activity of spontaneous 35 U neutron emission is two orders of magnitude lower than that or 238 U and six orders of magnitude lower than that of 240 Pu. However, 235 U has a sizable cross-section for the (n,f) reaction that can be exploited through interrogation with neutrons. In this work, AmLi sources were used to interrogate kilogram-scale masses of HEU at the Device Assembly Facility on the Nevada National Security Site. The response was recorded using the dual particle imaging system (DPI) developed at the University of Michigan, which is a combined Compton and neutron-scatter camera capable of spatial and spectral characterization of nuclear material. The response time of the liquid scintillators used in the DPI is on the order of nanoseconds, which is on the same time-scale of the fission chains induced in the HEU. This capability presents the possibility to directly analyze the fission chain dynamics to characterize the multiplication of the HEU samples. Results will show the discrimination of different quantities of HEU using the DPI.