Three-dimensional printing and casting materials were analyzed by prompt gamma-ray activation analysis (PGAA) to determine their suitability as human tissue surrogates for the fabrication of phantoms for medical imaging and radiation dosimetry applications. Measured elemental compositions and densities of five surrogate materials simulating soft tissue and bone were used to determine radiological properties (x-ray mass attenuation coefficient and electron stopping power). When compared with radiological properties of International Commission on Radiation Units and Measurements (ICRU) materials, it was determined that urethane rubber and PLA plastic yielded the best match for soft tissue, while silicone rubber and urethane resin best simulated the properties of bone.

A linear fast neutron beam chopper has been deployed at a prompt gamma activation analysis instrument, enabling in-beam activation analysis of short-lived (T-1/2 > similar to 10 ms) neutron capture products without the need for sample transfer. This article describes the design and operation of the chopper system and measurement results from test samples containing Se-77 (T-1/2 =17.36$ s) and Yb-175m (T-1/2 =0.0682$ s). The chopper timing was optimized for a specific isotope in each measurement, demonstrating an advantage of the programmable system. The linear response of the total gated counts in the decay phase to the incremented Se and Yb masses demonstrates the suitability of the system for quantitative analysis. Theoretical calculations based on the experimental parameters were in good agreement with the observed results. An example that illustrates the separation of overlapping energy peaks by the use of time-stamped list mode data acquisition is shown in a sample containing Yb-175m and Dy-165 (T-1/2 =75.4 s).

Instrumental neutron activation analysis may need to correct for matrix interferences caused by fast neutron induced threshold reactions. The very low Al mass fraction in bovine liver Standard Reference Material (SRM) has eluded certification due to such complications. Cold neutron beam irradiation absent fast neutrons provides a possible alternative, which has been applied for the first time to determine Al in bovine liver SRM 1577c. The Al mass fraction was determined to be (0.80 ± 0.15) mg/kg using the traditional single peak-fitting method, and (0.78 ± 0.23) mg/kg using a new physics model-based spectral fitting method.

The thermogenic transformation of kerogen into hydrocarbons accompanies the development of a pore network within the kerogen that serves as gas storage locations both in pore space and the surface area for adsorbed gas with source rocks. Therefore, the successful recovery of gas from these rocks depends on the accessible surface area, surface properties, and interconnectivity of the pore system. These parameters can be difficult to determine because of the nanoscale of the structures within source rocks. This study seeks to investigate the pore structure, surface heterogeneity, and composition of isolated kerogens with progressively increasing thermogenic maturities from source rocks at a middle-east reservoir. Prompt gamma-ray activation analysis (PGAA), nitrogen and methane volumetric gas sorption, and small-angle neutron scattering (SANS) are combined to explore the relationship between the chemical composition, pore structure, surface roughness, surface heterogeneity, and maturity. PGAA results indicate that more mature kerogens have lower hydrogen/carbon ratios. Nitrogen gas adsorption indicates that the pore volume and accessible specific surface area are higher for more mature kerogens. The methane isosteric heat at different methane uptakes in the kerogens is determined by methane isotherms and shows that approximately two types of binding sites are present in less mature kerogens while the binding sites are relatively homogeneous in the most mature kerogen. The hysteresis effects of the structure during the adsorption and desorption processes at different CD4 gas pressures are studied. An extended generalized Porod's scattering law method (GPSLM) is further developed here to analyze kerogens with fractal surfaces. This extended GPSLM quantifies the surface heterogeneity of the kerogens with a fractal surface and shows that more mature kerogen is chemically more homogeneous, consistent with the results from methane isosteric heat. SANS analysis also suggests a pronounced surface roughness in the more mature kerogens. A microporous region circling around the nanopores, which contributes to high surface roughness and methane storage, is shown to develop with maturity.

As a potential replacement for the National Bureau Standards Reactor (NBSR) at the U.S. National Institute of Standards and Technology (NIST), a conceptual design of a new reactor with a horizontally-split core has recently been studied using low-enriched uranium (LEU) silicide dispersion (U3Si2/Al) fuel. In this paper, the neutronics calculations of the proposed NIST reactor with other two low-enriched U-Mo fuels (U-10Mo monolithic fuel and U-7Mo/Al dispersion fuel) were performed, and the results were compared to that of the U3Si2/Al fuel, with the objective of identifying the best fuel candidate for the reactor cycle length and maximum cold neutron production. To make consistent comparisons, fuel inventories for multi-cycle equilibrium cores were produced for each fuel based on a 30 d reactor cycle at 20 MW thermal power. With its very high uranium density, the potential to load more uranium in the core with U-10Mo monolithic fuel was explored with test cases using an alternate fuel management scheme, a higher power level (30 MW), or a longer cycle (45 d). The research results indicate similar neutronics performance characteristics of the three LEU fuel options in the proposed NIST reactor with the same power level. However, the ability to load more fuel in the reactor with the U-10Mo option allows additional flexibility in the reactor design and could lead to other optimizations that maximize cold neutron production.

This work explores a novel tomographic approach to PGAA that is both quantitative and spatially resolved, adapted from a clinical "proton beam range finder" in which MeV gamma rays are imaged by coincidence measurements of Compton scattered gamma rays with multi-detector arrays. We performed preliminary measurements using a Compton camera made with CdZnTe detector arrays on a series of test samples with high-energy (> 1 MeV) gamma emission lines. 3D image reconstructions were performed on the 2.2 MeV peak from H. The image reconstruction methods were also evaluated using the emission data generated by Monte Carlo simulations under ideal conditions.

Understanding the chemical durability of neutron shielding materials is necessary when assessing their long-term service potential. In this study, the chemical durability of a 6Li enriched neutron shielding glass that has been exposed to natural, near-operational conditions is assessed by Prompt Gamma Activation Analysis (PGAA) and Neutron Depth Profiling (NDP). These non-destructive, nuclear analysis techniques are sensitive to 6Li, and PGAA is uniquely able to detect H in low quantities in solids. It was determined that the enriched alumino-silicate glass can alter within 2 months of exposure to the natural environment. This exposure resulted in an average surface alteration layer thickness of ≈22 μm. The alteration layer contained ≈47% less 6Li than the bulk glass. Alternatively, a 3 years exposed sample of the glass had a surface alteration depth of ≈30 μm and 6Li depletion levels in the alteration layer were between 47% and 75% less 6Li than the bulk glass. When the alteration layer on the 3 years sample was removed, the H content of the glass's surface was nearly eliminated. This sample also showed variable Li concentrations throughout the alteration volume, which contrasts with near static Li concentration in the alteration volume of the 2 months sample. From these findings it was determined that the depletion in Li at the surface of the glass will not affect the glass's neutron shielding properties, but it may change the mechanical stability of the glass's surface and, due to increased H content in the alteration layer, make it an inappropriate material for the lining of certain neutron analysis instruments.

In ancient China during the Shang dynasty, ca. 1600 BCE-1046 BCE, intricate jade and bronze dagger-axes (ge) were made and used by the elite as ritual symbols of power and prestige. These meticulously crafted ceremonial weapons consist of a nephrite jade [Ca-2(Mg, Fe)(5)Si8O22(OH)(2)] blade mounted in a bronze haft. Several examples of these dagger-axes are included in collections in the United States. There has been recent interest in applying neutron radiography to image the jade tang hidden within the bronze haft and uncover early manufacturing techniques. As a preliminary test of feasibility, a simulated dagger was made using a modern replica blade carved from nephrite from Wyoming, USA, and copper alloy plates. The simulated dagger was first imaged by X-ray and neutron radiography at the NIST Neutron Imaging Facility. The details of the jade tang within the copper alloy haft were clearly visible. Subsequently, the nephrite blade and the copper alloy plates were analyzed by cold prompt gamma neutron activation (PGAA) to evaluate the feasibility of this technique for identifying the nephrite source. The PGAA was performed at the Cold Neutron PGAA station at NIST. Three nephrite specimens in the Smithsonian collection from China, Siberia and Taiwan, characterized previously by electron microprobe analysis, were used as comparative standards. The major nephrite elements - Ca, Mg, Fe and Si - were analyzed with uncertainties in the range of 0.3%-0.4%. Three of the trace elements conventionally used for sourcing, Cr, Mn and Ni, were analyzed with similar uncertainties. The residual radioactivity of the objects was below the NRC exempt limits.

A new γγ coincidence system has been set up at NIST. It is operated with a digital data finder supported by new software developed at NIST. The system is used to explore possible enhancements in instrumental neutron activation analysis (INAA) and study applicability to neutron capture prompt gamma activation analysis (PGAA). The performance of the system is tested with certified reference materials for efficiency calibration and quantitative performance. Comparisons of INAA results based on conventional gamma-ray spectrometry data with INAA results based on coincidence data obtained from the same samples show improvements in the counting uncertainties and demonstrates the quantitative accuracy of the new system.

The neutron-capture reaction is fundamental for identifying and analyzing the γ-ray spectrum from an unknown assembly because it provides unambiguous information on the neutron-absorbing isotopes. Nondestructive-assay applications may exploit this phenomenon passively, for example, in the presence of spontaneous-fission neutrons, or actively where an external neutron source is used as a probe. There are known gaps in the Evaluated Nuclear Data File libraries corresponding to neutron-capture γ-ray data that otherwise limit transport-modeling applications. In this work, we describe how new thermal neutron-capture data are being used to improve information in the neutron-data libraries for isotopes relevant to nonproliferation applications. We address this problem by providing new experimentally-deduced partial and total neutron-capture reaction cross sections and then evaluate these data by comparison with statistical-model calculations.

The accuracy of low-level hydrogen measurements with prompt gamma-ray activation analysis (PGAA) depends on identifying and accounting for all background H signals, including interfering signals. At the cold-neutron (CN)PGAA facility at the NIST Center for Neutron Research, the sources of background H signals were investigated in the context of titanium-based matrices containing low-levels of H (<300 mg H per kg Ti) with the measurements of prepared standards (mixtures of polyvinyl chloride and titanium oxide) and Ti alloy (Ti6Al4V) samples. The sensitivity ratio, defined as the ratio of the H signal to the Ti signal per unit mass ratio of H in Ti, was determined (1) with the measurements of prepared standards and (2) based on partial gamma-ray production cross sections and full-energy detection efficiencies. The resulting calibrations from these two approaches agreed within experimental uncertainty. A series of Ti alloy NIST Standard Reference Materials (SRMs) previously certified for the H content (SRMs 2452, 2453, 2453a, 2454) were used as test cases, with the mass fractions determined based on the sensitivity ratios derived from method 1 and method 2, respectively. The results agreed with the certified values within experimental uncertainties, validating the analysis performed on the new instrument with newly-prepared standards at low H mass fractions (method 1), and with the standard-independent analysis (method 2). Various sample mounting improvements were made to lower the background H signal. Spectral interferences near the H peak were identified as potential sources of bias and as a limiting factor in the detection limit of H in Ti alloy samples.

Prompt gamma-ray activation analysis facilities with high neutron currents (≥109 s−1) generate triton-induced fast neutrons from 6Li-loaded collimators and beam stops at rates that damage gamma-ray detectors. We develop an alternative beam stop design using 3He gas that produces negligible gamma-ray and fast-neutron background following neutron absorption. Replacing a 6Li glass beam stop with a test cell containing 2.5 MPa cm of 3He reduced fast neutron production by 73 %. An optimal 3He beam stop design with a 100-µm-thick entrance window, modeled using MCNP6, enables operation of detectors closer to the beam stop.