Rick L. Paul

Prompt Gamma-Ray Activation Analysis: Fundamentals and Applications

Prompt gamma-ray activation analysis (PGAA) is an important nuclear analytical technique that complements conventional neutron activation analysis (NAA). When a target is placed in a beam of neutrons, gamma-rays emitted upon neutron capture are measured by a shielded germanium detector, yielding quantitative elemental analysis. The radiation is penetrating and the analysis both nondestructive and independent of the chemical form of the element(s) being measured. The technique is most useful for measurement of light elements (H, B, C, N, Si, P, S, Cl) which can not be easily measured by other methods. Best sensitivity is achieved with neutron beams from research reactors. Although sample preparation is minimal, care must be taken to select proper standards and blanks, and numerous corrections must sometimes be applied to the data from the complex spectra. PGAA has proven useful for multielement analysis of a wide variety of different materials spanning a broad range of scientific disciplines. Of particular importance has been the measurement of hydrogen in materials.

Prompt-Gamma Activation Analysis using the k0 Approach

Applying thek0 standardization method to prompt-gamma activation analysis (PGAA) offers similar benefits as in instrumental neutron activation analysis. It has been demonstrated that under constant flux conditionsk0-factors obtained by normalizing to a titanium comparator, measured separately, yield consistent analytical sensitivity ratios. The ratio method has been generalized by using stoichiometric compounds for the determination ofk0-factors. Since chlorine forms compounds with essentially everyelement and it also serves as a detector efficiency standard,k0 values have been determined relative to chlorine as an internal standard for several analytically important elements in two reactor facilities: the thermal guided beam at the BRR in Budapest and the cold-neutron beams at the NBSR at NIST.

Cold neutron prompt gamma-ray activation analysis at NIST — Recent developments

The cold neutron capture prompt γ-ray activation analysis (CNPGAA) spectrometer located in the Cold Neutron Research Facility (CNRF) at NIST has proven useful for the analysis of hydrogen and other elements in a wide variety of materials. Modifications of the instrument and the CNRF have resulted in improved measurement capabilities for PGAA. The addition of an atmosphere-controlled sample chamber and Compton suppression have reduced γ-ray background and increased signal-to-noise ratio. More recent revisions are expected to yield still further improvement in analytical capabilities. Replacement of the D2O ice cold source with a liquid H2 moderator is expected to yield a 5–10 fold increase in neutron capture rate, and improved neutron and γ-ray shielding will result in further reduction of the background. Other modifications to the instrument allow easier sample mounting and more precise positioning of samples in the neutron beam. Significant improvements in detection limits and analytical accuracy are expected.

The use of element ratios to eliminate analytical bias in cold neutron prompt gamma-ray activation analysis

Analytical bias due to neutron scattering and absorption in cold neutron prompt gamma-ray activation analysis (CNPGAA) is largely eliminated for homogeneous samples when element ratios are measured. Application of sensitivity ratios (measured relative to titanium) to the multielement analysis of the Allende meteorite increases both the speed and accuracy of the measurement. Greater measurement accuracy is achieved for some samples when ratios of element concentrations are reported. Problems are encountered when applying the ratio method to measurement of elements which deviate from 1/v behavior, and when gamma-ray attenuation or sample heterogeneity are significant.

Solution-phase structures of gallium-containing pyrogallol[4]arene scaffolds.

The variations in architecture of gallium-seamed (PgC4Ga) and gallium-zinc-seamed (PgC4GaZn) C-butylpyrogallol[4]arene nanoassemblies in solution (SANS/NMR) versus the solid state (XRD) have been investigated. Rearrangement from the solid-state spheroidal to the solution-phase toroidal shape differentiates the gallium-containing pyrogallol[4]arene nanoassemblies from all other PgCnM nanocapsules studied thus far. Different structural arrangements of the metals and arenes of PgC4Ga versus PgC4GaZn have been deduced from the different toroidal dimensions, C–H proton environments and guest encapsulation of the two toroids. PGAA of mixed-metal hexamers reveals a decrease in gallium-to-metal ratio as the second metal varies from cobalt to zinc. Overall, the combined study demonstrates the versatility of gallium in directing the selfassembly of pyrogallol[4]arenes into novel nanoarchitectures.

Measuring hydrogen by cold-neutron prompt-gamma activation analysis

By irradiating with cold neutrons and avoiding hydrogenous materials of construction, we have developed a PGAA instrument at the Cold Neutron Research Facility at NIST with hydrogen detection limits in the microgram range in many materials. Quantities of 5–10 μg H/g are presently measurable in gram-sized samples of silicon or quartz, and of order 0.01 wt % can be quantitatively measured in complex silicate rocks.

Neutron scattering by hydrogen in cold neutron prompt gamma-activation analysis

The effects of neutron scattering by hydrogen within targets for cold neutron prompt γ-ray activation analysis (CNPGAA) have been characterized. For most targets studied, the probability for neutron absorption, and hence CNPGAA sensitivities (counts·s−1·mg−1), decrease with increasing H content and with target thickness. Comparisons with results from thermal neutron PGAA indicate that the effects of cold neutron scattering differ from those of thermal neutron scattering. CNPGAA sensitivities for “l/v” nuclides show similar sensitivity decreases, while Sm sensitivities show smaller decreases.

Solution-phase and magnetic approach towards understanding iron gall ink-like nanoassemblies.

The degradative oxidation of Leonardo da Vinci s oeuvre, the works of Galileo, and many other imperiled ancient manuscripts is, ironically, catalyzed by the very ink that was used to write them. Historical artifacts such as these are characterized by the extensive use of “iron gall ink”, an ink commonly used prior to the twentieth century. Regardless of the specific composition, iron gall inks are complexes of various polyphenolic gallic acids, a class of tannins, and ferric/ ferrous ions, along with other agents such as gypsum and gum arabic. In the interest of preserving such invaluable works of ancient prose, Fe complexes with polyphenolic compounds, such as gallic acid, catechin, ellagic acid and pyrogallol, have been extensively studied by IR, ESR, NMR, XANES and Mçssbauer spectroscopy. However, structural elucidation of these complexes has proven difficult. Our interest in this field stems from the difficulty in characterizing similar Fe-polyphenolic complexes, namely the complexes with the bowlshaped pyrogallol[4]arenes (PgCn, n = alkyl chain length). Compared to other PgC-transition metal capsular entities, which have been thoroughly studied and characterized by XRD, the structure of the PgCnFe complexes has largely remained a mystery, much like that of the chemically similar gall inks. Herein, we present a new approach towards the characterization of these unique complexes through the combination of solid-state magnetic and in situ neutron scattering methods. In our previous studies, solid-state properties aided our understanding of solution-phase behavior. For example, solid-state PgCnM entities (M = Zn, Cu, Ni, Co) are spherical, and our small-angle neutron scattering (SANS) experiments indicate that they retain that architecture when dissolved in non-polar solvents. In contrast, solid-state PgC4Ga and PgC4GaZn entities have rugby-ball and spherical shapes, respectively, which convert to toroids of different metric dimensions in solution. Thus, SANS allows differentiation between architectures of similar metric dimensions and between varying metric dimensions of similar architectures. 6] The current study addresses our three-fold interest in investigating solution structures of magnetically interesting self-assembled frameworks, obtaining solid-state insight from solution-phase studies and exploring the parameters that direct self-assembly. Specifically, the stability, elemental ratios and magnetic properties of Fe-containing C-alkylpyrogallol[4]arene (PgCnFe) nanoassemblies were examined. PgC1Fe or PgC3Fe was synthesized by mixing 4 equiv of Fe(NO3)3 with 1 equiv of PgCn and 14 equiv of C5H5N (Py) in a variety of solvent systems. The blue-black precipitates obtained could not be crystallized; thus, structural studies were conducted using SANS. The solid-state magnetic behavior of these entities was investigated using a SQUID magnetometer. The composition of these nanoframeworks was measured with prompt gamma activation analysis (PGAA). The PGAA results for solid-state PgC1Fe and PgC3Fe reveal C:Fe ratios of 27.8:1 and 28.5:1 and C:N ratios of 28.1:1 and 29.2:1, respectively (see Supporting Information). The 1:1 ratio between Fe and C5H5N-derived nitrogen agrees with the metal:Py ratios typically found in metal-seamed pyrogallol[4]arene dimeric host capsules. However, in contrast to the typical capsular metal:PgCn ratio of 4:1, the Fe:PgCn ratio was unexpectedly deduced to be 1.3:1. This ratio also differs from those for the tubular and dimeric PgC1 ferrocene (PgC1 Fc) hydrogen-bonded inclusion complexes, for which the Fc:PgC1 ratios are 1:3 and 1:2, respectively. [4c,7] In the SANS study, the [D6]DMSO-solubilized PgC1Fe (3% mass fraction) was measured on the NG7 30 m SANS instrument at the NIST Center for Neutron Research (NCNR) in Gaithersburg, MD and analyzed with IGOR Pro. The scattering length density (SLD) of PgC1Fe was calculated at the molar ratio of 1:1.3:1.3 (PgC1:Fe:Py) obtained from the PGAA results, and the scattering data was fitted to spherical, cylindrical and ellipsoidal models. The data analysis revealed distinct structural differences between previously investigated metal-seamed spherical nanoassemblies and the Fe-containing pyrogallol[4]arene nanoassemblies (Figure 1). The scattering for PgC1Fe was higher at low scattering angles (q) and fitted [*] Dr. H. Kumari, A. V. Mossine, Prof. C. A. Deakyne, Prof. J. L. Atwood Department of Chemistry, University of Missouri-Columbia 601 S. College Avenue, Columbia, MO 65211 (USA) E-mail: deakynec@missouri.edu atwoodj@missouri.edu

Prompt gamma activation analysis enhanced by a neutron focusing capillary lens

Abstract A focusing neutron lens using glass polycapillary fibers has been introduced successfully into a prompt gamma activation analysis (PGAA) instrument placed at the exit of a cold neutron guide. The neutron current density gain of the lens is 80, averaged over the focused beam size of 0.53 mm diameter. PGAA measurements have been made on submillimeter particles of gadolinium and cadmium. The results indicate that elemental sensitivities of measurements are increased by ∼ 60, and that particles of sizes smaller than 0.5 mm can be discerned using the focusing lens. The measured gain in prompt gamma signals for these particles is less than anticipated, probably due to alignment difficulties. Gamma ray background associated with the lens is discussed and improvements are suggested.

Elemental analysis of a single-wall carbon nanotube candidate reference material

A material containing single-wall carbon nanotubes (SWCNTs) with other carbon species, catalyst residues, and trace element contaminants has been prepared by the National Institute of Standards and Technology for characterization and distribution as Standard Reference Material SRM 2483 Carbon Nanotube Soot. Neutron activation analysis (NAA) and inductively coupled plasma mass spectrometry (ICP–MS) were selected to characterize the elemental composition. Catalyst residues at percentage mass fraction level were determined with independent NAA procedures and a number of trace elements, including selected rare earth elements, were determined with NAA and ICP–MS procedures. The results of the investigated materials agreed well among the NAA and ICP–MS procedures and good agreement of measured values with certified values was found in selected SRMs included in the analyses. Based on this work mass fraction values for catalyst and trace elements were assigned to the candidate SRM.