A comparison between cold and thermal neutron prompt gamma activation analysis in the determination of carbon,nitrogen, and phosphorus in cattail

Methods for the determination of total carbon (TC), total nitrogen (TN), and total phosphorus (TP) in cattail (Typha domingensis) using cold and thermal neutron prompt gamma activation analysis (CNPGAA and TNPGAA) have been developed in the CNPGAA and TNPGAA facilities at the National Institute of Standards and Technology (NIST), and evaluated through the analysis of Standard Reference Materials (SRMs) in previous studies. There are major issues that impact the sensitivities of C, N and P using CNPGAA and TNPGAA including the effects of hydrogen content in samples, sample thickness and Compton scattering. However, interference from chlorine (Cl) in cattails has a major impact on the detection of P. This paper compares the effects of hydrogen content, sample thickness and Compton scattering on the sensitivities of C, N, and P between CNPGAA and TNPGAA and provides a resolution to the Cl interference on P in cattail.     

Determination of carbon, nitrogen, and phosphorus in cattail using thermal neutron prompt gamma activation analysis

A previous study on the determination of carbon, nitrogen, and phosphorus in cattail using cold neutron prompt gamma activation analysis (CNPGAA) demonstrates that the results of numerous cattail samples showed favorable comparison to results from an elemental analyzer (EA) for C and N. However, the results for P overestimated the results from a UV-VIS spectrophotometer because of the interference of chlorine existing in cattail collected from an estuarine environment. To compare to CNPGAA, a method for the determination of C, N, and P in cattail using thermal neutron prompt gamma activation analysis (TNPGAA) has been developed in the TNPGAA facility at the National Institute of Standards and Technology (NIST) and evaluated through the analysis of standard reference materials (SRMs).     

Prompt gamma-ray activation analysis for certification of sulfur in fuel oil SRMs

A combination of cold neutron prompt gamma-ray activation analysis (CNPGAA) and thermal neutron (TN) PGAA was used to determine sulfur in fuel oils to develop a method to provide values for certification. CNPGAA was used to measure S/H mass ratios, and TNPGAA to measure hydrogen mass fractions. Measurements were combined to determine sulfur mass fractions (with expanded uncertainties) of 2.159 ± 0.072 % for SRM 1622e, 0.7066 ± 0.0120 % for SRM 1619b, and 0.1266 ± 0.0030 % for SRM 1617b, in agreement with certified values. The results validate the method as suitable for certification of sulfur at mass fractions ≥0.1 %.

     

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

The effects of neutron scattering by hydrogen within targets for cold neutron prompt -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.     

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

The cold neutron capture prompt -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 -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 -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.     

Neutron beam preparation with Am–Be source for analysis of biological samples with PGNAA method

Material analysis with prompt gamma neutron activation analysis (PGNAA) requires a proper geometrical arrangement for equipments in laboratory. Application of PGNAA in analysis of biological samples, due to small size of sample, needs attention to the dimension of neutron beam. In our work, neutron source has been made of ²⁴¹Am–Be type. Activity of ²⁴¹Am was 20 Ci which lead to neutron source strength of 4.4 × 10⁷ neutrons per second. Water has been considered as the basic shielding material for the neutron source. The effect of various concentration of boric acid in the reduction of intensity of fast and thermal components of the neutron beam and gamma ray has been investigated. Gamma ray is produced by (α, n) reaction in Am–Be source (4.483 MeV), neutron capture by hydrogen (2.224 MeV), and neutron capture by boron (0.483 MeV). Various types of neutron and gamma ray dosimeters have been employed including BF₃ and NE-213 detectors to detect fast and thermal neutrons. BGO scintillation detector has been used for gamma ray spectroscopy. It is shown that the gamma and neutron radiation dose due to direct beam is of the same magnitude as the dose due to radiation scattered in the laboratory ambient. It is concluded that 14 kg boric acid dissolved in 1,000 kg water is the optimum solution to surround the neutron source. The experimental results have been compared with Monte Carlo simulation.     

Combinatorial study of thin film metal hydride by prompt gamma activation analysis

Cold neutron prompt gamma activation analysis (PGAA) was used to determine the mass of hydrogen in Mg hydride thin films with varying hydrogenation times. The results suggest that hydrogenation of the Mg thin films remains unsaturated even after 48 h of treatment, contrary to the indications of inferential hydrogen measurement methods. To demonstrate PGAA as an effective combinatorial methodology for hydride thin films, a continuously varying composition gradient of thin Mg_yTi_(1−y) hydride film with y ranging from 0.65 to 0.94 was prepared and analyzed by both PGAA and instrumental neutron activation analysis (INAA). The variation in the molar ratio of Mg, Ti, and H was obtained for nine 5 mm wide segments of the film.     

Xenon diffusion studies with prompt gamma activation analysis

Developing a better understanding of xenon transport through porous systems is critical to predicting how this gas will enter the atmosphere after a below ground nuclear weapons test. Radioxenon monitoring is a vital part of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) International Monitoring System. This work details the development of prompt gamma activation analysis for measuring the diffusion rates of xenon and argon gases through a porous medium. The University of Texas at Austin maintains a prompt gamma activation analysis facility with a peak neutron flux of ~1.5 × 10⁷ cm⁻² s⁻¹ and a beam diameter of 1 cm. Due to the relatively large prompt gamma cross sections of many stable xenon isotopes at thermal and sub-thermal neutron energies, prompt gamma activation analysis is a suitable technique for in situ non-destructive analysis of natural xenon. A test chamber has been designed and constructed to utilize prompt gamma activation analysis to measure xenon and argon diffusion through geological materials (e.g., sand, soil, etc.). Initial experiments have been conducted to determine the detection limits for stable gas measurements. The results from these experiments will be utilized to benchmark parts of a xenon transport model that is being used to determine diffusion coefficients for xenon and argon.     

Analysis of hydrogen concentration in low-alloy steel

The hydrogen concentration for the reference material low-alloy steel was estimated using prompt gamma neutron activation analysis. The hydrogen concentration had a range of 0.91–1.14 mg/kg as measured by the prompt gamma activation method but the reference value was about 1.0 mg/kg as measured by the ICP/AES method. The relative error appeared to be 14% and the differences showed as the background by blank. The standard reference material (NIST SRM) was used for the analytical control.     

K 0-prompt gamma ray activation analysis for estimation of boron and cadmium in aqueous solutions

K ₀-PGAA (prompt gamma activation analysis) has been used to estimate boron and cadmium contents in industrial and environmental materials from some different local areas as well as, samples from different locations in Burullus and Qarun Lakes. A high efficiency Compton background suppression gamma-ray spectrometry by anti-coincidence counting with a NaI(Tl) shield around a central HPGe detector for in beam PGAA using a ²⁵²Cf neutron source has been calibrated and described in this paper. The facility is principally designed for measurement of the prompt gamma-ray spectra obtained due to thermal neutron capture using 1951.14 keV gamma line of ³⁵Cl as the internal mono-standard comparator. A calibration curves were developed in which a set of boron and cadmium standards were tested and the count rate to boron and cadmium mass curves were determined. This set of boron and cadmium measurements was compared with a method for determining composition using K ₀-PGAA. Conventional prompt neutrons capture Gamma-ray results were in a good agreement with the data obtained by K ₀-PGAA method. Detection limits and self-shielding study are presented.     

U determination in environmental samples by delayed neutron activation analysis in Korea

A delayed neutron counting system has been implemented at the HANARO research reactor in 2007. Thermal neutron flux measured at the NAA #2 irradiation hole coupled to the delayed counting system, was higher than 3 × 10¹³ n cm⁻² s⁻¹. The delayed neutron counting system is composed of 18 ³He detectors which are divided into three groups with six detectors and the collected signals of each group are processed to a digital signal. The count numbers were measured with the uranium mass by using NIST SRMs under fixed analytical condition and their correlation could be determined. Finally, delayed neutron activation analysis has been carried out for the determination of uranium mass fraction in the collected environmental samples.     

Development of a procedure for measuring nitrogen by cold neutron prompt gamma-ray activation analysis

An instrument for cold neutron prompt gamma-ray activation analysis (CNPGAA) has been used for the nondestructive determination of nitrogen. The samples were analyzed in an evacuated box to minimize background from neutron capture by atmospheric nitrogen. The system features lower background and lower detection limits than obtainable with the University of Maryland-National Institute of Standards and Technology (NIST) thermal neutron PGAA instrument. CNPGAA has been used to measure nitrogen in standard reference materials which included biological materials and soils; the results are in agreement with certified values. The detection limit for nitrogen in most biological and geological samples is near 1000 mg kg-1.     

N-Cα bond cleavage of the peptide backbone via hydrogen abstraction

The specific cleavage of N-C_α bonds on the peptide backbone to form the so-called ‘c’ and ‘z + 2’ products, which can be used for the rapid determination of protein amino-acid sequences, has been examined to clarify the mechanism(s) that occur during hydrogen abstraction induced by bombardment with 337-nm laser photons in matrix-assisted laser desorption/ionization (MALDI) method. Intramolecular hydrogen abstraction, which results from the hydrogen(s) on the C_α or C_β carbon, did not occur with a deuterium-labeled dodecapeptide. To confirm a proposition that intermolecular hydrogen abstraction occurs between the peptide and the MALDI matrix, a deuterium dodecapeptide embedded in a deuterium 2,5-dihydroxybenzoic acid matrix at a molar ratio of 1:7000 was analyzed. The resulting deuterium c product ions suggested that c ions form via intermolecular hydrogen abstraction, although the results obtained did not deny any other possibilities such as intramolecular transfer of labile hydrogen. A mechanism for the N-C_α bond cleavage has been proposed that the formation of hypervalent radical species and subsequent prompt bond cleavages occur. The proposed mechanism successfully rationalizes the formation of both the z + 2 and the c product ions.     

Neutron diffraction in D2O salt solutions is no cause of sensitivity changes in prompt gamma or instrumental neutron activation analysis

When slab-shaped samples are analyzed with prompt gamma neutron activation analysis (PGNAA), sensitivity changes can be observed depending on the hydrogen content of the sample. In PGNAA experiments performed by MACKEY at NIST where D₂O solutions were compared to H₂O solutions, it was found that most elements tested showed statistically equal sensitivity increases with increasing hydrogen concentration. Outliers were manganese, hydrogen and samarium. For manganese, it is hypothesized in this paper that the sensitivity for ions dissolved in D₂O is influenced by neutron diffraction due to local ordering of hydrogen bonds. This hypothesis is tested with instrumental activation analysis and falsified.     

Stabilities in Polar Nonaqueous Solvents and Transfer Activity Coefficients from Water to Nonaqueous Solvents of 19-Crown-6-Alkali Metal Ion Complexes

Formation constants of 1 : 1 19-crown-6(19C6) complexes with alkali metal ions weredetermined conductometrically at 25 °Cin acetonitrile(AN), propylene carbonate (PC), methanol, DMF, andDMSO. 19C6 always forms the most stable complex withK⁺. The selectivity order of 19C6 forheavy alkali metal ions isK⁺ > Rb⁺ > Cs⁺.The selectivity for Na⁺ varies withthe solvent; that for Li⁺ is the second lowest(AN, DMSO) or the lowest (PC). Transfer activity coefficients(^SH _{2 O}) of19C6 from water to the nonaqueous solvents (S) weremeasured at 25 °C. The contributions of a methylenegroup and an ether oxygen atom to thelog ^SH _{2 O}value of a crown ether wereobtained. The ^SH _{2 O}values of the 19C6–alkali metal ion complexes(^SH _{2 O} (ML⁺)) werecalculated, M⁺ and L denoting an alkali metal ionand a crown ether, respectively. For AN, PC, andCH₃OH, although the M⁺ ion is more stronglysolvated by water than by AN, PC, or CH₃OH, thelog ^SH _{2 O} (ML⁺) islarger than the correspondinglog ^SH _{2 O} (L)expect for the case of M⁺ = Li⁺.The higher lipophilicity of the19C6 complex ion is attributed to an enforcement ofthe hydrogen-bonded structure of water for the complexion caused by the greatly decreased hydrogen bondingbetween ether oxygen atoms and water uponcomplexation. For DMF and DMSO, thelog ^SH _{2 O} (ML⁺) is also greater thanthe correspondinglog ^SH _{2 O} (L).It was concluded from thisfinding that the unexpectedly lowest stability of the19C6 complex ion in water is due to the hydrogenbonding between 19C6 and water. The stabilities and thelog ^SH _{2 O}of 19C6–alkali metal ion complexes were compared with those of 18C6complexes.     

A New Gamma-Ray Spectrum Catalog for PGAA

A major obstacle to the use of the prompt gamma activation analysis (PGAA) method has so far been the lack of a suitable library. Therefore, new measurements have been performed at the PGAA facility at Budapest Research Reactor (BRR) in order to create a prompt -ray catalog for qualitative and quantitative analysis. Prompt -ray energies and associated k ₀-factors have been determined by internal standardization. The resulting catalog contains prompt -ray data from neutron capture and other reactions such as (n,), and decay -ray data from short-lived reaction products. Data have been measured for nearly all stable elements, from hydrogen to uranium. Generally, data for several isotopes are given, to enable isotopic analysis as well.     

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.     

Structural and dynamic properties of the polyanionic hydrides SrAlGeH and BaAlGeH

The quaternary aluminium hydrides SrAlGeH and BaAlGeH were synthesized from either hydrogenating the intermetallic AlB₂-type precursors SrAlGe and BaAlGe or reacting SrH₂ with a mixture of Al and Ge in the presence of pressurized hydrogen. Their structures were characterized by X-ray and neutron powder diffraction of the corresponding deuterides. The compounds crystallize with the trigonal SrAlSiH structure type (space group P3m1, Z = 1, a = 4.2435(2) and 4.3450(2) Å, c = 4.9710(3) and 5.2130(4) Å for SrAlGeH and BaAlGeH, respectively) and feature a two-dimensional polyanion [AlGeH]²⁻ which represents a corrugated hexagon layer built from three-bonded Al and Ge atoms. H is terminally attached to Al. Polyanions [AlGeH]²⁻ are electron precise and, according to electronic structure calculations, the quaternary hydrides display band gaps with sizes between 0.7 and 0.8 eV. Infrared and inelastic neutron scattering spectroscopy show Al–H stretching and bending mode frequencies at around 1250 and 870 cm⁻¹, respectively. SrAlGeH and BaAlGeH are thermally stable up to at least 500 °C. When exposed to air the hydrides decompose rapidly to amorphous, orange colored materials.     

Cations Insertion in Molybdenum Cluster Compounds: Electronic Structure and Electrochemical Study Using Cavity Microelectrode

Owing to the high lability of cations in the three-dimensional framework of K_1+x Mo₁₂S₁₄ (0 ≤ x ≤ 1.6), first-principles calculations and electrochemical methods have been carried out to study the insertion of cations in the empty channels of this compound. The cavity microelectrode that is a suitable electrode for powder material analysis has been used in voltammetric experiments. Results obtained for Li⁺, Na⁺, Rb⁺, K⁺, Cs⁺ and NH₄ ⁺ cations are presented and discussed.