Comparison of neutron activation analysis methods

This work shows that nuclear data and neutron flux parameters can be applied to calculate directly the elemental concentrations. Techniques for extracting neutron flux parameters pertaining to the irradiation positions and nuclear data pertaining to the isotopes concerned from measured reaction rates have been previously developed. This method is compared to the comparator and relative methods of activation analysis. The principles, advantages and disadvantages of each method and sources of errors are discussed. It also briefly discusses other factors such as accuracy and precision, sensitivity, detection limits and limit of quantitative determination. The three methods are applied to the analysis of five environmental reference materials. The concentrations of more than 20 elements are determined. The results show a good agreement with the certified and/or literature values.     

Single-comparator methods in reactor neutron activation analysis

A critical evaluation of different comparator methods is given. The method based on experimentally-determined and convertible comparison factors (k) is considered to be the most suitable for general use. An alternative method is proposed, introducing generalized k₀ factors which are independent of irradiation and measuring conditions. This approach combines the simplicity of the absolute methods with nearly the same accuracy attained by the relative ones. It is suggested that k₀ factors be compiled in all cases when using single-comparator methods, to allow a continuous re-evaluation.     

Compton suppression neutron activation methods in environmental analysis

An overview of Compton suppression neutron activation analysis (NAA) is given. The basic theory of the technique, its experimental design, uniqueness, and limitations are discussed. Experimental data showing its usefulness for the low level determination of several key elements in environmental samples that can not be obtained using conventional non-destructive neutron activation is deliberated.     

Shielding optimization in an in vivo activation analysis setup

A neutron irradiation setup for in vivo activation analysis was characterized to optimize its spectrum measurements. In order to have a reliable spectrum, some shields and filters were defined to eliminate as much background possible. Since there were different types of background sources, different states were studied and necessary shields were employed to optimize the spectrum. To use a neutron shield, boron carbide layers (3 cm) around 3 × 3 in. sodium iodine detectors were employed. To prevent γ-ray background, bismuth (2 cm) is used under the detectors.     

A comparison of three activation analysis methods for iodine determination in foodstuffs

Activation analysis methods based on the nuclear reactions ¹²⁷ I(n, ) ¹²⁸ I, ¹²⁷ I(n,2n)¹²⁶I and ¹²⁷ I( ,n)¹²⁶ I were tested in terms ofdetection limits, accuracy and precision, and applicability for iodine determinationat various levels in foodstuffs. Both nondestructive and radiochemical modeswere employed using one of the two separation procedures developed (alkaline-oxidativefusion and iodine separation either by extraction of elemental iodine in chloroformor AgI precipitation). The lowest iodine detection limit of 1 ng . g ^—1 was achieved by employing the first nuclear reaction followedby separation of elemental iodine. Accuracy and precision of the proceduresdeveloped at largely different iodine levels were proved by analyses of biologicalreference materials having composition similar to that of various food items.     

Large sample activation analysis: Monitoring of photovoltaic module recycling using radioanalytical methods

Photovoltaic modules are the most promising sources of renewable energy. According to respective EU Directives both the glass carrier material and the photo-active layer (CdTe in this study) shall be re-used. Analyses for process surveillance and control have to be carried out, primarily for monitoring of the separation efficiency and the enrichment level of CdTe. For on-site measurement and process control energy-dispersive X-ray fluorescence spectrometry was used. Instrumental photon activation analysis using an electron linear accelerator as activating bremsstrahlung source is intended to be used later for iterative intermediate analysis of random samples for quality control. In this work, the feasibility of both methods for this task was studied.     

Halogen determination in Arctic aerosols by neutron activation analysis with Compton suppression methods

The study of halogens particularly bromine and chlorine in Arctic aerosolshas received a great deal of attention in the past decade in ozone depletionduring polar sunrise studies. Iodine has also been studied as part of geochemicalcycling. We have shown that all three of the above elements can be determinedsimultaneously with very low detection limits using epithermal NAA in conjunctionwith Compton suppression methods. Besides lowering the background considerably,Compton suppression can eliminate or minimize the overlapping peak of the620 keV photopeak arising form the 1642 keV double escape peak of ³⁸Cl interfering with the 616.9 keV photopeak of ⁷⁹Br(n,) ⁸⁰ Br reaction. Iodine is ideally determined by epithermal NAAbecause of its very good resonance integral cross-section. Although chlorineis usually determined using thermal neutrons via the ³⁷Cl(n,) ³⁸Cl reactions, epithermal NAA is still feasible for the Arcticaerosol, since it has a major sea-salt component.     

Gas chemical methods for the separation of elements in radioisotope production and activation analysis

Summary Liquid-liquid extraction is used for one-atom-at-a-time separations of transactinide elements from heavy-ion reaction product mixtures. It is suitable for this purpose because it is fast, provided that a chemical system with negligible kinetics is used, and it can be used for continuous separations. It is, however, not quite easy to determine the uncertainties of the measured distribution coefficients or complex formation constants. In this paper methods for such estimates will be discussed.     

Neutron activation analysis of reference materials by the k0-standardization and relative methods

Instrumental neutron activation analysis with the k₀-standardization method was applied to eight geological, environmental and biological reference materials, including leaves, blood, fish, sediments, soil and limestone. To a first approximation, the results were normally distributed around the certified values with a standard deviation of 10%. Results obtained by using the relative method based on well characterized multi-element standards for IAEA CRM Soil-7 are reported.     

Comparison of optimization methods in planar chromatography

Summary The optimisation of chromatographic parameters is studied. The Window Diagram and Overlapping Resolution Map (ORM) methods are compared. The latter is developed to take into account the spot diameter and the average plate height variations with the development distance, in order to improve the reliability of the method.     

Geometry optimization in density functional methods

The geometry optimization in delocalized internal coordinates is discussed within the framework of the density functional theory program deMon. A new algorithm for the selection of primitive coordinates according to their contribution to the nonredundant coordinate space is presented. With this new selection algorithm the excessive increase in computational time and the deterioration of the performance of the geometry optimization for floppy molecules and systems with high average coordination numbers is avoided. A new step selection based on the Cartesian geometry change is introduced. It combines the trust radius and line search method. The structure of the new geometry optimizer is described. The influence of the SCF convergence criteria and the grid accuracy on the geometry optimization are discussed. A performance analysis of the new geometry optimizer using different start Hessian matrices, basis sets and grid accuracies is given.     

Design and optimization of modified simplex methods

Four test functions are used to evaluate different modifications of the modified simplex method and to optimize the contraction, reflection and expansion parameters. The evaluation shows that some of the modifications and the optimization of the parameters result in a simplex method that has gained in convergence ability, with some improvement in speed.     

Flow injection analysis simulations and diffusion coefficient determination by stochastic and deterministic optimization methods

Stochastic and deterministic simulations of dispersion in cylindrical channels on the Poiseuille flow have been presented. The random walk (stochastic) and the uniform dispersion (deterministic) models have been used for computations of flow injection analysis responses. These methods coupled with the genetic algorithm and the Levenberg–Marquardt optimization methods, respectively, have been applied for determination of diffusion coefficients. The diffusion coefficients of fluorescein sodium, potassium hexacyanoferrate and potassium dichromate have been determined by means of the presented methods and FIA responses that are available in literature. The best-fit results agree with each other and with experimental data thus validating both presented approaches.     

Study of diffusion of impurities in semiconductor silicon by activation analysis and nuclear reaction methods

Diffusion of impurities is a basic operation in the technology of modern semiconductor devices especially integrated circuits. A knowledge of diffusion parameters such as concentration profile, diffusion depth and diffusion coefficient makes the optimization of a technological process possible. A destructive method of the step-by-step removal of the layers from the silicon single crystal slices irradiated in a nuclear reactor with subsequent determination of impurity contents was used in the diffusion studies of phosphorus, arsenic and gold. Phosphorus was determined by the beta counting of³²P after a separation, arsenic and gold were determined by means of a Ge(Li) gamma-ray spectroscopy without any chemical separation. The diffusion of boron was studied by the nondestructive method of the deconvolution of the energy spectrum of prompt alpha radiation from¹⁰B(n, α)⁷Li after thermal neutron irradiation.     

Analysis of high-purity material—A comparison of photon activation analysis with other instrumental methods

Photon activation analysis (PAA) has been a well-established analytical method for about two decades. In this paper, its particular features are outlined in comparison with other modern instrumental techniques, especially radioanalytical methods, e.g. activation analysis with thermal or fast neutrons (NAA) or charged particles (CPA). Emphasis is placed upon the determination of trace components of high-purity material; iron, copper and lead matrix were selected as examples. In PAA, both instrumental and radiochemical procedures were applied. The results are presented in comparison with those obtained by other laboratories using a large variety of different techniques.     

Multielement analysis of groundwater samples by neutron activation with comparison between direct and evaporation methods

This report is the results of the studies on NAA of groundwater samples with comparison between direct and evaporation methods, and of the applications for the samples of alluvium plain composed with granitiform soils and of Neogene strata composed with clay minerals.     

Comparison of activation methods for mercury determination

A comparative study is made on different activation methods for mercury analysis. Mercury concentrations down to 0.1 ppm were determined instrumentally via the isotopes¹⁹⁷Hg (T=65 h) and²⁰³Hg (T=47 d). A high-resolution Ge(Li) detector was used in measuring the activity and a small computer for data reduction. Up to 500 samples were measured daily. Chemical separations were performed on samples with low mercury concentrations. Sensitivity of 0.01 ppm was attained by precipitating HgS from basic solutions and counting¹⁹⁷Hg on NaI(T1) detector. A new rapid instrumental method was also developed based on^199mHg (T=43 min). This short-lived isotope was activated with resonance neutrons. The sensitivity of the method is 0.5 μg and it requires only 1 hr.     

A comparative analysis of various gradient methods for geometry optimization at the ab initio SCF-MO level

Various techniques, already suggested for improving the computational efficiency of the force relaxation method, have been applied here in conjunction with the variable metric algorithms suggested by Murtagh and Sargent and by Fletcher. Various tests have been performed to assess the computational efficiency of the resulting minimization procedures. In addition, procedures where the steps are defined in terms of a parabolic fitting of the forces in approximated normal coordinates have been tested. In all cases the forces are computed analytically.     

Dynamic lattice searching methods for optimization of clusters

Global optimization of clusters is a subject of intense interest in computational chemistry. Especially for large clusters, locating the global minima is a challenging problem. Two strategies are generally used for the problem, i.e., the stochastic optimization and the static modeling strategy. The former is known as unbiased global optimization method, while the latter is more efficient but biased. This review describes the development of a dynamic lattice searching (DLS) approach. In DLS, the lattices are constructed dynamically and optimization is achieved by searching these lattices. Therefore, DLS possesses the characteristics of both the stochastic and static methods. With the aim of improving the efficiency of DLS for optimization of large clusters, several variants of the method have been developed. The results show that DLS methods may be promising tools for fast modeling of large clusters. With this review, greater interests are expected for global optimization of atomic or molecular clusters.