Neutron activation for precise nuclear data

Techniques for extracting neutron flux parameters pertaining to the irradiation positions and nuclear data pertaining to the isotopes concemed from measured reaction rates have been developed. An adjustment procedure based on the generalized least squares method with incorporates the, evaluated literature nuclear data, estimates of flux parameters and the experimentally measured reaction rates using high resolution -ray spectrometry is applied. More precise values of the nuclear data, i.e., thermal neutron cross sections, resonance integrals and -ray emission probabilities are thus generated. The irradiation of a multi-element standard (MES) containing 24 elements is carried out in six diffieret positions in two diverse types of reactor in the UK and Russia. It is shown that the improvement in nuclear data is revelaed in moire than 90% of the cases.     

Neutron activation analysis without multi-element standards

The paper shows how, from the neutron irradiation of multi-element standards, one can derive neutron flux parameters for the irradiation position and, at the same time, greatly improve knowledge of nuclear data parameters, such as thermal cross sections, resonance integrals and gamma raz emission probabilities, for the nuclides concerned. It is then shown how the resulting neutron flux parameters and nuclear data parameters can be used to carry out neutron activation analysis without further irradiation of multi-element standards. The technique is applied to the analysis of Chinese geochemical reference material     

Some aspects of quality assessment of the k0-based method of NAA

Neutron activation analysis is one of the analytical techniques often used for certification of reference materials. The k₀-based method of instrumental neutron activation analysis can also be applied in intercomparison runs in the certification process and therefore it is desirable to know its accuracy in advance. Possible systematic errors related to the application of nuclear data at given neutron flux rate parameters, that can affect the uncertainties of the results obtained by this specific method, are elucidated and error propagation factors calculated for a typical irradiation position in the TRIGA Mark II reactor of the Jozef Stefan Institute. It was found that these uncertainties are at the level of 1–2% on the average.     

Neutron activation analysis without multielement standards

To carry out neutron activation analysis without using multielement standards requires knowledge of (i) the absolute photopeak efficiency as a function of energy of the gamma-ray detector, (ii) nuclear data for each reaction used, and (iii) neutron flux parameters for the irradiation position. The present paper discusses each of these topics and shows an example of the determination of flux parameters and improved nuclear data.     

Fast neutron activation analysis of rare-earth elements in monazite from Korea

The relative ans single comparator methods have been applied to determine 7 rare-earth elements and U, Th in Korean Monazites by 14.5 MeV neutron activation analysis. The (n, 2n) nuclear reactions are used for all elements except La, for which (n, p) reaction is used. Al is used as a flux monitor for the relative method and as a singlle comparator for the single comparator method. The analytical results obtained by the two methods agree well within 3% deviation except for Sm and Gd. These results are also compared with the result obtained by a single comparator method using reactor neutron.     

Reevaluation of neutron flux characterization parameters for NIST RT-2 facility

It has been difficult to characterize the thermal to epithermal neutron flux ratio (f) and the measure of the nonideal epithermal neutron flux distribution (α) for the RT-2 pneumatic rabbit facility at the NIST National Bureau of Standards Reactor (NBSR). In a previous paper, only cadmium-covered irradiations yielded physically reasonable parameters. New measurements were performed using chromium, manganese, cobalt, zinc, zirconium, molybdenum, antimony, gadolinium, lutetium, and gold. The neutron temperature (T _n ) in RT-2 measured using bare lutetium and gold foils gave unphysical values. The bare foil methods for measuring f and α gave inconsistent results. The underlying reasons are demonstrated via MCNP simulation results for cumulative reaction rates of selected isotopes. To determine expected intervals for f, α and T, parametric methods were explored. Measured reaction rate probability per target atom (R _p ) values for the listed elements were fitted to a modified Westcott curve using an iterative least-squares method to verify consistency of measurements and nuclear data. An advanced parametric approach using a detailed MCNP model of the NBSR was used to calculate neutron flux characterization parameters.     

A PC-Program for the Calculation of Neutron Flux and Element Contents Using the ki-Method of Neutron Activation Analysis

A computer program is described, which calculates the induced activities of isotopes after irradiation in a known neutron field, thermal and epithermal neutron fluxes from the measured induced activities and from nuclear data of 2–4 monitor nuclides as well as the element concentrations in samples irradiated together with the monitors. The program was developed for operation in Windows 3.1 (or higher). The application of the program for neutron activation analysis allows to simplify the experimental procedure and to reduce the time. The program was tested by measuring different types of standard reference materials at the FRJ-2 (Research Centre, Jülich, Germany) and Triga (University Mainz, Germany) reactors. Comparison of neutron flux parameters calculated by this program with those calculated by a VAX program developed at the Research Centre, Jülich was done. The results of testing seem to be satisfactory.     

Standardization of NAA Using the Activation Equation and Isotope-Related Constants

A standardization method for NAA has been developed based on the activation equation and on isotope-related k _i -values that are composite nuclear constants. This approach combines the simplicity of the absolute methods with nearly the same accuracy as that obtained by the relative ones. The k _i -constants are calculated from the well measured k ₀-factors and the neutron flux is determined as with the k ₀-method. A computer program was developed for the determination of the neutron flux parameters and the concentration of the trace element. These calculations start with the measured activity of the radionuclides.     

Determination of uranium element concentration and235U isotope abundance by neutron activation analysis

A new neutron activation technique has been developed for the determination of uranium element concentration and²³⁵U isotope abundance in nuclear safeguards and reference material samples based on the activation of bare and cadmium-covered samples with different thermal to epithermal neutron flux ratios and on the combination of the two corre-sponding delayed-fission neutron measurements. The principle of the new technique can be applied also to improve multi-element neutron activation analysis.     

Neutron spectrum parameters in irradiation channels of the Nigeria Research Reactor-1 (NIRR-1) for the <Emphasis Type="Italic">k</Emphasis><Subscript>0</Subscript>-NAA standardization

Summary The first nuclear research reactor in Nigeria has been commissioned for neutron activation analysis and limited radioisotope production. In order to extend its utilization to include the k₀-standardization method, the following neutron spectrum parameters in inner and outer irradiation channels were determined by the “Cd-ratio for multi-monitor method”: the thermal-to-epithermal flux ratio, f, and the epithermal flux shape factor, α. Neutron spectrum parameters determined in the inner irradiation channel B2, are: α = -0.052±0.002 and f = 19.2±0.5. For the outer irradiation channel B4, the neutron spectrum parameters were found to be α = +0.029±0.003 and f = 48.3±3.3. The results are compared with the neutron spectrum parameters of other reactor facilities with similar core configuration such as the Slowpoke and Miniature Neutron Source Reactor facilities available in the literature.     

On the determination of silicon in pottery

To study the nature of ancient and modern pottery it is necessary to be able to determine the concentrations of the major constituent elements. For such studies, mass balances calculated from these elemental concentrations cluster around 100% for a silicate-silica matrix and around 80–90% in highly calcarious pottery which has a silicate-carbonate (silica) matrix. This work requires experimental measurements with coefficients of variation wihtin approximately ±5%. The high concentrations of silicon in pottery may reasonably be determined by neutron activation analysis using the nuclear reaction²⁹Si(n, p)²⁹ Al. Aluminium-29 has a half life of 6.56 minutes and a γ-ray energy of 1272 keV. Epithermal neutron irradiations of samples in cadmium foil are required to minimize the quantities of thermal neutron induced radionuclides. The method of analysis developed for the low flux SLOWPOKE reactor is described and the accuracy and precision of the technique is discussed by allusion to analyses of standard reference materials. The application of this technique to two different archaeological problems is addressed. The first is a case of carbonate dilution found in neolithic pottery from Iran and the second case is a problem of silica dilution in Roman pottery from Germany.     

Investigation of gamma-spectra of biological materials irradiated in the epithermal range of the reactor neutron spectrum

Irradiation of various biological materials in the nuclear reactor neutron epithermal flux allows the increase of the selectivity of the pure instrumental method of activation analysis for the definite set of tracers either not determined instrumentally during sample irradiation by the thermal neutron flux or determined with low sensitivity. The given paper describes a method of increasing the sensitivity of the instrumental neutron activation determination of As, Br, Mo, Cd, Sb, W and Au in biological materials at the level of the whole blood, tissue and subcellular components.     

Measurement of neutron spectrum parameters for NAA irradiation holes in the Jordan research and training reactor

Three irradiation holes coupled to a pneumatic transfer system were installed for neutron activation analysis in the Jordan Research and Training Reactor (JRTR), which is the first research reactor in Jordan. To perform instrumental neutron activation analysis, neutron spectrum parameters, such as thermal neutron flux, α and f for the irradiation holes, should be measured. The Cd-ratio method was applied for the determination of the aforementioned parameters. For this purpose, 0.1% Au–Al wires and Zr foils were irradiated with and without Cd-cover, and the Cd ratios were determined for Au-198, Zr-95, and Zr-97/Nb-97m nuclides. Then, the parameters were calculated and determined at three irradiation holes.

     

The epithermal neutron flux distribution in a nuclear reactor and its effect on epithermal neutron activation analysis

The shape of the epithermal neutron energy distribution has been determined in two irradiation positions of the University of London CONSORT II reactor. The method applied involves cadmium ratio measurements using a series of resonance detectors. Principles of the method and some considerations relative to epithermal neutron activation analysis in connection with the deviation of the epithermal neutron flux distribution from the 1/E law are given.     

Implementation of a methodology to analyse cylindrical 5-g sample by neutron activation technique,k 0 method,at CDTN/CNEN,Belo Horizonte,Brazil

Neutron activation analysis (NAA) is routinely applied to geometrical point-source or small samples because there are technical and theoretical difficulties to analyse larger samples weighing more than 0.5 g. The analysis of larger samples is very advantageous, because the analytical procedure will be less time consuming, it may be possible to reach lower detection limit for several elements, it decreases cost and overcomes the difficulties related to the representativeness of the sample when dealing with inhomogeneous volume or several small samples. Thus, increasing the amount of sample is a way to compensate for low flux of neutrons. This paper is about the establishment of a method at Laboratory for Neutron Activation Analysis, CDTN/CNEN, to determine the elemental concentrations in 5 g-samples, 25 times larger than usual samples analysed by neutron activation, k ₀ method, keeping the current irradiation and gamma spectrometry facilities. To develop this method, several aspects were evaluated such as detector efficiency over the volume source, neutron self-shielding during neutron irradiation, axial neutron flux gradient and gamma ray attenuation within the sample during counting. The results suggest that if an appropriate adjustment of the above mentioned parameters is done, the k ₀ method of NAA can provide satisfactory results also for larger samples than the samples typically used in NAA. The KayWin software proved to be a robust program analysing the larger samples weighing 5 g and cylindrical geometry as if it were a small cylindrical sample, producing reliable results. It was successfully implemented at Belo Horizonte, Brazil, fully following the basic principles of the k ₀ standardization method.     

Using synthetic multi-element standards (SMELS) for calibration and quality control of the irradiation facilities in the BR1 reactor

The thermal to epithermal neutron flux ratio (f) and the deviation of the epithermal neutron spectrum from the 1/E shape (α) are essential parameters for the correct application of k ₀-standardized neutron activation analysis. Several methods are applied for the determination of f and α. They are based on Cd-covered multi-monitor or on bare-irradiations methods. The recently developed and characterized synthetic multi-element standards (SMELS) were designed as a validation tool for the proper implementation of the k ₀-NAA method in a laboratory. In particular, SMELS Type III contains Au and Zr, thus allowing the direct determination of f and α. It could, therefore, replace the traditional flux monitors. Furthermore, it could be used as a quality control material to monitor the stability of the irradiation facility and the detector. This paper presents the accuracy of the f and α determination and the feasibility of quality control using SMELS for irradiation channel Y4 of the BR1 reactor.     

Analysis of Brazilian snake venoms by neutron activation analysis

Instrumental neutron activation analysis (INAA) has been applied to multielemental determinations of Brazilian snake venoms from the species: Bothrops jararacussu, Crotalus durissus terrificus and Bothrops jararaca. Concentrations of Br, Ca, Cl, Cs, K, Mg, Na, Rb, Sb, Se and Zn have been determined in lyophilized venoms by using short and long irradiations in the IEA-RI nuclear reactor under a thermal neutron flux of 10¹¹ to 10¹³ n⁰ ·cm^–2·s^–1. The reference materials NIST bovine Liver 1577 and IUPAC Bowen's Kale were also analyzed simultaneously with the venoms to evaluate the accuracy and the reproducibility of the method. The concentrations of the elements found in snake venoms from different species were compared. The Crotalus durissus terrificus venoms presented high concentration of Se but low concentrations of Zn when these results are compared with those obtained from genera Bothrops venoms.     

The determination of Eu and Sm by application of X-ray spectrometry to isotope-source activation analysis

The use, for analysis, of K X-rays proceeding from metastable and beta-emitting radionuclides—produced by neutron activation—is an undeveloped aspect of activation analysis. Consequently, this study examines the feasibility of applying X-ray spectrometry to isotope source activation analysis and in particular the determination of Sm and Eu at low levels is described. Generally speaking, the sensitivity is seriously restricted by the thermal neutron flux of the source but in certain specific cases the analysis is favoured by the nuclear properties of the elements concerned. Therefore, the selection of elements was based largely on their demand for analysis and their ability to produce a practicable yield. Optimum signalto-noise ratios were attained by employing a detector that was particularly sensitive at energies below 150 keV. Analytical conditions are demonstrated for the elements of interest over a wide range of concentrations in small samples, and ultimately the applicability of the method was tested by the analysis of a typical monazite sample. The analytical potential of the method is thoroughly appraised and possible practical applications are discussed.     

ko-measurements and related nuclear data compilation for (n, γ) reactor neutron activation analysis

k_o-factors of 35 isotopes used in reactor neutron activation analysis were measured with a high degree of accuracy (1–2%). To minimize systematic errors, measurements were carried out using different reactor types, irradiation conditions (18 < Φ_s/Φ_e), Ge(Li) detectors, sample detector geometry, etc. Analyst-oriented tabulations including all necessary nuclear data, “best values”, as well as recommended k_o-values are given to facilitate analytical work with the new method. Some practical aspects as well as limitations of the k_o-method are also outlined together with the applied neutron flux and cross-section conventions. Research associate of the National Fonds voor Wetenschappelijk Onderzoek, Belgium     

The determination of ko factors and their application to the neutron activation analysis of pure zirconium metal by the single-comparator method

The procedures followed for the determination of standardization factors and their use in the single-comparator method for multi-element reactor neutron activation analysis are described. The method is applied to the determination of Cr, Mn, Fe, Ni and Mo in pure zirconium metal. The matrix could serve as the single comparator and as a bi-isotopic thermal-to-epithermal neutron flux ratio monitor.