Mathematical study to improve the sensitivity in the neutron activation analysis of fluorspar

A mathematical method to design a neutron activation procedure for analyzing a sample of fluorspar from a concentration plant has been created. The neutron activation is used to bombard a sample with neutrons and record the spectrum radioactive produced. The fluorspar grade is directly proportional to the fluorine content of the sample, and this in turn is directly proportional to the amount of 16N, the radioactive product produced in the bombardment. The ratio between both concentrations has been shown but the aim is to determine the activation parameters that increase the sensitivity of the method. This work discusses the mathematical models used and whose use is expected to increase the sensitivity of the reading.     

Calculated gamma-ray and/or neutron attenuation coefficients in 14 MeV neutron activation analysis

A program of theoretical and experimental work was performed to develop a mathematical model which can represent all the parameters of the neutron activation analysis operation. However, we avoid absolute technique by using a sample containing the same element as the reference but having a different geometry and weight. In the present work we apply this model to determine the neutron, gamma-ray and neutron-and-gamma-ray self-shielding of the sample in relation with the number of counts in the total absorption peak. However the results obtained should be applied within the limitations of the method.     

A study of flux monitoring for instrumental neutron activation analysis

The accuracy of instrumental neutron activation analysis is dependent upon measurement of the neutron flux to which samples are submitted. Flux monitoring techniques described and evaluated include: target cooling water activity, BFa counters, plastic scintillators, and a simple reference sample system. Evaluation is made on the basis of results obtained for oxygen analysis using primary standard materials. The superiority of the reference sample system for flux measurement during irregular neutron flux production is shown.     

New perspectives for very short-lived neutron activation analysis

Recent attempts to improve the performance of very short-lived neutron activation analysis are reviewed. It is shown that the combination of an intense cold neutron beam from a research reactor with a beam chopper offers higher signal-to-background ratio, more accurate timing and much simpler sample handling than conventional cyclic activation analysis. Application of a digital spectrum analyzer in data list mode allows for easy determination of the half-life. Hence, time-resolved activation analysis utilizing energy and time information becomes practical. This revised version was published online in July 2006 with corrections to the Cover Date.     

Instrumental neutron absorption activation analysis

We present and discuss a modification of instrumental neutron activation analysis (INAA) that is sensitive for nuclides that do not yield (suitable) activation products but have high cross sections for neutron absorption. Their presence in a sample may thwart INAA by neutron flux suppression inside the sample, but they remain undetected and thus unnoticed by the analyst. In particular, this refers to Li, B, Cd and Gd. The proposed method—instrumental neutron absorption activation analysis (INAAA)—takes advantage of the flux depression inside the sample caused by the neutron absorbers. It is made visible by addition of an activatable nuclide (indicator). The concentration of the neutron absorber (analyte) causes a decrease in activity of the indicator. The activity difference between a mixed sample (sample plus indicator) and the pure indicator carries the analytical information. The calibration curve hence follows a reciprocal exponential function. In a proof-of-principle experiment, the applicability for the quantification of boron was exemplified. In presence of only one neutron absorber (whose nature is known), INAAA can be applied easily for quantification of the analyte in powdered or liquid samples. Although INAAA is no trace sensitive method, it has the potential to increase the reliability of INAA analyses by fast and straightforward quality control (even in presence of two or more neutron absorbing nuclides). It is especially suited for research reactors that do not operate a prompt gamma neutron activation analysis (PGNAA) station.     

Cyclic activation analysis—A review

The setting of cyclic activation and its development in a historical context is attempted and a case is made for the usefulness of the method in the analysis of materials via short-lived isotopes. The theory of cyclic activation analysis is presented and the resulting detection limits shown to be superior to the conventional one-shot irradiation-counting sequence. The possibility of determining the half-life of an isotope from the data accumulated in the same experiment is also highlighted. Problems and applications are illustrated by experimental results obtained from a variety of sample matrices, mainly biological and environmental, with the use of a reactor irradiation facility, since the use of both isotopic neutron sources and neutron generators are discussed elsewhere. It should therefore be viewed as a personal and selective review of the field. Epithermal cyclic neutron activation analysis is suggested as an area demanding consideration and a new cyclic activation system is introduced. Dedicated to the memory of Dr. BASILEIOS GEORGOUDIS, surgeon and gynaecologist (d 2nd January 1980).     

Small sample in-vivo neutron activation analysis using californium sources

This work describes an in vivo neutron activation analysis facility for small samples, such as rats or human hand, using two 100 g²⁵²Cf neutron sources. The irradiation area is a cylindrical space, of 12 cm diameter and about 15 cm length, with fairly uniform neutron flux distribution. Experimental data on the reproducibility, effects of volume and other conditions for in vivo measurements are given. Comparative atomic absorption data on calcium measurements on rats are reported. The facility is now used for animal experiments as well as human hand irradiations in clinical investigations involving calcium metabolism and bone diseases.     

Evaluation of reactor pulsing activation analysis enhancement in reference materials

The enhancement factors possible by using reactor pulsing during neutron activation analysis have been shown to exceed 1.0 for half-lives shorter than 42 seconds. Studies published to date have applied the technique to pure elemental standards in order to evaluate the advantage. In addition, its use has been demonstrated for the analysis of several environmental samples. This paper describes experimental work performed in an effort to demonstrate the utilization of the technique on a wide range of real sample materials. The purpose of the investigation was to assess the value of the methodology to the neutron activation analysis program of our laboratory in support of university research. A series of sample matrices including reference materials, fossil fuels, geological specimen, ecologically important materials and animal and plant tissues have been subjected to pulsed neutron irradiation. Resulting gamma emissions were detected from samples after each irradiation. Representative spectral data are presented. Suitability of the technique for trace analysis are discussed.     

Improving neutron activation analysis accuracy for the measurement of gold in the characterization of heterogeneous catalysts using a TRIGA reactor

To measure the gold content of a catalyst accurately, neutron activation analysis (NAA) is one of the methods of choice. NAA is preferred for such heterogeneous catalysts because: (1) it requires minimal sample preparation; (2) NAA provides consistent and accurate results; and (3) in most cases results are obtained much quicker than competing methods. NAA is also used as a referee for the other elemental techniques when results do not fall within expected statistical uncertainties. However, at very high gold concentrations, applying NAA to determine the gold in a heterogeneous catalyst is more challenging than a routine NAA procedure. On the one hand, the neutron absorption cross section for gold is very high, resulting in significant self-shielding related errors. On the other hand, gold exhibits low energy resonance neutron absorptions. In this application the self-shielding minimization effort was handled more rigorously than the classic suppression of neutron flux within a specimen. This non-routine approach was used because: (1) for most applications, high accuracy, <3 % relative, is desired, (2) the low energy resonances of gold make its neutron reaction rate complex and (3) the TRIGA reactor flux profile used in this study contains both thermal and significant epithermal neutron fluxes. Accuracy and precision, using this new approach, are expected to improve from 15 % to better than 3 % relative uncertainty. This has been accomplished through a rigorous assessment of the observed effects of low energy resonance on the neutron flux spectral shape within the sample and designing an experiment to minimize the effects.     

Correction for neutron self-shielding in large-sample prompt-gamma neutron activation analysis

When a sample is analysed with neutron activation analysis (NAA) neutron self-shielding and gamma self-absorption affect the accuracy. Both effects become even more important when the mass of a sample analysed is changed from small (say, 1 g) to large (say 30 kg). Therefore, corrections have to be carried out. In this article only the correction method for neutron self-shielding is considered for a thermal neutron beam irradiating large homogeneous samples for prompt-gamma NAA (PGNAA). The correction method depends on the macroscopic scattering and absorption cross sections of the sample. To avoid doing experiments with samples with different macroscopic scattering and absorption cross sections, the Monte Carlo model MCNP is applied in the development of the correction method. The computational development of the method to determine these cross sections through flux monitoring outside the sample is described.     

Multielement determination in water by instrumental neutron activation analysis

Instrumental neutron activation analysis /INAA/ technique has been applied to a water sample to determine the elemental concentrations. The sample was irradiated with a neutron flux of 1.2×10¹² n cm^–2 s^–1 for two different periods followed by counting at three different decay times, using two coaxial type high-resolution Ge/Li/ detectors. The dominant elements determined in the water sample are Ca, Cl, Na, Mg, and K present in ppm-level while Co, I, Mn, Sm, and Sb are present in smaller amounts, approximately on the average 0.01 ppm. Only traces of other elements such as the rare-earth elements, Ag, As, Ba, Cu, Cd, Fe, Sr, W, Zn, seem to be present in the water sample.     

Instrumental neutron activation analysis as a means of evaluation of reference sample homogeneity

Homogeneity is the first and most essential prerequisite of a reference material which merits this marking. A possible way to do homogeneity tests on many elements is offered by instrumental neutron activation analysis. It is shown that this method allows the quick evaluation of sample homogeneity on very small samples and for many elements simultaneosly, regarding macro as well as ultra trace elements.     

A comparison of neutron activation and alpha spectroscopy analyses of thorium in crystalline rocks

The concentrations of Th in samples of crystalline rock from three drillcore sections were analysed independently by instrumental neutron activation analysis and by chemical separation and alpha spectroscopy. The two methods show good general agreement over an approximate concentration range of 1 to 100 ppm Th. Variations in results between the two methods are not of a systematic nature and probably arise from sample heterogeneity. The results confirm the reliability of both methods and provide a useful comparison of the standards and reference materials used. The study indicates that, in cases where Th isotopic information is not required, the simpler and more rapid neutron activation analysis provides a satisfactory method.     

NAAPRO: A code for predicting results and performance of neutron activation analysis

Summary A code is described for predicting the results and main characteristics of neutron activation analysis (NAA) on the basis of a simulated gamma-ray spectrum of activation products, calculated for the specified analysis conditions. These are analysis time mode, analyzed sample mass and elemental composition, characteristics of the irradiating neutron flux and irradiation conditions, gamma-spectrometry measurement geometry and background conditions, as well as detector and spectrometry system parameters. Gamma-ray dose rates for different points of time after sample irradiation and input count rate of the spectrometry system are also predicted.     

Multielemental determination in water by neutron activation analysis

Instrumental neutron activation analysis (INAA) technique has been applied to a water sample to determine the elemental concentrations. The sample was irradiated at a neutron flux of 1.2·10¹² n·cm^–2·s^–1 for two different periods followed by counting at three different decay times, using two coaxial type high-resolution Ge(Li) detectors. The dominant elements detected in the water sample are Ca, Cl, Na, Mg, and K present in levels while Co, I, Mn, Sm, and Sb are present in smaller amounts approximately on the average 0.01 ppm. Only traces of other elements such as rare-earth elements, Ag, As, Ba, Cu, Cd, Fe, Sr, W, Zn, seem to be present in the water samples.     

Neutron activation analysis for 39 elements in small or precious geologic samples

A procedure has been developed for the determination of 39 elements in a single sample of rock or mineral by neutron activation analysis. After the sample has been irradiated with neutrons, it is separated chemically into 12 groups for radioassay with one NaI(Tl) and two Ge(Li) detectors. Three chemists can complete the separations and sample preparations within 7 hrs after the end of the neutron irradiation. Carrier is added and a chemical yield is determined for each element. About two months are required to obtain complete data for a particular sample, but several samples can be analyzed concurrently. In a sample of typical igneous material weighing 0.5 g, half of the elements can be determined to better than ±5 per cent precision and accuracy, and fewer than a fifth with precision and accuracy poorer than ±25 per cent.     

Determination of Na, Mg, Al, Si, K, Cl, Ca and Fe in cigarette tobacco by fast neutron activation analysis

FNAA has been, for many years, a technique for the non-destructive analysis of a wide variety of sample materials-liquids, solids and powders. The important advantages of fast neutron activation analysis are good analytical sensitivity without sample preparation, accuracy and total analysis in a short time. In our work, the concentrations of the elements Na, Mg, Al, Si, K, Cl, Ca and Fe, were determined in cigarette tobacco of two brands commercially available in, Turkey using 14.6 MeV neutron activation analysis.     

Substoichiometry in neutron activation analysis technique

Neutron activation analysis has become one of the most sensitive and selective analytical technique for the determination of trace elements in a wide variety of matrices. Neutron activation involves the irradiation of the test sample and a standard of the element to be determined with thermal neutrons in a reactor, followed by dissolution of the test sample in the presence of carrier of the element to be determined. The carrier and radioisotopes are separated from the bulk of other induced activities (employing precipitation, solvent extraction, ion exchange etc.) and then the activity induced in the sample is measured on a suitable detector. The standard is treated identically. From the ratios of the activity of the sample and standard and the weight of the standard irradiated, the concentration of element in the test sample is calculated. A rapid, selective and sensitive method of radiochemical separation is subtoichiometric extraction in which the same amount of carrier is added to the irradiated test sample and standard. Exactly the same amount of reagent is added to both the sample and standard but in substoichiometric amounts, followed by the separation of the species formed by extracting it with an organic solvent. The activities of the extracts are measured. The amount of element present in the sample is calculated with help of the ratio of the activities and the weight of the standard taken. The advantages of the method are discussed. Application of substoichiometry in neutron activation has been elucidated with reference to the determination of Au in various samples by substoichiometric neutron activation analysis.     

Neutron activation analysis of very pure silicon wafers

Procedure and results of instrumental neutron activation analysis of very pure silicon wafers of 12.5 and 15 cm diameter are described. It is shown that the determined contaminations are mainly present on the surface or in a surface layer of the wafers. With the method outlined here very low limits of detection for a large number of elements are obtained. The analyses also confirmed that two routine process cleaning procedures do not contaminate the surface of the waters.