Non-destructive analysis of objects using neutron resonance capture

Summary In this paper we discuss an analytical method, neutron-resonance-capture-analysis (NRCA), which uses the energies of resonances to recognize elements. These resonances are observed by the time-of-flight technique using a pulsed neutron beam at the GELINA facility in Geel (Belgium). The prompt γ-radiation, emitted after a neutron is captured, is used to signal that capture occurred. Areas of resonance peaks are used to evaluate the elemental composition of objects and materials quantitatively. A comparison of NRCA and PGAA is made and elucidated with experimental data resulting from measurements of bronze artefacts at GELINA and at the Budapest Neutron Centre.     

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.     

Neutron resonance capture analysis: Improvements for resonances above 3 keV and new applications

In neutron resonance capture analysis (NRCA), the detection of resonances in the neutron cross section as a function of neutron energy with the time of flight (TOF) technique is used to identify and quantify isotopes. In this paper the improvements of the measuring conditions are discussed in order to reduce the high background, especially in the keV range. The detection efficiency was generally improved, especially for resonances above 3 keV, which allowed the detection of lighter elements, usually having resonances only in the keV region. This broadened the number of applications that could be targeted with NRCA.     

Performance of flowing sample neutron activation analysis technique for determination of multi-elemental content

In order to measure and obtain reliable data about elements via measuring of short-lived isotopes in liquid samples using neutron activation analysis, experiments have been carried out using a flowing sample neutron activation analysis method. During this work the performance and reliability for the proposed method have been tested using synthetic multi elements liquid samples. The radionuclide, associated elements and their concentration level have been measured using a counting system with a fixed dead-time. An HPGe spectrometry system was used to determine the most important γ-ray energies and intensities of these isotopes. The obtained results demonstrated the reliability and accuracy (bias <5 % and zeta score <2) of the tested method for determination the elemental content of liquid samples.     

The diminution of the neutron odd-even effect due to prompt neutron emission in low energy nuclear fission

Two fissioning systems [²³⁵U(n_th, f) and²⁵²Cf(sf)] have been studied to see the consequence of prompt neutron emission on odd-even effects. Monte Carlo calculations have been carried out for prompt neutron emission. Fragment and product yields have been analyzed by the WahlZ _p Model, the Method of Differences and Model Independent Calculations. The result of the wash-out effect of prompt neutron emission on the neutron odd-even effect has been shown; however, this was not seen with the protons.     

Determination of cadmium in zinc ores by thermal neutron absorption analysis

A method has been developed for routine determination of cadmium in zinc ores by thermal neutron absorption analysis, based on the attenuation of a thermal neutron flux passing through a neutron absorbing material. The thermal neutron flux is related to the⁵²V-activity induced in a vanadium detector, surrounded by pellets pressed from a mixture of powdered material with graphite. Besides cadmium, also the major constitutents zinc, iron and sulfur contribute significantly to the total attenuation of the thermal neutron flux. Calibration lines for these elements are worked out. All irradiations are carried out for 200 s in the partially thermalized neutron flux of a 5 Ci²²⁷Ac—Be isotope neutron source. After a decay of 30 s, the⁵²V-activity of the vanadium detector is measured for 400 s with a NaI(T1) scintillation detector. The analysis sequence, including the computation of the results from the counting data, is automated by means of a LSI—11 microprocessor with 12K×16 bit memory. Zinc ores, containing 0.02 to 1.45% cadmium, have been analyzed with a precision ranging from 12.6% to 0.54% relative. As a test for the reliability of the method, two NBS standard reference materials were analyzed in the same way as the zinc ore samples.     

Rare-earth elemental analysis of banded iron-formations by instrumental neutron activation analysis

Representative banded iron-formations (BIFs) from various locations of the eastern Indian geological belt were investigated by instrumental neutron activation analysis (INAA). After pre-concentration, irradiation was carried out using a neutron flux of 5.1·10¹⁶ m⁻²·s⁻¹, 1.0·10¹⁵ m⁻²·s⁻¹ and 3.7·10¹⁵ m⁻²s⁻¹, with thermal, epi-thermal and fast neutrons, respectively. The activities in these samples were measured by a HPGe detector. Ten rare-earth elements, such as La, Ce, Nd, Sm, Eu, Tb, Ho, Tm, Yb and Lu, have been qualitatively identified and quantitatively estimated in these samples. The present investigation is an example of employing a pre-concentration method for high iron-containing ores prior to neutron activation analysis.     

Determination of uranium in thorium matrixes by epithermal neutron activation analysis

Uranium in thorium matrixes or in minerals and ores containing thorium is determined by epithermal neutron activation analysis (ENAA). In some minerals and ores, such as monazite sands, the analysis can be carried out by purely instrumental means with no chemical separation of uranium or thorium from the irradiated matrix. For thorium compound matrixes with very low uranium contents, a rapid radiochemical separation method, based on the retention of uranyl ion on anion-exchange resins, is first carried out, before counting the gamma-ray peaks for²³⁹U in multichannel analysers coupled to NaI(Tl) scintillators or to Ge(Li) detectors.     

Capillary neutron optics for prompt-gamma activation analysis

A neutron lens has been constructed to focus cold neutrons from the exit of a⁵⁸Ni neutron guide, which delivers a beam to the Prompt-Gamma Activation Analysis (PGAA) station at the NIST Cold Neutron Research Facility. The lens compresses a neutron beam of cross section 50 mm× 45 mm onto a focal spot of diameter 0.53 mm (fwhm) wich an average gain of 80 in neutron current density. PGAA measurements have been performed to demonstrate the enhanced sensitivity and detection limits for various elements and the spatial resolution in one transverse dimension. For the two test particles (a gadolinium glass bead and cadmium metal of sizes less than 0.5 mm), the gain in the -count rate with the lens is a factor of 60, and the detection limit is improved by a factor of 20. The system can be used for two-dimensional mapping of samples on a sub-millimeter scale to complement other analytical techniques such as neutron depth profiling (NDP).     

Rare earth element (REE) in surface mangrove sediment by instrumental neutron activation analysis

A study is carried out on the concentrations of rare earth element (REE) elements present in surface mangrove sediments from 10 locations throughout west coast Malaysia. In carrying out the analysis, the best and most convenient method being the instrumental neutron activation analysis (INAA). Samples were obtained, dried, crushed to powdery form and samples prepared for INAA. All the samples for analysis were weighted approximately 150 mg for short irradiation and 200 mg for long irradiation time. As calibration and quality control procedures, blank samples, standard reference material SL-1 were then irradiated with thermal neutron flux of 4 × 10¹² cm^?2 s^?1 at the MINT TRIGA Mark II research reactor which operated at 750 kW by using a pneumatic transport facility. The REE elements of surface sediment samples in this study are Dy, Sm, Eu,Yb, Lu, Tb, La and Ce. It was found that the level of concentrations of all the REE elements varies in the range (0.35–117.4 mg/kg). The geochemical behavior of REEs in surface sediments and normalized pattern (chondrite and shale) has been studied. The degree of sediments contaminations were computed using an enrichment factor. The results showed that the enrichment factor varied in the range (0.75–6.75).     

Determination of characteristic neutron flux parameters of nuclear reactors for thermal neutron activation analysis

The characteristic neutron spectrum parameters for thermal neutron activation analysis have been determined for the most important irradiation positions of the reactors BR1 and BR2 at Mol (Belgium), HFR at Petten (Netherlands) and FRJ2 at Jülich (Federal Republic of Germany). The method of determination is described.     

Measuremets of presolar grain in meteorite using neutron activation analysis with multi-parameter coincidence method

Neutron activation analysis with multi-parameter coincidence method was developed at the Japan Atomic Energy Agency (JAEA) and a non-destructive, ultra-high sensitive multi-elemental determination has been realized. The multi-parameter coincidence method is carried out with an array of 19 germanium detectors, GEMINI-II. Using this system, very weak γ-rays emitted from trace amounts of elements can be detected. The presolar grains were extracted from the Allende meteorite. Trace elements in the presolar diamonds were measured by neutron activation analysis with multi-parameter coincidence method.     

Experimental and Monte Carlo simulation on new manganese bath facility for absolute neutron source emission rate measurement at ENEA-INMRI

This work deals with the absolute measurement of the neutron emission rate from a ²⁴¹Am–Be source by means of the manganese sulphate bath technique, which is the principal method for the absolute determination of the neutron emission rate from radionuclide neutron sources. The facility consists of a spherical container filled with an aqueous solution of manganese sulphate with a ²⁴¹Am–Be neutron source placed at the center. As well known, neutrons from the source, after having been thermalized by the aqueous solution, undergo neutron capture by hydrogen, manganese, sulphur, and oxygen nuclei, thus inducing a certain activity to the solution. Subsequent gamma spectrometry measurements of ⁵⁶Mn activity generated by ⁵⁵Mn neutron activation allows to determine the neutron emission rate of the source, The experimental activity has involved a variety of measurement techniques and calculation procedures, ranging from neutron reactor activation to liquid scintillation counting and Monte Carlo calculations. Neutron activations of ⁵⁵Mn samples has been carried out with the TRIGA reactor of the ENEA-Casaccia Research Centre, and ⁵⁶Mn activated samples were subsequently characterized by liquid scintillation counting, in order to obtain reference standards for the calibration of the NaI(Tl) scintillation detectors utilized to record gamma-ray emission from ⁵⁶Mn. Monte Carlo calculations, carried out by the MCNPX code, were required to calculate neutron transport within the sulphate manganese bath, in particular to determine ⁵⁵Mn neutron capture probability, and (n, α) and (n, p) concurrent reactions, as well as the neutron leakage. Such a procedure has allowed to maintaining the neutron emission rate uncertainty well below 1 %. All the measurements have been carried out at the ENEA-Casaccia Research Centre by the Italian National Institute of Ionizing Radiation Metrology.     

Neutron self-shielding correction for prompt gamma neutron activation analysis of large samples

Summary Facilities and methods for INAA of large samples (up to 30 kg) have been developed and successfully tested at IRI, Delft. The methods encompass corrections for neutron self shielding in an isotropic neutron field and gamma self-absorption. The sample’s neutron absorption and scattering characteristics are determined by monitoring the neutron fluence rate around the sample and comparing the neutron densities measured with unperturbed fluence rates. We report on the possibility of developing similar methods for PGNAA. Relative self-shielding factors were measured as well as obtained from Monte Carlo computations. The agreement is good except for the most extreme case, with respect to absorption, attempted (CCl₄).     

Instrumental neutron activation analysis of gunpowder residues

A procedure has been developed for the detection of gunpowder residues deposited on the hand of a person firing a gun. The method is based on neutron activation analysis of the antimony level on the surface of the hand. The surface materials are removed by a film made by spraying a 4% solution of cellulose acetate in acetone, which sets to form a thin film that can readily be stripped off. This technique was found to be preferable to the paraffin-lift technique which is in common use. Following neutron activation of the film in a nuclear reactor, antimony is assayed by high-resolution Ge(Li) spectrometry without prior chemical processing. The sensitivity of the method is about 5·10⁻⁹ g Sb, with a precision of about ±10% at a neutron flux of 5·10¹³ n·cm⁻²·sec⁻¹. Analysis of twenty samples taken from the hands of persons who had fired a pistol gave Sb levels of 0.4±0.2 μg, compared with 0.024±0.013 μg found on the hands of persons who had not fired a revolver. The possible extension of the present technique to include the determination of additional elements is discussed. Project carried out with the support of the Office of the Chief Scientist to the Ministry of Defense and with the collaboration of the Israel Police.     

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.     

Influence of the effective mass on the relative neutron density distribution inside a large sample in prompt-gamma neutron activation analysis

A Monte Carlo study was carried out to determine the influence of the effective scattering mass (M _e) of the atoms on the neutron density profile inside and outside the sample illuminated by a thermal neutron beam as in large-sample prompt-gamma neutron activation analysis (LS-PGNAA). From theory it is known that the spatial neutron density distribution (n(r)) inside a large sample is not the same for atoms with the same macroscopic scattering and absorption cross-section (Σ _s and Σ _a) but different M _e, due to anisotropic scattering at low M _e. The probability of neutron absorption in the sample was found to be the same for materials with equal Σ _s and Σ _a but different M _e, even though the neutron density distribution in the sample was found to change slightly. In view of typical sample, collimator and detector dimensions, it is concluded that M _e does not need to be taken into account in a correction method for neutron self-shielding in LS-PGNAA.     

Performance comparison of an 241Am-Be neutron source-based PGNAA setup with the KFUPM PGNAA setup

Monte Carlo calculations have been carried out to compare the performance of an ²⁴¹Am-Be neutron source-based prompt gamma neutron activation analysis (PGNAA) setup with that of the 2.8 MeV neutron-based PGNAA setup at King Fahd University of Petroleum and Minerals (KFUPM) to analyze Portland cement samples. This work is a part of a wide Monte Carlo studies being conducted at KFUPM in search of a more efficient neutron source for its 2.8 MeV neutrons, from the D(d,n) reaction, based PGNAA facility. In this study an ²⁴¹Am-Be neutron source-based PGNAA setup was simulated. For comparison, the diameter of a cylindrical external moderator of the ²⁴¹Am-Be neutron source, based PGNAA setup, was assumed to be similar to that used in the KFUPM PGNAA setup. The results of this study revealed that although the optimum geometry of the ²⁴¹Am-Be neutron source-based setup is similar to that of the KFUPM PGNAA facility, the performance of the ²⁴¹Am-Be neutron source-based setup is slightly poorer than that of the 2.8 MeV neutron-based setup. This revised version was published online in July 2006 with corrections to the Cover Date.     

H*(10) and neutron spectra around linacs

Neutron spectrum and ambient dose equivalent has been measured around two 10 MV linear accelerators. Measurements were carried out in Mevatron and Primus model linacs, both made by Siemens. Main differences between those models are the beam collimator and the vault room. Here, Bonner sphere spectrometer with a passive thermal neutron detector has been utilized to measure the neutron spectrum inside the vault. Using an active detector the neutron spectrum was measured at the vault’s door of both accelerators. With a neutron area monitor the dose equivalent was measured by the door at both vault doors. Neutron strength, total fluence rate and ambient dose equivalent were compared, from this was found that shielding conditions are better in the Primus model.     

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.