Fast neutron activation analysis using short-lived radionuclides

Fast neutron activation analysis experiments were performed to investigate the analytical possibilities and prospective utilization of short-lived activation products. A rapid pneumatic transfer system for use with neutron generators has been installed and applied for detecting radionuclides with a half-life from 300 ms to 20 s. The transport time for samples of total mass of 1–4 g is between 130 and 160 ms for pressurized air of 0.1–0.4 MPa. The reproducibility of transport times is less than 2%. The employed method of correcting time-dependent counting losses is based on the virtual pulse generator principle. The measuring equipment consists of CAMAC modules and a special gating circuit. Typical time distributions of counting losses are presented. The same 14 elements were studied by the conventional activation method (single irradiation and single counting) by both a typical pneumatic transport system (run time 3 s) and the fast pneumatic transport facility. Furthermore, the influence of the cyclic activation technique on the elemental sensitivities was investigated.     

14 MeV neutron activation analysis using short-lived products

Cyclic activation using pneumatic shuttling system and switch off and on the neutron source and detector are described in order to eliminate some uncertainties by the provision of more accurate timing, the measurement of the effective activating neutron flux and the correction for the detection system dead time.     

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.     

Epithermal neutron activation analysis of short-lived nuclides in geological material

The cadmium ratios of 52 short-lived nuclides have been measured. Epithermal neutron irradiation reduces the activities of²⁰F,²⁷Mg,²⁸Al,³⁸Cl,⁴⁹Ca,^46mSc,⁵¹Ti,⁵⁶Mn and⁶⁶Cu by factors of 20–30. The calculated improvements in detection limits for Ga, Br, Rb, Y, Mo, Rh, Pd, Ag, In, Sn, Sb, I, Ba, Nd, Sm, Gd, Dy, Er, Yb, Hf, W, Re, Pt, Au, Th and U are in the range 1–6. Hafnium was measured in USGS rocks: AGV-1 (4.9 μg g⁻¹), G-2 (7.5 μg g⁻¹) and GSP-1 (14.7 μg g⁻¹) and IAEA standards: SOIL-5 (6.3 μg g⁻¹ and SL-1 (4.6 μg g⁻¹). CCRMP reference concentrates PTC and PTM were analysed for rhodium (1.1 and 0.75 μg g⁻¹, respectively) and silver (69 and 5.8 μg g⁻¹, respectively).     

Precision high-speed neutron activation analysis via very short-lived activities

An automated, highly reproducible fast-transfer system (0.36±0.01 sec) has been developed, an oscillator/multichannel-scaling system has been developed to follow rapidly-changing spectrometer dead-times, the mathematics has been developed, and the data processing steps have been computerized. Three induced activities have been studied in some detail:^207mPb (0.80 sec),^38mCl (0.70 sec) and²⁰F (11.03 sec). Both steady-state and high-intensity pulsed reactor irradiations, with and without a Cd liner, have been used. The dead-time correction method has been carefully checked, and the overall method has been tested by the analysis of two NBS Standard Reference Materials: orchard leaves (for Cl and Pb) and bovine liver (for Cl). This paper represents a portion of the Ph. D. thesis of D. A. MILLER.     

Errors in instrumental neutron activation analysis based on short-lived radionuclides

Accuracy and precision of instrumental neutron activation analysis, based on short-lived radionuclides, are considered. Equations are derived for both quantitives as a function of concentration and features of the γ-ray spectrum. The determination of vanadium by way of⁵²V(T=3.75 min) is used as an example.     

Recent instrumental neutron activation analysis studies utilizing very short-lived activities

The advantages and disadvantages of utilizing very short-lived induced activities (half lives in the range of 0.3 sec to 60 sec) in neutron activation analysis are reviewed. Results obtained with the recently described fast-transfer system and deadtime correction system are presented. They include results for 12 elements (F, S, Cl, Sc, Ge, Se, Br, Y, Zr, Ba, W, Pb), in both steady-state and high-intensity pulse irradiations, in both unlined and cadmium-lined irradiation positions. The half lives of the 12 species studied range from 0.3 sec to 18.7 sec. Some further aspects of the deadtime correction system are discussed.     

Accounting for flux decrease in neutron activation analysis

The problem of neutron flux decrease during the activation of samples is discussed. Data by different authors on the weight of spherical samples corresponding to a flux decrease of 10% are reported. The self-shielding effect due to the presence of constituents with large absorption cross-sections is discussed.     

Compton suppression spectrometry for analysis of short-lived neutron activation products in foods

Compton suppression spectrometry was used to analyze foods for elements with short-lived neutron activation products (half-lives of about 2 minutes to 1.5 days). Analysis conditions were optimized to provide quality assurance analyses for iodine in FDA’s Total Diet Study. Iodine mass fractions (0.075 to 2.03 mg/kg) were measured in 19 of 42 foods analyzed, with limits of detection (LODs) ranging from 0.03 to 1.4 mg/kg, mostly depending on NaCl content. LODs were lowered by up to a factor of 2 for 16 elements. Suppression factors ranged from about 2 to 8 over the energy range 400 to 3200 keV.     

Radiochemical separation of mostly short-lived neutron activation products

Journal of Radioanalytical and Nuclear Chemistry - It is demonstrated that radiochemical separation in neutron activation analysis (RNAA) is usually the most effective means of optimization of the...     

Routine multielement instrumental neutron activation analysis of silicate rocks using short-lived radionuclides

By means of thermal neutron activation and countings on the small planar and large coaxial Ge(Li) detector, 13 elements are determined in various silicate rocks, using short-lived radionuclides with half-lives from 2 min to 15 h. A method of routine analysis with simple dead-time and pile-up correction is described and tested with the standard rocks AGV-1 and G-2.     

An automated neutron activation system for short-lived nuclides

The present paper describes an attempt to create a reliable and easy to use system for neutron activation with short-lived nuclides, suitable to be used with several irradiation and counting procedures in instrumental neutron activation analysis (INAA) at the Portuguese Research Reactor. This system can become very useful for a large community within LNETI, as well as other national institutes and universities, particularly for those involved in studies where the analysis of a large number of samples is required. Selenium determination has been performed, through the short-lived nuclide^77mSe, as an example of the reliability of the system.     

Measurement of the neutron flux distribution in a research reactor by neutron activation analysis

Journal of Radioanalytical and Nuclear Chemistry - The neutron flux distribution in the core region of the JRTR was measured through the neutron activation analysis method. Sample mount and a tool...     

Validation of a loss-free counting system for neutron activation analysis with short-lived indicators

The use of loss-free counting systems makes possible the exact correction for pile-up and dead-time losses during counting of a mixture of short-lived radionuclides even at very high count rates. However, counting statistics cannot be calculated by taking into account only the Poisson distribution of the incoming -quanta, such as is done in existing computer programs for -spectrometry. At moderate count rates Müller statistic was found to account for the observed variability between duplicate countings; however, at higher count rates the variability of weighing factors was found to be significant in comparison with the Müller statistic. While counting statistics could not be correctly estimated for short-lived species, experiments showed excellent accuracy for initial dead times up to 90%.     

Beta spectroscopy of short-lived radionuclides using in-cocktail neutron activation

Low-flux, in-cocktail neutron activation analysis was used to quantify atmospheric tracer aerosols. Liquid scintillation was necessary to reduce the experimental uncertainty due to low sample activity. Activation of the tracers was performed in-cocktail since the short half lives of the nuclides did not allow time for sample preparation. Most of the tested cocktails proved inadequate for such analysis because of activation of their components. Neutron and gamma induced luminescence, and neutron generated bubbles in suspensions introduced further difficulties. Only a few cocktails showed activity levels after irradiation comparable to background. This method allowed substantial cost reduction by eliminating the need for irradiation in a research reactor.     

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.     

A software counting loss correction method for neutron activation analysis with short-lived radionuclides

A method has been developed for the correction of counting losses in NAA for the case of a mixture of short-lived radionuclides. It is applicable to systems with Ge detectors and Wilkinson or successive approximation ADC's and will correct losses from pulse pileup and ADC dead time up to 90%. The losses are modeled as a constant plus time-dependent terms expressed as a fourth order polynomial function of the count rates of the short-lived radionuclides. The correction factors are calculated iteratively using the peak areas of the short-lived radionuclides in the spectrum and the average losses as given by the difference between the live time and true time clocks of the MCA. To calibrate the system a measurement is performed for each short-lived nuclide. In a test where the dead time varied from 70% at the start of the measurement to 13% at the end, the measured activities were corrected with an accuracy of 1%.     

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.     

Cumulative neutron activation of short-lived radionuclides for the analysis of large samples in mineral exploration

Cumulative neutron activation is used to determine mg/kg concentrations of gold in rock samples, weighing 10 g. Twenty 0.5 g replicates are irradiated in an epithermal-neutron flux for 5 s and counted for 14 s. The twenty spectra are summed, and gold is measured with the 279 keV gamma-ray of^197mAu (7.2 s). Total analysis time for a 10 g sample is 400 s and detection limits around 2 mg/kg are achieved in gold ores, sulfides and silicates.