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.     

Design and simulation of neutron radiography system based on 241Am–Be source

Neutron imaging is extended rapidly as a means of non-destructive testing (NDT) of materials. Various effective parameters on the image quality are needed to be studied for neutron radiography system with good resolution. In the present study a portable system of neutron radiography has been designed using ²⁴¹Am–Be neutron source. The effective collimator parameters were calculated to obtain relatively pure, collimated and uniform neutron beam. All simulations were carried out in two stages using MCNPX Monte Carlo code. In the first stage, different collimator configurations were investigated and the appropriate design was selected based on maximum intensity and uniformity of neutron flux at the image plane in the outlet of collimator. Then, the overall system including source, collimator and sample was simulated for achieving radiographic images of standard samples. Normalized thermal neutron fluence of 2.61×10^?5 cm^?2 per source particle with n/γ ratio of 1.92×10⁵ cm^?2 μSv^?1 could be obtained at beam port of the designed collimator. Quality of images was assessed for two standard samples, using radiographic imaging capability in MCNPX. The collimated neutron beam in the designed system could be useful in a transportable exposure module for neutron radiography application.     

Monte Carlo calculations and neutron spectrometry in quantitative prompt gamma neutron activation analysis (PGNAA) of bulk samples using an isotopic neutron source

An activation analysis facility based on an isotopic neutron source (185 GBq ²⁴¹Am/Be) which can perform both prompt and cyclic activation analysis on bulk samples, has been used for more than 20 years in many applications including 'in vivo' activation analysis and the determination of the composition of bio-environmental samples, such as, landfill waste and coal. Although the comparator method is often employed, because of the variety in shape, size and elemental composition of these bulk samples, it is often difficult and time consuming to construct appropriate comparator samples for reference. One of the obvious problems is the distribution and energy of the neutron flux in these bulk and comparator samples. In recent years, we have attempted to adopt the absolute method based on a monostandard and to make calculations using a Monte Carlo code (MCNP4C2) to explore this further. In particular, a model of the irradiation facility has been made using the MCNP4C2 code in order to investigate the factors contributing to the quantitative determination of the elemental concentrations through prompt gamma neutron activation analysis (PGNAA) and most importantly, to estimate how the neutron energy spectrum and neutron dose vary with penetration depth into the sample. This simulation is compared against the scattered and transmitted neutron energy spectra that are experimentally and empirically determined using a portable neutron spectrometry system. This revised version was published online in August 2006 with corrections to the Cover Date.     

Development of automated analytical systems for large throughput

The need to be able to handle a large throughput of samples for neutron activation analysis has led to the development of automated counting and sample handling systems. These are coupled with available computer-assisted INAA techniques to perform a wide range of analytical services on a commercial basis. A fully automated delayed neutron counting system and a computer-controlled pneumatic transfer for INAA use are described, as is a multi-detector gamma spectroscopy system.     

An optimized setup for determining the bitumen content in asphalt concrete by the neutron reflection method

Summary A setup was developed in order to determine the bitumen content in asphalt concrete by neutron reflection. The main parts of this setup are an Am-Be neutron source of 37 GBq and a ³He neutron detector. To avoid geometrical effects of the measurements, the optimum volume of the samples was determined by plotting the reflected neutron response versus the sample volume. For the calibration of the system, the American Standard Method (ASTM) was used when preparing aggregate and asphalt concrete samples. The method was then compared to the conventional chemical method by analyzing unknown samples taken from an asphalt comcrete producing plant and finally the method was assessed in terms of reproducibility, sensitivity and precision.     

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.     

Development of a procedure for measuring nitrogen by cold neutron prompt gamma-ray activation analysis

An instrument for cold neutron prompt gamma-ray activation analysis (CNPGAA) has been used for the nondestructive determination of nitrogen. The samples were analyzed in an evacuated box to minimize background from neutron capture by atmospheric nitrogen. The system features lower background and lower detection limits than obtainable with the University of Maryland-National Institute of Standards and Technology (NIST) thermal neutron PGAA instrument. CNPGAA has been used to measure nitrogen in standard reference materials which included biological materials and soils; the results are in agreement with certified values. The detection limit for nitrogen in most biological and geological samples is near 1000 mg kg-1.     

Progress in Activation Analysis at the Atomic Institute Vienna

Fields in which progress has been achieved at the Atomic Institute Vienna are discussed briefly. The use of short and medium lived nuclides for activation analysis (AA). Construction of out-core parts of a fast transfer system. Combined sample catcher sample-changer for AA allowing cyclic and pseudocyclic AA. A system for discontinuous or continuous irradiation of liquid samples. Counting with an anticompton shield and quantitative correction of random coincidences. Screening analyses with the fast irradiation and measurement system. Improved peak-evaluation of gamma-spectra. Epithermal AA by a neutron converter. Following short-lived nuclide measurement, automatic sample transfer into a well-type detector enabling sensitive counting of nuclides with short half-lives. Use of large samples to increase the detection limits of ng to pg at activation with moderate neutron flux. Flux determination by liquid scintillation counting.     

Analysis of selenium in environmental and biological samples by neutron activation

This paper presents the results of the application of a fast rabbit system and a high thermal neutron flux to neutron activation analysis of selenium in environmental samples. The short-lived radionuclide^77mSe, (17.5s) is used for analysis. Results are presented for selenium in sea-water and rain-water, in biological reference materials, in food-stuffs and milk powder, in human hair and human blood-serum.     

Advances in 14 MeV neutron activation analysis by means of a new intense neutron source

A new intense 14 MeV neutron generator with cylindrical acceleration structure has been put in operation at the GKSS Research Center Geesthacht. The sealed neutron tube is combined with a fast pneumatic rabbit system with particular capabilities for neutron activation analysis involving shortlived reaction products. The sample transfer time is less than 140 ms. The maximum neutron flux available for activation is 5.2·10¹⁰ n/cm²s. Theoretical sensitivity predictions made in a previous study have been verified for some important trace elements. As a first application, samples of freeze-dried suspended matter and fishes of the Elbe river were analyzed.     

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.     

Monte Carlo simulation of PGNAA system for determining element content in the rock sample

The element content in rock sample can be determined by prompt gamma ray activation analysis technology. The neutron distributions under the conditions with different moderating materials, moderator size and distance from neutron source to lead-out hole were simulated using Monte Carlo method, and then the optimal structure parameters to get the highest thermal neutron flux was obtained. The PGNAA system with optimal parameters based on ²⁵²Cf neutron source was founded. In addition, the rock and element standard samples were irradiated by thermal neutron in this system. Moreover, the element content was calculated by processing gamma ray spectroscopy recorded, and it is in agreement with result of XRF method.     

First Results from the Prompt Gamma-Ray Activation Facility at PSI

Since October 1997, a new Prompt Gamma-ray Activation (PGA) Facility at the neutron spallation source SINQ of the Paul Scherrer Institute (PSI) in Villigen in Switzerland has been operational. The detection system includes a Compton-suppression and a pair spectrometer. Of particular interest is the availability of a neutron focusing lens. During the beam periods 1997 and 1998, measurements were performed to characterize the PGA facility (gamma-ray background, profile and flux of the neutron beam, efficiencies of the two spectrometers, analytical sensitivities and detection limits for several elements, elemental analysis of standards). Scanning of samples and nuclear spectroscopy of isotopes having small capture cross section were also performed using the neutron focusing lens.     

Phosphorus determination in milk and bone samples by neutron activation analysis

The determination of phosphorus in milk and bone samples by both radiochemical and instrumental neutron activation analysis is described. The radiochemical method consists of thermal neutron irradiation of samples and standards, sample dissolution, phosphorus precipitation as ammonium phosphomolyb-date, use of zinc holdback carrier and counting of the phosphorus-32 ß-activity. The instrumental method involves thermal neutron irradiation of samples and standards, waiting for a decay time and ß-counting. The methods were applied to commercial samples and reference materials.     

A combined set of automatic and interactive programs for instrumental neutron activation analysis

A set of several automatic and interactive computer programs for instrumental neutron activation analysis is discussed. The system allows fast and precise evaluation of data obtained from counting samples on a large number of different Ge(Li) detectors.     

Performance comparison of 2.8 MeV and <Superscript>241</Superscript>Am-Be neutrons based moisture measurement setups

Performance of a ²⁴¹Am-Be neutron source-based and 2.8 MeV neutrons-based moisture measurement setups have been compared using Monte Carlo simulation. In the setup fast neutrons transmitted through the sample were detected by a fast neutron detector, which was placed behind a massive long double truncated collimator. The setup geometry was optimized to detect maximum effect of 1–7 wt.% moisture on the neutron intensity transmitted through the sample. The yield of neutrons transmitted through concrete, coal, wood and soil samples containing 1–7 wt.% moisture was calculated for 2.8 MeV neutrons and neutrons from an ²⁴¹Am-Be source. The slopes of the fast neutron intensities transmitted through the samples vs. their moisture contents are very sensitive to the neutron energy and the sample composition. Higher slopes have been observed for the samples with larger bulk density. The slopes of fast neutron yield show dependence on the incident neutron energy. Larger slopes have been observed for neutrons with samller energy. Due to the overall large slopes of the transmitted intensity data of the samples for 2.8 MeV neutrons, it is expected to achieve better sensitivity in moisture measurements for a 2.8 MeV neutrons based moisture setup.     

Fast-neutron analysis at the Swedish Defence Research Agency

A fast-neutron analysis (FNA) system is being developed at the Swedish Defence Research Agency. The experimental set up, consisting of a SODERN Genie 16 (D-T) 14 MeV neutron generator, a HPGe-detector, a neutron monitor detector and related electronics, is described in some detail. Results from preliminary measurements on bulk samples containing mainly carbon and nitrogen are presented. Finally future objectives are discussed.     

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.     

Analysis of uranium using delayed neutron emission

Analyses of uranium and thorium by delayed neutron counting are described.The experimental system comprises an automatic pneumatic transfer system associatedwith a device made of twelve BF 3 neutron counters. Using homogeneous preparedsolutions of the samples, the analyses were based on a triple cycle each including60-second irradiation followed by 1-second cooling and 60-second counting.In these conditions, the limit of detection of uranium is about 0.3 µgwith a precision of measurement better than 10%. The contributions of possibleperturbations from 17 O(n,p)17 N reactions, followed by simultaneous –disintegration and neutron emission and from (n) reactions, have been studied.     

Analytical applications of delayed and instrumental neutron activation analysis

Delayed neutron activation analysis (DNAA) is a rapid and sensitive analytical method for the determination of fissile elements in a variety of samples. The present work describes two different analytical applications of delayed neutron activation for the analysis of biological and environmental samples, respectively. In the first application, DNAA was utilized to determine the natural uranium content in NIST standard reference materials (SRM) 1547 peach leaves and 1573a tomato leaves. Measured uranium mass fractions are comparable to the non-certified values listed on the certificates for these materials. In the second application, delayed neutron activation is coupled with instrumental neutron activation analysis (INAA) for the measurement of rare earth elements (REE) (cerium, lanthanum, neodymium, and ytterbium) in NIST SRM 2586, Trace Elements in Soil Containing Lead from Paint. DNAA was utilized to determine the uranium mass fraction in SRM 2586 for the subsequent application of a correction factor to account for cerium, lanthanum, and neodymium produced as part of the INAA irradiation. Measured and corrected mass fractions for the REEs described here are all within the uncertainty limits provided on the NIST certificate for SRM 2586. These results and the demonstrated sensitivity of the DNAA system establish and validate the use of this method for the determination of REEs and for potential nuclear forensics applications.