Status of JRC reference measurements of radioactivity to realise the becquerel

The Radionuclide Metrology group of JRC performs reference measurements of activity and nuclear decay characteristics in support of the common measurement system for radioactivity, as stipulated in Art. 8 of the Euratom Treaty. Primary standardisations of activity performed at the JRC and National Metrology Institutes (NMI) are used at a global scale to establish radioactivity measurements in a traceable manner to the SI unit becquerel. To ensure equivalence of national standards, the NMIs and the JRC participate in key comparisons of their primary standardisation measurements of a common mononuclidic solution. The Bureau International des Poids et Mesures (BIPM) in Sèvres (F) issues a report in which the Key Comparison Reference Value (KCRV) is calculated from a mean of the laboratory results, as well as a Degree of Equivalence (DoE) of each participating laboratory to the KCRV. Thus, the SI unit becquerel is established and international equivalence is demonstrated. Recently, the BIPM published final reports on various key comparisons, which validates standardisation work performed earlier at the JRC and other NMIs. This report gives an overview the corresponding key comparison results.JRC.G.2-Standards for Nuclear Safety, Security and Safeguard

Determination of a reference value, associated standard uncertainty and degrees of equivalence for CCRI(II) key comparison data

CCRI(II) key comparison data consist of a measured value of activity concentration, independently obtained, and the associated standard uncertainty for each laboratory participating in a key comparison. A method is proposed for calculating a key comparison reference value (KCRV), its associated standard uncertainty, and degrees of equivalence (DoE) for the laboratories. The estimator has the property that for increasingly discrepant data sets there is a smooth transition of the KCRV from the weighted mean to the arithmetic mean. It is a good compromise between efficiency and robustness, while providing also a reliable uncertainty. A suitable statistical criterion is provided to identify extreme data.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

Evaluation of EC Measurement Comparison for 137Cs, 40K and 90Sr in Milk Powder

This report describes the full life cycle of the measurement comparison of 137Cs, 40K and 90Sr in milk powder among 60 European laboratories monitoring radioactivity in food and the environment. An available IAEA reference material was re-processed at IRMM into suitable intercomparison samples and the homogeneity of the distributed samples together with other quality parameters was determined. Reference values of the three radionuclides under study in this intercomparison were determined at IRMM using tracer techniques and standardised radionuclide solutions and are thus traceable to the SI units. The sample preparation and measurement processes applied in the participating laboratories are described and the results of the intercomparison are presented and discussed in detail. Whereas, in general, the measurement results for 137Cs and 40K show good agreement with the reference value, the results of this comparison point at problems of 90Sr determination in about one third of the laboratories. The corresponding participants should investigate and revise their analytical methods, next to many laboratories needing to improve their estimation of measurement uncertainty.JRC.D.4-Isotope measurement

Quantification of uranium-238 in environmental samples using gamma-ray spectrometry

A large number of environmental samples are routinely measured world-wide using gamma-ray spectrometry some of its assets being easy sample preparation and comprehensive data for many radionu-clides in one analysis. Although other techniques can be considered more suitable for analysing 238U in environmental samples, it is also routinely done by gamma-ray spectrometry. One mainly uses γ-ray emissions following the decay of the first daughter, 234Th, for determining the 238U activity. However, the low-energy gamma-rays at 63 keV and 92.5 keV are very difficult to quantify in a robust way due to high attenuation and interferences. This paper quantifies parameters affecting the possibility of making robust quantification of 238U via 234Th using gamma-ray spectrometry. It addresses the use of correct decay data, suitable detectors, optimised sample size, enhanced spectral amplification, correction for peak interferences and control of background.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

Preparation and certification of 243Am spike reference material: IRMM-0243 : Certified reference material for the amount content of 243Am and n(241Am)7n(243Am) isotope amount ratio

This report describes the preparation and certification of IRMM-0243, a 243Am spike reference material. It is certified for the amount content of 243Am and the isotope amount ratios of n(241Am)/n(243Am) and n(242mAm)/n(243Am). Furthermore, the material is certified for the amount contents of 241Am and total Am, the mass fractions of 243Am, 241Am and total Am, the isotope amount and mass fractions (e.g. isotopic composition) and the molar mass of Am. The material was produced in compliance with ISO/IEC 17034:2016 and certified in accordance with ISO Guide 35:2006. The material was prepared by dilution of an americium starting solution in nitric acid and dispensing of the solution into glass ampoules. In total 587 units were produced. Between-unit homogeneity was quantified and stability during dispatch and storage were assessed in accordance with ISO Guide 35:2006. The characterisation of the amount content of 243Am was performed by Isotope Dilution Mass Spectrometry (IDMS) using a 241Am spike, produced from highly enriched 241Pu material. The isotope amount ratios of n(241Am)/n(243Am) and n(242mAm)/n(243Am) were measured by Thermal Ionisation Mass Spectrometry (TIMS). The certified values were verified by alpha particle spectrometry, alpha particle counting at a defined solid angle (DSA) and high-resolution gamma-ray spectrometry as independent verification methods. The uncertainties of the certified values were estimated in compliance with the Guide to the Expression of Uncertainty in Measurement (GUM) [ ] and include uncertainties related to possible inhomogeneity, instability and characterisation. The main purpose of this material is for use as a spike isotopic reference material for the quantification of americium by IDMS in unknown samples. A unit of IRMM-0243 consists of a glass ampoule with a screw cap containing about 3.5 mL nitric acid solution (c = 1 mol/L) with an americium mass fraction of about 1.5 μg/g solution. The material is a true solution; therefore there is no recommended minimum sample intake to be taken into account.JRC.G.2-Standards for Nuclear Safety, Security and Safeguard

Comments on "A Comparison of Different Analytical Methods of Determining the Solid Angle of a Circular Coaxial Source-detector System"

This paper gives some critical remarks on a recent publication concerning analytical methods to calculate the solid angle subtended by a disk-shaped detector for a coaxial, homogeneous, disk-shaped source (Galiano and Rodrigues, 2006). The mentioned paper lacks rigour and contains numerous incorrect statements, including mistakes in the mathematical expressions of the eight equations that are being scrutinised, as well as computational errors. The conclusions of the paper are incorrect and bear little scientific relevancy. In particular, the authors are wrong concerning the alleged ‘unphysical results’ obtained from Pommé’s series expansion of Ruby’s formula (Pommé, 2004). Some of the paper’s shortcomings are briefly discussed.JRC.D.4-Isotope measurement

The uncertainty of the half-life

Half-life measurements of radionuclides are unjustly perceived as easy and the experimental uncertainties are commonly underestimated. Data evaluators, scanning the literature, are faced with bad documentation, lack of traceability, incomplete uncertainty budgets and discrepant results. Poor control of uncertainties has its implications in the end user community, varying from limitations to the accuracy and reliability of nuclear-based analytical techniques to the fundamental question whether half-lives are invariable or not. This paper addresses some issues from the viewpoints of the user community and of the decay data provider. It addresses the propagation of the uncertainty of the half-life in activity measurements and discusses different types of half-life measurements, typical parameters influencing their uncertainty, a tool to propagate the uncertainties and suggestions for a more complete reporting style. Problems and solutions are illustrated with striking examples from literature.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

The Solid Angle Subtended by a Circular Detector for a Linear Source

An exact solution is presented for the solid angle subtended by a circular detector for a linear source. It is based on Conway's equation for a non-coaxial point source, involving an integral of Bessel functions (Conway, 2006). Some numerical examples are calculated and compared with the results obtained with a recently published closed formula (Galiano and Pagnutti, 2006), which is incorrect but can be improved by applying a correction factor.JRC.D.4-Isotope measurement

Dead Time, Pile-up and Counting Statistics

An overview is presented on recent progress in the field of nuclear counting statistics; theoretical expressions are shown to predict deviations from Poisson statistics due to non-random count loss in the spectrometer set-up. Frequently encountered misconceptions in the literature and in daily practice are uncovered: the unconditional belief in the general validity of Poisson statistics, the neglect of the dependency of counting statistics on the considered fraction of the pulse spectrum, the mix-up between pulse pile-up and extending dead time, the unawareness of the influence of pile-up rejection on the counting statistics in fixed live-time measurements and also with ‘loss-free counting’. Insight is provided into the statistical properties of spectra taken with ‘loss-free counting’ and ‘zero dead time’ counting, as well as the ‘variance spectrum’ provided with the latter. Uncertainty formulas are also presented for more conventional nuclear spectrometry measurements, with different types of count loss.JRC.D.4-Isotope measurement

Cascades of Pile-up and Dead Time

Count loss through a cascade of pile-up and dead time is studied. Time interval density distribution functions and throughput factors are presented for counters with a series arrangement of pile-up with extending or non-extending dead time. A counter is considered where an artifcial dead time is imposed on every counted event, in order to control the length and type of dead time. For such system, it is relatively easy to determine an average count loss correction factor via a live-time clock gated by the imposed dead time signal ('live-time mode'), or otherwise to apply a correction factor based on the inversion of the throughput function ('real-time mode'). However, these techniques do not account for additional loss through pulse pile-up. In this work, counting errors associated with neglecting cascade effects are calculated for measurements in live-time and real-time mode.JRC.D.4-Isotope measurement