Evidence against solar influence on nuclear decay constants.

The hypothesis that proximity to the Sun causes variation of decay constants at permille level has been tested and disproved. Repeated activity measurements of mono-radionuclide sources were performed over periods from 200 days up to four decades at 14 laboratories across the globe. Residuals from the exponential nuclear decay curves were inspected for annual oscillations. Systematic deviations from a purely exponential decay curve differ from one data set to another and are attributable to instabilities in the instrumentation and measurement conditions. The most stable activity measurements of alpha, beta-minus, electron capture, and beta-plus decaying sources set an upper limit of 0.0006% to 0.008% to the amplitude of annual oscillations in the decay rate. Oscillations in phase with Earth's orbital distance to the Sun could not be observed within a 10-6 to 10-5 range of precision. There are also no apparent modulations over periods of weeks or months. Consequently, there is no indication of a natural impediment against sub-permille accuracy in half-life determinations, renormalisation of activity to a distant reference date, application of nuclear dating for archaeology, geo- and cosmochronology, nor in establishing the SI unit becquerel and seeking international equivalence of activity standards.

Radioactivity determination of individual radionuclides in a mixture by liquid scintillation spectra deconvolution.

A spectrum deconvolution method has been applied to logarithmic liquid scintillation spectra containing more than one radionuclide, to determine the activity of the individual components. The shape of the sample spectrum has been used for resolving the various components in the sample by fitting the reference spectrum of each component to the measured composite spectrum. The procedure has been applied to environmental samples containing (90)Sr, (90)Y and (85)Sr, added as a tracer for the chemical recovery determination. Comparisons are made with results obtained using a modified window-based method and results obtained by measuring the (85)Sr activity by gamma-ray spectrometry and subtracting its contribution to the LS count rate. It is demonstrated that this treatment of the beta-particle spectra represents a rapid and reliable method of determining these radionuclides in a mixture, even at low activity levels.

Characterisation of the IAEA-152 milk powder reference material for radioactivity with assigned values traceable to the SI units.

The Institute for Reference Materials and Measurements (IRMM) participated in a research project initiated by the International Atomic Energy Agency (IAEA) to upgrade some of its existing reference materials (RMs). The aim of the project is to improve the RM metrological status by establishing traceability of their assigned values to SI units. The purpose of the work described in this article was to establish traceability to the International System of Units (SI) of the activity concentrations of the radionuclides (134)Cs, (137)Cs, (40)K, and (90)Sr in the IAEA-152 milk powder RM. The choice of the particular RM was based on the concern about radioactivity levels in foodstuff. The sample preparation and the assaying of the activity concentrations in the milk powder, the methods used to achieve instrument calibrations and measurements traceable to the SI units, the data reduction and analysis, and finally, the results obtained are presented.

Measurement comparisons of radioactivity among European monitoring laboratories for the environment and food stuff.

For more than 15 years, European Union (EU) laboratories monitoring environmental radioactivity have been obliged to participate in measurement comparisons organised by the European Commission. After a short review of comparisons conducted during the 1990s, the approach of IRMM organising these comparisons since 2003 is presented. It relies on the provision of comparison samples with reference values traceable to the International Reference System for radionuclides (SIR). The results of the most recent comparison, the determination of (40)K, (90)Sr and (137)Cs in milk powder, are presented. The influence of repetitive participation in measurement comparisons on laboratory performance is studied on the basis of data from more than 20 laboratories having participated in several exercises during the last 15 years.

alpha-Particle and gamma-ray spectrometry of a plutonium solution for impurity determination.

A highly enriched (240)Pu solution was measured by alpha-particle and gamma-ray spectrometry to determine other radionuclides present in the material as impurities. Low activities of (238)Pu, (241)Am, (243)Cm and (244)Cm were determined by measuring thin sources, made from the original solution, in a high-resolution alpha-particle spectrometer. The sources were prepared by evaporating the plutonium solution on quartz plates in a vacuum chamber. From the ingrowth of (241)Am in the original solution, the amount of (241)Pu could be calculated. After radiochemical separation of (241)Am, the plutonium was measured by high-efficiency alpha-particle spectrometry to determine the amount of (238)Pu. The enriched (240)Pu material was also measured by high-resolution gamma-ray spectrometry, using two different HPGe detectors to determine the impurities of (239)Pu and (241)Am. The preparation of the sources and the measurement methods are described and discussed. The measured impurities, given in % of the (240)Pu activity, are compared with the values on the certificate.

Is decay constant?

Some authors have raised doubt about the invariability of decay constants, which would invalidate the exponential-decay law and the foundation on which the common measurement system for radioactivity is based. Claims were made about a new interaction - the fifth force - by which neutrinos could affect decay constants, thus predicting changes in decay rates in correlation with the variations of the solar neutrino flux. Their argument is based on the observation of permille-sized annual modulations in particular decay rate measurements, as well as transient oscillations at frequencies near 11 year-1 and 12.7 year-1 which they speculatively associate with dynamics of the solar interior. In this work, 12 data sets of precise long-term decay rate measurements have been investigated for the presence of systematic modulations at frequencies between 0.08 and 20 year-1. Besides small annual effects, no common oscillations could be observed among α, ß-, ß+ or EC decaying nuclides. The amplitudes of fitted oscillations to residuals from exponential decay do not exceed 3 times their standard uncertainty, which varies from 0.00023 % to 0.023 %. This contradicts the assertion that 'neutrino-induced' beta decay provides information about the deep solar interior.

EC intercomparisons for laboratories monitoring environmental radioactivity.

International measurement comparisons are organised regularly for EU laboratories involved in monitoring radioactivity, with emphasis on meeting routine measurement conditions. Using the recent comparison of 137Cs in air filters as an example, the whole cycle is described: establishment of traceable reference values, spiking of individual filters for the comparison and their quality assurance, treatment and measurement of filters in the participating laboratories and evaluation of comparison results. The treatment of an individual result, deviating widely from the reference value, is discussed. Monte-Carlo simulations allow to estimate the maximum errors possibly made due to a non-suitable measurement geometry.

Characterisation of the IAEA-375 Soil Reference Material for radioactivity.

The Joint Research Centre Institute for Reference Materials and Measurements (JRC-IRMM) participated in a research project initiated by the International Atomic Energy Agency (IAEA) to upgrade some of its existing reference materials (RMs). The aim of the work described in this article was to determine the activity concentration of a series of radionuclides in the IAEA-375 soil RM with values traceable to the SI units. The radionuclides (40)K, (134)Cs, (137)Cs, (212)Pb, (212)Bi, (214)Pb and (214)Bi were measured by γ-ray spectrometry after drying the sample and placing it in a suitable container. The (90)Sr was assessed by liquid scintillation counting after dissolution of the soil by wet digestion and chemical separation of Sr by extraction chromatography. This soil RM was used later as basis for the 2010 EC Interlaboratory Comparison on Radionuclides in Soil.

Spiked environmental matrix for use as a reference material for gamma-ray spectrometry: Production and homogeneity test.

The application of a spiking method for reference material production and its utilisation for a food matrix is presented. The raw rice powder was tested by means of γ-ray spectrometry and spiked with a (137)Cs solution. The spiked material was mixed and tested for homogeneity. The future use of the rice powder reference material after the entire characterisation cycle will be for γ-ray spectrometry method validation.

Evaluation of the 2014 EC measurement comparison on (137)Cs in air filters.

In 2014, the Joint Research Centre organised an interlaboratory comparison of (137)Cs measurement in air filters. This paper describes the context of the European measurement comparisons, as well as the technical implementation. Furthermore, sample treatment and measurements performed by participating laboratories are discussed and finally the evaluation of comparison results is presented. The intercomparison results are such that 71 out of the 76 laboratories (i.e. 93.4%) reported values within ±33% range of the reference value.

Analysis of triple-label samples by liquid scintillation spectrometry.

The simultaneous analysis of three radionuclides present in a sample is demonstrated. The approach requires fitting a series of counting data acquired over a period of time to a triple exponential mathematical expression. The use and advantages of the method are illustrated for the case of the standardisation of 32P in the presence of 33P and 35S by 4pi-beta liquid scintillation spectrometry. The effects of sample preparation, chemical quenching and different activity ratios were studied.

Low-level radioactivity measurements in an ocean shellfish matrix.

Reference marine biological samples are necessary to test the performance of the analytical methods employed in surveying and monitoring radioactive materials in the sea. The measurement of artificial and natural radionuclide activity concentrations in ocean shellfish material by nondestructive ultra low-level gamma-ray spectrometry in an underground laboratory is reported. The material analysed, a composite material made of Irish Sea and White Sea mussel and Japan Sea oyster, was prepared by the National Institute of Standards and Technology (NIST).

Standardisation of low-activity actinide solutions by alpha-particle counting at a defined solid angle.

In the frame of an IAEA co-ordinated research project 232U, 236Pu and 229Th tracers were standardised by alpha-particle counting at a defined solid angle. The aim of this project was to assign property values that are traceable to the International System of Units (SI). This article depicts the standardisation of the tracers, elaborating on problems related to low activity and interference from daughter nuclides. The activity concentrations of 232U, 236Pu and 229Th were found to be, respectively (734+/-4), (142.0+/-0.9) and (528.2+/-2.6) Bq g(-1), at reference date (1 January 2003).

Measurement of 90Sr in bone ash.

Bone is a critical organ for the accumulation of many radionuclides, including 90Sr. A bone ash has been prepared by NIST to become a standard reference material and the assaying of the 90Sr activity concentration is reported. A radiochemical procedure, based on extraction chromatography using a crown ether, has been developed to separate Sr from the bone ash. The natural Sr content of the bone ash has been determined by isotope dilution inductively coupled plasma mass spectrometry. The 90Sr activity concentration has been measured by low-level liquid scintillation counting.

Self-absorption correction in standardisation of 204Tl.

In the frame of the CCRI(II) (Consultative Committee for Ionizing Radiation at the BIPM) working group on 204Tl, dedicated to investigate the problem and explain the reasons for the discrepancy between methods in standardisation of 204Tl, IRMM was assigned the task of studying the influence of self-absorption in solid sources of 204Tl. This was investigated by measuring the activity of quantitative solid sources of different carrier content, prepared by employing a special source drying technique, developed at IRMM. The activity of the solid sources was determined using a 4pi pressurised proportional counter. The self-absorption was calculated using the activity determined from liquid scintillation spectrometry, unaffected by self-absorption, as the reference value. After correction for foil absorption and non-detected X-rays, the self-absorption showed a linear relation to the logarithm of the dry mass of the source. For a typical source, the self-absorption correction for 204Tl was found to be as high as 8%. This article describes a way to minimise and correct for self-absorption in solid sources of 204Tl or nuclides with similar decay properties.

Standardisation of 89Sr using three different methods.

Three independent methods were used to standardise a 89Sr solution within the frame of an international comparison organised by BIPM/CCRI(II): 4pi liquid scintillation (LS) spectrometry, based on the CIEMAT/NIST efficiency tracing method, and direct activity measurements with a windowless 4pi CsI(Tl)-sandwich spectrometer and a 4pi pressurised gas proportional counter. Quantitative solid point-like sources were prepared paying special attention during the source drying phase. The impurities in the original solution were measured and corrected for in the results of all three methods. The activity concentration of 89Sr was found to be 26.21 +/- 0.08 kBq g(-1). A new half-life value for 89Sr of 50.61 +/- 0.05 days was determined from the LS measurements. The measurement methods, including the impurity measurements, are described and the results compared. considering the advantages and disadvantages of each method.

Validation of methods for the determination of radium in waters and soil.

This article describes the advantages and disadvantages of several analytical methods used to prepare the alpha-particle source. As a result of this study, a new method combining commercial extraction and ion chromatography prior to a final co-precipitation step is proposed. This method has been applied and validated on several matrices (soil, waters) in the framework of international intercomparisons. The integration of this method in a global procedure to analyze actinoids and radium from a single solution (or digested soil) is also described.

Aspects of sample preparation for the determination of actinoids in soil.

In the frame of the re-certification measurements of the IAEA-375 soil reference material the activity concentration of four actinoids (U, Th, Pu and Am) was determined. The method is based on microwave digestion or aqua regia leaching, separation by extraction chromatography and alpha-particle counting source preparation by co-precipitation or electro-deposition. This article describes some of the problems encountered during the validation of the sample preparation method and their solutions.

Characterisation of reference materials for radioactivity with assigned values traceable to the SI units.

The Institute for Reference Materials and Measurements participates in an IAEA co-ordinated project to upgrade Analytical Quality Control Services intercomparison materials to reference materials. The principal objective is to ensure that assigned property values are traceable to the International System of Units. General principles and practical mechanisms for achieving this traceability are described.

Development of the NIST bone ash standard reference material for environmental radioactivity measurement.

The bone ash standard reference material (SRM), a blend of 4% contaminated human bone and 96% diluent bovine bone, has been developed for radiochemical method validation and quality control for radio-bone analysis. The massic activities of 90Sr, 226Ra, 230Th, 232Th, 234U, 235U, 238U, 238Pu, (239 + 240)Pu and (243 + 244)Cm were certified using a variety of radiochemical procedures and detection methods. Measurements confirmed undetectable radionuclide heterogeneity down to a sample size of 5 g. thereby implying adequate blending of particulate materials with dilution factors of up to 17,900. The results among most of the intercomparison laboratories and their methods were consistent. Disequilibrium was observed for decay chains: 234U(0.67 mBq/g)-230Th(0.47 mBq/g)-226Ra(15.1 mBq/g)-210Pb(23 mBq/g)-210Po(13 mBq/g) and 232Th(0.99 mBq/g)-228 Ra(6.1 mBq/g)-228Th(7.1 mBq/g). The disequilibria were the results of mixing occupationally contaminated human bone with natural bovine bone and the fractionation during internal biological processes. The massic activity of 210Pb, 228Th and 241Am were not certified because of insufficient 228Ra and 241Pu data and lack of knowledge in how 222Rn and its daughters will be fractionated in the SRM bottle over time.