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.

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.

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.

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.

Measurement of the 230U half-life.

The (230)U half-life was determined by measuring the decay curve of (230)U sources by various nuclear detection techniques: α-particle counting at a defined small solid angle; 4πα+ß counting with a windowless CsI sandwich spectrometer, a liquid scintillation counter and a pressurised proportional counter; gamma-ray spectrometry with a HPGe detector and nearly-2π α-particle counting with an ion-implanted silicon detector. Depending on the technique, the decay was followed for 100-200 d, which is 5-10 times the (230)U half-life. The measurement results of the various techniques were in good mutual agreement. The mean value, T(1/2)((230)U)=20.23 (2) d, is lower than the literature value which is based on one measurement in 1948 and resulted in a half-life value of 20.8d without statement of uncertainty. A correction for the ingrowth of the long-lived (210)Pb and its daughter products may have been overlooked in the past.

Determination of natural and anthropogenic radionuclides in soil--results of an European Union comparison.

A comparison was organised by the Institute for Reference Materials and Measurements (IRMM) among 73 environmental radioactivity monitoring laboratories for the determination of 15 radionuclides in soil. The reference material IAEA-375 soil was reprocessed to provide the comparison material and reference values traceable to SI units and SIR were established at IRMM. The analytical procedures used by participants laboratories are discussed and performances evaluted using relative deviations and E(n) numbers. This paper focuses on the results for (40)K, (90)Sr, (137)Cs, (226)Ra, (234)U, (235)U and (238)U. Worst performance was found for (90)Sr and (235)U, where 65% and 78% of the results, respectively, deviate more than 20% from the reference values. The use of the E(n) criterion revealed that the uncertainty estimation of many participants is poor.

Results of an international comparison for the activity measurement of 177Lu.

An international Key Comparison of (177)Lu has recently been carried out. Twelve laboratories performed assays for radioactivity content on aliquots of a common master solution of (177)Lu, leading to eleven results submitted for entry into the Key Comparison Database of the Mutual Recognition Arrangement. A proposed Comparison Reference Value (CRV) was calculated to be 3.288(4)MBq/g using all eleven results. Degrees of equivalence and their uncertainties were calculated for each laboratory based on the CRV. Most of the values reported by the participating laboratories were within 0.6% of the CRV.

Results of an international comparison for the determination of radionuclide activity in bilberry material.

Radioactivity found in wild food products has assumed greater importance when assessing the total exposure of the population. For this reason, IRMM has been developing a reference material for the activity concentration of three radionuclides in bilberry samples. In order to characterise this new material, a CCRI(II) supplementary comparison was organised. The difficulties encountered in this comparison are discussed, in particular the efficiency calibration for volume sources of gamma-ray emitters, and comparison reference values for (137)Cs and (40)K are calculated.

Intercomparison of methods for coincidence summing corrections in gamma-ray spectrometry--part II (volume sources).

The second part of an intercomparison of the coincidence summing correction methods is presented. This exercise concerned three volume sources, filled with liquid radioactive solution. The same experimental spectra, decay scheme and photon emission intensities were used by all the participants. The results were expressed as coincidence summing corrective factors for several energies of (152)Eu and (134)Cs, and different source-to-detector distances. They are presented and discussed.

Measurement of the 177Lu half-life.

The half-life of (177)Lu was measured by following the decay of sources with an ionisation chamber and with a liquid scintillation counter over a period of 85 and 42 days, respectively. The detector signals showed three major components: decay of (177)Lu, decay of (177m)Lu and the background signal. The half-life value, T(1/2)((177)Lu)=6.6465 (50)d, was obtained by a least-squares fit of two exponential decay functions and a constant background to the experimental data. The data was corrected for decay during the measurements. Residuals of the fit and uncertainty budgets are presented in this paper. The result is in agreement with three out of seven previously published measurement results and the currently recommended value of 6.647 (4) d (Kondev, 2002). A new value of T(1/2)((177)Lu)=6.6465 (32) d was obtained from a weighted mean of our results with the selected literature data.

Intercomparison of methods for coincidence summing corrections in gamma-ray spectrometry.

A comparison of the coincidence summing correction methods is presented. Since there are several ways for computing these corrections, each method has advantages and drawbacks that could be compared. This part of the comparison was restricted to point sources. The same experimental spectra, decay scheme and photon emission intensities were used by all the participants. The results were expressed as coincidence summing correction factors for several energies of (152)Eu and (134)Cs, and three source-to-detector distances. They are presented and discussed.

International exercise on 124Sb activity measurements.

An international exercise, registered as EUROMET project no. 907, was launched to measure both the activity of a solution of (124)Sb and the photon emission intensities of its decay. The same solution was sent by LNE-LNHB to eight participating laboratories. In order to identify possible biases, the participants were asked to use all possible activity measurement methods available in their laboratory and then to determine their reference value for comparison. Thus, measurement results from 4pibeta-gamma coincidence/anti-coincidence counting, CIEMAT/NIST liquid-scintillation counting, 4pigamma counting with well-type ionization chambers and well-type crystal detectors were given. The results are compared and show a maximum discrepancy of about 1.6%: possible explanations are proposed.

International exercise on 124Sb photon emission intensities determination.

An international exercise, registered as EUROMET project no. 907, was launched to measure both the activity of a solution of (124)Sb and the photon emission intensities of its decay. The same solution was sent by LNE-LNHB to eight participating laboratories, six of which sent results for photon emission intensities both in absolute and in relative terms. From these results and including previous published values, a consistent decay scheme was worked out, proving that problems in activity measurements have not been due to decay scheme data.

Half-life measurement of 124Sb.

The half-life of (124)Sb was determined experimentally by following the decay of a source from a radiopure solution with a Centronic IG12 ionisation chamber. Thousands of measurements were performed over a period of 358 days, i.e. about six half-life periods. However, the data analysis was restricted to the first 221 days, in order to limit the dominant uncertainty component associated with the hypothetical possibility of a systematic error on background subtraction. The resulting value for the (124)Sb half-life, 60.212 (11) days, is found to be in very good agreement with published values, but carries a lower uncertainty. Major uncertainty contributions pertain to possible systematic errors in background correction, long-term changes in source-detector geometry and medium- and long-term instability of the instrument. Additional measurements were performed with a high-purity germanium detector to confirm the above value.

Alpha-particle emission probabilities in the decay of 240Pu.

Sources of enriched (240)Pu were prepared by vacuum evaporation on quartz substrates. High-resolution alpha-particle spectrometry of (240)Pu was performed with high statistical accuracy using silicon detectors and with low statistical accuracy using a bolometer. The alpha-particle emission probabilities of six transitions were derived from the spectra and compared with literature values. Additionally, some alpha-particle emission probabilities were derived from gamma-ray intensity measurements with a high-purity germanium detector. The alpha-particle emission probabilities of the three main transitions at 5168.1, 5123.6 and 5021.2 keV were derived from seven aggregate spectra analysed with five different fit functions and the results were compatible with evaluated data. Two additional weak peaks at 4863.5 and 4492.0 keV were fitted separately, using the exponential of a polynomial function to represent the underlying tailing of the larger peaks. The peak at 4655 keV could not be detected by alpha-particle spectrometry, while gamma-ray spectrometry confirms that its intensity is much lower than expected from literature.

EC comparison on the determination of 226Ra, 228Ra, 234U and 238U in water among European monitoring laboratories.

In anticipation of new European requirements for monitoring radioactivity concentration in drinking water, IRMM organized an interlaboratory comparison on the determination of low levels of activity concentrations (about 10-100 mBq L(-1)) of the naturally occurring radionuclides (226)Ra, (228)Ra, (234)U and (238)U in three commercially available mineral waters. Using two or three different methods with traceability to the International System of Reference (SIR), the reference values of the water samples were determined prior to the proficiency test within combined standard uncertainties of the order of 3%-10%. An overview of radiochemical separation and measurement methods used by the 45 participating laboratories are given. The results of the participants are evaluated versus the reference values. Several of the participants' results deviate by more than a factor of two from the reference values, in particular for the radium isotopes. Such erroneous analysis results may lead to a crucial omission of remedial actions on drinking water supplies or to economic loss by an unjustified action.

Comparison of triple-to-double coincidence ratio (TDCR) efficiency calculations and uncertainty assessments for 99Tc.

A comparison exercise for data analysis was recently conducted by the Liquid Scintillation Counting Working Group (LSCWG) of the International Committee on Radionuclide Metrology (ICRM) to evaluate the uncertainties involved in applying different analysis methodologies (including computer programs) for the triple-to-double coincidence ratio (TDCR) method. The goals of the comparison were to (1) study differences in calculation results from different TDCR analysis programs, (2) investigate differences in analysis techniques and uncertainty assessment philosophies between laboratories, and (3) study the effect of not taking asymmetry of photomultiplier tube (PMT) efficiencies into account on the calculated activity. To achieve this, a single set of TDCR data for the pure beta emitter (99)Tc, was distributed to the participants, who analyzed the data according to their normal procedures and report the activity concentration of the (99)Tc solution from their results. The results indicate that the presently used programs are generally able to calculate the same activity values, assuming that the correct input parameters are used and that not taking PMT asymmetry into account in the calculations can lead to significant (0.6% for (99)Tc) errors in reported results. The comparison also highlighted the need for a more rigorous approach to estimating and reporting uncertainties.

226Ra and 228Ra determination in mineral waters--comparison of methods.

A comparison of different radiochemical separation procedures and measurement techniques used to determine the activity concentration of (226)Ra and (228)Ra in water is made with respect to accuracy, detection limits and turn-around time. Radium-226 activity concentration was determined by the radon emanation technique, alpha-particle and gamma-ray spectrometry. To determine the (228)Ra activity concentration, four different techniques were used: low-level liquid scintillation counting, low-level proportional counting, alpha-particle and low-level gamma-ray spectrometry.

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.