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.

Evaluation of a proficiency test of 131I, 134Cs and 137Cs in maize powder.

In 2017, JRC-Geel organised a proficiency test for 120 participants on the massic activity determination of 131I, 134Cs and 137Cs in maize powder. The proficiency test reference material was produced by spiking blank maize powder. The material was characterised for its radioactive content using γ-ray spectrometry. The z scores of the reported massic activity were acceptable in 92% of the results for 131I, and in 94% for both 134Cs and 137Cs. The ζ scores were consistent with the reference value for 66% of the results for 131I, 56% for 134Cs and 68% for 137Cs.

The 55Fe half-life measured with a pressurised proportional counter.

The half-life of 55Fe has been measured accurately by following the decay curve of three sources with a large pressurised proportional counter. An argon(90%)-methane(10%) mixture was used as counter gas, at atmospheric pressure (∼1 × 105 Pa) and at enhanced pressures of 5 × 105 Pa and 8 × 105 Pa (for 1 source), respectively. The first measurements were performed in 2001, but the experiment was executed more systematically between 2005 and 2018, covering a period of about 5 half-lives. The residuals from an exponential decay curve were of the order of 0.1% to 0.2% at 1 × 105 Pa, and 0.03% at 5 × 105 Pa and 8 × 105 Pa. The gain of stability with increased gas pressure was due to asymptotically reaching the maximum counting efficiency, resulting in lower sensitivity to pressure variations. The deduced half-life value of T1/2(55Fe) = 1006.70 (15) d or 2.7563 (4) a is more accurate than other data in literature, which are mutually discrepant. It is consistent with previous measurements at JRC with an X-ray defined solid angle counter.

Measurement of absolute γ-ray emission probabilities in the decay of 227Ac in equilibrium with its progeny.

The emission probabilities of γ rays produced in the 227Ac decay series were determined by high-resolution γ-ray spectrometry of sources with standardised activity. The sources were prepared quantitatively on glass discs by drop deposition of a solution with 227Ac in radioactive equilibrium with its daughter nuclides. Their activity was measured by a primary standardisation technique based on alpha-particle counting at a defined low solid angle. Four laboratories performed γ-ray spectrometry and derived absolute γ-ray intensities. Mean values were calculated and compared with literature data and the currently recommended evaluated data. New values on certain γ-ray emission probabilities are proposed.

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.

Linearity check of an ionisation chamber through 99 mTc half-life measurements.

The half-life of 99 mTc was measured at the JRC using the ionisation chamber 'IC1' (type Centronic IG12). The result, T1/2(99 mTc) = 6.00660 (18) h, is in good agreement with literature data. One experiment was performed in IC1's default set-up with the ionisation current being integrated over an air capacitor and read out as a voltage increase over time. This ensured excellent linearity and precision throughout the dynamic range, but the maximum current was limited to 2 nA. In a second test, the current was directly read out with a Keithley 6517 A electrometer. Applying correction factors for the automatic range switching of the electrometer, an acceptable linearity was demonstrated over a range of 12 half-life periods starting at 20 nA. Range switching and autocorrelation of the current readout increase the systematic and random error propagation factors. Piecewise fitting of the decay curve over periods of 6 h yields the same 99 mTc half-life value within 0.04% (0.0025 h) standard deviation over an activity range spanning at least 10 half-life periods (3 orders of magnitude).

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.

Measurement of absolute γ-ray emission probabilities in the decay of 235U.

Accurate measurements were performed of the photon emission probabilities following the α decay of 235U to 231Th. Sources of highly enriched 235U were characterised in terms of isotopic composition by mass spectrometry and their activities were standardised by means of alpha-particle counting at a low defined solid angle. The standardised sources were subsequently measured by high-resolution γ-ray spectrometry with calibrated high-purity germanium detectors to determine the photon emission probabilities. Four laboratories participated in this work and reported emission probabilities for 33 γ-ray lines. Most of them agree with previously published evaluated data. In addition, new values are proposed for γ-lines which have been measured only once in the past.

Direct measurement of alpha emission probabilities in the decay of 226Ra.

High-resolution alpha-particle spectrometry was performed to determine the main alpha-particle emission probabilities in the decay of 226Ra. Thin, homogeneous sources were prepared by electrodeposition on stainless steel disks. Alpha spectra with an energy resolution of 20keV were obtained in three laboratories and analysed with different deconvolution algorithms. In two set-ups, a magnet system was used to deflect conversion electrons to avoid their coincidental detection with the alpha particles. Spectra taken at close range without a magnet system yielded biased results which cannot be fully compensated by statistical corrections for coincidence summing. The derived emission probabilities of the three main alpha decays are 94.07 (1)%, 5.93 (1)%, and 0.0059 (15)%, respectively. They are in excellent agreement with calculated values derived from the P(γ+ce) decay scheme balance, which solves the existing discrepancy problem with two previous direct measurements published in literature.

New high-throughput measurement systems for radioactive wastes segregation and free release.

This paper addresses the measurement facilities for pre-selection of waste materials prior to measurement for repository acceptance or possible free release (segregation measurement system); and free release (free release measurement system), based on a single standardized concept characterized by unique, patented lead-free shielding. The key objective is to improve the throughput, accuracy, reliability, modularity and mobility of segregation and free-release measurement. This will result in a more reliable decision-making with regard to the safe release and disposal of radioactive wastes into the environment and, resulting in positive economic outcomes. The research was carried out within "Metrology for Decommissioning Nuclear Facilities" (MetroDecom) project.

Half-life measurement of 55Fe.

The half-life of 55Fe was determined experimentally and compared with literature values. A solid 55Fe source was prepared by electrodeposition and repeatedly measured in an X-ray detector at a constant, low solid angle. In total, more than six hundred measurements were performed over a period of ten months, corresponding to one third of the half-life. The experimental method and the corresponding uncertainty budget are presented. The result of 1005.2+/-1.4 days is slightly higher than the current recommended values.

Experimental verification of the half-life of 65Zn.

The half-life of 65Zn was determined experimentally and compared with literature values. Two ampoules of a 65Zn solution were measured at regular intervals in two ionisation chambers over a period corresponding to nearly two half-lives. The result of 243.8+/-0.3 days is in agreement with the value of 244.0+/-0.2 days obtained by De Roost et al. (Z. Phys. 250 (1975) 395) at the same institute (previously called CBNM) in 1972. Moreover, this half-life is supported by most of the other measurements reported in the literature.

Alpha-particle emission probabilities of ²³6U obtained by alpha spectrometry.

High-resolution alpha-particle spectrometry was performed with an ion-implanted silicon detector in vacuum on a homogeneously electrodeposited (236)U source. The source was measured at different solid angles subtended by the detector, varying between 0.8% and 2.4% of 4π sr, to assess the influence of coincidental detection of alpha-particles and conversion electrons on the measured alpha-particle emission probabilities. Additional measurements were performed using a bending magnet to eliminate conversion electrons, the results of which coincide with normal measurements extrapolated to an infinitely small solid angle. The measured alpha emission probabilities for the three main peaks - 74.20 (5)%, 25.68 (5)% and 0.123 (5)%, respectively - are consistent with literature data, but their precision has been improved by at least one order of magnitude in this work.

A magnet system for the suppression of conversion electrons in alpha spectrometry.

A new magnet system has been designed and constructed to reduce coincidence effects between alpha particles and conversion electrons in high-resolution alpha-particle spectrometry. By means of a magnetic field, the conversion electrons are deflected away from the PIPS(®) detector. Compared to existing magnet systems, the new system is not restricted to point sources and can accommodate source diameters up to about 30 mm. Two yokes were built, allowing for configurations with 20 mm or 36 mm distance between the magnets. The effectiveness of both configurations is demonstrated by measuring the conversion electron spectrum of a (237)Np source. The magnet system effectively rejects 93 (7)% of electrons up to 85 keV (36 mm) and 90 (9)% of electrons up to 320 keV (20 mm). It has been successfully applied in the alpha-particle spectrometry of the long-lived nuclides (236)U and (238)U, resulting in significant improvement of the accuracy of alpha emission probabilities.

High-resolution alpha-particle spectrometry of ²³8U.

The alpha-particle emission probabilities associated with the three main alpha transitions of (238)U were measured by high-resolution alpha-particle spectrometry. Highly enriched (238)U material was used and its isotopic composition characterised by mass spectrometry. Source production through electrodeposition was optimised to reconcile conflicting demands for good spectral resolution and statistical precision. Measurements were performed at IRMM and CIEMAT for 1-2 years in three different set-ups. A new magnet system was put into use to largely eliminate true coincidence effects with low-energy conversion electrons. Finally the accuracy and precision of the relative emission probabilities for the three transitions - 77.01 (10)%, 22.92 (10)% and 0.068 (10)%, respectively - have been improved significantly.

Half-lives of 221Fr, 217At, 213Bi, 213Po and 209Pb from the 225Ac decay series.

The half-lives of (221)Fr, (217)At, (213)Bi, (213)Po, and (209)Pb were measured by means of an ion-implanted planar Si detector for alpha and beta particles emitted from weak (225)Ac sources or from recoil sources, which were placed in a quasi-2π counting geometry. Recoil sources were prepared by collecting atoms from an open (225)Ac source onto a glass substrate. The (221)Fr and (213)Bi half-lives were determined by following the alpha particle emission rate of recoil sources as a function of time. Similarly, the (209)Pb half-life was determined from the beta particle count rate. The shorter half-lives of (217)At and (213)Po were deduced from delayed coincidence measurements on weak (225)Ac sources using digital data acquisition in list mode. The resulting values: T1/2((221)Fr)=4.806 (6) min, T1/2((217)At)=32.8 (3)ms, T1/2((213)Bi)=45.62 (6)min, T1/2((213)Po)=3.708 (8) µs, and T1/2((209)Pb)=3.232 (5)h were in agreement only with the best literature data.

Measurements of the half-life of 214Po and 218Rn using digital electronics.

The half-lives of (214)Po and (218)Rn have been measured. The radionuclides were produced in the decay of a (230)U source and the emitted alpha-particles were measured in nearly-2π geometry with an ion-implanted planar silicon detector. The data acquisition was performed with a digitiser operated in list mode, saving the energy and time of detection (10 ns precision timestamp) of each event. The half-lives were deduced from the time differences between the alpha-decays populating the nuclide of interest and those corresponding to its decay. Different methods were applied, based on delayed coincidence counting and time-interval distribution analysis. The resulting half-lives are 33.75 (15) ms for (218)Rn and 164.2 (6) µs for (214)Po, both in agreement with some of the literature values, and obtained with higher precision in this work.

Measurement of the 226Th and 222Ra half-lives.

The half-lives of (226)Th and (222)Ra were measured by counting alpha-particle emissions from sources as a function of time. The (226)Th sources were prepared from an open (230)U source, capturing recoil atoms after alpha-particle decay on glass disks or even directly onto a detector. Similarly, the (222)Ra sources were obtained by self-transfer of recoil atoms from (226)Th sources. The activity measurements were performed in (nearly-)2π geometry with an ion-implanted silicon detector. The decay curves were analysed in different ways, incl. moment analysis, trying to avoid the pitfalls of bias of least-squares fits to Poisson distributed data. The observed half-life values are T(1/2)((226)Th)=30.70 (3) min and T(1/2)((222)Ra)=33.6 (4) s. Literature values show some inconsistency.

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.

High-resolution alpha-particle spectrometry of the 230U decay series.

High-resolution alpha-particle spectrometry was performed on the (230)U decay series. A (230)U source was prepared on a stainless steel disc by electrodeposition in an ammonium nitrate solution. Spectrometry of the alpha-particle energy spectrum was performed with ion-implanted planar silicon detectors in vacuum. A set of alpha emission probabilities is presented for (230)U and (226)Th. The measured peak intensities were corrected mathematically for coincidental detection of alpha-particles and conversion electrons emitted in the same decay. A good agreement with literature data was observed. The uncertainty budget and the correlation matrix are presented. The validity of the alpha-particle energies was tested and could be confirmed for most peaks within a few keV, but discrepancies were found for the 2nd peak of (226)Th and the main peak of (218)Rn.