Three uncertainty propagation methods were evaluated for the radiochronometry of the long-lived parent–daughter pair 234U-230Th for nuclear forensics applications. The methods considered were: (1) explicit analytical uncertainty propagation formulas derived by Pommé et al.; (2) linear uncertainty propagation using sensitivity coefficients generated with the GUM Workbench, in accordance with the Guide to the Expression of Uncertainty in Measurement; and (3) the Kragten method, which numerically estimates sensitivity coefficients by varying the input parameters of the nuclear dating equation. All three approaches yielded closely consistent results, thereby validating the analytical uncertainty propagation equations of the first method.

The response of a discrete ion counter is not perfectly linear due to count loss caused by dead time and pulse pileup. As a result, the output rate of the counter does not scale linearly with the input rate of ions reaching the detector. The value of a stable input rate can be determined from the measured output rate by inverting the throughput formula of the ion counter. However, when the input rate varies during the measurement, a mismatch between the average input rate and the average output rate becomes apparent. The resulting bias can be particularly significant when measuring transient signals. A correction procedure is proposed to calculate a better estimate of the average input rate from the observed mean and variance of the output rate. Implementation of this refined throughput formula is recommended to improve accuracy of mass spectrometry utilising discrete ion counters.

This study explores the relationship between experimentally observed inter-event time-interval distributions (TIDs) and measured dead times for pulses generated by a HPGe gamma detector and processed by a digital spectrometer. The system utilizes a fast and slow channel, pileup rejector, trapezoid filtering, and flash analog-to-digital converter. The experimentally derived TIDs were compared with theory for validation. The results demonstrate that the theoretical model reliably describes measured TIDs up to 40% dead time. However, significant distortion effects become increasingly pronounced at higher input rates. It appears that the deviation between the measured and calculated TIDs take the shape of higher-order convolutions of the TID caused by an imperfection in the timing resolution in the pileup rejection circuit. In this work, theoretical functions for the TID are expanded to reproduce measurements up to 90% dead time. This refinement in the interpretation and treatment of the measured TIDs provides improved accuracy and precision in the prediction of the true event rate and measured dead time in the counter with results comparable to those from traditional methods. Although the new theoretical TIDs reflect processes that are specific to the experimental set-up, it is expected that similar adjustments are applicable to other gamma detection systems as well.

A simplified theoretical model is developed to predict counting statistics for a stationary Poisson process passing through a spectrometer with pulse-pileup rejection. The model is applicable to digital counters used for spectrometry as well as set-ups utilising analogue electronics for pulse shaping and pileup rejection. In comparison with an existing model for a perfect pileup rejector, the new model addresses the common imperfection of having a finite time resolution of the fast channel, allowing quasi-coincident signals to pass through the pile-up rejector. From the Laplace transform of a simplified interval-density distribution, approximate expressions are derived for the throughput factor and the variance of the number of counted events. The results are compared with computer simulations of a cascade of extending dead time and subsequent pileup rejection. In addition, a rigorous throughput factor is derived from probabilistic reasoning, as well as an effective throughput factor for singular and coincident events.

The introduction over the last decade of radionuclide therapy based on 223Ra and 227Th has reawakened interest in the radionuclides of the 235U decay series (the 4n+3 decay chain). This has coincided with a requirement for improved accuracy in dating of long-lived radionuclides for nuclear forensic and for geological purposes. Thus, 231Pa has become the subject of revived interest in recent years. The short-term ingrowth of the decay progeny is of interest to nuclear forensic science since it enables the direct calculation of the separation age of enriched 235U[1]; separation times based on the 234U-230Th chain may also be calculated, but are more complex due to the reliance on the 238U-234Th-234mPa-234U-230Th decay family. Furthermore, since protactinium fluorides are non-volatile at ordinary temperature, the build-up of 231Pa in fuel enrichment facilities may provide information on throughput of separation units as well as the whole plant. In the longer term, the characterisation of sedimentation rates is facilitated by a range of natural nuclear chronometers that include 231Pa/235U to provide information concerning sediment formation, and the measurement of 231Pa:230Th mass ratios (as well as 231Pa:235U and 230Th:234U mass ratios) may also provide information of global temperature trends over the 100-200 ka range[2]. This report summarises the results of an international comparison of the activity per unit mass of the same 231Pa solution along with a new half-life determination[3]. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/. The final report has been peer-reviewed and approved for publication by the CCRI, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

A mathematical model is presented to calculate the expected throughput rate in a discrete ion counter with imposed non-extending dead time.

When verifying the validity of the exponential-decay law through 137 precise decay rate measurement series at various nuclear laboratories, minor violations have been observed in the shape of annual cycles in the residuals with different amplitudes and phase shifts. The timing and amplitude of these deviations have been compared with local weather data and it appears that ambient humidity is highly correlated with the observed instabilities in these radioactivity measurements. In fact, when compensating the residuals for a linear relationship with absolute humidity in air, most of the annual cycles are no longer statistically significant. As a result, the validity of the exponential-decay law can now be demonstrated with even higher fidelity.

A liquid scintillation background sample was measured daily in a custom-built TDCR counter for more than 17 months. The double and triple coincidence counting rates exhibit an annual sinusoidal fluctuation with a maximum in winter and a minimum in summer. Possible correlations with air temperature, air humidity, radon concentration and secondary cosmic radiation were investigated. The observation of a correlation with the ambient dose equivalent rate H-center dot & lowast;(10)SCR originating from the charged component of secondary cosmic radiation and an anti-correlation with the effective atmospheric temperature Teff suggest that the seasonal fluctuations in the background counting rate may be primarily driven by temporal variations in the muon flux at ground level. Additionally, a correlation was found with the indoor 222Rn concentration in air.

The half-life of 22Na has been determined through 523 activity measurements over 14.5 years with a re-entrant ionisation chamber in a temperature-stabilised room. The data were selected and aggregated from in total 127 830 ionisation current measurements with a reproducibility of the order of 0.065% standard deviation. The ionisation current was collected over an air capacitor and measured as a change of voltage over time, which warrants excellent linearity with activity throughout the experiment. The residuals to an exponential decay curve show a distinct annual cycle of 0.0055 (5)% amplitude, which reduces to 0.0012 (5)% after compensation for a correlation with ambient humidity. The data confirm the validity of the exponential-decay law, in absence of cyclic perturbations at daily, monthly, and multi-annual scale at the level of 0.0005% standard deviation. The 22Na half-life value obtained in this study is 950.68 (12) d or 2.60290 (34) a, with a relative standard uncertainty of 0.013%. This value is consistent with the mean value of other measurement results in the literature, although the latter have an issue with incomplete uncertainty budgets. Owing to the high statistical accuracy and conservative uncertainty estimate of systematic errors, the result obtained in this work can be recommended as a reliable reference value.

For a single radionuclide being measured in an ionisation chamber, a calibration factor can be established that relates the ionisation current to the source activity. The same applies to a decay chain in secular equilibrium, for which the calibration factor comprises the ionisation current produced by the parent and progeny nuclei combined. The calibration of an ionisation chamber for non-equilibrated parent-progeny decay poses a problem because the activity ratio of the parent and progeny nuclei varies with time. This study examines cases in which the half-lives of the parent and only one of the progenies in the decay series are significantly long. Thus, two calibration factors are involved, which combine differently as a function of time. By means of nuclear dating of the material, the parent-progeny activity ratio can be determined, and the respective contributions to the ionisation current can be unambiguously distinguished. Once the ionisation chamber is calibrated, two measurements taken at different times are sufficient to determine the activity and age of the parent-progeny mixture. This study presents equations for calculating the calibration factors and propagating uncertainties, illustrated with a case study focusing on the 227 Th/ 223 Ra decay chain used in alpha-immunotherapy.

This study compares the theoretical formula for time interval density distributions (TIDs) in counters with pileup rejection to TIDs measured using a digital spectrometer with trapezoid filtering. The TIDs were derived from successive time stamps of 60Co and 133Ba γ rays detected with a HPGe spectrometer. The results demonstrate that the theoretical formula is applicable up to 40% count loss and can be used to retrospectively deduce the incident event rate.

A 100-nm-thick gadolinium layer deposited on a pixelated silicon sensor was activated in a neutron field to measure the internal conversion electron (ICE) spectrum generated by neutron capture products of 155Gd and 157Gd. The experiment was performed at the ISIS neutron and muon facility, using a bespoke version of the HEXITEC spectroscopic imaging camera. Signals originating from internal conversion electrons, Auger electrons, x rays and gamma rays up to 150 keV were identified. The ICE spectrum has an energy resolution of 1.8-1.9 keV at 72 keV and shows peaks from the K, L, M, N+ ICEs of the 79.51 keV and 88.967 keV 2+-0+ gamma transitions from the first excited states in 158Gd and 156Gd, respectively, as well as the K ICEs of the 4+-2+ transitions at 181.931 keV and 199.213 keV from the respective second excited states. Spectrum analysis was performed using a convolution of a Gaussian with exponential functions at the low and high energy side as the peak shaping function. Relative ICE intensities were derived from the fitted peak areas and compared with internal conversion coefficient (ICC) values calculated from the BrIcc database. Relative to the dominant L shell contribution, the K ICE intensity conforms to BrIcc and the M, N, O+ ICE intensities are somewhat higher than expected.

There has been scientific debate about speculations that ‘neutrino-induced’ radioactive decay causes apparent violations of the exponential-decay law. Sturrock and others repeatedly publish papers asserting influences by solar and cosmic neutrinos on radioactive decay measurements and therefrom draw conclusions about space science that are highly speculative. Recurrent themes in their work are claims that the solar neutrino flux reveals oscillations at a monthly rate which can be linked to solar rotation, that annual and monthly oscillations occur in radioactive decay rates or directionality of emitted radiation which can be linked to variations in solar and cosmic neutrino flux hitting Earth’s surface, and that unstable radioactivity measurements can be used as a source of information about the interior of the Sun and dark matter. Radionuclide metrologists have extensively investigated and refuted their arguments. Metrological evidence shows that radioactive decay does not violate the exponential-decay law and is not a probe for variations in solar neutrino flux. In this review paper, the main arguments of Sturrock are listed and counterarguments are presented. Reference is made to earlier published work in which the evidence has been scrutinised in detail.

Parkhomov published decay rate measurements of Sr-90/Y-90 and Co-60 beta decay sources with Geiger-Muller counters which showed annual cyclic deviations with less than 0.2% amplitude from a purely exponential slope. He investigated instrument instability induced by environmental parameters, yet did not find a clear coincidence with local temperature, atmospheric pressure, and relative humidity. Parkhomov hypothesised that gravitationally-focussed 'slow' cosmic neutrinos influenced beta decay. In the current work, environmental conditions in the Moscow area at the time of the experiment are presented. There appears to be a resemblance of the shape of the annual Sr-90/Y-90 decay rate anomalies with the inverse of the absolute air humidity, albeit with an apparent time shift of 0.05-0.15 year. Humidity may have influenced the range of beta particles in air, as well as geometric and electronic properties of the detection set-up, however causality could not be unambiguously demonstrated. The instabilities in the Co-60 data were more difficult to correlate with environmental data, except for some similarities with temperature and external dew point.

Parkhomov published decay rate measurements of 90 Sr/ 90 Y and 60 Co beta decay sources with Geiger–Müller counters which showed annual cyclic deviations with less than 0.2% amplitude from a purely exponential slope. He investigated instrument instability induced by environmental parameters, yet did not find a clear coincidence with local temperature, atmospheric pressure, and relative humidity. Parkhomov hypothesised that gravitationally-focussed ‘slow’ cosmic neutrinos influenced beta decay. In the current work, environmental conditions in the Moscow area at the time of the experiment are presented. There appears to be a resemblance of the shape of the annual 90 Sr/ 90 Y decay rate anomalies with the inverse of the absolute air humidity, albeit with an apparent time shift of 0.05–0.15 year. Humidity may have influenced the range of beta particles in air, as well as geometric and electronic properties of the detection set-up, however causality could not be unambiguously demonstrated. The instabilities in the 60 Co data were more difficult to correlate with environmental data, except for some similarities with temperature and external dew point.

In recent work, Milian-Sanchez et al. observed fluctuations in radioactive decay rate measurement series, and after excluding environmental influences (measured indoors) as root causes, they looked for possible correlations with astrophysical variables. In spite of the authors efforts to investigate possible influences of environmental parameters (such as ambient temperature, pressure and humidity) on the detectors stability, it turns out that the influence of ambient humidity on the instrumentation has been underestimated.

Radionuclides, whether naturally occurring or artificially produced, are readily detected through their particle and photon emissions following nuclear decay. Radioanalytical techniques use the radiation as a looking glass into the composition of materials, thus providing valuable information to various scientific disciplines. Absolute quantification of the measurand often relies on accurate knowledge of nuclear decay data and detector calibrations traceable to the SI units. Behind the scenes of the radioanalytical world, there is a small community of radionuclide metrologists who provide the vital tools to convert detection rates into activity values. They perform highly accurate primary standardisations of activity to establish the SI-derived unit becquerel for the most relevant radionuclides, and demonstrate international equivalence of their standards through key comparisons. The trustworthiness of their metrological work crucially depends on painstaking scrutiny of their methods and the elaboration of comprehensive uncertainty budgets. Through meticulous methodology, rigorous data analysis, performance of reference measurements, technological innovation, education and training, and organisation of proficiency tests, they help the user community to achieve confidence in measurements for policy support, science, and trade. The author dedicates the George Hevesy Medal Award 2020 to the current and previous generations of radionuclide metrologists who have devoted their professional lives to this noble endeavour.

The 32Si decay rate measurement data of Alburger et al. obtained in 1982–1986 at Brookhaven National Laboratory have been presented repeatedly as evidence for solar neutrino-induced beta decay. The count rates show an annual sinusoidal oscillation of about 0.1% amplitude and maximum at February–March. Several authors have claimed that the annual oscillations could not be explained by environmental influences on the set-up, and they questioned the invariability of the decay constant. They hypothesised a correlation with changes in the solar neutrino flux due to annual variations in the Earth-Sun distance, in spite of an obvious mismatch in amplitude and phase. In this work, environmental conditions at the time of the experiment are presented. The 32Si decay rate measurements appear to be inversely correlated with the dew point in a nearby weather station. Susceptibility of the detection set-up to local temperature and humidity conditions is a likely cause of the observed instabilities in the measured decay rates. Similar conclusions apply to 36Cl decay rates measured at Ohio State University in 2005–2012.

The half-life of 171 Tm was measured by gamma-ray spectrometry, using the reference source method. The measured point-like source was prepared by homogenous mixing of 44 Ti with 171 Tm in order to obtain an identical detection geometry for both nuclides. The half-life of 171 Tm was determined by following the count-rate ratio between the gamma-ray emissions at 66.73 keV ( 171 Tm) and 67.9/78.3 keV ( 44 Ti). The measurement campaign consisted of 280 measurements acquired over a period of 449 days, corresponding to about 60% of the half-life value. The result for the 171 Tm half-life, 702.4 (70) d, is consistent with the currently recommended value, however the uncertainty assessment of the latter is poorly documented.

The half-life of 145Sm has been measured by means of the reference source method with a HPGe detector. The long-lived radionuclide 44Ti was mixed into the source for reference. The time-dependency of the 145Sm/44Ti activity ratio was followed by assessing the count-rate ratio of their characteristic gamma-ray emissions at 61.2 keV (145Sm) and 67.9/78.3 keV (44Ti) in spectra recorded over periods of typically one day. In total, 220 measurements were performed over a period of 384 days or about one half-life period. The experiment and ensuing uncertainty budget are discussed in detail. Different error propagation is applied for random uncertainties, autocorrelated structures in the fit residuals, and potential systematic errors. The result for the 145Sm half-life, 345 (16) d, is compatible with the scarce literature values, however the experimental details of the old measurements were barely documented.