The Joint Committee for Guides in Metrology, Working Group 1, JCGM-WG1, is currently revising the ‘Guide to the Expression of Uncertainty in Measurement’. In this communication, the motivation for undertaking such a revision is given and the main changes with respect to the current, 2008 edition are outlined.

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.

Claims that proximity to the Sun causes variation of decay constants at permille level have been investigated for alpha decaying nuclides. Repeated decay rate measurements of 209 Po, 226 Ra, 228 Th, 230 U, and 241 Am sources were performed over periods of 200 d up to two decades at various nuclear metrology institutes around the globe. Residuals from the exponential decay curves were inspected for annual oscillations. Systematic deviations from a purely exponential decay curve differ in amplitude and phase from one data set to another and appear attributable to instabilities in the instrumentation and measurement conditions. The most stable activity measurements of α decaying sources set an upper limit between 0.0006% and 0.006% to the amplitude of annual oscillations in the decay rate. There are no apparent indications for systematic oscillations at a level of weeks or months. Oscillations in phase with Earth's orbital distance to the sun could not be observed within 10 −5 -10 −6 range precision.

Claims that proximity to the Sun causes variations of decay constants at the permille level have been investigated for beta-minus decaying nuclides. Repeated activity measurements of 3 H, 14 C, 60 Co, 85 Kr, 90 Sr, 124 Sb, 134 Cs, 137 Cs, and 154 Eu sources were performed over periods of 259 d up to 5 decades at various nuclear metrology institutes. Residuals from the exponential decay curves were inspected for annual oscillations. Systematic deviations from a purely exponential decay curve differ in amplitude and phase from one data set to another and appear attributable to instabilities in the instrumentation and measurement conditions. Oscillations in phase with Earth's orbital distance to the Sun could not be observed within 10 −4 -10 −5 range precision. The most stable activity measurements of β − decaying sources set an upper limit of 0.003%-0.007% to the amplitude of annual oscillations in the decay rate. There are no apparent indications for systematic oscillations at a level of weeks or months.

The contents of the current edition, JCGM 100:2008, of the Guide to the Expression of Uncertainty in Measurement (GUM) and its Supplements are reviewed and remarks made concerning a proposed revision of the GUM. A committee draft of the revision was circulated to member organizations of the Joint Committee for Guides in Metrology (JCGM) and all national metrology institutes in December 2014. The motivation for the proposed changes is given and reactions to the committee draft are summarized. Some of the contents of this paper are solely an expression by the authors and do not constitute an official statement by the JCGM.

The hypothesis that seasonal changes in proximity to the Sun cause variation of decay constants at permille level has been tested for radionuclides disintegrating through electron capture and beta plus decay. Activity measurements of 22

The international reference system for radioactivity relies on stable instrumentation for comparing primary standards of radioactivity; these instruments enable national metrology institutes to demonstrate the equivalence of their primary standards. The international bureau of weights and measures (BIPM) holds two such instruments for gamma-ray emitting radionuclides, and is working with the POLATOM, LNE/LNHB, NIM, NPL and PTB to develop a new system called the 'extended international reference system' (ESIR). The ESIR will address pure beta-particle emitting radionuclides as well as other radionuclides (such as 55 Fe) which cannot be measured in the established international systems. The ESIR will be a liquid scintillation system based on three photon-counting channels operating in coincidence. This article reports the results from validation studies carried out using solutions of 3 H, 55 Fe, 63 Ni and 14 C to assess the reproducibility of the results. Several key comparison indicators have been developed and tested to find the best way to obtain a robust and reproducible international reference value. An important conclusion is that the new ESIR can deliver accurate comparison values, immune from changes in detection efficiency or asymmetry of the counting channels. A relative uncertainty better than 0.2% can be expected for radionuclides emitting beta particles with an end-point energy above 150 keV while using commercial liquid scintillation cocktails.

Detailed uncertainty reporting is imperative for proficiency tests and comparison exercises because uncertainties need to be comparable and trusted by all the participants. Even though participants do their best to follow the Guide to the Expression of Uncertainty in Measurement, ambiguities and divergences about uncertainty evaluation remain. Consequently, to analyze the situation, the CCRI (II) Uncertainties Working Group proposed a comparison exercise (CCRI(II)-S7) about the uncertainty evaluation of a relatively simple primary activity measurement: the standardization of a 60Co source by coincidence counting. To be able to understand how various NMIs calculate coincidence counting uncertainties, our study focused on two of the dominant uncertainty components commonly quoted for 4πβ-γ coincidence counting in the International Reference System (SIR) submissions and Key Comparison exercises: efficiency-extrapolation and weighing. Participants from twelve different laboratories were sent the same set of measurement data from the analysis of a 60Co solution standardized at the National Physical Laboratory (NPL). Our study demonstrated the extent of the different interpretations of the uncertainty components. Some factors causing large discrepancies were isolated and are discussed. Further studies of other techniques using a similar approach would be beneficial for the metrology community. Main text. 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 kcdb.bipm.org/.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).