The International Bureau of Weights and Measures (French: Bureau international des poids et mesures, BIPM) is an intergovernmental organisation, through which its 59 member-states act together on measurement standards in four areas: chemistry, ionising radiation, physical metrology, and coordinated universal time. It is based in Saint-Cloud, Paris, France. The organisation has been referred to as IBWM (from its name in English) in older literature.
This paper reports on the development and validation of a new 4πβ(TDCR)-γ digital coincidence counting system at the International Bureau of Weights and Measures (BIPM) for absolute, high-precision disintegration rate determinations. The system integrates a triple-photomultiplier liquid scintillation system operating on the Triple-to-Double Coincidence Ratio (TDCR) principle. It constitutes the beta channel of the system in which a NaI(Tl) detector acts as the gamma channel, and with four input signals are processed by a CAEN digitizer. We introduce the DigiCoinc-2025 software, which implements a revised version of the classical Müller correction model for accidental coincidences. This new approach replaces theoretical parametric approximations with transmission factors derived directly from measured digital live times, resulting in superior stability and accuracy. The system was validated through measurements of 60Co sources, demonstrating agreement within 0.4
Abstract In order to fulfill its mission to ensure and promote the global comparability of measurements, the BIPM operates laboratories in the fields of physical metrology, ionizing radiation, chemistry and time. These laboratories act as centers for scientific and technical collaboration between Member States providing capabilities for international measurement comparisons on a shared cost basis. They coordinate international comparisons of national measurement standards agreed to be of the highest priority, and they establish and maintain appropriate reference standards for use as the foundation of key international comparisons at the highest level and provide selected calibrations from them. In the following sections, we provide highlights of the work the laboratories have undertaken during 2025.
The participants of the 12th International Atomic Energy Agency (IAEA) meeting on stable isotope reference materials reached a consensus, acknowledging the existence and use of two carbon isotope delta scales: the VPDB (Vienna Peedee belemnite) scale and the VPDB-LSVEC (LSVEC - lithium carbonate prepared by H. J. Svec). Conversion models between the two scales can be established and used but introduce uncertainty. A format for isotope delta scale definition was agreed upon and was used to define the two carbon isotope delta scales and the two main oxygen isotope delta scales, VSMOW-SLAP (Vienna Standard Mean Ocean Water-Standard Light Antarctic Precipitation) and VPDB. Confirmation or identification of a second-scale-defining point is still necessary for the nitrogen and sulfur isotope delta scales. Efforts are encouraged to improve consistency among laboratories in the isotopic analysis of "non-exchangeable hydrogen" in bulk organic materials and oxygen in carbonates using the phosphoric acid reaction. Additional topics discussed include (1) need for improvement in reference materials for accurate greenhouse gas isotopic analyses; (2) reference materials under production by the IAEA, the US Geological Survey (USGS), and the US National Institute of Standards and Technology (NIST); (3) methods for value and uncertainty assignment of reference materials; and (4) calculation of carbon-13 isotope delta and oxygen-18 isotope delta of CO2 measured by dual-inlet isotope ratio mass spectrometry.
Optical clocks provide ultra-precise frequency references that are vital for international metrology as well as for tests of fundamental physics. To investigate the level of agreement between different clocks, we simultaneously measured the frequency ratios between ten optical clocks in six different countries, using fiber and satellite links. This is the largest coordinated comparison to date, from which we present a subset of 38 optical frequency ratios and an evaluation of the correlations between them. Four ratios were measured directly for the first time, while others had significantly lower uncertainties than previously achieved, supporting the advance towards a redefinition of the second and the use of optical standards for international time scales.
Measurements play a crucial role in our daily lives; and we rely on metrology to ensure that measurements are accurate and comparable. Celebrating the 150th anniversary of the beginning of the global measurement system, we look into its future.
