The international reference system for beta-particle emitting radionuclides: Validation through the pilot study CCRI(II)-P1.Co-60

The Bureau International des Poids et Mesures (BIPM) is developing a new transfer instrument to extend its centralized services for assessing the international equivalence of radioactive standards to new radionuclides. A liquid scintillation counter using the triple/double coincidence ratio method is being studied and tested in the CCRI(II)-P1.Co-60 pilot study. The pilot study, involving 13 participating laboratories with primary calibration capabilities, validated the approach against the original international reference system based on ionization chambers, which has been in operation since 1976. The results are in agreement and an accuracy suitable for purpose, below 5×10-4, is achieved. The pilot study also reveals an issue when impurities emitting low-energy electrons are present in the standard solution, which have a different impact on liquid scintillation counting compared to other primary measurement methods.

Liquid scintillation counting of electrons from internal conversion for quantifying 109Cd activity using a 3-photomultiplier tube system and a specific spectral unfolding

As part of the international key comparison CCRI(II)-K2.Cd-109.2021, the BIPM has implemented a specific method to measure the activity of ¹⁰⁹Cd solution - a key radionuclide in the calibration of gamma-ray spectrometers. The counting of electrons from internal conversion was carried out using a liquid scintillation counter based on 3-photomultiplier tubes. In this technique, a major part of the uncertainty comes from the overlap between the conversion electrons peak and the peak at lower energy from the other products of the decay. As a result, the energy resolution that the liquid scintillation system can achieve is the most critical challenge to achieving a precise measurement. The study demonstrates the advantage of producing a sum of the signal from the three photomultipliers to enhance the energy resolution and limit the peak overlap. In addition, the spectrum has been processed by a specific unfolding approach to properly separate the spectral components. Thanks to the method introduced in this study, an activity estimation has been realized with a relative standard uncertainty of 0.5%.

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.