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%.

Test of a digitizer to process the pulse signal from the 3 photomultiplier tubes of a TDCR liquid scintillation counter.

The BIPM is developing a new service for international key comparisons in radionuclide metrology. The system, called ESIR, is based on a liquid scintillation counter using the Triple-to-Double Coincidence Ratio (TDCR) technique. The aim is to produce an international reference that can be reproduced over several decades of time in order to compare the calibration capabilities of National Metrology Institutes (NMIs). The maintainability of the electronics performing the signal processing is a challenge. To ensure the long-term sustainability of the electronics, the strategy is to set up redundant systems including at least one digital electronics module. The analogue modules developed in the 1990s and 2000s are less and less maintained and digital electronics are increasingly available on the market. In this context, a digitizer was tested and its suitability for the TDCR measurements compared to the currently used module based on an analogue front-end. This first implementation directly linking the photomultiplier anode to the CAEN digitizer without any analogue preconditioning shows a significant loss of detection efficiency and a lower signal to noise ratio observed on distributions of single photoelectrons. Although the TDCR method can correct for these efficiency losses, the loss of symmetry between the channels is too great to provide a sufficiently robust measurement. The use of low-pass filters upstream of the ADC will be considered to make this digital measurement system more reliable.

Half-life determination and comparison of activity standards of 231Pa.

The results of an international comparison of activity measurements of a solution of 231Pa are reported and analysed. Prior to this, no known standardisation of 231Pa by activity measurement had been carried out. The comparison was run in 2017-2018 involving eight laboratories, and returned results with no identifiable inconsistencies between methods or laboratories. The results, including one mass determination, gave a231Pa activity concentration of 41.461(48) kBq g-1 and a231Pa atom concentration of 61.48(23) × 1015 atoms g-1, from which a half-life value of 32 570(130) years was derived.