The international bureau of weights and measures (BIPM) is the centre for governments to act in common on matters relating to metrology, co-ordinating the world-wide measurement system and providing capabilities for international measurement comparisons on a shared-cost basis. The ionizing radiation department maintains and develops high-precision instrumentation to enable national metrology institutes to demonstrate the equivalence of primary standards of radioactivity. The scope of this service is expanding: the ‘extended international reference system (ESIR)’ project aims to provide a stable instrument capable of comparing standards of pure beta-emitting radionuclides, complementing existing instruments for gamma-emitters. In 2019, the members of the ESIR project devised a liquid scintillation system that is capable of producing very stable reference values over a long timescale, using a method inspired by the ‘triple-to-double coincidence ratio’ (TDCR) technique. This article sets out the approach that will be used for evaluating uncertainties in future comparison exercises using the ESIR. The method adopted was based on the guide to the expression of uncertainty in measurement published by the joint committee for guides in metrology. Particular care has been taken in choosing the evaluation method: the response of the TDCR instrument cannot be linearized, so a Monte-Carlo method was implemented and a technique to manage correlated input data was developed. A novel sampling technique was used for the input multi-variate distribution of coincident count rates; this approach may also be relevant to primary standards realized using the TDCR technique. Calculation have shown that standard uncertainties of the order of 0.1 % will be achievable for a typical radionuclide.

国际度量衡局（BIPM）是各国政府就计量相关事宜采取共同行动，协调全球度量系统并为共享度量基础上的国际度量比较提供能力的中心。电离辐射部门维护并开发了高精度仪器，以使国家计量机构能够证明放射性基本标准的等效性。该服务的范围正在扩大：“扩展的国际参考系统（ESIR）”项目旨在提供一种稳定的仪器，能够比较纯β发射放射性核素的标准，以补充现有的伽马发射器仪器。在2019年 ESIR项目的成员设计了一种液体闪烁系统，该系统能够使用“三重对偶符合率”（TDCR）技术启发的方法，在很长的时间内产生非常稳定的参考值。本文列出了在将来使用ESIR进行比较时，将用于评估不确定性的方法。所采用的方法基于计量指南联合委员会发布的测量不确定度表达指南。选择评估方法时要格外小心：TDCR仪器的响应无法线性化，因此实施了蒙特卡洛方法，并开发了一种用于管理相关输入数据的技术。一种新颖的采样技术被用于同时计数率的输入多变量分布。这种方法也可能与使用TDCR技术实现的主要标准有关。计算表明，典型放射性核素的标准不确定度可达到0.1％左右。