Absolute standardization of carbon-14 by the CIEMAT/NIST method with empirical determination of the Birks parameter

Abstract

This study presents the implementation and validation of the CIEMAT/NIST efficiency tracing method for the absolute standardization of Carbon-14 using an ultra-low-level Quantulus 1220 liquid scintillation counter, toluene-based cocktails, and the EFFY-9 code. Due to the absence of a dedicated profile for classical toluene cocktails in the software library, Ultima Gold was used as a substitute. To achieve this, a universal curve was constructed using a series of tritiated standards as a tracer, correlating the instrumental quench index with the model’s free parameter. Subsequently, the activity of two certified Carbon-14 standards was computed across a range of Birks parameter (kB) values from 0.004 to 0.014 cm.MeV−1. The results exhibited excellent agreement with certified values, with relative deviations consistently remaining below 2.1%. Detailed analysis indicated that the minimum bias corresponds to kB=0.004 cm.MeV−1. This finding confirms that, in this specific configuration, kB serves as an effective parameter, compensating for the residual mismatch between the actual properties of toluene and the surrogate computational profile. This research emphasizes the necessity of experimentally determining the Birks parameter for each specific laboratory setup to ensure maximum accuracy.