Preparation of radon-free thoron sources based on ²²⁸Th separated from industrial thorium nitrate

To improve the metrological traceability of thoron activity concentration, we separated ²²⁸Th from industrial thorium nitrate and produced flow-through radon-free thoron sources using two different methods. Four flow-through sources with different activities ranging from 120 Bq to 1200 Bq were prepared, and the activity, thoron emanation coefficient and stability were evaluated. Results show that loading the ²²⁸Th solution onto a TEVA resin column gave significantly higher ²²⁸Th recovery rates and better stability than immersing TEVA resin directly into the solution. The thoron emanation coefficient increases as absolute humidity increases, which can be used to realize different thoron concentration environments. The average emanation coefficient for the four different thoron sources was 96.37% ± 2.38% at absolute humidity above 20 g/m³.