Abstract
A new triple to double coincidence ratio (TDCR) system has been established at NPL. The system incorporates a spherical optical chamber, a manual sample changing facility and an integral light-tight housing. A 6 in diameter NaI(Tl) detector has been incorporated to allow 4pibeta-gamma coincidence measurements to be performed in parallel. Details of the detectors, the electronics and the overall TDCR system are given. Validation measurements using suitable low-energy beta and electron capture nuclides, i.e. (3)H and (55)Fe have been performed. The highest efficiency achieved with a (3)H solution in Ultima Gold AB in a glass vial was 53% and in a high-efficiency LS cocktail 65%. This indicates that the optical chamber is performing well. After varying the efficiency by de-focusing the PM tubes, the activity of the sources was determined. The activity concentration determined with TDCR agreed with certified values within the range of uncertainties. Further results from validation measurements and the corresponding uncertainty budgets are presented.
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