Nuclear metrology at NPL

Orateur : Peter IVANOV & Simon JEROME (National Physical Laboratory, Hampton Road, TW11 0LW Teddington, UK)

The absolute determination of radioactivity is central to the dissemination of measurement traceability from radioactivity measurements, whether they be for medical, environmental, electricity generation or security purposes. An overview of techniques employed at NPL will be described, covering coincidence counting, liquid scintillation techniques and calibration of secondary systems. Protactinium-231 is one of the lesser known actinide nuclides, yet the measurement of this radionuclide is central to dating studies in both paleoclimate and nuclear forensics measurements ; furthermore, it is important as the immediate parent nuclide of the 227Ac decay chain. 231Pa presents some unique measurement challenges, due to the nature of its chemical behavior which hasn’t been extensively studied, the complexity of its decay scheme and the generally unsatisfactory state of the decay data of this radionuclide. There are challenges to the long term use of multi-MBq 226Ra sources as ionisation chamber check sources as some regulatory authorities place strict limits on the use of such sources, radiation damage to the container and increased internal gas pressure may eventually result in the source container rupturing, and the effective half-life depends on the equilibrium state of the 226Ra decay chain. This talk describes investigative work into a replacement radionuclide for 226Ra. We conclude that 166mHo is a suitable candidate due to its decay characteristics. The plan for production of suitable quantities of 166mHo is described, as well as the subsequent chemical purification techniques employed, based on cation exchange and extraction chromatography to deliver high activity sources of 166mHo with less than 0.01% impurities (by activity) of other radionuclides.

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