Optimisation of a batch thermal combustion method using a tube furnace oxidation system (pyrolyser) and LSC for carbon-14 determination in environmental matrices

Accelerator mass spectrometry and benzene synthesis coupled with liquid scintillation spectrometry are often used for accurate measurements of ¹⁴C activity in the environmental matrices. Thermal oxidation is one of the methods employed for ¹⁴C determination in environmental matrices. In this method, the sample is oxidised at high temperature (600-900 °C) to convert carbon species to CO₂ and trapped in an amine-based absorber for determining the activity in a liquid scintillation counting (LSC) system. In this study, the performance of a commercially available tube furnace system (pyrolyser), for batch combustion of samples, was evaluated for the determination of ¹⁴C specific activity in terrestrial biota samples. Significant improvements over the manufacturer specified method, which is primarily designed for analysis of samples with activity well above the environmental background level, was implemented to achieve accurate determination of ¹⁴C specific activity at ambient background level. In the improved method, the CO₂ produced from the combustion of the sample was isolated from the combustion products through cryogenic trapping and then absorbed in the absorber (Carbo-Sorb E) through a simple off-line transfer process. This allowed (i) optimisation of CO₂ absorption by the absorber (2.2477 g of CO₂/10 mL), (ii) achieving good accuracy and precision in the measurements, and a minimum detectable activity value of 13 Bq kg^-1C for a counting time of 300 min (7 Bq kg^-1C for 1000 min), (iii) avoiding uncertainty associated with the determination of recovery of ¹⁴C in the combustion and trapping process, and (iv) elimination of the need for an independent determination of carbon content (%) for expressing the results in terms of ¹⁴C specific activity. The method is capable of yielding accurate results with a deviation of <2.4% from the target value for IAEA C3 quality assurance reference material (with a relative standard deviation of 1.40%, and relative error of 0.34%). The combined uncertainty (1σ) associated with the measurements was computed to be 3.4%. Upon optimisation, the suitability of the method for the determination of ¹⁴C specific activity in typical terrestrial biota samples of clean air region (region not affected by local anthropogenic sources) and for the quantification of a small increase in the ¹⁴C activity above ambient levels in the vicinity of a nuclear power plant is demonstrated.

Natural and artificial radionuclides in a marine core. First results of 236U in North Atlantic Ocean sediments

There are very few data available of ²³⁶U in marine sediment cores. In this study we present the results from the first oceanic depth profile of ²³⁶U in a sediment core sampled in the North Atlantic Ocean, at the PAP site (4500 m depth, Porcupine Abyssal Plain (PAP) site, 49°0' N, 16°30' W). Additionally, the sediment core was radiologically characterized through the measurement of anthropogenic ¹³⁷Cs, ²³⁹Pu, ²⁴⁰Pu, ¹²⁹I and ¹⁴C and natural ²¹⁰Pb, ⁴⁰K and ²²⁶Ra. The measured ²³⁶U concentrations decrease from about 90·10⁶ at g^-1 at the seafloor down to 0.5·10⁶ at g^-1 at 6 cm depth. They are several orders of magnitude lower than the reported values for soils from the Northern Hemisphere solely influenced by global fallout (i.e. from 2700·10⁶ to 7500·10⁶ at g^-1). ²³⁶U/²³⁸U atom ratios measured are at least three orders of magnitude above the estimated level for the naturally occurring dissolved uranium. The obtained inventories are 1·10¹² at m^-2 for ²³⁶U, 80 Bq m^-2 for ¹³⁷Cs, 45 Bq m^-2 for ^239+240Pu and 2.6·10¹² at m^-2 for ¹²⁹I. Atomic ratios for ²³⁶U/²³⁹Pu, ¹³⁷Cs/²³⁶U and ¹²⁹I/²³⁶U, obtained from the inventories are 0.036, 0.11 and 2.5 respectively. Concentration profiles show mobilization probably due to bioturbation from the abundant detritivore holothurian species living at the PAP site sea-floor. The range of ²³⁶U, ¹³⁷Cs, ^239+240Pu and ¹²⁹I values, inventories and ratios of these anthropogenic radionuclides are more similar to the values due to fall-out than values from a contribution from the Nuclear Fuel Reprocessing Plants dispersed to the south-west of the North Atlantic Ocean. However, signs of an additional source are detected and might be associated to the nuclear wastes dumped on the Eastern North Atlantic Ocean.