Depth profile of ²³⁶U/²³⁸U in soil samples in La Palma, Canary Islands

The vertical distribution of the (236)U/(238)U isotopic ratio was investigated in soil samples from three different locations on La Palma (one of the seven Canary Islands, Spain). Additionally the (240)Pu/(239)Pu atomic ratio, as it is a well establish tool for the source identification, was determined. The radiochemical procedure consisted of a U separation step by extraction chromatography using UTEVA Resin (Eichrom Technologies, Inc.). Afterwards Pu was separated from Th and Np by anion exchange using Dowex 1 x 2 (Dow Chemical Co.). Furthermore a new chemical procedure with tandem columns to separate Pu and U from the matrix was tested. For the determination of the uranium and plutonium isotopes by alpha spectrometry thin sources were prepared by microprecipitation techniques. Additionally these fractions separated from the soil samples were measured by Accelerator Mass Spectrometry (AMS) to get information on the isotopic ratios (236)U/(238)U, (240)Pu/(239)Pu and (236)U/(239)Pu, respectively. The (236)U concentrations [atoms/g] in each surface layer (∼2 cm) were surprisingly high compared to deeper layers where values around two orders of magnitude smaller were found. Since the isotopic ratio (240)Pu/(239)Pu indicated a global fallout signature we assume the same origin as the probable source for (236)U. Our measured (236)U/(239)Pu value of around 0.2 is within the expected range for this contamination source.