Neptunium and americium distribution and diffusion measurements in deep-ocean sediment samples

Abstract

Three sediment samples from the Madeira Abyssal Plain containing 8 wt.%, 55 wt.% and 85 wt.% CaCO 3 were received from the Geologic Survey of The Netherlands. The least oxic sample, an olive grey turbiditic ooze, was used for distribution measurements under anoxic and oxic conditions, but no difference in the behaviour of neptunium was found. The distribution coefficients for 237Np, 239Np and 243Am were measured over a period of 500 days. The increase in the distribution coefficients during that period is due to diffusion into the sediment particles. The low concentration of the nuclides in the seawater fraction necessitates the separation of 243Am from the natural alpha-emitting isotopes. The measurement of the effective diffusion coefficient was carried out on the three samples and for neptunium a linear relationship was found between the CaCO 3 content and the rootmean square displacement of neptunium at a given time. The effective diffusion coefficient of americium was too small to be measured in a 350 day period.