Knie et al [1] have measured 60Fe in a deep sea Mn crust, and interpreted an observed peak as evidence of matter from a supernova explosion deposited on the earth ~2.8 My ago. If this interpretation is correct, there should be evidence of other supernova products deposited at the same time. Using the Gif-sur-Yvette AMS facility, we are looking for evidence of 244Pu in deep sea sediments. In addition to confirming the origin of the 60Fe signal, the observation of 244Pu would add important information on r-process nucleosynthesis in supernovae. An advantage of marine sediments for such studies is that they potentially permit a much better time resolution, and thus both an enhanced and more reliable flux estimate, compared to Mn crusts. We will describe both the chemical and AMS procedures developed for these studies. A novel aspect of the AMS procedure is the use of various Mo isotopes as “pilot” beams, which allow us to tune the machine for the various Pu isotopes, without the necessity of using enriched Pu standards, thus minimizing the risk of 244Pu contamination. These techniques have been tested on an IAEA marine standard containing nuclear weapons contaminated 239Pu and 240Pu. In a first application on an Indian Ocean sediment, we have obtained tentative limits of < 1x104 atoms/g of 244Pu in four continuous 100 g samples in the time range 2.0 to 3.6 My. From these we can get an upper limit of <4 atoms/cm2y for the flux of 244Pu for each of these samples. This is about an order of magnitude higher than expected based on an estimate of the flux of 244Pu relative to 60Fe, and the estimated 60Fe fluence of Knie at al. [1]. However, using samples representing shorter time intervals and/or sediments with a slower deposition rate, the sensitivity achieved here would be sufficient to test the expected 244Pu flux.
[1] Knie, K., Korschinek, G. Faestermann, T. Dorfi, E.A., Rugel, R.&Wallner, A. 60Fe anomaly in a deep-sea manganese crust and implications for a nearby supernova source. Phys. Rev. Lett. 93, 171103 (2004).
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