Search for supernova-produced 60Fe in a marine sediment

An 60Fe peak in a deep-sea FeMn crust has been interpreted as due to the signature left by the ejecta of a supernova explosion close to the solar system 2.8 +/- 0.4 Myr ago [Knie et al., Phys. Rev. Lett. 93, 171103 (2004)]. To confirm this interpretation with better time resolution and obtain a more direct flux estimate, we measured 60Fe concentrations along a dated marine sediment. We find no 60Fe peak at the expected level from 1.7 to 3.2 Myr ago. However, applying the same chemistry used for the sediment, we confirm the 60Fe signal in the FeMn crust. The cause of the discrepancy is discussed.Comment: 15 pages, 5 figures, submitted to PR

First Measurement of the 64Ni(gamma,n)63Ni Cross Section

Copyright owned by the author(s) under the terms of the Creative Commons Attribution-Non Commercial-ShareAlike LicenceIn the past 10 years new and more accurate stellar neutron capture cross section measurements have changed and improved the abundance predictions of the weak s process. Among other elements in the region between iron and strontium, most of the copper abundance observed today in the solar system distribution was produced by the s process in massive stars. However, experimental data for the stellar 63Ni(n,gamma)64Ni cross section are still missing, but is strongly required for a reliable prediction of the copper abundances. 63Ni (t1/2 =101.2 a) is a branching point and also bottleneck in the weak s process flow, and abehaves differently during core He and shell C burning. During core He burning the reaction flow proceeds via beta-decay to 63Cu, and a change of the 63Ni(n,gamma)64Ni cross section would have no influence. However, this behavior changes at higher temperatures and neutron densities during the shell C burning phase. Under these conditions, a significant amount of the s process nucleosynthesis flow is passing through the channel 62Ni(n,gamma)63Ni(n,gamma)64Ni. At present only theoretical estimates are available for the 63Ni(n,gamma)64Ni cross section. The corresponding uncertainty affects the production of 63Cu in present s process nucleosynthesis calculations and propagates to the abundances of the heavier species up to A=70. So far, experimental information is also missing for the inverse 64Ni(gamma,n) channel. We have measured for the first time the 64Ni(gamma,n)63Ni cross section and also combined for the first time successfully the photoactivation technique with subsequent Accelerator Mass Spectrometry (AMS). The activations at the ELBE facility in Dresden-Rossendorf were followed by the 63Ni/64Ni determination with AMS at the MLL accelerator laboratory in Garching. First results indicate that theoretical predictions have overestimated this cross section up to now. If this also holds for the inverse channel 63Ni(n,gamma)64Ni, more 63Ni is accumulated during the high neutron density regime of the C shell that will contribute to the final abundance of 63Cu by radiogenic decay. In this case, also a lower s process efficiency is expected for the heavier species along the neutron capture path up to the Ga-Ge regio

Solving the stellar 62Ni problem with AMS

An accurate knowledge of the neutron capture cross sections of 62,63Ni is crucial since both isotopes take key positions which affect the whole reaction flow in the weak s process up to A=90. No experimental value for the 63Ni(n,gamma) cross section exists so far, and until recently the experimental values for 62Ni(n,gamma) at stellar temperatures (kT=30 keV) ranged between 12 and 37 mb. This latter discrepancy could now be solved by two activations with following AMS using the GAMS setup at the Munich tandem accelerator which are also in perfect agreement with a recent time-of-flight measurement. The resulting (preliminary) Maxwellian cross section at kT=30 keV was determined to be 30keV = 23.4 +/- 4.6 mb. Additionally, we have measured the 64Ni(gamma,n)63Ni cross section close to threshold. Photoactivations at 13.5 MeV, 11.4 MeV and 10.3 MeV were carried out with the ELBE accelerator at Forschungszentrum Dresden-Rossendorf. A first AMS measurement of the sample activated at 13.5 MeV revealed a cross section smaller by more than a factor of 2 compared to NON-SMOKER predictions.Comment: Proceedings of the 11th International Conference on Accelerator Mass Spectrometry in Rome, Sept. 14-19, 2008; to be published in Nucl. Instr. Meth.

Measurement of the stellar Ni 58 (n,γ) Ni 59 cross section with accelerator mass spectrometry

The Ni58(n,γ)Ni59 cross section was measured with a combination of the activation technique and accelerator mass spectrometry (AMS). The neutron activations were performed at the Karlsruhe 3.7 MV Van de Graaff accelerator using the quasistellar neutron spectrum at kT=25 keV produced by the Li7(p,n)Be7 reaction. The subsequent AMS measurements were carried out at the 14 MV tandem accelerator of the Maier-Leibnitz Laboratory in Garching using the gas-filled analyzing magnet system (GAMS). Three individual samples were measured, yielding a Maxwellian-averaged cross section at kT=30 keV of (σ)30keV = 30.4 (23)syst(9)stat mbarn. This value is slightly lower than two recently published measurements using the time-of-flight (TOF) method, but agrees within the uncertainties. Our new results also resolve the large discrepancy between older TOF measurements and our previous value

Supernova-Produced 53Mn on Earth

For the time period from 1.5 to 4 Myr before the present we found in deep ocean ferromanganese crusts a Mn53 excess concentration in terms of Mn53/Mn of about 4×10-14 over that expected for cosmogenic production. We conclude that this Mn53 is of supernova origin because it is detected in the same time window, about 2.5 Myr ago, where Fe60 has been found earlier. This overabundance confirms the supernova origin of that Fe60. For the first time, supernova-formed Mn53 has been detected and it is the second positively identified radioisotope from the same supernova. The ratio Mn53/Fe60 of about 14 is consistent with that expected for a SN with a 11-25 M progenitor mass and solar metallicity.Fil: Korschinek, G.. Universitat Technical Zu Munich; AlemaniaFil: Faestermann, T.. Universitat Technical Zu Munich; AlemaniaFil: Poutivtsev, M.. Universitat Technical Zu Munich; AlemaniaFil: Arazi, Andres. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica; ArgentinaFil: Knie, K.. Universitat Technical Zu Munich; AlemaniaFil: Rugel, G.. Universitat Technical Zu Munich; Alemania. Helmholtz-Zentrum Dresden Rossendorf; AlemaniaFil: Wallner, A.. Universitat Technical Zu Munich; Alemania. Helmholtz-Zentrum Dresden Rossendorf; Alemani

Highly sensitive AMS measurements of 53Mn

Our AMS system, with the gas-filled detector system GAMS, has been optimized for measurements with 53Mn. A high sensitivity has been achieved. A newly installed cesium sputter ion source yields an improved emittance, and thus a higher mass resolution. B

Attempt to detect primordial ²⁴⁴Pu on Earth.

With a half-life of 81.1 Myr, (244)Pu could be both the heaviest and the shortest-lived nuclide present on Earth as a relic of the last supernova(e) that occurred before the formation of the Solar System. Hoffman et al. [Nature (London) 234, 132 (1971)] reported on the detection of this nuclide (1.0 x 10(-18) g (244)Pu/g) in the rare-earth mineral bastnasite with the use of a mass spectrometer. Up to now these findings were never reassessed. We describe the search for primordial (244)Pu in a sample of bastnasite with the method of accelerator mass spectrometry (AMS). It was performed with a highly sensitive setup, identifying the ions by the determination of their time-of-flight and energy. Using AMS, the stripping to high charge states allows the suppression of any molecular interference. During our measurements we observed no event of (244)Pu. Therefore, we can give an upper limit for the abundance of (244)Pu in our sample of the mineral bastnasite of 370 atoms per gram (1.5 x 10(-19) g (244)Pu/g). The concentration of (244)Pu in our sample of bastnasite is significantly lower than the previously determined value