Nucleosynthesis in the lab-neutron-capture on Fe and Ni

The proposed project aims at precision measurements of neutron capture cross sections of iron and nickel and should provide an essential contribution to clarify the recently found discrepancy of s-process nucleosynthesis at lower mass nuclei (A<120). The discrepancy is related to observations of r-process elements in ultra metal-poor stars. In such stars the abundance for elements heavier than barium scales exactly with the r-process abundances found in the solar system, while those lighter than barium (A<120) show a systematic deviation of the order of 20%. A similar discrepancy is found for s-only isotopes lighter than barium. These facts hint to some deficiency in the standard description of the s-process nucleosynthesis. Either there is a systematic offset in the experimental data (mainly in neutron-capture cross sections) or there are different presently not considered mechanisms. The former can be tested by precision measurements of neutron capture cross sections, where up-to-date corrections for the neutron sensitivity of the detectors may lead to a systematic shift from the actual values. In order to address this question, neutron-capture measurements on 54Fe and 62Ni are proposed, which are both at the beginning of the s-process path. To identify systematic uncertainties, measurements by two independent techniques are envisaged, i.e. (1) direct measurements using neutron time-of-flight technique and (2) measurements using activation technique combined with accelerator mass spectrometry (AMS). The direct neutron-capture measurements will be performed at the n_TOF facility at CERN, where specially prepared (low neutron sensitivity) C6 D6 liquid scintillator detectors in combination with a 4pBaF2 total absorption calorimeter array are available to detect capture events in the energy range from 0.1 to 500keV. The authors of this proposal are familiar with the facility as they participate in the n_TOF Collaboration since its start in 2000. The present proposal is an essential part of the envisaged measurement programm at n_TOF, which is already accepted by the INTC committee at CERN and should be performed at the next measurement campaign envisaged for 2008. In order to identify systematic errors, complimentary neutron capture measurements on 54Fe will be performed by activation technique. Here, the 54Fe samples will be irradiated at astrophysically relevant energies using the 3.7 MV Van de Graaff accelerator at the Forschungszentrum Karlsruhe and analysed with high precision via AMS at the VERA facility in Vienna. Independently from this proposal a 62Ni sample will be irradiated at Forschungszentrum Karlsruhe and analysed at the AMS facility in Munich. It is expected that the combination of the proposed measurements will lead to the most reliable neutron capture cross section on 54Fe and 62Ni, thus elucidating the current discrepancies within the s-process path. Apart from the astrophysical relevance these measurements will provide a precise 55Fe standard for AMS measurements in general and are also of interest for materials research in nuclear technology.

The proposed project aims at precision measurements of neutron capture cross sections of iron and nickel and should provide an essential contribution to clarify the recently found discrepancy of s-process nucleosynthesis at lower mass nuclei (A<120). The discrepancy is related to observations of r-process elements in ultra metal-poor stars. In such stars the abundance for elements heavier than barium scales exactly with the r-process abundances found in the solar system, while those lighter than barium (A<120) show a systematic deviation of the order of 20%. A similar discrepancy is found for s-only isotopes lighter than barium. These facts hint to some deficiency in the standard description of the s-process nucleosynthesis. Either there is a systematic offset in the experimental data (mainly in neutron-capture cross sections) or there are different presently not considered mechanisms. The former can be tested by precision measurements of neutron capture cross sections, where up-to-date corrections for the neutron sensitivity of the detectors may lead to a systematic shift from the actual values. In order to address this question, neutron-capture measurements on 54Fe and 62Ni are proposed, which are both at the beginning of the s-process path. To identify systematic uncertainties, measurements by two independent techniques are envisaged, i.e. (1) direct measurements using neutron time-of-flight technique and (2) measurements using activation technique combined with accelerator mass spectrometry (AMS). The direct neutron-capture measurements will be performed at the n_TOF facility at CERN, where specially prepared (low neutron sensitivity) C6D6 liquid scintillator detectors in combination with a 4 BaF2 total absorption calorimeter array are available to detect capture events in the energy range from 0.1 to 500keV. The authors of this proposal are familiar with the facility as they participate in the n_TOF Collaboration since its start in 2000. The present proposal is an essential part of the envisaged measurement programm at n_TOF, which is already accepted by the INTC committee at CERN and should be performed at the next measurement campaign envisaged for 2008. In order to identify systematic errors, complimentary neutron capture measurements on 54Fe will be performed by activation technique. Here, the 54Fe samples will be irradiated at astrophysically relevant energies using the 3.7 MV Van de Graaff accelerator at the Forschungszentrum Karlsruhe and analysed with high precision via AMS at the VERA facility in Vienna. Independently from this proposal a 62Ni sample will be irradiated at Forschungszentrum Karlsruhe and analysed at the AMS facility in Munich. It is expected that the combination of the proposed measurements will lead to the most reliable neutron capture cross section on 54Fe and 62Ni, thus elucidating the current discrepancies within the s-process path. Apart from the astrophysical relevance these measurements will provide a precise 55Fe standard for AMS measurements in general and are also of interest for materials research in nuclear technology.