Reaction cross sections 54Fe(n,γ)55Fe and 35Cl(n,γ)36Cl at keV neutron energies investigated by Accelerator Mass Spectrometry
Dr Zuzana Slavkovska1, Prof. Anton Wallner1,3, R. Reifarth2, L. Bott2, B. Brückner2, P. Erbacher2, Prof. Keith Fifield1, Dr Michaela Froehlich1, K. Göbel2, K. Al-Khasawneh2, Mr Dominik Koll1, Johannes Lachner3, Silke Merchel3,4, Dr Stefan Pavetich1, M. Reich2, G. Rugel3, B. Thomas2, S. G. Tims1, M. Volknandt2, M. Weigand2
1Research School of Physics, The Australian National University, , Australia, 2Institute of Applied Physics, Goethe University, Frankfurt, Germany, 33Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany, 44Faculty of Physics, University of Vienna, Vienna, Austria
Typical neutron energies for the astrophysical s-process follow the Maxwell-Boltzmann distribution in the keV energy range. Neutron capture cross sections highly relevant for modelling the s-process can be experimentally determined by using the Time-of-Flight (ToF) method  or by the activation technique. If the reaction product is a long-lived radionuclide (t1/2 ~ yr -100 Myr), the cross section can be determined by activation with a quasi-stellar neutron distribution (typically kT = 25 keV) and a subsequent accelerator mass spectrometry (AMS) measurement of the reaction product . Comparison of a number of such neutron capture cross sections shows a systematic bias, i.e. AMS data being lower than the ToF data [3, 4].
To investigate this discrepancy, we repeated experiments for two reactions that allow for highly precise AMS data: Maxwellian-averaged cross sections for the reactions 54Fe(n,γ)55Fe and 35Cl(n,γ)36Cl were investigated with dedicated activations at the Frankfurt Neutron Source (FRANZ) in Germany  and AMS measurements at two independent facilities. Analogously to previous activations, a quasi-stellar neutron spectrum of kT = 25 keV was produced via the 7Li(p,n) reaction, but at a different neutron-producing facility. Furthermore, to complement existing ToF and AMS data, an additional neutron activation of 54Fe and 35Cl at a proton energy of 2 MeV was performed, yielding data in the not-yet explored kT = 90 keV region.
The irradiated metallic Fe foil and NaCl pellet (both of natural isotopic composition) were chemically processed and converted to AMS targets (Fe2O3 and AgCl) together with non-irradiated blanks. The subsequent AMS measurements of both radionuclides, 36Cl and 55Fe, were performed at two complementary AMS facilities, the Heavy Ion Accelerator Facility (HIAF) at the Australian National University  and at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Germany . AMS allows a direct measurement of the 55Fe/54Fe and 36Cl/35Cl conversion ratios that result from the irradiation. The cross section is then deduced from the isotope ratio and the neutron fluence, which is determined using Au monitor foils.
The new experiment was designed to produce highly accurate data and, owing to the two independent AMS measurements, it minimizes unrecognized sources of uncertainties in the AMS technique. The new preliminary data obtained in this work seem to confirm the previous AMS results. Consequently, the systematic discrepancy between AMS and ToF data remains unresolved.
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Zuzana received her PhD degree in Physics in 2018 at Goethe University Frankfurt, Germany, where she was a member of the Experimental Nuclear Astrophysics Group. Her main research focus was on nucleosynthesis of heavy nuclides, in particular using storage rings at the GSI Helmoholtz Centre for Heavy Ion Research in Darmstadt, Germany.
– She joined the Accelerator Mass Spectromectry Group at the ANU in February 2019 as a Postdoctoral Fellow, continuing with heavy elements nucleosynthesis research.