Reaction cross sections ⁵⁴Fe(n,γ)⁵⁵Fe and ³⁵Cl(n,γ)³⁶Cl at keV neutron energies investigated by Accelerator Mass Spectrometry

Full text: 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 [1] 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 [2]. 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 [5] 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 [6] and at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Germany [7]. 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.