Measurement of the stellar cross sections for the reactions⁹Be(n,γ)¹⁰Be and¹³C(n,γ)¹⁴C via AMS

Abstract

We started a series of experiments, aiming at obtaining neutron-capture cross sections from measurements of the production of both long-lived nuclides 10Be and 14C, in the energy region of interest for astrophysical applications. Neutron-rich scenarios lead to the production of the long-lived radionuclides 10Be (t1/2 = 1.51 Myr) and 14C (t1/2 = 5730 yr) via neutron-capture reactions on 9Be and 13C, respectively. Data for both neutron-capture reactions are sparse and disagree in the astrophysical relevant energy region between 10 and 250 keV. Since (n,γ) cross sections in this mass and energy range are expected to be very small (some tens of µbarn), the combination of the activation technique and accelerator mass spectrometry (AMS) offers the powerful experimental approach which is necessary for such investigations. Beryllium oxide and enriched 13C graphite samples have been irradiated in a quasi-stellar neutron spectrum of kT = 25 keV at Forschungszentrum Karlsruhe. The subsequent AMS measurements were performed at the VERA (Vienna Environmental Research Accelerator) laboratory. The first AMS results confirm the expected low cross sections of about 10 µbarn for both reactions.

Stellar and thermal neutron capture cross section of Be9

“…The isotope ratio 10 Be/ 9 Be is eventually obtained by a current measurement of the stable 9 Be ions in a Faraday cup followed by a count-rate measurement of the rare isotope 10 Be in the energy detectors, either the Gas-absorber-E-detector (method a) [33,46,51] or the E-E detector (method b) in combination with a degrader foil for 10 B separation [44,45].…”

Accelerator mass spectrometry measurements of theC13(n,γ)C14andWallner1, Bichler2, Buczak3 et al. 2016Phys. Rev. C281170View full textAdd to dashboardCite

“…These test runs were also used to study whether the highly enriched 13 C sample gave rise to an enhanced AMS background compared to measurements on natural graphite (which is assumed to be 14 C-free). It could be demonstrated that the amount of 14 C produced in the activations at keV neutron energies was high enough that this background did not jeopardize the final uncertainties [38,43,44].…”

Nuclear astrophysics and AMS – Probing nucleosynthesis in the lab

“…However, up to date, no experimental values can be found for 9 Be(n, c) 10 Be neutron capture above thermal energies. Adopted values for keV energies are extrapolations from the only existing thermal values and they scatter by more than a factor of 10 [32].…”

“…Besides the above mentioned reactions, several additional neutron-induced reactions of relevance to nuclear astrophysics are studied (see also [32,33]). The production of 14 C via 14 N(n, p) has been recognized as an important reaction in the CNO cycle.…”