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Status and future directions for direct cross-section measurements of the 13C(a,n)16O reaction for astrophysics

L. Csedreki, Gy. Gyürky, D. Rapagnani, G. F. Ciani, M. Aliotta, C. Anannad, L. Barbieri, F. Barile, D. Bemmerer, A. Best, A. Boeltzig, C. Broggini, C. G. Bruno, A. Caciolli, F. Casaburom, F. Cavannak, P. Colombetti, A. Compagnucci, P. Corvisiero, T. Davinson, R. Depalo, A. Di Leva, Z. Elekes, F. Ferraro, A. Formicola, Zs. Fülöpa, G. Gervino, A. Guglielmetti, C. Gustavino, G. Imbriani, M. Junker, M. Lugaro, P. Marigo, J. Marsh, E. Masha, R. Menegazzo, D. Mercogliano, V. Paticchio, R. Perrino, D. Piatti, P. Prati, D. Robb, L. Schiavulli, R. S. Sidhu, J. Skowronski, O. Straniero, T. Szücsa, S. Zavatarelli

Abstract

The 13C(a,n)16O reaction is the main neutron source of the s-process taking place in thermally pulsing AGB stars and it is one of the main candidate sources of neutrons for the i-process in the astrophysical sites proposed so far. Therefore, its rate is crucial to understand the production of the nuclei heavier than iron in the Universe. For the first time, the LUNA collaboration was able to measure the 13C(a,n)16O cross section at Ec.m.=0.23-0.3 MeV drastically reducing the uncertainty of the S(E)-factor in the astrophysically relevant energy range. In this paper, we provide details and critical thoughts about the LUNA measurement and compare them with the current understanding of the 13C(a,n)16O reaction in view of future prospect for higher energy measurements. The two very recent results (from the University of Notre Dame and the JUNA collaboration) published after the LUNA data represent an important step forward. There is, however, still room for a lot of improvement in the experimental study of the 13C(a,n)16O reaction, as emphasized in the present manuscript. We conclude that to provide significantly better constraints on the low-energy extrapolation, experimental data need to be provided over a wide energy range, which overlaps with the energy range of current measurements. Furthermore, future experiments need to focus on the proper target characterisation, the determination of neutron detection efficiency having more nuclear physics input, such as angular distribution of the 13C(a,n)16O reaction below Ea<0.8 MeV and study of nuclear properties of monoenergetic neutron sources and/or via the study of sharp resonances of 13C(a,n)16O. Moreover, comprehensive, multichannel R-matrix analysis with a proper estimate of uncertainty budget of experimental data are still required.

Status and future directions for direct cross-section measurements of the 13C(a,n)16O reaction for astrophysics

Abstract

The 13C(a,n)16O reaction is the main neutron source of the s-process taking place in thermally pulsing AGB stars and it is one of the main candidate sources of neutrons for the i-process in the astrophysical sites proposed so far. Therefore, its rate is crucial to understand the production of the nuclei heavier than iron in the Universe. For the first time, the LUNA collaboration was able to measure the 13C(a,n)16O cross section at Ec.m.=0.23-0.3 MeV drastically reducing the uncertainty of the S(E)-factor in the astrophysically relevant energy range. In this paper, we provide details and critical thoughts about the LUNA measurement and compare them with the current understanding of the 13C(a,n)16O reaction in view of future prospect for higher energy measurements. The two very recent results (from the University of Notre Dame and the JUNA collaboration) published after the LUNA data represent an important step forward. There is, however, still room for a lot of improvement in the experimental study of the 13C(a,n)16O reaction, as emphasized in the present manuscript. We conclude that to provide significantly better constraints on the low-energy extrapolation, experimental data need to be provided over a wide energy range, which overlaps with the energy range of current measurements. Furthermore, future experiments need to focus on the proper target characterisation, the determination of neutron detection efficiency having more nuclear physics input, such as angular distribution of the 13C(a,n)16O reaction below Ea<0.8 MeV and study of nuclear properties of monoenergetic neutron sources and/or via the study of sharp resonances of 13C(a,n)16O. Moreover, comprehensive, multichannel R-matrix analysis with a proper estimate of uncertainty budget of experimental data are still required.

Paper Structure

This paper contains 14 sections, 2 equations, 9 figures, 3 tables.

Table of Contents

  1. Introduction
  2. The $^{13}$C($\alpha$,$n$)$^{16}$O cross section: state of the art
  3. Experimental aspects
  4. Experimental apparatus and its background
  5. Neutron detection efficiency
  6. Target characterisation
  7. Methods of target characterisation
  8. Stoichiometry of carbon target
  9. Light element contamination
  10. Effect of high dose $\alpha$ implantation on the experimental yield
  11. Summary of currently available direct measurement data
  12. R-matrix approach for low-energy extrapolations
  13. Summary
  14. Acknowledgement

Figures (9)

  • Figure 1: (Colour online) Level scheme of $^{17}$O. Widths of the horizontal lines reflect the widths of the excited states. The sharp 5/2 levels with altering parities at 6.861, 7.166, 7.379 and 7.382 MeV are omitted, because their effect over the cross section at stellar energies is marginal.
  • Figure 2: (Colour online) Experimental $S(E)$-factor data of the $^{13}$C($\alpha$,$n$)$^{16}$O reaction combined with theoretical calculations with and without the effect of the near-threshold resonance. The grey area represents the Gamow-window of the reaction for about T=0.1 GK.
  • Figure 3: (Colour online) Experimental efficiencies (filled symbols) and the rescaled simulated efficiency curve (dashed line) obtained using the vertical (upper panel) and horizontal (lower panel) setups. The simulated and the experimental efficiencies related to the inner (green squares and dotted lines) and outer (blue triangles and dash-dotted lines) rings of the setups are also presented. The interpolated efficiency value at $E_{\text{n}}$=2.4 MeV are shown as green half empty dot.
  • Figure 4: (Colour online) Reaction yield of $^{13}$C(p,$\gamma$)$^{14}$N as a function of accumulated charge. Blue squares represent the experimental yields, while red triangles show the calculated values based on cross section taken from KING1994 and corrected for sputtering effect from SRIM calculation. The green stars represent the calculated yields based on the NRRA scans.
  • Figure 5: (Colour online) Resonance yield profiles measured on targets with different accumulated ($\alpha$-beam) charge. Lines are drawn to guide the eye.
  • ...and 4 more figures