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The neutron source for the weak component of the s-process: latest experimental results

The neutron source for the weak component of the s-process: latest experimental results. Claudio Ugalde University of North Carolina at Chapel Hill and Triangle Universities Nuclear Laboratory. OUTLINE. Synthesis of nuclei beyond iron in stars: the s-process

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The neutron source for the weak component of the s-process: latest experimental results

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  1. The neutron source for the weak component of the s-process: latest experimental results Claudio Ugalde University of North Carolina at Chapel Hill and Triangle Universities Nuclear Laboratory

  2. OUTLINE • Synthesis of nuclei beyond iron in stars: the s-process • The main and weak components of the s-process • The 22Ne(a,n)25Mg as a neutron source • The current status of the reaction rate • The 22Ne(6Li,d) experiment and results • Conclusions

  3. The S-PROCESS

  4. The slow neutron capture process (s-process) is responsible for the synthesis of most nuclei heavier than iron.

  5. The s-process involves neutron captures with the emission of gamma radiation (n,g). • The captures occur at a SLOW rate compared to the beta decay rate. Slooow Z (n,g ) N Stable FAST! Unstable b - • Therefore, the s-process follows the path of • beta stable nuclei.

  6. Charged-particle reactions synthesize nuclei in the low-mass region of the B/A curve by exoergic processes up to the iron-like nuclei, where the nucleon binding energy has a maximum. Beyond iron, nuclear processes become endoergic. The result is an abundance peak around A=58.

  7. ( n,g ) The Coulomb barrier hinders charged particle reactions at these high Z, but ...

  8. Neutron captures are favored for N>30. In a nutshell, the s-process is a series of neutron captures along the valley of stability that requires iron-like nuclei as a seed. But where do neutrons come from?

  9. AGB stars (6 > MSun > 0.8) NGC 6543, HST Massive stars (M > 13 MSun) Betelgeuse, HST THE SITES OF THE S-PROCESS

  10. AGB stars Karakas, Ph.D. Thesis 2003 M3, NOAO

  11. mixing but ... 14N(n,p)14C 12C(p,g)13N Neutron source in AGB stars 12C(a,g)16O Pulse H mixing Convective envelope H burning He burning H envelope Convective pocket He intershell C-O core C-O core (bn)13C (a,n)16O

  12. For nuclei with A>90 the phenomenological sN (s = cross section, N = s-only nuclei abundance) curve describes the data fairly well. However, for 60<A<90 TWO contributions (each with its own neutron exposure) are needed. The s-process abundance pattern has contributions from two components: a) Main component (A>90) AGB stars, 13C(a,n)16O b) Weak component (60<A<90) Massive stars

  13. Betelgeuse, HST The weak component of the s-process

  14. It has been proposed that the site of the weak component of the s-process is stars with M>13Msun. The weak component helps to constrain the contribution of the main component to the nucleosynthesis of nuclei with 60<A<90. It also depends very strongly on the initial metallicity of the star, so it may be used to study the role of massive stars is the early phase of the chemical evolution of the galaxy.

  15. 22Ne(a,n)25Mg 14C(a,g)18O(a,g)22Ne or 22Ne(a,g)26Mg but ... 25Mg(n,g)26Mg Red giant in hydra supercluster It was proposed in 1968 by Peters (ApJ 154, 224) that the main neutron source triggering the s-process in massive stars is the 22Ne(a,n)25Mg reaction. The chain proceeds as follows: First, the CNO cycle (main mechanism of hydrogen burning in massive stars) enriches the core of the massive star with 14N. 14N(a,g)18F(bn)18O(a,g)22Ne

  16. The rates for 22Ne(a,g)26Mg and 22Ne(a,n)25Mg Both reactions are in competition with each other at low temperatures. There is very limited experimental and theoretical information about possible natural parity resonances in 26Mg in the energy of relevance to neutron production for the s-process. Both rates carry considerable uncertainties. Both reactions are important producers of the magnesium isotopes (25Mg and 26Mg). However, at least we are lucky in that Mg is one of the few elements for which we can obtain isotopic information from stellar spectroscopy. It is also possible to obtain isotopic abundances from analyzing presolar grains.

  17. From Karakas et al., astro-ph/0601645

  18. The rate for 22Ne(a,n)25Mg In the temperature range between 0.3 and 0.5 GK the rate is dominated by the Ex=11.328 MeV resonance, measured by Jaeger et al, 2001. For T< 0.3 GK the rate is dominated by the threshold states (still unmeasured). The largest uncertainty in the rate is associated with this low temperature range (~1 order of magnitude). The uncertainty depends mainly on the spectroscopic a-strengths of the threshold resonances. NGC4526

  19. The rate for 22Ne(a,g)26Mg Most of the subneutron threshold information used to evaluate the rate comes from a 22Ne(6Li,d)26Mg experiment at Notre Dame. The deuteron spectra resolution came out to be 120 keV. The largest uncertainty in the rate comes from the spin-parity values of the Ecm=330 keV resonance (Ex=10.95 MeV). Unluckily, this is the most important resonance in the rate. Possible contributions to the rate may also come from the Ecm=538 keV, 568 keV, and 711 keV resonances.

  20. Cross over region

  21. To give an idea on how the current situation is for 22Ne(a,g)26Mg below the neutron threshold... Resonances reported by Endt 1990 below the neutron threshold A lot of experimental work is urgent!

  22. What is needed a) Resolve states in 26Mg below the neutron threshold by improving the energy resolution of previous experiments. b) Determine the quantum numbers of 26Mg states around the neutron threshold. Of special interest is the state at 10.95 MeV.

  23. The experiment A plausible solution would be to study the 22Ne(6Li,2H)26Mg transfer reaction at lab energies where the direct reaction mechanism is dominant (say 30-40 MeV) and populate states in 26Mg. The 6Li beam could be accelerated without problem by a Tandem and the target could be prepared by implanting 22Ne on a thin carbon foil. The reaction products can then be analyzed with a split pole spectrometer positioned at several angles.

  24. NGC 6543, HST The experiment (continued) The excitation energy could be reconstructed from the energy of the deuterons detected at the focal plane, the reaction kinematics, and the energy losses in the target. On the other hand, we shall try to obtain the spins of 26Mg states by measuring angular distributions moving the spectrometer to different angles and then analyzing in terms of DWBA.

  25. Produces stable beams from 20 keV to 200 keV with a mass resolution Dm/m ~ 0.01. Beam currents can be obtained at hundreds of mA Target preparation with the Eaton ion implanter at North Carolina

  26. 22Ne targets • 40mg/cm212C-enriched foils • Implanted on both sides, two energies each • Dose ~ 20 mC per target • Targets are very, very fragile. Substrates can withstand up to 400 nA of 22Ne beam

  27. The Wright Nuclear Structure Laboratory floorplan

  28. ESTU-1 Tandem Van de Graaff Accelerator at Yale Vmax= 22.5 MV

  29. Enge split-pole spectrometer Bmax ~ 14-15 kG Wmax = 12.8 msr

  30. Focal plane detector Position resolution ~ 1mm Gas filled (isobutane @150 Torr) DE (cathode), E (Plastic scintillator), position (FW and BW)

  31. 22Ne(6Li,d)26Mg Focal plane coincident with the front wire. 6Li beam, @ 30 MeV BEnge = 13.0 kG qEnge = 6o WEnge = 1.5 msr Red giant in hydra supercluster Particles enter the detector at 45o relative to the wires DE~80 keV (as opposed to ~120 keV in Giesen et al.1994)

  32. 16O 16O 22Ne implanted on 12C deuteron spectrum 26Mg – 10.95 ?, 10.82 MeV 16O – 6.92, 7.12 MeV 16O - 6.05, 6.13 MeV 12C, deuteron spectrum

  33. Target content analysis Elastic scattering experiment 6Li beam, @ 30 MeV BEnge = 7.7 kG qEnge = 20o WEnge = 1.5 msr

  34. 12C substrate, 6Li spectrum 56Fe 16O 27Al 35Cl? 12C 22Ne-implanted, 6Li spectrum 27Al 16O 22Ne 56Fe 35Cl? 12C

  35. Offline focus Trajectory Focal plane Solving for x and y Shapira et al., NIM 129(1975),123 S = 3.5 cm

  36. before focus after (S/H=2)

  37. Old situation

  38. New situation

  39. Conclusions Both the 22Ne(a,n)25Mg and 22Ne(a,g)26Mg reactions hold large uncertainties at temperatures of relevance to the s-process. We observed the 10.82 MeV state in 26Mg; it is likely to have natural parity, thus would contribute significantly to the rate of the 22Ne(a,g)26Mg reaction. We failed to measure the spin and parity of the 10.95 MeV state in 26Mg. We’ll try next time.

  40. Thank you! Yale Jason Clark Catherine Deibel Anuj Parikh Peter Parker Chris Wrede North Carolina Art Champagne Stephen Daigle Christian Iliadis Joseph Newton Eliza Osenbaugh Eta Carinae University of Colorado & NASA

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