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 22 Ne( ,n) 25 Mg as a neutron source ● The current status of the reaction rate ● The 22 Ne( 6 Li,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,  ). ● The captures occur at a SLOW rate compared to the beta decay rate. (n,  )  - FAST! Slooow Stable Unstable Z N ● 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. The Coulomb barrier hinders charged particle reactions at these high Z, but... ( n,  )

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

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

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

12. For nuclei with A>90 the phenomenological  N (  = 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, 13 C( ,n) 16 O 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. It was proposed in 1968 by Peters (ApJ 154, 224) that the main neutron source triggering the s-process in massive stars is the 22 Ne( ,n) 25 Mg reaction. 14 N(  ) 18 F(  ) 18 O(  ) 22 Ne or 14 C(  ) 18 O(  ) 22 Ne but... 25 Mg(n  ) 26 Mg 22 Ne(  ) 26 Mg 22 Ne(  n) 25 Mg 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 14 N. Red giant in hydra supercluster

17. The rates for 22 Ne(  ) 26 Mg and 22 Ne( ,n) 25 Mg Both reactions are in competition with each other at low temperatures. It is also possible to obtain isotopic abundances from analyzing presolar grains. There is very limited experimental and theoretical information about possible natural parity resonances in 26 Mg 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.

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

19. The rate for 22 Ne( ,n) 25 Mg 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  -strengths of the threshold resonances. NGC4526

21. The rate for 22 Ne(  ) 26 Mg Most of the subneutron threshold information used to evaluate the rate comes from a 22 Ne( 6 Li,d) 26 Mg 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 E cm =330 keV resonance (E x =10.95 MeV). Unluckily, this is the most important resonance in the rate. Possible contributions to the rate may also come from the E cm =538 keV, 568 keV, and 711 keV resonances.

24. Cross over region

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

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

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

28. NGC 6543, HST 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 26 Mg states by measuring angular distributions moving the spectrometer to different angles and then analyzing in terms of DWBA. The experiment (continued)

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

30. 22 Ne targets 40  g/cm 2 12 C-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 22 Ne beam

32. The Wright Nuclear Structure Laboratory floorplan

33. ESTU-1 Tandem Van de Graaff Accelerator at Yale V max = 22.5 MV

35. Enge split-pole spectrometer B max ~ 14-15 kG  max = 12.8 msr

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

38. 22 Ne( 6 Li,d) 26 Mg 6 Li beam, @ 30 MeV B Enge = 13.0 kG  Enge  o Focal plane coincident with the front wire.  Enge = 1.5 msr Particles enter the detector at 45 o relative to the wires  E~80 keV (as opposed to ~120 keV in Giesen et al.1994) Red giant in hydra supercluster

39. 22 Ne implanted on 12 C deuteron spectrum 12 C, deuteron spectrum 16 O 16 O - 6.05, 6.13 MeV 16 O – 6.92, 7.12 MeV 16 O 26 Mg – 10.95 ?, 10.82 MeV

40. Target content analysis 6 Li beam, @ 30 MeV B Enge = 7.7 kG  Enge  o Elastic scattering experiment  Enge = 1.5 msr

41. 12 C substrate, 6 Li spectrum 22 Ne-implanted, 6 Li spectrum 12 C 22 Ne 27 Al 35 Cl? 56 Fe 16 O 12 C 16 O 27 Al 35 Cl? 56 Fe

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

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

44. Old situation

45. New situation

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

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