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Brian Bucher University of Notre Dame

Searching for the Low-Energy Resonances in the 12 C( 12 C,n) 23 Mg Reaction Cross Section Relevant for S-Process Nucleosynthesis. Brian Bucher University of Notre Dame. Outline. 12 C( 12 C,n) 23 Mg in the weak s-process & rate uncertainty Measurement at ND via 23 Mg decays

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Brian Bucher University of Notre Dame

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  1. Searching for the Low-Energy Resonances in the 12C(12C,n)23Mg Reaction Cross Section Relevant for S-Process Nucleosynthesis Brian Bucher University of Notre Dame

  2. Outline • 12C(12C,n)23Mg in the weak s-process & rate uncertainty • Measurement at ND via 23Mg decays • Prediction based on mirror system 12C(12C,p)23Na • Measurement at ND via n-detection • Results & astrophysical implications

  3. Effect on Carbon-Shell Yields Pignatari, Priv. Comm. • T9=1.1 • Results are model-dependent 12C(12C,n) rate varied by factors 2, 5, 10

  4. Uncertainty in Reaction Rate 1977 1969 ? Qa=+4.6 MeV Qp=+2.2 MeV Qn=-2.6 MeV 12C(12C,n)23Mg • Becker et al. 1981 • Spillane et al. 2007 Low E resonances measured in total fusion x-section 12C+12C→a+20Ne →p+23Na →n+23Mg 12C+12C Resonances form important contribution to excitation function Resonance structure continues to lowest energies Current rate cannot account for resonances Typical C-shell burning

  5. 1st Measurement of 12C(12C,n)23Mg at Notre Dame Detect b+ from 23Mg decay (t1/2=11s) Our Setup Online g-rays for p & a channel

  6. Experimental Results • Measured finer step size over large energy range • Consistent with others • Difficult to measure lower • Low cross-section • Increasing background due to reactions with H/D PRELIMINARY Note consistent resonance energies

  7. p2 n2 24Mg* p1 12C 12C n1 p0 Mirror Nuclei n0 Low-Energy Extrapolation • n-channel difficult to measure at low E • Can p-channel(s) provide useful information for n-channel Becker et al. Z. Phys. A303, 305-312 (1981)

  8. n-channel prediction using Becker et al. p-channel measurements • Gross structure reproduced • Energy & strength mismatch • Low-E resonance predicted Perform measurement at ND to check these results…..

  9. Focus on: 12C(12C, p)23Na 0.5 pmA 12C beam from FN tandem YY1 detector target YY1 detector The backward angle θLab: 113.5° - 163.5° θcm: 122.5° - 166.3° Solid angle calibrated by mixed alpha source 2.59% 3 MeV<Ecm<5.7 MeV More details are available in Thursday, May 31, Session 27: Nuclear Astrophysics-3, Room 3: PECAN 5:50-6:10 pm Experimental Investigations Of Stellar 12C+12C Fusion Toward Extremely Low Energies by Direct And Indirect Methods Xiao Fang, University of Notre Dame, Notre Dame, Indiana, USA

  10. Extrapolation Using New Measurement Assumed isotropic angular distribution • Better overall agreement • Low-E resonance not as strong PRELIMINARY

  11. Central bore for beam tube LN2 reservoir Beam Target Polyethylene moderator 3He proportional counter Turbo Pump 2nd Measurement: Direct detection of neutrons using 3He array

  12. Improved detection efficiency (>40%) • Low-E measurements hindered by D(12C,n) Highly Ordered Pyrolytic Graphite (renewable surface) HOPG graphite much cleaner! Flat component presumably D(12C,n) (target or beam line) Use 13C beam to measure background component 13C(12C,n) PRELIMINARY Thick Target Neutron Yield Curves

  13. 3.4 MeV resonance confirmed as predicted (p0 & p1) Detector efficiency from Geant4 Good agreement with prediction! Results PRELIMINARY

  14. n0 prediction (n1 closed) Astrophysical Rate Sensitivity to Remaining Excitation Function Predict a maximum of 2x enhancement for weak s-process due to potential resonances at lower energies Typical shell-carbon burning • Resonance below 3.0 MeV from Zickefoose PhD thesis, UConn 2010 • Measured p0+p1 from HOPG • Assumed isotropic distribution • Here we assume full strength to p0 channel

  15. Summary • 12C(12C,n) can be an important component to the weak s-process • Reaction measured via 2 different methods at ND • 1st measurement at energies of astrophysical relevance • Mirror system-based prediction provides good agreement • Uncertainty in rate is reduced with new measurements

  16. Thank You! Collaborators: X. Fang, J. Browne, A. Alongi, C. Cahillane, E. Dahlstrom, A. Moncion, W. Tan, M. Notani, X.D. Tang Nuclear Science Lab: S. Almaraz-Calderon, A. Ayangeakaa, A. Best, M. Couder, J. DeBoer, W. Lu, D. Patel, N. Paul, A. Roberts, R. Talwar, A. Kontos, M. Smith, S. Lyons, Q. Li, K. Smith, A. Long, M. Beard, M. Wiescher

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