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 Prediction based on mirror system 12 C( 12 C,p) 23 Na Measurement at ND via n-detection Results & astrophysical implications

Effect on Carbon-Shell Yields Pignatari, Priv. Comm. 12 C( 12 C,n) rate varied by factors 2, 5, 10 T 9 =1.1 Results are model- dependent

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

1 st Measurement of 12 C( 12 C,n) 23 Mg at Notre Dame Online  -rays for p &  channel Our Setup Detect  + from 23 Mg decay (t 1/2 =11s)

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

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

n-channel prediction using Becker et al. p-channel measurements 1.Gross structure reproduced 2.Energy & strength mismatch 3.Low-E resonance predicted Perform measurement at ND to check these results…..

The backward angle θ Lab : 113.5° ° θ cm : 122.5° ° Solid angle calibrated by mixed alpha source 2.59% 3 MeV<E cm <5.7 MeV 0.5 p  A 12 C beam from FN tandem target YY1 detector Focus on: 12 C( 12 C, p) 23 Na 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

Extrapolation Using New Measurement PRELIMINARY Assumed isotropic angular distribution 1.Better overall agreement 2.Low-E resonance not as strong

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

1.Improved detection efficiency (>40%) 2.Low-E measurements hindered by D( 12 C,n) HOPG graphite much cleaner! Highly Ordered Pyrolytic Graphite (renewable surface) Thick Target Neutron Yield Curves Use 13 C beam to measure background component 13 C( 12 C,n) Flat component presumably D( 12 C,n) (target or beam line) PRELIMINARY

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

Astrophysical Rate Sensitivity to Remaining Excitation Function 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 n0 prediction (n1 closed) Typical shell- carbon burning Predict a maximum of 2x enhancement for weak s-process due to potential resonances at lower energies

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

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