Astrophysical Factor for the CNO Cycle Reaction 15 N(p, g ) 16 O

1. Astrophysical Factor for the CNO Cycle Reaction 15N(p,g)16O Adele Plunkett, Middlebury College REU 2007, Cyclotron Institute, Texas A&M University Mentor: A.M. Mukhamedzhanov "For my own part, I declare I know nothing whatever about it, but looking at the stars always makes me dream, as simply as I dream over the black dots representing villages and towns on a map. Why, I ask myself, shouldn't the shining dots of the sky be as accessible as the black dots on the map of France?" Vincent Van Gogh, 1889

2. p(p,e+,ν)d d(p,γ)3He 3He(3He,2p)4He 3He(α,γ)7Be 7Be(e-,ν)7Li 7Li(p, α)4He 7Be(p, γ)8B 8B(e+, ν)8Be 8Be(α)4He Why the stars shineEnergy production and nucleosynthesis in stars Proton-proton chain Hydrogen burning in the stars produces energy by two cycles, with the same net result 4p  4He + 2e+ + 2n + Q (Q=26.73 Mev). The fusion of hydrogen into helium fuels the luminosity of stars, and produces energy to make the stars shine. In first-generation stars, energy is produced by burning hydrogen via the proton-proton (p-p) chain. In second-generation stars, energy is produced by hydrogen burning of heavier elements in the CN cycle. Carbon and nitrogen are heavier elements than helium with relatively small Coulomb barriers and high abundances compared to heavier elements. 86% 14% 14% .02%

3. CNO Cycles (p,γ) (p,α) (p,γ) (p,γ) (e+,ν) (e+,ν) II (e+,ν) I III (p,γ) (e+,ν) (p,γ) (p,γ) (p,γ) (p,α) (p,γ) IV (p,α) (p,α) IMPORTANT LEAK REACTION Catalytic material is lost from the process via the leak reaction 15N(p,g)16O. Subsequent reactions restore the catalytic material to the cycle, creating oxygen-16 and heavier elements. The nucleosynthesis of heavier elements can be explained by the relative abundance and reaction rates of CN elements. The important leak reaction 15N(p,g)16O is the subject of this research. Rolfs, C., and Rodney, W.S., Cauldrons in the Cosmos (The University of Chicago Press, Chicago, 1988)

4. Mechanics The leak reaction involves proton radiative capture. An impinging proton experiences two forces: the long-range electromagnetic force (Coulomb potential) outside of the nucleus, and the short-range strong force (nuclear potential) inside the nucleus. In classical mechanics, E=T+V, and thus rcl is the classical closest approach distance. In quantum mechanics, due to the tunnel effect, a proton can penetrate the Coulomb barrier with energy Ep<Ec. At astrophysical energies, the Coulomb barrier is the main obstacle for the reaction to occur, and the probability to penetrate the barrier is small. Proton Capture Nonresonant reactions: In nonresonant (direct-capture) reactions, a proton goes directly to the state in the final compound nucleus, and g-radiation is emitted. This process can occur for all proton energies. Resonant reactions: In resonant reactions, an excited state of the compound nucleus is first formed, and then g-decays to the final compound state. This is a two-step process which occurs at fixed proton energies. Energy Coulomb Barrier p Resonant Capture Direct Capture Radius Nuclear Potential

5. Astrophysical S(E) Factor The cross section s is the effective geometrical area of a projectile and target nucleus interaction, and it is directly related to the probability of the reaction under consideration. For projectile energies below the Coulomb barrier, such as astrophysical energies, the cross section drops by many orders of magnitude with decreasing energy. We use the astrophysical S(E) factor in our theory to extrapolate to lower astrophysical energies, essentially to zero energy. The astrophysical S(E) factor has units [Energy-Area][keV-b]. • Our Analysis • In our analysis, the astrophysical S(E) factor is contributed by two resonances AND direct captures. • The S(E) factor is determined by resonant parameters and asymptotic normalization coefficients (ANCs). • The resonant parameters were fit for both channels 15N(p,a)12C and 15N(p,g)16O. • The ANCs were recently measured in Prague • Nuclear Physics Institute, Czech Republic • Catania National Lab, Italy • Cyclotron Institute, TAMU, USA.

6. Experimental S(E) factor for 15N(p,γ)16O (p,γ) (e+,ν) (e+,ν) II (e+,ν) I III (p,γ) (e+,ν) (p,γ) (p,γ) (p,γ) (p,α) (p,γ) C. Rolfs and W. S. Rodney, Nucl. Phys. A235 (1974) 450. Rolfs and Rodney (RR) measured at higher energies and extrapolated data to zero energy. From their direct measurements, the S(E) factor is contributed by two Jp=1- resonances at Ecm=317 keV and 964 keV and nonresonant captures. Their result was very sensitive to direct capture; the direct capture amplitude was varied arbitrarily to fit the data. They conclude S(0keV)=64±6 keV b, thus 1 leak per 880 CN cycles. D.F. Hebbard, Nucl. Phys. 15 (1960) 289. Hebbard measured an S(E) factor contributed by two resonances at Ecm=317 keV and 947 keV. The experimental data were renormalized at the second resonance with 42% error, contributing 18% uncertainty at lower energies. Hebbard concludes S(25keV)=32±5.76 keV b, thus 1 leak per 2200 CN cycles. IV (p,α) (p,α) IMPORTANT LEAK REACTION C. Angulo,Nucl. Phys. A656 (1999) 3-183.

7. Resonances For the reaction 15N(p,g)16O: 15N has spin ½, proton has spin ½, and relative orbital angular momentum li is 0. The angular momentum Jf of the resonant state 16O is 1. 15N(1/2) + p(1/2) + li(0) = Jf(1) 15N has parity -, proton has parity +, and parity of li is +. Parity of 16O is -. (-) (+) (+) = (-) Interference Interference between resonant and direct amplitudes depends on selection rules – angular momentum and parity conservation laws. Total angular momentum is the sum of channel spins and relative orbital angular momentum. Resonances and direct capture amplitudes interfere if they have the same quantum numbers in the initial state.

8. Energy Levels 16O

9. Fit of Direct Data Resonant Parameters α To find the resonant parameters, we fit direct data for 15N(p,a)12C with two interfering Jp=1- resonances at Ecm=312 keV and 960 keV. Direct data from A. Redder et al., Z. Phys. A305, 325 (1982) from 15N(p,a)12C from 15N(p,g)16O *Inelastic electron scattering Gg2=31±8 eV

10. R-Matrix Approach In the R-Matrix approach, the configuration space is divided into an internal and an external region, divided at a radius r0. The nuclear parameters inside combine resonant and nonresonant capture. The collision matrix outside includes only nonresonant capture from r0∞. The advantage of the R-Matrix approach is the essential independence of astrophysical results on the radius r0 division of configuration space.

11. S(E) Factor Expression for 15N(p,g)16O remove the Gamow factor from the cross section s proton partial resonant width capture amplitude (through resonance i) radiative width total width direct capture amplitude to bound state i kinematical factor

12. Fitting the Data for 15N(p,γ)16O DC Gg for the 1-st and 2-nd resonances are fixed by the resonant peaks - - - Nonresonant part ■ Rolfs, C., and Rodney, W.S., Nucl. Phys. A235 (1974) 450. ▲ D.F. Hebbard, Nucl. Phys. 15 (1960) 289.

13. Reaction Rates for 15N(p,γ)16O Reaction rates are used to determine relative abundance of elements in the CNO cycle. Because the theoretical S(E) factor is lower than experimental data at lower energies, theoretical reaction rates (RRTotal) are also lower than experimental (NACRE). At 0.3 T9, reaction rates increase due to a narrow resonance. More resonances contribute to the hot CNO cycle (above 0.1 T9). New measurements at low energies and new calculations of reaction rates should consider all resonances. (NACRE) C. Angulo et al., Nucl. Phys. A 656 (1999) 3-183.

14. Zone 28 p Zone 1 15N p 16O Zone 28 Zone 8 Zone 4 16O Zone 8 Zone 1 Zone 28 15N Zone 1 Abundance of elements in the sun www.nucastrodata.org

15. Considerations We consider factors which may contribute to discrepancies with experimental and theoretical data. • Stopping power and straggling has little effect on proton energy when a thin target is used. • The Rolfs & Rodney (RR) analysis contains inconsistent use of lab and center of mass systems. • Systematic uncertainties of resonant S(E) factors are not given. We estimated uncertainties of 15%. • The RR analysis theory is not specified, and a significant overestimation of the nonresonant capture may be due to incorrect division of configuration space; channel radius is not specified. Conclusions The calculated astrophysical factor S(0keV)=38 keV b. The result does not depend on channel radius. The ratio of S(E) factors for 15N(p,a)12C and 15N(p,g)16O is 1436:1. Our analysis calls for new measurements of 15N(p,g)16O.

16. Paper Results of this analysis included in the paper to be submitted to Phys Rev C Asymptotic Normalization Coefficients From the Reaction and Astrophysical S Factor for A. M. Mukhamedzhanov, C. A. Gagliardi, A. Plunkett, L. Trache, R. E. Tribble, Cyclotron Institute, Texas A&M University, College Station, TX 77843 P. Bem, V. Burjan Z. Hons, V. Kroha, J. Novak, S. Piskor, E. Simeckova, F. Vesely, J.Vincour, Nuclear Physics Institute, Czech Academy of Sciences, 250 68 Rez near Prague, Czech Republic M. La Cognata, R. G. Pizzone, S. Romano, C. Spitaleri, Universitádi Catania and INFN LaboratoriNazionali del Sud, Catania, Italy

17. THANKS! • Dr. Zhanov – physics and soccer advisor. • Changbo Fu, Dr. Goldberg, Dr. Tribble – insightful experimentalists.