Direct measurements of the total fusion cross section for <math><mmultiscripts><mrow><mtext>B</mtext></mrow><mprescripts></mprescripts><none></none><mrow><mn>8</mn></mrow></mmultiscripts><mo>+</mo><msup><mrow></mrow><mrow><mn>40</mn></mrow></msup><mtext>Ar</mtext></math> were achieved with the active target technique. The fusion excitation function was extracted at energies near the Coulomb barrier. The cross section is well described by a coupled reaction channels calculation. The data were compared with previous <math><mmultiscripts><mrow><mtext>B</mtext></mrow><mprescripts></mprescripts><none></none><mrow><mn>8</mn></mrow></mmultiscripts></math> fusion experiments on <math><mmultiscripts><mrow><mtext>Si</mtext></mrow><mprescripts></mprescripts><none></none><mrow><mn>28</mn></mrow></mmultiscripts></math> and <math><mmultiscripts><mrow><mtext>Ni</mtext></mrow><mprescripts></mprescripts><none></none><mrow><mn>58</mn></mrow></mmultiscripts></math> targets. No evidence of striking enhancement of the total fusion cross section at near the Coulomb barrier, that was previously reported for the <math><mmultiscripts><mrow><mtext>B</mtext></mrow><mprescripts></mprescripts><none></none><mrow><mn>8</mn></mrow></mmultiscripts><mo>+</mo><msup><mrow></mrow><mrow><mn>58</mn></mrow></msup><mtext>Ni</mtext></math> system, was observed in these direct measurements. The present data are systematically consistent with the results for <math><mmultiscripts><mrow><mtext>B</mtext></mrow><mprescripts></mprescripts><none></none><mrow><mn>8</mn></mrow></mmultiscripts><mo>+</mo><msup><mrow></mrow><mrow><mn>28</mn></mrow></msup><mtext>Si</mtext></math> at higher energies and with other weakly-bound systems at near-barrier energies.

Brazil

J.C. Zamora

V. Guimaraes

United States

G.V. Rogachev

S. Ahn

J. Lubian

E.N. Cardozo

E. Aboud

M. Assuncao

M. Barbui

J. Bishop

A. Bosh

J. Hooker

C. Hunt

H. Jayatissa

E. Koshchiy

JINR

S. Lukyanov

R. O'Dwyer

Y. Penionzhkevich

B.T. Roeder

A. Saastamoinen

S. Upadhyayula

Direct fusion measurement of the B proton-halo nucleus at near-barrier energies

The $^{22}$Ne(α,γ)$^{26}$Mg and $^{22}$Ne(α,n)$^{25}$Mg reactions play an important role in astrophysics because they have significant influence on the neutron flux during the weak branch of the s-process. We constrain the astrophysical rates for these reactions by measuring partial α-widths of resonances in $^{26}$Mg located in the Gamow window for the <math><mmultiscripts><mrow><mi>Ne</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>22</mn></mrow></mmultiscripts><mo>+</mo><mi>α</mi></math> capture. These resonances were populated using $^{22}$Ne($^{6}$Li,d)$^{26}$Mg and $^{22}$Ne($^{7}$Li,t)$^{26}$Mg reactions at energies near the Coulomb barrier. At these low energies α-transfer reactions favor population of low spin states and the extracted partial α-widths for the observed resonances exhibit only minor dependence on the model parameters. The astrophysical rates for both the $^{22}$Ne(α,γ)$^{26}$Mg and the $^{22}$Ne(α,n)$^{25}$Mg reactions are shown to be significantly different than the previously suggested values.

V.Z. Goldberg

G. Christian

S. Ota

Italy

O. Trippella

E. Uberseder

Constraining the Ne( , ) Mg and Ne( ,n) Mg reaction rates using sub-Coulomb -transfer Reactions

The astrophysical s-process is one of the two main processes forming elements heavier than iron. A key outstanding uncertainty surrounding s-process nucleosynthesis is the neutron flux generated by the <math><mmultiscripts><mrow><mi>Ne</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>22</mn></mrow></mmultiscripts><mo>(</mo><mi>α</mi><mo>,</mo><mi>n</mi><mo>)</mo><msup><mrow></mrow><mrow><mn>25</mn></mrow></msup><mrow><mi>Mg</mi></mrow></math> reaction during the He-core and C-shell burning phases of massive stars. This reaction, as well as the competing <math><mmultiscripts><mrow><mi>Ne</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>22</mn></mrow></mmultiscripts><mo>(</mo><mi>α</mi><mo>,</mo><mi>γ</mi><mo>)</mo><msup><mrow></mrow><mrow><mn>26</mn></mrow></msup><mrow><mi>Mg</mi></mrow></math> reaction, is not well constrained in the important temperature regime from ∼0.2–0.4 GK, owing to uncertainties in the nuclear properties of resonances lying within the Gamow window. To address these uncertainties, we have performed a new measurement of the <math><mmultiscripts><mrow><mi>Ne</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>22</mn></mrow></mmultiscripts><mo>(</mo><mmultiscripts><mrow><mi>Li</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>6</mn></mrow></mmultiscripts><mo>,</mo><mi>d</mi><mo>)</mo><msup><mrow></mrow><mrow><mn>26</mn></mrow></msup><mrow><mi>Mg</mi></mrow></math> reaction in inverse kinematics, detecting the outgoing deuterons and <math><mmultiscripts><mrow><mi>Mg</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>25</mn><mo>,</mo><mn>26</mn></mrow></mmultiscripts></math> recoils in coincidence. We have established a new <math><mi>n</mi><mo>/</mo><mi>γ</mi></math> decay branching ratio of <math><mn>1.14</mn><mo>(</mo><mn>26</mn><mo>)</mo></math> for the key <math><msub><mrow><mi>E</mi></mrow><mrow><mi>x</mi></mrow></msub><mo>=</mo><mn>11.32</mn></math> MeV resonance in <math><mmultiscripts><mrow><mi>Mg</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>26</mn></mrow></mmultiscripts></math>, which results in a new <math><mo>(</mo><mi>α</mi><mo>,</mo><mi>n</mi><mo>)</mo></math> strength for this resonance of <math><mn>42</mn><mo>(</mo><mn>11</mn><mo>)</mo><mspace width="0.25em"></mspace><mrow><mi>μ</mi><mtext>eV</mtext></mrow></math> when combined with the well-established <math><mo>(</mo><mi>α</mi><mo>,</mo><mi>γ</mi><mo>)</mo></math> strength of this resonance. We have also determined new upper limits on the α partial widths of neutron-unbound resonances at <math><msub><mrow><mi>E</mi></mrow><mrow><mi>x</mi></mrow></msub><mo>=</mo><mn>11.112</mn></math>, 11.163, 11.169, and 11.171 MeV. Monte-Carlo calculations of the stellar <math><mmultiscripts><mrow><mi>Ne</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>22</mn></mrow></mmultiscripts><mo>(</mo><mi>α</mi><mo>,</mo><mi>n</mi><mo>)</mo><msup><mrow></mrow><mrow><mn>25</mn></mrow></msup><mrow><mi>Mg</mi></mrow></math> and <math><mmultiscripts><mrow><mi>Ne</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>22</mn></mrow></mmultiscripts><mo>(</mo><mi>α</mi><mo>,</mo><mi>γ</mi><mo>)</mo><msup><mrow></mrow><mrow><mn>26</mn></mrow></msup><mrow><mi>Mg</mi></mrow></math> rates, which incorporate these results, indicate that both rates are substantially lower than previously thought in the temperature range from ∼0.2–0.4 GK.

United Kingdom

G. Lotay

W.N. Catford

E.A. Bennett

S. Dede

D.T. Doherty

S. Hallam

A. Matta

M. Moukaddam

J.A. Tostevin

R. Wilkinson

Decay properties of Ne + Resonances and Their Impact on -process Nucleosynthesis

The structure of exotic nucleus 10 N was studied using 9 C+p resonance scattering. Two ℓ=0 resonances were found to be the lowest states in 10 N. The ground state of 10 N is unbound with respect to proton decay by 2.2(2) or 1.9(2) MeV depending on the 2− or 1− spin-parity assignment, and the first excited state is unbound by 2.8(2) MeV.

C. Magana

Structure of 10 N in 9 C+p resonance scattering

The level structure of the very neutron rich and unbound 9 He nucleus has been the subject of significant experimental and theoretical study. Many recent works have claimed that the two lowest energy 9 He states exist with spins Jπ=1/2+ and Jπ=1/2− and widths on the order of 100–200 keV. These findings cannot be reconciled with our contemporary understanding of nuclear structure. The present work is the first high-resolution study with low statistical uncertainty of the relevant excitation energy range in the 8 He+n system, performed via a search for the T=5/2 isobaric analog states in 9 Li populated through 8 He+p elastic scattering. The present data show no indication of any narrow structures. Instead, we find evidence for a broad Jπ=1/2+ state in 9 He located approximately 3 MeV above the neutron decay threshold.

Canada

M. Alcorta

G. Chubarian

B. Davids

China

C. Fu

D. Melconian

R.E. Tribble

Nuclear structure beyond the neutron drip line: The lowest energy states in 9 He via their T = 5/2 isobaric analogs in 9 Li