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At low stellar temperatures the energy release due to the CN cycle is regulated by its slowest reaction 14N(p,γ)15O, the rate of which strongly depends on the subthreshold resonance at Ec.m. = -504 keV, which corresponds to the 6.79 MeV state in 15O. By using the Doppler-shift attenuation method and the ³He(16O,α)15O reaction to populate the state, we obtained an upper limit on the lifetime of this state of τ < 1.8 fs [68.3% confidence level (C.L.)]. In addition we measured the lifetimes of the 6.18 and 6.86 MeV states to be τ < 2.5 and τ = 13.3-1.2+0.9 fs, respectively (68.3% C.L.), in good agreement with the literature. [ABSTRACT FROM AUTHOR]

Proton capture on the excited isomeric state of 26Al strongly influences the abundance of 26Mg ejected in explosive astronomical events and, as such, plays a critical role in determining the initial content of radiogenic 26Al in presolar grains. This reaction also affects the temperature range for thermal equilibrium between the ground and isomeric levels. We present a novel technique, which exploits the isospin symmetry of the nuclear force, to address the long-standing challenge of determining proton-capture rates on excited nuclear levels. Such a technique has in-built tests that strongly support its veracity and, for the first time, we have experimentally constrained the strengths of resonances that dominate the astrophysical 26mAl(p,γ)27Si reaction. These constraints demonstrate that the rate is at least a factor ∼8 lower than previously expected, indicating an increase in the stellar production of 26Mg and a possible need to reinvestigate sensitivity studies involving the thermal equilibration of 26Al. [ABSTRACT FROM AUTHOR]

Excited states in 58,60,62Ni were populated via inelastic proton scattering at the Australian National University as well as via inelastic neutron scattering at the University of Kentucky Accelerator Laboratory. The Super-e electron spectrometer and the CAESAR Compton-suppressed HPGe array were used in complementary experiments to measure conversion coefficients and δ(E2/M1) mixing ratios, respectively, for a number of 2+→2+ transitions. The data obtained were combined with lifetimes and branching ratios to determine E0,M1, and E2 transition strengths between 2+ states. The E0 transition strengths between 0+ states were measured using internal conversion electron spectroscopy and compare well to previous results from internal pair formation spectroscopy. The E0 transition strengths between the lowest-lying 2+ states were found to be consistently large for the isotopes studied. [ABSTRACT FROM AUTHOR]

Background: The evolution of nuclear magic numbers at extremes of isospin is a topic at the forefront of contemporary nuclear physics. N=50 is a prime example, with increasing experimental data coming to light on potentially doubly magic 100Sn and 78Ni at the proton-rich and proton-deficient extremes, respectively; however, experimental discrepancies exist in the data for less exotic systems. Purpose: In 86Kr the B(E2;2+1→0+1) value--a key indicator of shell evolution--has been experimentally determined by two different methodologies, with the results deviating by 3σ. Here, we report on a new high-precision measurement of this value, as well as the first measured lifetimes and hence transition strengths for the 2+2 and 3-(2) states in the nucleus. Methods: The Doppler-shift attenuation method was implemented using the TRIUMF-ISAC γ-ray escape-suppressed spectrometer (TIGRESS) γ-ray spectrometer and the TIGRESS integrated plunger device. High-statistics Monte Carlo simulations were utilized to extract lifetimes in accordance with state-of-the-art methodologies. Results: Lifetimes of τ(2+1)=336±4(stat.)±20(sys.) fs, τ(2+2)=263±9(stat.)±19(sys.) fs, and τ(3-(2))=73±6(stat.)±32(sys.) fs were extracted. This yields a transition strength for the first-excited state of B(E2;2+1→0+1)=259±3(stat.)±16(sys.) e² fm4. Conclusions: The measured lifetime disagrees with the previous Doppler-shift attenuation method measurement by more than 3σ, while agreeing well with a previous value extracted from Coulomb excitation. The newly extracted B(E2;2+1→0+1) value indicates a more significant reduction in the N=50 isotones approaching Z=40. [ABSTRACT FROM AUTHOR]

A high-precision half-life measurement for the superallowed β+ transition between the isospin T=1/2 mirror nuclei 21Na and 21Ne has been performed at the TRIUMF-ISAC radioactive ion beam facility yielding T1/2=22.4506(33) s, a result that is a factor of 4 more precise than the previous world-average half-life for 21Na and represents the single most precisely determined half-life for a transition between mirror nuclei to date. The contribution to the uncertainty in the 21Na Ftmirror value due to the half-life is now reduced to the level of the nuclear-structure-dependent theoretical corrections, leaving the branching ratio as the dominant experimental uncertainty. [ABSTRACT FROM AUTHOR]

A high precision lifetime measurement of the 21+ state in 94Sr was performed at TRIUMF's ISAC-II facility by coupling the recoil distance method implemented via the TIGRESS integrated plunger with unsafe Coulomb excitation in inverse kinematics. Due to limited statistics imposed by the use of a radioactive 94Sr beam, a likelihood ratio χ² method was derived and used to compare experimental data to Geant4 simulations. The B(E2; 21+ → 01+) value extracted from the lifetime measurement of 7.80-0.4+00.50 (stat.) ± 0.07 (sys.) ps is approximately 25% larger than previously reported while the relative error has been reduced by a factor of approximately 8. A baseline deformation has been established for Sr isotopes with N ≤ 58 which is a necessary condition for the quantum phase transition interpretation of the onset of deformation in this region. A comparison to existing theoretical models is presented. [ABSTRACT FROM AUTHOR]

The ^{12}C+^{12}C fusion reaction plays a critical role in the evolution of massive stars and also strongly impacts various explosive astrophysical scenarios. The presence of resonances in this reaction at energies around and below the Coulomb barrier makes it impossible to carry out a simple extrapolation down to the Gamow window-the energy regime relevant to carbon burning in massive stars. The ^{12}C+^{12}C system forms a unique laboratory for challenging the contemporary picture of deep sub-barrier fusion (possible sub-barrier hindrance) and its interplay with nuclear structure (sub-barrier resonances). Here, we show that direct measurements of the ^{12}C+^{12}C fusion cross section may be made into the Gamow window using an advanced particle-gamma coincidence technique. The sensitivity of this technique effectively removes ambiguities in existing measurements made with gamma ray or charged-particle detection alone. The present cross-section data span over 8 orders of magnitude and support the fusion-hindrance model at deep sub-barrier energies.

Angular distributions of the elastic, inelastic, and breakup cross sections of the halo nucleus ^{11}Be on ^{197}Au were measured at energies below (E&#95;{lab}=31.9  MeV) and around (39.6 MeV) the Coulomb barrier. These three channels were unambiguously separated for the first time for reactions of ^{11}Be on a high-Z target at low energies. The experiment was performed at TRIUMF (Vancouver, Canada). The differential cross sections were compared with three different calculations: semiclassical, inert-core continuum-coupled-channels and continuum-coupled-channels ones with including core deformation. These results show conclusively that the elastic and inelastic differential cross sections can only be accounted for if core-excited admixtures are taken into account. The cross sections for these channels strongly depend on the B(E1) distribution in ^{11}Be, and the reaction mechanism is sensitive to the entanglement of core and halo degrees of freedom in ^{11}Be.

Energies and spectroscopic factors of the first 7/2-, 3/2-, 1/2-, and 5/2- states in the (35)Si21 nucleus were determined by means of the (d, p) transfer reaction in inverse kinematics at GANIL using the MUST2 and EXOGAM detectors. By comparing the spectroscopic information on the Si35 and S37 isotones, a reduction of the p3/2-p1/2 spin-orbit splitting by about 25% is proposed, while the f7/2-f5/2 spin-orbit splitting seems to remain constant. These features, derived after having unfolded nuclear correlations using shell model calculations, have been attributed to the properties of the two-body spin-orbit interaction, the amplitude of which is derived for the first time in an atomic nucleus. The present results, remarkably well reproduced by using several realistic nucleon-nucleon forces, provide a unique touchstone for the modeling of the spin-orbit interaction in atomic nuclei.