Remeasurement of the 193 keV resonance in O17(p,α)N14

A recently discovered resonance at 193 keV determines the thermonuclear rates of the O17 +p reactions at temperatures important for the nucleosynthesis in classical novae (T=0.1-0.4 GK). We report on a remeasurement of this resonance in the O17(p,α)N14 reaction by using a different kind of target compared to the previous study. Special emphasis is placed on Monte Carlo simulations of the experiment in order to better understand certain effects that have been disregarded previously. Our measured value of the resonance strength amounts to (ωγ)pα=(1.66±0.17)×10-3 eV, in agreement with the previously reported result. As a byproduct of our study, we find that the inhomogeneity of the foil placed in front of the α-particle detector determines the resolution in the pulse-height spectrum, and thus constrains the signal-to-noise ratio in searches of very weak (p,α) resonances

Measurement of O17(p,γ)F18 between the narrow resonances at Erlab=193 and 519keV

The O17(p,γ)F18 reaction sensitively influences hydrogen burning nucleosynthesis in a number of stellar sites, including classical novae. These thermonuclear explosions, taking place in close binary star systems, produce peak temperatures in the range of T=100-400 MK. Recent results indicate that the thermonuclear rates for this reaction in this particular temperature range are dominated by the direct capture process. We report on the measurement of the O17(p,γ)F18 cross section between the narrow resonances at Erlab=193 and 519keV, where the S factor is expected to vary smoothly with energy. We extract the direct capture contribution from the total cross section and demonstrate that earlier data are inconsistent with our results

Gamow peak in thermonuclear reactions at high temperatures

The Gamow peak represents one of the most important concepts in the study of thermonuclear reactions in stars. It is widely used in order to determine, at a given plasma temperature, the effective stellar energy region in which most charged-particle induced nuclear reactions occur. It is of importance in the design of nuclear astrophysics measurements, including those involving radioactive ion beams, and for the determination of stellar reaction rates. We demonstrate that the Gamow peak concept breaks down under certain conditions if a nuclear reaction proceeds through narrow resonances at elevated temperatures. It is shown that an effective stellar energy window does indeed exist in which most thermonuclear reactions take place at a given temperature, but that this energy window can differ significantly from the commonly used Gamow peak. We expect that these findings are especially important for thermonuclear reactions in the advanced burning stages of massive stars and in explosive stellar environments

Resonance strength in Ne22(p,γ)Na23 from depth profiling in aluminum

A novel method for extracting absolute resonance strengths has been investigated. By implanting Ne22 ions into a thick aluminum backing and simultaneously measuring the Ne22+p and Al27+p reactions, the strength of the Erlab=479 keV resonance in Ne22(p,γ)Na23 was determined to be ωγ=0.524(51) eV. This result has significantly reduced uncertainties compared to earlier work. Our results are important for the absolute normalizations of resonance strengths in the Ne22(p,γ)Na23 hydrogen-burning reaction and in the Ne22+α s-process neutron-source reactions

New measurements of low-energy resonances in the Ne 22 (p,γ) Na 23 reaction

The Ne22(p,γ)Na23 reaction is one of the most uncertain reactions in the NeNa cycle and plays a crucial role in the creation of Na23, the only stable Na isotope. Uncertainties in the low-energy rates of this and other reactions in the NeNa cycle lead to ambiguities in the nucleosynthesis predicted from models of thermally pulsing asymptotic giant branch (AGB) stars. This in turn complicates the interpretation of anomalous Na-O trends in globular cluster evolutionary scenarios. Previous studies of the Ne22(p,γ)Na23, Ne22(He3,d)Na23, and C12(C12,p)Na23 reactions disagree on the strengths, spins, and parities of low-energy resonances in Na23 and the direct-capture Ne22(p,γ)Na23 reaction rate contains large uncertainties as well. In this work we present new measurements of resonances at Erc.m.=417, 178, and 151 keV and of the direct-capture process in the Ne22(p,γ)Na23 reaction. The resulting total Ne22(p,γ)Na23 rate is approximately a factor of 20 higher than the rate listed in a recent compilation at temperatures relevant to hot-bottom burning in AGB stars. Although our rate is close to that derived from a recent Ne22(p,γ)Na23 measurement by Cavanna et al. in 2015, we find that this large rate increase results in only a modest 18% increase in the Na23 abundance predicted from a 5 M thermally pulsing AGB star model from Ventura and D'Antona (2005). The estimated astrophysical impact of this rate increase is in marked contrast to the factor of ∼3 increase in Na23 abundance predicted by Cavanna et al. and is attributed to the interplay between the Na23(p,α)Ne20 and Ne20(p,γ)Na21 reactions, both of which remain fairly uncertain at the relevant temperature range

Measurement of the e r c.m. = 138 keV resonance in the 23 Na(p, γ) 24 Mg reaction and the abundance of sodium in AGB stars

Globular clusters represent some of the oldest stellar aggregations in the universe. As such, they are used as testing grounds for theories of stellar evolution and nucleosynthesis. Astronomical observations have shown star-to-star abundance variations in light-mass elements in all galactic globular clusters that are not predicted by standard stellar evolution models. In particular, there exists a pronounced anticorrelation between Na and O in the cluster stars that is not observed in field stars of similar evolutionary state. The abundance of Na is regulated in part by the 23Na+p reaction, which is also a bridge between the NeNa and the MgAl mass regions, but the 23Na(p,γ)24Mg reaction rate is very uncertain for burning temperatures relevant to stars on the red giant and asymptotic giant branches. This uncertainty arises from an expected but unobserved resonance at Erc.m. = 138 keV. The resonance strength upper limit has been determined to be ωγUL(138 keV) ≤5.17×10-9 eV with indications of a signal at the 90% confidence level. New reaction rates have been calculated for the 23Na(p,γ)24Mg and 23Na(p,α)20Ne reactions and the recommended value for the 23Na(p,γ)24Mg rate has been reduced by over an order of magnitude at T9 = 0.07. This will have implications for the processing of material between the NeNa and MgAl mass regions

Development of a variable-energy, high-intensity, pulsed-mode ion source for low-energy nuclear astrophysics studies

The primary challenge in directly measuring nuclear reaction rates near stellar energies is their small cross sections. The signal-to-background ratio in these complex experiments can be significantly improved by employing high-current (mA-range) beams and novel detection techniques. Therefore, the electron cyclotron resonance ion source at the Laboratory for Experimental Nuclear Astrophysics underwent a complete upgrade of its acceleration column and microwave system to obtain high-intensity, pulsed proton beams. The new column uses a compression design with O-ring seals for vacuum integrity. Its voltage gradient between electrode sections is produced by the parallel resistance of channels of chilled, deionized water. It also incorporates alternating, transverse magnetic fields for electron suppression and an axially adjustable beam extraction system. Following this upgrade, the operational bremsstrahlung radiation levels and high-voltage stability of the source were vastly improved, over 3.5 mA of target beam current was achieved, and an order-of-magnitude increase in normalized brightness was measured. Beam optics calculations, structural design, and further performance results for this source are presented

Search for W' bosons decaying to an electron and a neutrino with the D0 detector

This Letter describes the search for a new heavy charged gauge boson W' decaying into an electron and a neutrino. The data were collected with the D0 detector at the Fermilab Tevatron proton-antiproton Collider at a center-of-mass energy of 1.96 TeV, and correspond to an integrated luminosity of about 1 inverse femtobarn. Lacking any significant excess in the data in comparison with known processes, an upper limit is set on the production cross section times branching fraction, and a W' boson with mass below 1.00 TeV can be excluded at the 95% C.L., assuming standard-model-like couplings to fermions. This result significantly improves upon previous limits, and is the most stringent to date.Comment: submitted to Phys. Rev. Let

Measurement of the ratios of the Z/G* + >= n jet production cross sections to the total inclusive Z/G* cross section in ppbar collisions at sqrt(s) = 1.96 TeV

We present a study of events with Z bosons and jets produced at the Fermilab Tevatron Collider in ppbar collisions at a center of mass energy of 1.96 TeV. The data sample consists of nearly 14,000 Z/G* -> e+e- candidates corresponding to the integrated luminosity of 0.4 fb-1 collected using the D0 detector. Ratios of the Z/G* + >= n jet cross sections to the total inclusive Z/G* cross section have been measured for n = 1 to 4 jet events. Our measurements are found to be in good agreement with a next-to-leading order QCD calculation and with a tree-level QCD prediction with parton shower simulation and hadronization.Comment: 7 pages, 2 figures, slightly modified, submitted to Phys. Lett.

Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in √s = 7 TeV pp collisions with the ATLAS detector

A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fb−1 of proton–proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results