New recommended ωγ for the Er c. m. =458 keV resonance in Ne 22 (p,γ) Na 23

The Erc.m.=458 keV resonance in Ne22(p,γ)Na23 is an ideal reference resonance for measurements of cross sections and resonance strengths in noble gas targets. We report on a new measurement of the strength of this resonance. Data analysis employed the TFractionFitter class of root combined with geant simulations of potential decay cascades from this resonance. This approach allowed us to extract precise primary branching ratios for decays from the resonant state, including a new primary branch to the 7082-keV state in Na23. Our new resonance strength of ωγ(458 keV) = 0.583(43) eV is more than 1σ higher than a recent high-precision result that relied on literature branching ratios

Measurement of the Er c. m. =259 keV resonance in the N 14 (p,γ) O 15 reaction MEASUREMENT of the Er c. m. =259 keV

The N14(p,γ)O15 reaction regulates the power generated by the CN cycle and thus impacts the structure and evolution of every star at some point in its life. The lowest positive-energy resonance in this reaction is located at Erc.m.=259 keV, too high in energy to strongly influence quiescent stellar burning. However, the strength of this resonance is used as a cross-section normalization for lower-energy measurements of this reaction. We report on new measurements of the energy, strength, and γ-ray branching ratios for the 259-keV resonance, using different detection and data-analysis schemes. We have also reevaluated previous results, where possible. Our new recommended strength of ωγ=12.6(3) meV is in agreement with the previous value of 13.1(6) meV, but is more precise and thus provides a more reliable normalization for low-energy (p,γ) measurements

Improved thermonuclear reaction rate for 18O(p,γ) 19F

For 0.8 Mȯ ≤ M ≤ 8.0 Mȯ stars, the final phase of nucleosynthesis occurs during the asymptotic giant branch (AGB) stage. Grain condensation and significant mass loss transpires during this stellar evolutionary period, and presolar grains recovered from comet and meteorite samples can often be attributed to this unique stellar environment. A subset of presolar oxide grain specimens exhibit dramatic 18O depletion that cannot be explained by standard AGB stellar burning stages and dredge-up models. An extra mixing process, referred to as cool bottom processing (CBP), was proposed for low-mass AGB stars to explain similar isotopic anomalies. The 18O depletion observed within certain stellar environments and within presolar grain samples may result from the 18O+p processes during CBP, and we report here on a study of the 18O(p,γ)19F reaction at low energies. The (p,γ) reaction rate at low temperatures was found to not be affected by a low-energy, unobserved, narrow resonance-ElabR = 95 keV-near the CBP Gamow peak. A new strength upper limit measurement was performed at TUNL's Laboratory for Experimental Nuclear Astrophysics, and an improved reaction rate was calculated. In addition, non-resonant cross section and astrophysical S-factor upper limits were measured at low bombarding energies

Thermonuclear reaction rate of 18O(p,γ)19F

For stars with 0.8 M⊙ ≤ M ≤ 8.0 M⊙, nucleosynthesis enters its final phase during the asymptotic giant branch (AGB) stage. During this evolutionary period, grain condensation occurs in the stellar atmosphere, and the star experiences significant mass loss. The production of presolar grains can often be attributed to this unique stellar environment. A subset of presolar oxide grains features dramatic 18O depletion that cannot be explained by the standard AGB star burning stages and dredge-up models. An extra mixing process, referred to as cool bottom processing (CBP), was proposed for low-mass AGB stars. The 18O depletion observed within certain stellar environments and within presolar grain samples may result from the 18O+p processes during CBP. We report here on a study of the 18O(p,γ)19F reaction at low energies. Based on our new results, we found that the resonance at ERlab=95 keV has a negligible affect on the reaction rate at the temperatures associated with CBP. We also determined that the direct capture S factor is almost a factor of 2 lower than the previously recommended value at low energies. An improved thermonuclear reaction rate for 18O(p,γ)19F is presented

High-intensity-beam study of O 17 (p,γ) F 18 and thermonuclear reaction rates for O 17 +p

Hydrogen burning of the oxygen isotopes takes place in low-mass stars, asymptotic giant branch stars, and classical novae. Observations of oxygen elemental and isotopic abundances in stellar spectra or in presolar grains provide strong constraints for stellar models if reliable thermonuclear reaction rates for hydrogen burning of oxygen are available. We present the results of a new measurement of the 17O(p,γ)18F reaction in the laboratory bombarding energy range of 170-530 keV. The measurement is performed with significantly higher beam intensities (Imax ≈ 2 mA) compared to previous work and by employing a sophisticated γ-ray coincidence spectrometer. We measured the cross section at much lower energies than previous in-beam experiments. We also apply a novel data-analysis technique that is based on the decomposition of different contributions to the measured pulse-height spectrum. Our measured strengths of the low-energy resonances amount to ωγpres(193keV)=(1.86±0.13)×10-6 eV and ωγpres(518keV)=(13.70±0.96)×10-3 eV. For the direct capture S factor at zero energy, we find a value of SDCpres(0) = 4.82±0.41 keV b. We also present new thermonuclear rates for the 17O+p reactions, taking into account all consistent results from previous measurements

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

First direct observation of enhanced octupole collectivity in 146Ba

The octupole strength present in the neutron-rich, radiocative nucleus 146Ba has been experimentally determined for the first time using Coulomb excitation. To achieve this, A=146 fission fragments from CARIBU were post-accelerated by the Argonne Tandem Linac Accelerator System (ATLAS) and impinged on a thin 208Pb target. Using the GRETINA γ-ray spectrometer and the CHICO2 heavy-ion counter, the reduced transition probability B(E3; 3-→0+) was determined as 48(+21-29) W.u. The new result provides further experimental evidence for the presence of a region of octupole deformation surrounding the neutron-rich barium isotopes

242Pu absolute neutron-capture cross section measurement

The absolute neutron-capture cross section of 242Pu was measured at the Los Alamos Neutron Science Center using the Detector for Advanced Neutron-Capture Experiments array along with a compact parallel-plate avalanche counter for fission-fragment detection. During target fabrication, a small amount of 239Pu was added to the active target so that the absolute scale of the 242Pu(n,γ) cross section could be set according to the known 239Pu(n,f) resonance at En,R = 7.83 eV. The relative scale of the 242Pu(n,γ) cross section covers four orders of magnitude for incident neutron energies from thermal to ≈ 40 keV. The cross section reported in ENDF/B-VII.1 for the 242Pu(n,γ) En,R = 2.68 eV resonance was found to be 2.4% lower than the new absolute 242Pu(n,γ) cross section

γ-Ray spectroscopy using a binned likelihood approach

The measurement of a reaction cross section from a pulse height spectrum is a ubiquitous problem in experimental nuclear physics. In γ-ray spectroscopy, this is accomplished frequently by measuring the intensity of full-energy primary transition peaks and correcting the intensities for experimental artifacts, such as detection efficiencies and angular correlations. Implicit in this procedure is the assumption that full-energy peaks do not overlap with any secondary peaks, escape peaks, or environmental backgrounds. However, for complex γ-ray cascades, this is often not the case. Furthermore, this technique is difficult to adapt for coincidence spectroscopy, where intensities depend not only on the detection efficiency, but also the detailed decay scheme. We present a method that incorporates the intensities of the entire spectrum (e.g., primary and secondary transition peaks, escape peaks, Compton continua, etc.) into a statistical model, where the transition intensities and branching ratios can be determined using Bayesian statistical inference. This new method provides an elegant solution to the difficulties associated with analyzing coincidence spectra. We describe it in detail and examine its efficacy in the analysis of 18O(p,γ)19F and 25Mg(p,γ)26Al resonance data. For the 18O(p,γ)19F reaction, the measured branching ratios improve upon the literature values, with a factor of 3 reduction in the uncertainties