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Measurement of Radiative Proton Capture on 18F and Implications for Oxygen-Neon Novae
The rate of the 18F(p,g)19Ne reaction affects the final abundance of the gamma-ray observable radioisotope 18F, produced in novae. However, no successful measurement of this reaction exists and the rate used is
calculated from incomplete information on the contributing resonances. Of the two resonances thought to
play a significant role, one has a radiative width estimated from the assumed analogue state in the mirror
nucleus, 19F. The second does not have an analogue state assignment at all, resulting in an arbitrary
radiative width being assumed. Here, we report the first successful direct measurement of the
18F(p,g)19Ne reaction. The strength of the 665 keV resonance (Ex = 7.076 MeV) is found to be over
an order of magnitude weaker than currently assumed in nova models. Reaction rate calculations show
that this resonance therefore plays no significant role in the destruction of 18F at any astrophysical
energy
Journal: Phys. Rev. C
Strength of the Ec.m. = 1113 keV resonance in 20Ne(p, γ) 21Na
The 20Ne(p, γ)21Na reaction is the starting point of the NeNa cycle, which is an important process for the production of intermediate mass elements. The Ec.m. = 1113 keV resonance plays an important role in the determination of stellar rates for this reaction since it is used to normalize experimental direct capture yields at lower energies. The commonly accepted strength of this resonance, ωγ = 1.13±0.07 eV, has been misinterpreted as the strength in the center-of-mass frame when it is actually the strength in the laboratory frame. This has motivated a new measurement of the Ec.m. = 1113 keV resonance strength in 20Ne(p, γ)21Na using the DRAGON recoil mass spectrometer. The DRAGON result, 0.972 ± 0.11 eV, is in good agreement with the accepted value when both are calculated in the same frame of reference.
Journal: Phys. Rev. C, Brief Reports
Solar fusion cross sections II: the pp chain and CNO cycles
The available data on nuclear fusion cross sections important to energy generation in the Sun and other hydrogen-burning stars and to solar neutrino production are summarized and critically evaluated. Recommended values and uncertainties are provided for key cross sections, and a recommended spectrum is given for 8B solar neutrinos. Opportunities for further increasing the precision of key rates are also discussed, including new facilities, new experimental techniques, and improvements in theory. This review, which summarizes the conclusions of a workshop held at the Institute for Nuclear Theory, Seattle, in January 2009, is intended as a 10-year update and supplement to 1998, Rev. Mod. Phys. 70, 1265.
Journal: Review of Modern Physics, vol. 83, Issue 1, pp. 195-246
Direct Measurements of Na22(p,γ)Mg23 Resonances and Consequences for Na22 Production in Classical Novae
The radionuclide Na22 is a potential astronomical observable that is expected to be produced in classical novae in quantities that depend on the thermonuclear rate of the Na22(p,γ)Mg23 reaction. We have measured the strengths of low-energy Na22(p,γ)Mg23 resonances directly and absolutely using a radioactive Na22 target. We find the strengths of resonances at Ep=213, 288, 454, and 610 keV to be higher than previous measurements by factors of 2.4-3.2, and we exclude important contributions to the rate from proposed resonances at Ep=198, 209, and 232 keV. The Na22 abundances expected in the ejecta of classical novae are reduced by a factor of ≈2.
Journal: Physical Review Letters, vol. 105, Issue 15, id. 152501 (2010)
Absolute determination of the Na22(p,γ)Mg23 reaction rate in novae
Gamma-ray telescopes in orbit around the earth are searching for evidence of the elusive radionuclide Na22 produced in novae. Previously published uncertainties in the dominant destructive reaction, Na22(p,γ)Mg23, indicated new measurements in the proton energy range of 150 to 300 keV were needed to constrain predictions. We have measured the resonance strengths, energies, and branches directly and absolutely by using protons from the University of Washington accelerator with a specially designed beam line, which included beam rastering and cold vacuum protection of the Na22 implanted targets. The targets, fabricated at TRIUMF-ISAC, displayed minimal degradation over a ~20 C bombardment as a result of protective layers. We avoided the need to know the absolute stopping power, and hence the target composition, by extracting resonance strengths from excitation functions integrated over proton energy. Our measurements revealed that resonance strengths for Ep=213, 288, 454, and 610 keV are stronger by factors of 2.4-3.2 than previously reported. Upper limits have been placed on proposed resonances at 198, 209, and 232 keV. These substantially reduce the uncertainty in the reaction rate. We have re-evaluated the Na22(p,γ) reaction rate, and our measurements indicate the resonance at 213 keV makes the most significant contribution to Na22 destruction in novae. Hydrodynamic simulations including our rate indicate that the expected abundance of Na22 ejecta from a classical nova is reduced by factors between 1.5 and 2, depending on the mass of the white-dwarf star hosting the nova explosion.
Journal: Physical Review C, vol. 83, Issue 3, id. 034611 (2011)
Aspects of Nuclear Phenomena Under Explosive Astrophysical Condition
The breakout of the hot CNO cycle and the onset of the rapid-proton process are of signi cant importance to our understanding of the nucleosynthesis of proton-rich nuclei in our universe. In particular 15O( ; )19Ne and 21Na(p, )22Mg are both thought to be key reactions for these processes under explosive astrophysical conditions. In this work, an experiment has been carried out at Louvain-la-Neuve, Belgium, in order to test the feasibility of a measurement of the lifetime of the 4.033 MeV state in 19Ne, which is considered extremely important for the 15O( ; )19Ne reaction. Also, an elastic-scattering experiment was performed using a newly-developed 21Na beam at the ISAC post-accelerated radioactive beam facility in Vancouver,Canada. The experiment represents the rst scienti c result achieved with this facility. A centre-of-mass energy range of 0.4-1.5 MeV was investigated using a thick-target scan technique utilising polyethylene [CH2]n) foils. Data were collected using a silicon charged-particle detector array, enabling the identi cation of elastic and inelastic resonances in the 21Na+p system. Monte-Carlo simulations were used to estimate the experimental resolution e ects present in the experiment. These results were then incorporated into an analysis of the data using a single-channel ` = 0 R-matrix
code. An analysis of the data enabled the identi cation of four states in22Mg, one of which was previously unobserved. Resonance energies and widths were estimated for each of these states. A comparison of the resultswith states in the T=1 analogue system was made. The e ect a detailed knowledge of these resonances would have on the 21Na(p,)22Mg rate under extreme high temperature conditions was also investigated.
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Experimental developments for the study of explosive nucleosynthesis in stars
For several years now, the -SNS collaboration has been working to place a small neutrino detector at the Spallation Neutron Source at Oak Ridge National Lab. If successful, the experiment may produce the needed neutrino-nucleus cross sections on solid targets such as iron and aluminum. These reaction probabilites are of great interest for a number of reasons, including: neutrino astronomy, explosive nucleosynthesis, and nuclear structure.
However, success for this project requires a very efficient cosmic ray detector to exclude backgrounds. The system would need to be 99% efficient while remaining affordable in a difficult financial climate for basic science. The first half of this thesis addresses a prototype cosmic ray veto based on extruded scintillator with embedded wave-length-shifting fibers. This approach has been successfully used before, and may provide the performance needed for this project. However, our results suggest some additional research and development would be required to meet the requirements for the -SNS experiment.
The second half of this thesis relates to experimental work to study the resonance strength of the 23Mg(p,)24Al reaction. For this purpose a radioactive ion beam experiment has been conducted at TRIUMF using the DRAGON experiment. This reaction is thought to play an important role during explosive nucleosynthesis such as novae and X-ray bursts. If so, then accurate knowledge of this break-out reaction would help explain the isotopic abundances around that mass range in the universe.
Our results suggest the rate of this reaction at astrophysically relevant energies is lower than predicted and might further exclude explosive binary systems as the production site for such elements as 26Al.
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