Gamma-ray spectroscopy measurements for nuclear reactions in novae

The 23Mg(pγ)24Al and 26Al(pγ)27Si astrophysical reactions are expected to be of considerable importance in the nucleosynthesis of A≥20 nuclei in classical novae. Previous studies have indicated that both reactions are dominated by resonant capture to excited states, above the proton-emission thresholds, in the proton-rich nuclei 24Al and 27Si, respectively. Consequently, by determining the nuclear properties of such resonant states it is possible to estimate the 23Mg(pγ)24Al and 26Al(pγ)27Si stellar reaction rates. In this thesis work, excited states in 24Al and 27Si were populated via the 10B(16O, 2n) and 12C(16O, n) heavy-ion fusion-evaporation reactions, respectively. The beams of 16O ions were produced by the Argonne Tandem Linear Accelerator System and prompt electromagnetic radiation was detected using the GAMMASPHERE detector array, which, in the case of the 24Al experiment, was used in coincidence with recoil selection provided by the Argonne Fragment Mass Analyzer. The two γray spectroscopy studies performed in this work allowed level structure determinations below the respective proton-emission thresholds of 24Al and 27Si nuclei, with improved precision on previous work. In addition to this, these studies also allowed a determination of the nuclear properties of proton-unbound astrophysically important γ decaying states, which, in turn, were used to re-evaluate the 23Mg(pγ)24Al and 26Al(pγ)27Si stellar reaction rates. The improved precision of the level energies and unambiguous assignments of resonant states has reduced the relative uncertainties in both the 23Mg(pγ)24Al and 26Al(pγ)27Si stellar reaction rates, constraining the production of A≥20 nuclei in classical novae

First Measurement of the 96^{96}Ru(p,γ\gamma)97^{97}Rh Cross Section for the p-Process with a Storage Ring

This work presents a direct measurement of the $^{96}$Ru($p, \gamma$)$^{97}$Rh cross section via a novel technique using a storage ring, which opens opportunities for reaction measurements on unstable nuclei. A proof-of-principle experiment was performed at the storage ring ESR at GSI in Darmstadt, where circulating $^{96}$Ru ions interacted repeatedly with a hydrogen target. The $^{96}$Ru($p, \gamma$)$^{97}$Rh cross section between 9 and 11 MeV has been determined using two independent normalization methods. As key ingredients in Hauser-Feshbach calculations, the $\gamma$-ray strength function as well as the level density model can be pinned down with the measured ($p, \gamma$) cross section. Furthermore, the proton optical potential can be optimized after the uncertainties from the $\gamma$-ray strength function and the level density have been removed. As a result, a constrained $^{96}$Ru($p, \gamma$)$^{97}$Rh reaction rate over a wide temperature range is recommended for $p$-process network calculations.Comment: 10 pages, 7 figs, Accepted for publication at PR

Measurements of proton-induced reactions on ruthenium-96 in the ESR at GSI

8th International Conference on Nuclear Physics at Storage Rings Stori11, October 9-14, 2011 Laboratori Nazionale di Frascati, Italy. Storage rings offer the possibility of measuring proton- and alpha-induced reactions in inverse kinematics. The combination of this approachwith a radioactive beamfacility allows, in principle, the determination of the respective cross sections for radioactive isotopes. Such data are highly desired for a better understanding of astrophysical nucleosynthesis processes like the p-process. A pioneering experiment has been performed at the Experimental Storage Ring (ESR) at GSI using a stable 96Ru beam at 9-11 AMeV and a hydrogen target. Monte-Carlo simulations of the experiment were made using the Geant4 code. In these simulations, the experimental setup is described in detail and all reaction channels can be investigated. Based on the Geant4 simulations, a prediction of the shape of different spectral components can be performed. A comparison of simulated predictions with the experimental results shows a good agreement and allows the extraction of the cross section

Confirmation of a new resonance in Si 26 and contribution of classical novae to the galactic abundance of Al 26

The 25Al(p,¿) reaction has long been highlighted as a possible means to bypass the production of 26Al cosmic ¿ rays in classical nova explosions. However, uncertainties in the properties of key resonant states in 26Si have hindered our ability to accurately model the influence of this reaction in such environments. We report on a detailed ¿-ray spectroscopy study of 26Si and present evidence for the existence of a new, likely l=1, resonance in the 25Al + p system at Er=153.9(15) keV. This state is now expected to provide the dominant contribution to the 25Al(p,¿) stellar reaction rate over the temperature range, T˜0.1-0.2 GK. Despite a significant increase in the rate at low temperatures, we find that the final ejected abundance of 26Al from classical novae remains largely unaffected even if the reaction rate is artificially increased by a factor of 10. Based on new, galactic chemical evolution calculations, we estimate that the maximum contribution of novae to the observed galactic abundance of 26Al is ˜0.2M¿. Finally, we briefly highlight the important role that super-asymptotic giant branch stars may play in the production of 26Al.Postprint (published version

Gamma-ray spectroscopy measurements for nuclear reactions in novae

The 23Mg(pγ)24Al and 26Al(pγ)27Si astrophysical reactions are expected to be of considerable importance in the nucleosynthesis of A≥20 nuclei in classical novae. Previous studies have indicated that both reactions are dominated by resonant capture to excited states, above the proton-emission thresholds, in the proton-rich nuclei 24Al and 27Si, respectively. Consequently, by determining the nuclear properties of such resonant states it is possible to estimate the 23Mg(pγ)24Al and 26Al(pγ)27Si stellar reaction rates. In this thesis work, excited states in 24Al and 27Si were populated via the 10B(16O, 2n) and 12C(16O, n) heavy-ion fusion-evaporation reactions, respectively. The beams of 16O ions were produced by the Argonne Tandem Linear Accelerator System and prompt electromagnetic radiation was detected using the GAMMASPHERE detector array, which, in the case of the 24Al experiment, was used in coincidence with recoil selection provided by the Argonne Fragment Mass Analyzer. The two γray spectroscopy studies performed in this work allowed level structure determinations below the respective proton-emission thresholds of 24Al and 27Si nuclei, with improved precision on previous work. In addition to this, these studies also allowed a determination of the nuclear properties of proton-unbound astrophysically important γ decaying states, which, in turn, were used to re-evaluate the 23Mg(pγ)24Al and 26Al(pγ)27Si stellar reaction rates. The improved precision of the level energies and unambiguous assignments of resonant states has reduced the relative uncertainties in both the 23Mg(pγ)24Al and 26Al(pγ)27Si stellar reaction rates, constraining the production of A≥20 nuclei in classical novae.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Gamma-ray spectroscopy measurements for nuclear reactions in novae

The 23Mg(pγ)24Al and 26Al(pγ)27Si astrophysical reactions are expected to be of considerable importance in the nucleosynthesis of A≥20 nuclei in classical novae. Previous studies have indicated that both reactions are dominated by resonant capture to excited states, above the proton-emission thresholds, in the proton-rich nuclei 24Al and 27Si, respectively. Consequently, by determining the nuclear properties of such resonant states it is possible to estimate the 23Mg(pγ)24Al and 26Al(pγ)27Si stellar reaction rates. In this thesis work, excited states in 24Al and 27Si were populated via the 10B(16O, 2n) and 12C(16O, n) heavy-ion fusion-evaporation reactions, respectively. The beams of 16O ions were produced by the Argonne Tandem Linear Accelerator System and prompt electromagnetic radiation was detected using the GAMMASPHERE detector array, which, in the case of the 24Al experiment, was used in coincidence with recoil selection provided by the Argonne Fragment Mass Analyzer. The two γray spectroscopy studies performed in this work allowed level structure determinations below the respective proton-emission thresholds of 24Al and 27Si nuclei, with improved precision on previous work. In addition to this, these studies also allowed a determination of the nuclear properties of proton-unbound astrophysically important γ decaying states, which, in turn, were used to re-evaluate the 23Mg(pγ)24Al and 26Al(pγ)27Si stellar reaction rates. The improved precision of the level energies and unambiguous assignments of resonant states has reduced the relative uncertainties in both the 23Mg(pγ)24Al and 26Al(pγ)27Si stellar reaction rates, constraining the production of A≥20 nuclei in classical novae.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Direct measurement of astrophysically important resonances in 38K( p,γ )39Ca

Background: Classical novae are cataclysmic nuclear explosions occurring when a white dwarf in a binary system accretes hydrogen-rich material from its companion star. Novae are partially responsible for the galactic synthesis of a variety of nuclides up to the calcium ( A ∼ 40 ) region of the nuclear chart. Although the structure and dynamics of novae are thought to be relatively well understood, the predicted abundances of elements near the nucleosynthesis endpoint, in particular Ar and Ca, appear to sometimes be in disagreement with astronomical observations of the spectra of nova ejecta. Purpose: One possible source of the discrepancies between model predictions and astronomical observations is nuclear reaction data. Most reaction rates near the nova endpoint are estimated only from statistical model calculations, which carry large uncertainties. For certain key reactions, these rate uncertainties translate into large uncertainties in nucleosynthesis predictions. In particular, the 38 K ( p , γ ) 39 Ca reaction has been identified as having a significant influence on Ar, K, and Ca production. In order to constrain the rate of this reaction, we have performed a direct measurement of the strengths of three candidate ℓ = 0 resonances within the Gamow window for nova burning, at 386 ± 10 keV, 515 ± 10 keV, and 689 ± 10 keV. Method: The experiment was performed in inverse kinematics using a beam of unstable 38 K impinged on a windowless hydrogen gas target. The 39 Ca recoils and prompt γ rays from 38 K ( p , γ ) 39 Ca reactions were detected in coincidence using a recoil mass separator and a bismuth-germanate scintillator array, respectively. Results: For the 689 keV resonance, we observed a clear recoil- γ coincidence signal and extracted resonance strength and energy values of 120 + 50 − 30 ( stat . ) + 20 − 60 ( sys . ) meV and 679 + 2 − 1 ( stat . ) ± 1 ( sys . ) keV , respectively. We also performed a singles analysis of the recoil data alone, extracting a resonance strength of 120 ± 20 ( stat . ) ± 15 ( sys . ) meV, consistent with the coincidence result. For the 386 keV and 515 keV resonances, we extract 90 % confidence level upper limits of 2.54 meV and 18.4 meV, respectively. Conclusions: We have established a new recommended 38 K ( p , γ ) 39 Ca rate based on experimental information, which reduces overall uncertainties near the peak temperatures of nova burning by a factor of ∼ 250 . Using the rate obtained in this work in model calculations of the hottest oxygen-neon novae reduces overall uncertainties on Ar, K, and Ca synthesis to factors of 15 or less in all cases.</p