Papers

The 21Na(α,γ)22Mg Reaction from Ec.m.=200 to 1103 keV in Novae and X-Ray Bursts

The long-lived radioactive nuclide 22Na (t_1/2=2.6 yr) is an astronomical observable for understanding the physical processes of oxygen-neon novae. Yields of 22Na in these events are sensitive to the unknown total rate of the 21Na(ρ,ϒ)22Mg reation. Using high intensity 21Na beam at the TRIUMF-ISAC facility, the strengths of seven resonances in 22Mg, of potential astrophysical importance, have been directly measured at center of mass energies from E_c.m.=200 to 1103 keV. We report the results obtained for these resonances and their respective contributions to the 21Na(ρ,ϒ)22Mg rate in novae and x-ray bursts, and their impact on 22Na production in novae.

prc65803.pdf

Author: D'Auria, J. M. et al.

Journal: Physical Review C

The Influence of Uncertainties in the ¹⁵O(α,γ)¹⁹Ne Reaction Rate on Models of Type I X-Ray Bursts

We present a Monte Carlo calculation of the astrophysical rate of the ¹⁵O(α,γ)¹⁹Ne reaction based on an evaluation of published experimental data. By considering the likelihood distributions of individual resonance parameters derived from measurements, estimates of upper and lower limits on the reaction rate at the 99.73% confidence level are derived in addition to the recommended, median value. These three reaction rates are used as input for three separate calculations of Type I X-ray bursts (XRBs) using spherically symmetric, hydrodynamic simulations of an accreting neutron star. In this way the influence of the ¹⁵O(α,γ)¹⁹Ne reaction rate on the peak luminosity, recurrence time, and associated nucleosynthesis in models of Type I XRBs is studied. Contrary to previous findings, no substantial effect on any of these quantities is observed in a sequence of four bursts when varying the reaction rate between its lower and upper limits. Rather, the differences in these quantities are comparable to the burst-to-burst variations with a fixed reaction rate, indicating that uncertainties in the ¹⁵O(α,γ)¹⁹Ne reaction rate do not strongly affect the predictions of this Type I XRB model.

davids11.pdf

Author: Davids, Barry; Cyburt, Richard H.; José, Jordi; Mythili, Subramanian

Journal: The Astrophysical Journal 735, 40 (2011)

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 E_p=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 E_p=198, 209, and 232 keV. The Na22 abundances expected in the ejecta of classical novae are reduced by a factor of ≈2.

physrevlett.105.152501.pdf

Author: Sallaska, A. L.; Wrede, C.; García, A.; Storm, D. W.; Brown, T. A. D.; Ruiz, C.; Snover, K. A.; Ottewell, D. F.; Buchmann, L.; Vockenhuber, C.; Hutcheon, D. A.; Caggiano, J. A.

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 E_p=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.

physrevc.83.034611.pdf

Author: Sallaska, A. L.; Wrede, C.; García, A.; Storm, D. W.; Brown, T. A. D.; Ruiz, C.; Snover, K. A.; Ottewell, D. F.; Buchmann, L.; Vockenhuber, C.; Hutcheon, D. A.; Caggiano, J. A.; José, J.

Journal: Physical Review C, vol. 83, Issue 3, id. 034611 (2011)

Direct measurement of the 18F(p,α)15O reaction at nova temperatures

The ¹⁸F(p,α)¹⁵O reaction rate is crucial for understanding the final abundance of ¹⁸F predicted by nova models. The γ-ray emission in the first few hours after a nova outburst is expected to be dominated by 511 keV annihilation photons from the decay of ¹⁸F, and so understanding its production can provide important constraints on the conditions during the outburst when compared with observations. Results are presented from the lowest-energy direct measurement to date, performed at the Isotope Separator and Accelerator radioactive beam facility at the TRIUMF laboratory, Canada. Cross section measurements at center-of-mass energies of 250, 330, 453, and 673 keV are obtained and the results compared to previous data and R-matrix calculations. The implications for the overall reaction rate in the context of nova explosions have been discussed.

physrevc.83.042801_2.pdf

Author: C. E. Beer, A. M. Laird, A. St. J. Murphy, M. A. Bentley, L. Buchman, B. Davids, T. Davinson, C. A. Diget, S. P. Fox, B. R. Fulton, U. Hager, D. Howell, L. Martin, C. Ruiz, G. Ruprecht, P. Salter, C. Vockenhuber, and P. Walden

Journal: Phys. Rev. C 83, 042801(R) (2011) [4 pages]

First direct measurement of the ²³Mg(p,γ)²⁴Al reaction

The lowest-energy resonance in the ²³Mg(p,γ)²⁴Al reaction, which is dominant at classical nova temperatures, has been measured directly for the first time using the DRAGON recoil spectrometer. The experiment used a radioactive ²³Mg beam (mixed within a significantly stronger ²³Na beam) of peak intensity 5×10⁷ s^-1, at the ISAC facility at TRIUMF. We extract values of E_R=485.7_-1.8^+1.3keV and ωγ=38_-15⁺²¹ meV from our data (all values in the center-of-mass frame unless otherwise stated). In addition, the experiment prompted a recalculation of the Q value for this reaction based on a revision of the ²⁴Al mass. The effect on the uncertainties in the quantities of ejected ²²Na and ²⁶Al from oxygen-neon classical novae is discussed.

erikson_23mgpg_2010.pdf

Author: Erikson, L. et al.

Journal: Physical Review C

Measurement of the ⁴⁰Ca(α,γ) ⁴⁴Ti reaction relevant for supernova nucleosynthesis

The short-lived nuclide 44 Ti is an important nuclide for the understanding of explosive nucle-osynthesis. The main production reaction, 40 Ca(α, γ)44 Ti, has been studied in inverse kinematics with the recoil mass spectrometer DRAGON located at the TRIUMF-ISAC facility in Vancouver, Canada. The temperature range relevant for α-rich freeze-out during a core-collapse supernova has been covered entirely with a 40 Ca beam of 0.60 to 1.15 MeV/nucleon. All relevant quantities for the calculation of the astrophysical reaction rate have been measured directly. Due to many previously undiscovered resonances, the reaction rate derived from the energy dependent 44 Ti yield is higher than the one based on previous prompt γ-ray studies commonly used in supernova models. The presented new rate results in an increased 44 Ti production in supernovae.

VockenhuberPRC2007_40Ca_ag_44Ti_preprint.pdf

Author: See Paper Document

Journal: Phys. Rev. C 76 035801 (2007).

⁴⁰Ca(α,γ) ⁴⁴Ti and the production of ⁴⁴Ti in supernovae

The nuclide 44 Ti is predicted to be produced in signiﬁcant quantities in core-collapse supernovae, and indeed it has been observed in the supernova remnant Cassiopeia-A by space-based γ-ray telescopes. The main production of 44 Ti takes place in the α-rich freeze-out phase deep inside the supernova. The key reactions governing the 44 Ti abundance have been identiﬁed in an earlier sensitivity study. Using the recoil mass spectrometer DRAGON at the TRIUMF-ISAC facility in Vancouver, Canada, we measured the main production reaction 40 Ca(α,γ)44 Ti, resulting in an increased reaction rate compared to the rate derived from previous prompt γ-ray studies, which is commonly used in supernova models. The uncertainty of the 44 Ti production is now dominated by the rate of reactions with short-lived nuclides around 44 Ti, namely 45 V(p,γ)46 Cr, 44 Ti(α,p)47 V and 44 Ti(α,γ)48 Cr. The sensitivity of these reactions on the 44 Ti production has been revisited.

VockenhuberNPAIII_preprint.pdf