Gamma
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
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)
The DRAGON Facility at TRIUMF-ISAC: Development
The proposed DRAGON (Detector of Recoils And Gammas Of Nuclear reactions) facility at TRIUMF is uniquely designed to perform important nuclear astrophysics experiments with radioactive nuclear beams and the goal is to be operational when the new ISAC accelerated radioactive beams facility is operational in 2000. The projects required to meet this goal include the following:
a. Projects related to the development and commissioning of the facility, which are not covered by the TRIUMF infrastructure;
b. Projects related to developing and finalizing the concepts for the Phase II of the DRAGON facility, the Gamma Array;
c. Projects related to some specific experiments planned for the DRAGON.
A secondary goal is to develop a team of scientists dedicated to this new facility and knowledgeable of its operational idiosyncrasies.
This grant will fund the continuing work of the recoil group in commissioning the DRAGON and in the development of the required detectors, electronics, targets and beams.
Journal: Submitted to NSERC, Oct. 1997
A Gamma Ray Detector Array for DRAGON at ISAC
A prime justification for the accelerated beams facility (ISAC) at TRIUMF is the study of radiative capture reactions using radioactive beams. Some key reactions for heavy-element synthesis during explosive burning in stars involve radioactive reactants of short half-lives. Table 1 lists some of these astrophysically-interesting reactions of the initial ISAC program. The limited intensity of radioactive beams and the small cross sections for capture reactions require a detection system which has both high efficiency for detection of reaction products and highly effective suppression of beam background. The DRAGON system has been designed for radioactive beam capture studies at ISAC; with support from NSERC, TRIUMF and agencies outside Canada, construction is under way on a windowless gas target, electromagnetic mass separator (EMS) and recoil ion detector for DRAGON. This request is for an economical addition to DRAGON, a compact array of gamma-ray detectors, which will enhance its ability to suppress background and should provide additional information of significance both in performing the experiment and in interpreting the results.
Journal: Submitted to NSERC, Sept. 1999
The 40Ca(alpha,gamma)44Ti Reaction Using DRAGON
In our every day life we are surrounded by materials composed of the elements of the periodic table. Rarely does one ask where these elements came from. It has been a long process of discovery to understand the precise origin of many of the elements we consider commonplace. It is now believed that the Big-Bang produced only the lightest elements, primarily hydrogen and helium, and that heavier elements were synthesized as the product of nuclear reactions within stars. Occasionally the nuclear reactions that occur within stars produce an isotope of an element which is unstable, radioactive. When a radioactive species decays it emits radiation which is characteristic of the species that decayed. Satellites have been able to detect the characteristic radiation from the decay of several isotopes in the Milky Way. One in particular which has been detected is the isotope of titanium, 44Ti. The decay of 44Ti has been seen in the ashes of exploding stars, vast gas clouds termed supernova remnants. This isotope of titanium eventually decays to a stable isotope of calcium found everywhere on Earth from bones to chalk. It is believed that the bulk of the production in stars of 44Ti occurs as the star explodes, during the supernova. Calculations indicate that among the many possible reactions during a supernova, a particular nuclear reaction, where calcium captures a helium nucleus and fuses into titanium, is the main source of 44Ti. In this work it is detailed how using laboratory equipment on Earth one is able to shed light on the nuclear physics of this particular reaction governing the production of an isotope in our universe.
Journal:
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.
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.
Journal: J. Phys. G: Nucl. Part. Phys. 35 014034 (2008)
