Bulletin of the American Physical Society
2015 Fall Meeting of the APS Division of Nuclear Physics
Volume 60, Number 13
Wednesday–Saturday, October 28–31, 2015; Santa Fe, New Mexico
Session CC: Mini-Symposium on Nuclear Reactions for Astrophysics |
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Chair: Artemis Spyrou, Michigan State University Room: Sweeney Ballroom B |
Thursday, October 29, 2015 8:30AM - 9:06AM |
CC.00001: Direct measurements of radiative capture reactions with DRAGON Invited Speaker: Gregory Christian Direct measurements of radiative proton and alpha capture reactions are crucial for understanding nucleosynthesis in a variety of astrophysical environments, including classical novae, supernovae, X-Ray bursts, and quiescent stellar burning. Often the most important reactions have very low cross sections or involve unstable targets, making laboratory measurements extremely challenging. The detector of recoils and gammas of nuclear reactions (DRAGON) at TRIUMF is a recoil mass separator designed to measure radiative capture reactions in inverse kinematics, with beam suppression factors as high as $10^{16}$. When combined with the intense radioactive beams available at the ISAC-I facility, DRAGON's capabilities are unique and world-leading. In this talk, I will give a brief technical overview of DRAGON before presenting results from recent experiments. Some highlights include the first-ever direct measurement of $^{38}$K$(p,\gamma)^{39}$Ca, a crucial reaction for determining the endpoint of nova nucleosynthesis, and measurements of $^{76}$Se$(\alpha, \gamma)^{80}$Kr. The latter measurements determine the rate of the reverse reaction, $^{80}$Kr$(\gamma, \alpha)^{76}$Se, an important waiting point in the synthesis of the $p$-nuclei. I will also discuss future (and ongoing) developments at DRAGON, including the commissioning of a new chamber for high-precision elastic scattering measurements and plans to determine the $330$ keV resonance strength in $^{18}$F$(p,\gamma)^{19}$Ne via measurements of $^{15}$O$(\alpha,\gamma)^{19}$Ne and $^{15}$O$ + \alpha$ elastic scattering. [Preview Abstract] |
Thursday, October 29, 2015 9:06AM - 9:18AM |
CC.00002: T(T,4He)2n and 3He(3He,4He)2p Reactions and the Energy Dependence of Their Mechanisms Andrew Bacher, Dennis McNabb, Carl Brune, Dan Sayre, Gerry Hale, Johan Frenje, Maria Gatu Johnson We have studied the T(T,alpha)2n reaction because it is the charge symmetric analog to the 3He(3He,alpha)2p reaction which completes the most direct mode of the p-p chain in stellar interiors. These reactions lead to three-body final states whose energy spectrum shapes are dominated by the strong nucleon-alpha interaction and the weaker nucleon-nucleon interaction. These experiments were done at OMEGA at the University of Rochester and at the NIF at Lawrence Livermore Lab. We will focus on two features: (1) the excitation energy dependence of the reaction mechanism and (2) the center-of-mass energy dependence of the reaction mechanism. At stellar energies (OMEGA and the NIF) we find that the shape of the neutron spectrum peaks in the middle. The n-alpha 1/2- excited state is about two times stronger than the n-alpha 3/2- ground state. For the 3He+3He reaction (at CalTech), the proton spectrum peaks at the high end. The p-alpha 3/2- state is about two times stronger than the 1/2- state. This difference in the spectrum shape is explained by theoretical models which include the interference between the two identical fermions in the final state. At CalTech we have angular distributions of the 3He+3He reaction from 2 MeV to 18MeV. We see the p-wave strength increasing. [Preview Abstract] |
Thursday, October 29, 2015 9:18AM - 9:30AM |
CC.00003: $\alpha$-cluster ANCs at sub-Coulomb energies for nuclear astrophysics M.L. Avila, G.V. Rogachev, E. Koshchiy, L.T. Baby, J. Belarge, K.W. Kemper, A.N. Kuchera, A.M. Mukhamedzhanov, D. Santiago-Gonzalez, E. Uberseder Many important $\alpha$-particle induced reactions can only be measured indirectly due to small cross section at energies of astrophysical interest. Extracting the Asymptotic Normalization Coefficients (ANCs) using sub-Coulomb $\alpha$-transfer reactions can been used as an effective method to determine properties of near-threshold resonances to constrain and drastically limit the uncertainties related to extrapolations procedures for key astrophysical reactions. We have applied this valuable tool to investigate the $\alpha$-transfer reactions $^{16}$O($^6$Li,d)$^{20}$Ne, $^{13}$C($^6$Li,d)$^{17}$O and $^{12}$C($^6$Li,d)$^{16}$O. [Preview Abstract] |
Thursday, October 29, 2015 9:30AM - 9:42AM |
CC.00004: Studying $^{20}$Ne($\alpha$,p)$^{23}$Na directly with HELIOS Jianping Lai, Daniel Santiago-Gonzalez, Catherine Deibel, Amber Lauer, Liudmyla Afanasieva, Jeffrey Blackmon, Sergio Almaraz, Calem Hoffman, Benjamin Kay, Birger Back During nucleosynthesis ($\alpha$,p) reactions are important in a variety of astrophysical sites, including classical novae, X-ray bursts and supernovae. Direct measurements of these reaction rates are needed to reduce uncertainties and understand the nucleosynthesis in these stellar sites. Sensitivity studies indicate that the $^{20}$Ne($\alpha$,p)$^{23}$Na reaction contributes significantly to the energy output and nucleosynthesis abundances produced in Type Ia supernovae. Recently we performed a direct experimental study of the $^{20}$Ne($\alpha$,p)$^{23}$Na reaction with the HELIcal Orbit Spectrometer (HELIOS) at Argonne National Laboratory. A cryogenic gas target was implemented to produce a high-density $^{4}$He gas target and the heavy recoils were detected with a high counting rate gas ionization chamber in coincidence with the protons, which were detected in the HELIOS Si array. The reaction was measured through inverse kinematics with $^{20}$Ne beams at multiple energies. Promising results have been achieved. This experiment also serves as a stable beam proof-of-principle study for future direct measurements of other ($\alpha$,p) reactions using radioactive beam. Preliminary analysis will be presented. [Preview Abstract] |
Thursday, October 29, 2015 9:42AM - 9:54AM |
CC.00005: Constraint of the Astrophysical $^{26g}$Al(p,$\gamma$)$^{27}$Si Destruction Rate at Stellar Temperatures S.D. Pain The 1809-keV $\gamma$ ray from the beta decay of $^{26}$Al provides an unsurpassed opportunity for studying the ongoing nucleosynthesis within our Galaxy. A detailed understanding of the production and destruction rates for $^{26}$Al are required to quantitatively understand the $^{26}$Al signature; the $^{26}$Al(p,$\gamma$)$^{27}$Si reaction is a major destruction pathway at progenitor stellar temperatures. This reaction rate is determined by the properties of states near the proton threshold in $^{27}$Si, some of which are too low in energy for direct measurements of the $^{26}$Al(p,$\gamma$)$^{27}$Si rate with current beam intensities. We have measured mirror states in $^{27}$Al to inform the $^{27}$Si structure, via the $^{26}$Al(d,p)$^{27}$Al reaction in inverse kinematics using the ORRUBA and SIDAR arrays of silicon detectors. Spectroscopic information on the states populated in $^{27}$Al have been extracted and spectroscopic factors for the $^{27}$Si states have been determined by comparisons with shell-model-embedded-in-the-continuum calculations. Experimental results and the constrained reaction rate for massive-star nucleosynthesis will be presented.\\[4pt] [1]. S.D. Pain et al., PRL 114, 212501 (2015). [Preview Abstract] |
Thursday, October 29, 2015 9:54AM - 10:06AM |
CC.00006: Measurement of $^{67,68}$Zn Neutron Capture for the weak s-process Kevin T. Macon, Jeff C. Blackmon, B.C. Rasco, Aaron Couture, Shea Mosby, John M. O'Donnell, John L. Ullmann, Travis Baugher The observed abundance distributions for the heavy elements ($A>60$) are driven by neutron capture processes. The slow neutron capture process (the s-process) takes place on a timescale of tens of thousands of years and is responsible for the origin of about half the heavy elements. The weak s-process in particular occurs in massive stars and is responsible for the production of a major portion of the elements up to $A=90$. The s-process path follows close to the stable elements and most reactions can be directly studied in the laboratory using neutron beams. Precise measurements on specific isotopes with low neutron capture cross-sections ($\alt$100mb) in the mass $60 < A < 70$ region are important for abundance calculations. In the past decade, new capture measurements with calorimeters have seen large discrepancies with liquid scintillator time-of-flight measurements, requiring new measurements on isotopes with high scatter/capture cross-section ratios. I will present preliminary results from a recent measurement for $^{67,68}$Zn using the Detector for Advanced Neutron Capture Experiments at LANSCE. Improved capture cross-sections on these isotopes will significantly reduce uncertainties on the synthesis of elements in the weak s-process. [Preview Abstract] |
Thursday, October 29, 2015 10:06AM - 10:18AM |
CC.00007: Transfer Reaction Studies with JENSA P. Thompson, D.W. Bardayan, J.C. Blackmon, K.A. Chipps, U. Greife, L.E. Linhardt, A. Kontos, R.L. Kozub, M. Matos, F. Montes, S.D. Pain, S.T. Pittman, A. Sachs, H. Schatz, K.T. Schmitt, M.S. Smith The Jet Experiments in Nuclear Structure and Astrophysics (JENSA) gas jet target system was designed to provide a gas target that was pure, localized, and dense. Several commissioning experiments with the JENSA target, performed at Oak Ridge National Laboratory (ORNL), were undertaken to demonstrate the unique capability of JENSA for transfer reaction studies. JENSA has since completed its move from ORNL to the ReA3 reaccelerated beam hall at the National Superconducting Cyclotron Laboratory (NSCL). An overview of the JENSA design and operation will be presented, as well as a brief discussion of the experiments performed at ORNL with JENSA, with a focus on preliminary results from the 20Ne(p,t)18Ne commissioning experiment [Preview Abstract] |
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