Bulletin of the American Physical Society
2008 Annual Meeting of the Division of Nuclear Physics
Volume 53, Number 12
Thursday–Sunday, October 23–26, 2008; Oakland, California
Session MG: Astrophysics: Light Nuclei |
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Chair: Richard Cyburt, National Superconducting Cyclotron Laboratory Room: Jewett Ballroom C |
Sunday, October 26, 2008 10:30AM - 10:42AM |
MG.00001: Experiments to Further the Understanding of the Triple-$\alpha$ Process in Hot Astrophysical Scenarios N.R. Patel, U. Greife, K.E. Rehm, C.M. Deibel, J. Greene, D. Henderson, C.-L. Jiang, B.P. Kay, H.Y. Lee, M. Notani, R. Pardo, S.T. Marley, C.B. Segal, X.D. Tang The first 0$^{+}$ state of $^{12}$C at 7.654 MeV (the Hoyle state) is the most relevant in the triple-$\alpha$ process for carbon nucleosynthesis. In explosive scenarios such as supernovae, the interference of the Hoyle state with the second 0$^{+}$ state located at 10.3 MeV in $^{12}$C becomes significant. The recent NACRE listing assumes a 2$^{+}$ resonance at 9.117 MeV for which no experimental evidence exists. The states above 7.654 MeV level in $^{12}$C were populated through the $\beta$-decay of $^{12}$B and $^{12}$N produced at the ATLAS in-flight facility at ANL. The decay of $^{12}$C into three alphas is detected in a twin Frisch grid ionization chamber, acting as a calorimeter. This minimizes the effects of $\beta$-summing and allowed us to investigate the minimum above the Hoyle state with much higher accuracy than previously possible. A detailed data analysis will include an R-matrix fit to determine an upper limit on the 2$^{+}$ resonance. Work is supported by U.S. DOE, ONP under contracts DE-AC02-06CH11357 (ANL), DE-FG02-04R41320 (WMU), NSF grant PHY01-40324, and JINA NSF-PFC grant PHY02-16783. [Preview Abstract] |
Sunday, October 26, 2008 10:42AM - 10:54AM |
MG.00002: 12B(n,g) - The Influence on r-process Nucleosynthesis of Light Elements H.Y. Lee, C.L. Jiang, R.C. Pardo, K.E. Rehm, J.P. Schiffer, N.J. Goodman, J.C. Lighthall, S.T. Marley, A.H. Wuosmaa, M. Notani, X. Tang, N. Patel Astrophysical models predict enhanced abundances for heavy elements produced in the r-process by extending the reaction network to include light, neutron-rich nuclei (Z$\leq$10). The independent study in search for the most influential reactions producing the final r-process abundances emphasize the importance of improved experimental data, especially for (n,$\gamma$) reaction rates with unstable nuclei. Based on this analysis, $^{12}$B(n,$\gamma$)$^{13}$B is found to be one of most important light-mass nuclear reactions. We have measured the (d,p) reaction with radioactive $^{12}$B beam for the first time using the ATLAS in-flight facility at Argonne. The spectroscopic factors determined from this measurement will be used to compare to theoretical calculations and the astrophysical implications using experimentally determined reaction rates will be discussed. [Preview Abstract] |
Sunday, October 26, 2008 10:54AM - 11:06AM |
MG.00003: Reconstruction of $^{16}{\rm O}(\gamma,\alpha)^{12}{\rm C}$ Events in the HI$\gamma$S Optical Readout Time Projection Chamber S. Stave, M.W. Ahmed, E.R. Clinton, C.R. Howell, P.-N. Seo, H.R. Weller, M. Gai, A.H. Young, B. Bromberger, V. Dangendorf, K. Tittelmeier A new optical readout time projection chamber (O-TPC) is in use at the High Intensity $\gamma$-ray Source (HI$\gamma$S) located at the Free Electron Laser Laboratory on the Duke University campus. One application of the HI$\gamma$S O-TPC is the study of the inverse of the $^{12}{\rm C}(\alpha,\gamma)^{16}{\rm O}$ reaction which is important for stellar evolution theory. In the O-TPC, incoming $\gamma$-rays interact with an $^{16}{\rm O}$ nucleus producing an $\alpha$ and a $^{12}{\rm C}$ which then leave trails of ionization electrons along their path. The O-TPC then provides several signals each of which must be interpreted simultaneously to determine the different particle types, their energies and their directions. Part of the challenge of operating the O-TPC is integrating the fast signals from the photomultiplier tubes and drift chamber with the slow signals from the CCD camera which images the tracks. Automated routines have been developed to take all of the available track information and provide a reliable determination of the interaction cross section. Examples of the method and preliminary results will be presented. [Preview Abstract] |
Sunday, October 26, 2008 11:06AM - 11:18AM |
MG.00004: New Measurements of spectroscopic factors for low-lying $^{16}$N levels P.D. O'Malley, J.A. Cizewski, R. Hatarik, S.D. Pain, W.A. Peters, D.W. Bardayan, C.D. Nesaraja, M.S. Smith, J.C. Blackmon, K.L. Jones, S.T. Pittman, K.Y. Chae, S. Paulauskas, B.H. Moazen, R.L. Kozub, J.F. Shriner, Jr., C. Matei, K.A. Chipps The origin of galactic $^{19}$F is uncertain, but thought to be created via the $^{15}$N(a,g)$^{19}$F reaction. However, many of the reaction rates involved are not well known. In particular, the $^{15}$N(n,g)$^{16}$N reaction bypasses $^{19}$F production at a rate that depends on neutron spectroscopic factors of low-lying $^{16}$N levels. The previously measured values ($\sim $0.5) differed greatly from the expected values of near unity, based on Oxbash calculations for the closed neutron shell of $^{15}$N. Knowledge of these spectroscopic factors is also important for fine-tuning shell model calculations. We have performed a new study of the $^{15}$N(d,p)$^{16}$N reaction in inverse kinematics with 100-MeV $^{15}$N beam from the HRIBF on a CD$_{2}$ target. Reaction protons were detected using the SIDAR and ORRUBA silicon detector arrays. The data and analysis will be presented. *Work supported in part by DOE and NSF. [Preview Abstract] |
Sunday, October 26, 2008 11:18AM - 11:30AM |
MG.00005: Breakup of proton-rich nuclei $^{24}$Si, $^{23}$Al, $^{22}$Mg, $^{21}$Na at intermediate energies for reaction rates in explosive H-burning in novae A. Banu, L. Trache, R.E. Tribble, C.A. Gagliardi, N. Orr, M. Chartier, P. Roussel-Chomaz, F. Carstoiu, A. Bonaccorso We discuss the use of one proton-removal reactions of loosely bound nuclei at intermediate energies as an indirect method in nuclear astrophysics, with particular reference to the results of a GANIL experiment with a cocktail beam around $^{23}$Al at 50 MeV/nucleon. Momentum distributions of the core fragments, inclusive and in coincidence with gamma rays, were measured, from which we can determine mixing ratios in the structure of the ground states of the projectile nuclei. The method has the advantage that it can be used for beams of low quality, such as cocktail beams, and intensities as low as a few pps. The breakup reactions provide complementary information to the use of transfer reactions (the ANC method) which require radioactive beams of much better purity and intensity. Results on $^{23}$Al, $^{24}$Si, $^{22}$Mg and $^{21}$Na will be discussed. [Preview Abstract] |
Sunday, October 26, 2008 11:30AM - 11:42AM |
MG.00006: Study of the hindrance effect in sub-barrier fusion reactions Paul Davies, Ansel Hillmer, Antonios Kontos, Larry Lamm, Chi Ma, Edward Martin, Masahiro Notani, Francesco Raiola, Ed Stech, Wanpeng Tan, Xiaodong Tang In recent years, a hindrance model has been proposed to extrapolate heavy-ion fusion cross sections at sub-barrier bombarding energies. Compared to the single- and coupled-channel model, the hindrance model greatly reduces the fusion reaction rates in the stellar matter at temperatures T$\le $(3-10)x10$^{8}$ K. To test the hindrance model, we have measured the cross sections for the reaction $^{12}$C($^{13}$C,p)$^{24}$Na in the energy range E$_{c.m.}$= 3.08-4.80 MeV by counting the beta-gamma coincidence from the decay of $^{24}$Na which has a half life of 15 hours. Preliminary results will be presented. [Preview Abstract] |
Sunday, October 26, 2008 11:42AM - 11:54AM |
MG.00007: Probing Nucleosynthesis in Novae: $^{22}$Na(p,$\gamma$)$^{23}$Mg A.L. Sallaska, D.W. Storm, A. Garcia, T.A.D. Brown, K.A. Snover, C. Wrede, K. Deryckx, C. Ruiz, D.A. Hutcheon, L. Buchmann, D.F. Ottewell, C. Vockenhuber, J.A. Caggiano Orbiting gamma ray telescopes have yet to observe the elusive $^{22}$Na isotope. More sensitive observatories are planned, and present uncertainties in the dominant destructive reaction, $^{22}$Na(p,$\gamma$), suggest new measurements in the proton energy range of 150 to 300 keV are needed to clarify the predictions of the amount of $^{22}$Na expected in a nova explosion. In particular, a state in $^{23}$Mg reported by Jenkins [1] implies a possible resonance at E$_p$ = 198 keV which could be significant. We are in the process of measuring the $^{22}$Na(p,$\gamma$) reaction rate directly by using protons from the UW tandem on a specially designed beamline, which includes rastering and cold vacuum protection of the $^{22}$Na implanted targets, fabricated at TRIUMF. A multitude of target tests have been performed with stable $^{23}$Na, focusing on sodium stability with respect to sputtering and heating caused by our high intensity beam. Utilizing two 100$\%$ Ge detectors with anticoincidence shields to reduce cosmic backgrounds, preliminary measurements have been performed on known resonances of $^{22}$Na, as well as on the proposed new resonance. Results will be presented. [1] Jenkins $\it{et}$ $\it{al}$., PRL $\bf{92}$ (2004) 031101 [Preview Abstract] |
Sunday, October 26, 2008 11:54AM - 12:06PM |
MG.00008: Lowest l=0 Proton-Resonance in 26Si and Implications for Nucleosynthesis of 26Al P.N. Peplowski, L.T. Baby, E. Diffenderfer, P. Hoflich, N. Keeley, A. Rojas, A. Volya, I. Wiedenhover The first successful experiment to determine the 25Al (p,gamma)26Si reaction rate using a radioactive beam of 25Al is presented here. The experiment was carried out using the new in-flight radioactive beam production facility, known as RESOLUT, at The Florida State University. The analogous single proton transfer reaction d(25Al,26Si)n was measured. Details of the RESOLUT beamline and detection scheme for the experiment will be discussed. Results from this experiment, including implications for the rp-process and stellar nucleosynthesis of 26Al will be presented. [Preview Abstract] |
Sunday, October 26, 2008 12:06PM - 12:18PM |
MG.00009: Measurement of Low Energy Resonances in $^{31}$P(p,$\alpha$)$^{28}$Si B.H. Moazen, C. Matei, D.W. Bardayan, J.C. Blackmon, K.Y. Chae, K.A. Chipps, R. Hatarik, K.L. Grzywacz, R.W. Kapler, R.L. Kozub, M. Matos, C.D. Nesaraja, S.D. Pain, T. Pelham, W.A. Peters, S.T. Pittman, J.F. Shriner Jr., M.S. Smith The (p,$\alpha$) reactions on T=1/2 nuclei like $^{23}$Na, $^{27}$Al, $^{31}$P, and $^{35}$Cl, and the competing (p,$\gamma$) reactions are important for understanding the reaction flow to heavier elements in the rp-process. Previous rate calculations of the $^{31}$P(p,$\alpha$)$^{28}$Si reaction were based on indirect information gained from studies of the $^{31}$P($^{3}$He,d)$^{28}$S reaction [1]. At ORNL, we measured the energy and strength of the 371 and 599 keV resonances in $^{31}$P(p,$\alpha$)$^{28}$Si using a technique previously employed for an $^{17}$O(p,$\alpha$)$^{14}$N study[2]. A beam of $^{31}$P bombarded hydrogen gas which filled a large, differentially pumped scattering chamber at a pressure of 3 Torr. The alpha particle and $^{28}$Si recoil were detected in coincidence and the reaction vertex was determined using the relative kinematics of the reaction products. The experimental setup and preliminary results will be presented. [1] Ross $\it et$ $\it al$., Phys. Rev C 52, 1681 (1995) [2] B. H. Moazen $\it et$ $\it al$., Phys. Rev. C 75, 065801 (2006) ORNL is managed by UT-Battelle for the US DOE [Preview Abstract] |
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