### Session BE: Nuclear Astrophysics I

Chair: Richard Cyburt, NSCL Michigan State University
Room: Newport News Marriott at City Center Grand Salon V

 Thursday, October 11, 2007 2:00PM - 2:12PM BE.00001: Core Collapse Supernovae and the r-Process: An Amalgam of Current Ideas Matthew Meixner , Grant Mathews , James Wilson , Kaori Otsuki After several decades of research the sites for the rapid neutron capture process (i.e. the r-process) have not been unequivocally identified. However, it is generally agreed upon that the most likely candidates are type II supernovae. We will be using a renovated 1D core collapse supernova simulation that utilizes full general relativity and runs out to late times. In this talk we will discuss using this simulation in conjunction with the latest r-process nuclear abundance calculations. We will address the critical role played by the neutrino-energized high entropy bubble in the nucleosynthesis of heavy nuclei. Thursday, October 11, 2007 2:12PM - 2:24PM BE.00002: Neutrinos, Fission Cycling, and the r-process Joshua Beun , Gail McLaughlin , Rebecca Surman , Raph Hix Recent halo star abundance observations exhibit the presence of a consistent $r$-process pattern between the second, $A \approx 130$, and third, $A \approx 195$, peaks. This universal pattern is known as the main'' $r$-process. Using the neutrino-driven wind of the core-collapse supernova as our guide, we explore fission cycling and steady-$\beta$ flow as the driving mechanisms behind production of this main'' $r$-process. These mechanisms robustly produce the general structure of the main'' $r$-process. In the core-collapse supernova environment, neutrinos play an important role as they facilitate the explosion mechanism and influence the elemental composition of the outflow. As conditions must be more neutron-rich than current wind models predict for both fission cycling and steady-$\beta$ flow to occur, we examine wind environments under a variety of neutrino luminosities and effective temperatures. We find a reduction in the electron neutrino luminosity is necessary to allow for both fission cycling and steady-$\beta$ flow. This reduction may result from active-sterile neutrino oscillations or other new physics. Thursday, October 11, 2007 2:24PM - 2:36PM BE.00003: Big Bang Nucleosynthesis of $^6$Li and $^7$Li Grant Mathews , Motohico Kusakabe , Toshitaka Kajino , Takahashi Yoshida , Richard Boyd The $^6$Li abundance observed in metal poor halo stars exhibits a plateau similar to that for $^7$Li suggesting a primordial origin. However, the observed abundance of $^6$Li is a factor of $10^3$ larger and that of $^7$Li is a factor of 3 lower than the abundances predicted in the standard big bang when the baryon-to-photon ratio is fixed by WMAP. Here we show that both of these abundance anomalies can be explained by the existence of a long-lived massive, negatively-charged leptonic particle during nucleosynthesis. Such particles would capture onto the newly synthesized nuclei thereby reducing the reaction Coulomb barriers and opening new transfer reaction possibilities, which catalyze a second round of big bang nucleosynthesis. This novel solution to both of the Li problems can be achieved with or without the additional effects of stellar destruction. Thursday, October 11, 2007 2:36PM - 2:48PM BE.00004: Helium Burning Reaction Rate Uncertainties and Consequences for Supernovae C. Tur , A. Heger , S.M. Austin The triple alpha and $^{12}C(\alpha,\gamma)^{16}O$ reaction rates determine the carbon to oxygen ratio at the completion of core helium burning in stars, which, in turn, influences the later stellar burning stages. We explored the dependence of massive star evolution and nucleosynthesis yields on the experimental uncertainties in the triple alpha rate (10 to 12\%) and the $^{12}C(\alpha,\gamma)^{16}O$ rate (25 to 35\%) using full stellar models followed to core collapse and including supernova explosion. The production factors of medium-weight elements obtained by using the Lodders (2003) solar abundances for the initial star composition, rather than the abundances of Anders \& Grevesse (1989), provide a less stringent constraint on the $^{12}C(\alpha,\gamma)^{16}O$ rate. Variations within the current uncertainties in both reaction rates, however, induce significant changes in the central carbon abundance at core carbon ignition and in the mass of the supernova remnant. An experiment is being carried out by an NSCL/WMU collaboration to improve the accuracy of the triple alpha reaction rate. Thursday, October 11, 2007 2:48PM - 3:00PM BE.00005: Maxwellian-averaged cross sections and astrophysical reaction rates from ENDF/B-VII.0, JEFF-3.1, JENDL-3.3 and ENDF/B-VI.8 evaluated nuclear reaction data libraries Boris Pritychenko , Alejandro A. Sonzogni , Said F. Mughabghab Maxwellian-averaged cross sections and astrophysical reaction rates were calculated for (n,$\gamma )$, (n,p), (n,f), (n,$\alpha )$, (n,2n) and (n, t+2$\alpha )$ reactions from ENDF/B-VII.0, JEFF-3.1, JENDL-3.3 and ENDF/B-VI.8 evaluated nuclear reaction data libraries. Four major nuclear reaction libraries were processed under the same conditions for temperatures (kT) range from 1 keV to 1 MeV. Present results provide a set of independent benchmarks and complimentary nuclear data sets for the KADONIS nuclear astrophysics database that is currently under development. Current calculation of s-process nucleosynthesis nuclei is compared with previous data sets. Possible physics implications and differences between data sets are discussed. Thursday, October 11, 2007 3:00PM - 3:12PM BE.00006: A precision measurement of the $^{3}$He($\alpha$,$\gamma$)$^{7}$Be astrophysical S-factor T.A.D. Brown , K.A. Snover , D.W. Storm , C. Bordeanu , D. Melconian , A.L. Sallaska , S.K.L. Sjue , S. Triambak , A.M. Crisp , J.D. Lowrey , K. Michnicki , P. Peplowski , J. Sibille The $^{3}$He($\alpha$,$\gamma$)$^{7}$Be reaction is the gateway to the ppII and ppIII branches, providing the principle route to energetic neutrino production in the Sun. The uncertainty on the accepted value of S(0) for this reaction is currently the largest important nuclear physics uncertainty (+/- 10$\%$) in the Solar Model [1]. A more precise value of S$_{34}$(0) would bring an improvement in solar neutrino flux calculations, and in predictions of $^{7}$Li production in Big-Bang Nucleosynthesis which are currently significantly higher than observed $^{7}$Li abundances [2]. Precision measurements of S$_{34}$(E) have been made at eight different energies between E$_{CM}$ = 329 and 1235 keV, using the terminal ion source on the Van-de-Graaff accelerator at the University of Washington. The prompt gamma-ray yield and the $^{7}$Be activity have been measured at each energy in the same irradiation, permitting two different methods for determining S$_{34}$(E). This presentation discusses the experimental details of these measurements, the analysis of the data and our results for S$_{34}$(0). \newline \noindent [1] E. G. Adelberger \textit{et al.}, Rev. Mod. Phys. \textbf{70} 4 (1998) 1265 \newline [2] P. Bonifacio \textit{et al.}, Astron. Astrophys. \textbf{390} (2002) 91 \newline Thursday, October 11, 2007 3:12PM - 3:24PM BE.00007: Determination of the astrophysical S-factor for the $^{12}$N(p,$\gamma )^{13}$O reaction from ($^{12}$N,$^{13}$O) proton transfer reaction A. Banu , T. Al-Abdullah , C. Fu , C.A. Gagliardi , Y. Li , M. McCleskey , G. Tabacaru , L. Trache , R.E. Tribble , Y. Zhai , V. Burjan , F. Carstoiu The reaction rate for the radiative proton capture on the drip line nucleus $^{12}$N was determined using the indirect Asymptotic Normalization Coefficient (ANC) method. This reaction is important for studying the nucleosynthesis in Population III stars with low-metallicity. A 23 MeV/nucleon $^{12}$C primary beam from the K500 cyclotron at Texas A{\&}M University was employed. Secondary $^{12}$N beam of 2 $\times$ 10$^{5}$ pps was separated using the recoil spectrometer MARS. The $^{14}$N($^{12}$N,$^{13}$O)$^{13}$C proton transfer reaction at 12 MeV/nucleon was measured to extract the ANC for the virtual decay $^{13}$O-$>^{12}$N + p. The ANC was then used to determine the direct component of the astrophysical S-factor. The results of this measurement will be discussed. Thursday, October 11, 2007 3:24PM - 3:36PM BE.00008: New measurement of $\Gamma_\pi/\Gamma$ for the $0^+_2$ (7.65 MeV) state in $^{12}$C N.J. Goodman , J. Bos , J.C. Lighthall , S.T. Marley , J. Snyder , A.H. Wuosmaa , C. Tur , Sam M. Austin , E. Estrada , G. Lorusso The rate of $^{12}$C formation through the well known triple- alpha'' process is determined by the radiative partial width of the excited 0$^+$ state at 7.65 MeV in $^{12}$C. Experimentally, the uncertainty in this quantity is determined from the radiative branching ratio, the partial width for $e^+e^-$ decay, and the pair branching ratio. The current uncertainty in the 3-$\alpha$ rate is dominated by that for the $e^+e^-$ branching ratio which is 9.2\%. We have performed a new measurement of this quantity aimed at reducing this uncertainty to 5\%. 10.4 MeV protons from the Western Michigan University Tandem Van de Graaff accelerator bombarded a 100 $\mu$g/cm$^2$ $^{12}$C target, exciting the 0$^+$ 7.65 MeV state. Protons were detected at backward angles using two 1 mm thick silicon detectors, and coincident $e^+$ and $e^-$ were detected with an array of plastic-scintillator detectors where the sensitivity of the device to photons was reduced by dividing the detector into a thick outer block, and thin inner sleeve. The performance of the detector and preliminary results will be discussed. Work supported by the U. S. Department of Energy under contract number DE-FG02-ER41230 (WMU) and the U. S. National Science Foundation, contract numbers PHY06-06007 (MSU) and PHY02-16783 (JINA). Thursday, October 11, 2007 3:36PM - 3:48PM BE.00009: Measuring the Radiative Width of the Hoyle State in $^{12}$C S.A. Sheets , J.T. Burke , R.D. Hoffman , E.B. Norman , L.A. Bernstein , L.W. Phair , J. Gibelin , M. Wiedeking , R.M. Clark , E. Vieitez-Rodriguez , P. McMahan , I.Y. Lee , A.O. Macchiavelli , C. Beausang , S. Lesher , B. Darakchieva , M. Evtimova , B. Lyles , M. Dolinski , H. Ai In stellar nucleosynthesis the conversion of helium into heavier elements begins with the triple-$\alpha$ process, in which three $\alpha$ particles combine to form $^{12}$C. The rate of this process is governed by the 0$^+$ second excited state of $^{12}$C which provides a resonance for the $\alpha$+$^{8}$Be$\rightarrow$ $^{12}$C* at an excitation energy 7.65 MeV (the Hoyle state). Overwhelmingly, the 7.65 MeV state decays by $\alpha$ particle to $^8$Be which then breaks up into two $\alpha$ particles. However, there is a small radiative decay branch (approximately 4$\times10^{-4}$) which allows the excited $^{12}$C* nucleus to decay to its ground state. A new measurement of the ratio of the radiative width to the total width has been performed by the Lawrence Livermore National Laboratory and Lawrence Berkeley National Laboratory STARS/LIBERACE collaboration. Current results and the our experimental method will be presented. Thursday, October 11, 2007 3:48PM - 4:00PM BE.00010: The branching ratio of the sub-threshold 1$^{-}$ state in the $\beta$-decay of $^{16}$N K.E. Rehm , X.D. Tang , M. Carpenter , J.P. Greene , R.V.F. Janssens , L. Jisonna , C.L. Jiang , C.J. Lister , M. Notani , N. Patel , R.C. Pardo , G. Savard , J.P. Schiffer , R.E. Segel , A. Wuosmaa , S. Zhu A measurement of the $\beta$-delayed $\alpha$ decay of $^{16}$N can give information about the E1 component of the astrophysical S-factor of the $^{12}$C($\alpha$,$\gamma )^{16}$O reaction. The uncertainty in this measurement depends on many parameters, which are used in the fitting procedure. One of them is the ratio of the $\beta$ decay in $^{16}$N, populating the sub-threshold 1$^{-}$ state in $^{16}$O. We have performed a new measurement of this branching ratio using Gammasphere. The $^{16}$N particles were produced by bombarding a deuterium target with a $^{15}$N beam from the ATLAS accelerator at Argonne. From this experiment a new branching ratio has been obtained, which is about 10{\%} higher than the previous value. The implication of this new value on the S(E1) factor will be discussed. \textit{This work was supported in part by the U.S. Department of Energy, Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357 and by the NSF Grant No. PHY-02-16783 (Joint Institute for Nuclear Astrophysics).}