Bulletin of the American Physical Society
2006 Division of Nuclear Physics Annual Meeting
Wednesday–Saturday, October 25–28, 2006; Nashville, Tennessee
Session BE: Mini-symposium on Experimental Techniques in Low Energy Nuclear Astrophysics Studies |
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Sponsoring Units: DNP Chair: Michael Wiescher, University of Notre Dame Room: Gaylord Opryland Hermitage B |
Thursday, October 26, 2006 2:00PM - 2:36PM |
BE.00001: Experimental nuclear astrophysics underground Invited Speaker: Cross section measurements for quiescent stellar H and He burning are hampered mainly by extremely low counting rate and cosmic background. Some of the main reactions of H-burning phase have been measured at the LUNA facility (Laboratory for Underground Nuclear Astrophysics) taking advantage of the very low background environment of the Underground Gran Sasso National Laboratory in Italy. An overview of the adopted experimental techniques will be given together with the latest results on the $^{14}$N(p,$\gamma)^{15}$O reaction and the status of the ongoing $^{3}$He($^{4}$He,$\gamma )^{7}$Be experiment. Furthermore a brief summary of possible future experimental methods coupling low background environment and detector techniques will be presented. [Preview Abstract] |
Thursday, October 26, 2006 2:36PM - 2:48PM |
BE.00002: Cross sections for reactions in explosive H burning from indirect methods Livius Trache, T. Al-Abdullah, A. Banu, C. Fu, C.A. Gagliardi, J.C. Hardy, V.E. Iacob, M. McCleskey, A.M. Mukhamedzhanov, G. Tabacaru, R.E. Tribble, Y. Zhai We present results for the cross sections of radiative proton capture reactions relevant for explosive H burning in stars, extracted from a number of indirect techniques using stable or radioactive nuclear beams. We use or combine proton transfer reactions above the Coulomb barrier, breakup of loosely bound proton rich nuclei at intermediate energies, and beta-decay studies to extract nuclear information needed to determine capture cross sections at very low energies. The extraction of ANC from proton transfer reactions around 10 MeV/u will be briefly discussed with examples from the latest measurements at the K500 superconducting cyclotron. Studies of the breakup of $^{9}$C and $^{23}$Al will be used to exemplify the method and its spectroscopic power, and to assess the astrophysical S-factors for the $^{8}$B(p,$\gamma $)$^{9}$C and $% ^{22}$Mg(p,$\gamma $)$^{23}$Al reactions, respectively. Finally, we will show how the results of a $\beta $-decay study of pure samples of $^{23}$Al separated with MARS can be used to constrain the direct contribution to the reaction rate for $^{22}$Mg(p,$\gamma $)$^{23}$Al and to determine resonant contributions for the $^{22}$Na(p,$\gamma $)$^{23}$Mg. These reactions are considered candidates to explain why space-based gamma-ray telescopes do not observe $\gamma $-rays from the decay of long-lived $^{22}$Na formed in ONe novae explosions: flux is diverted from the A=22 into the A=23 mass chain. [Preview Abstract] |
Thursday, October 26, 2006 2:48PM - 3:00PM |
BE.00003: Measurement of capture reactions with the recoil mass spectrometer DRAGON Christof Vockenhuber DRAGON is a state-of-the-art recoil mass spectrometer located at the radioactive beam facility ISAC at TRIUMF in Vancouver/Canada. It is designed to measure proton and alpha-capture reactions of light nuclei in inverse kinematics. In the last few years several astrophysically important reactions have been successfully measured, among them $^{21}$Na(p,$\gamma )^{22}$Mg and $^{26g}$Al(p,$\gamma )^{27}$Si using the high intensity radioactive beams available at ISAC. In addition, reactions with stable beams have been also investigated. Besides the astrophysical context, the wider range of available beams and the higher intensities allow to explore the limits of the current setup. With the recently measured $^{40}$Ca($\alpha $,$\gamma )^{44}$Ti reaction we could demonstrate the capabilities of DRAGON in the mass 40 range. Another important reaction is $^{12}$C($\alpha $,$\gamma )^{16}$O which makes high demands on the acceptance of the recoil spectrometer. In this talk I will report on our experience with these for astrophysics important, but difficult measurements. [Preview Abstract] |
Thursday, October 26, 2006 3:00PM - 3:12PM |
BE.00004: Experimental techniques to investigate astrophysical key reactions at the extremes. Wolfgang Hammer In order to explore stellar evolution and nucleosynthesis the reaction rates of the relevant nuclear reactions must be determined at burning temperatures. This means investigating the reactions down to a pikobarn or subpikobarn level. To achieve this, all important experimental parameters have to be optimized, which are: high primary beam intensity, targets of high chemical and isotopic purity and high stability, high detection efficiency and resolution for the reaction products, appropriate means to reduce disturbing background of any kind, sufficient long measuring time. The key reaction $^{12}$C($\alpha,\gamma$)$^{16}$O was investigated using enriched solid targets, which could withstand beam powers of up to 10\,kW/cm$^2$ and beam currents of up to 800\,$\mu$A. The angular distributions of the $\gamma$'s were measured using a close 4$\pi$ Ge-detector array or the GANDI array of high efficient Ge-detectors on a turntable. The background was suppressed by active shielding. Thus the $E1$ and $E2$ S-factors could be determined in the energy range $E_{\rm c.m.}$\,=\,890\,--\,2800\,keV, enabling the extrapolation of the reaction rate with 25\,\% accuracy. \\ The astrophysical key neutron source $^{22}$Ne($\alpha,n$)$^{25}$Mg was investigated using the windowless recirculating gas target RHINOCEROS with gas purification in the main stream. The neutrons were detected by a 4$\pi$ detector with an absolute efficiency of 50\,\%. The excitation function was determined down to the threshold with a sensitivity at the 10$^{-11}$\,b level. The uncertainty of the reaction rate was reduced considerably. [Preview Abstract] |
Thursday, October 26, 2006 3:12PM - 3:24PM |
BE.00005: Measuring the Radiative Width of the Hoyle State in $^{12}$C J.T. Burke, R.D. Hoffman, E.B. Norman, L.A. Bernstein, R. Macri, 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, S. Sheets, H. Ai Helium burning is possibly the most important burning phase for stellar nucleosynthesis. The two main products are carbon, produced via the 3$\alpha $ reaction, and oxygen by $^{12}$C($\alpha $,$\gamma )^{16}$O. The 3$\alpha $ reaction represents the start of heavy element production in stars. The fortuitous resonance formed by $^{8}$Be and an alpha particle allows the creation of $^{12}$C$^{\ast }$ (the Hoyle state at 7.65 MeV). Overwhelmingly $^{12}$C$^{\ast }$ decays by emitting an alpha particle, followed by the break up of $^{8}$Be into two alpha particles. Fortunately, there is a small radiative decay branch (approximately 4 x 10$^{-4})$ which allows the excited $^{12}$C$^{\ast }$ 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. Our current results and experimental method will be presented. This work was sponsored by UC-LLNL under Contract No. W-7405-Eng-48 and Grant Nos. DE-FG-05NA25929, DE-FG52-06NA26206, and DE-FG02-05ER41379. [Preview Abstract] |
Thursday, October 26, 2006 3:24PM - 3:36PM |
BE.00006: Nuclear Astrophysics at the LENA facility: The $\gamma$-ray detection system. Richard Longland, Christian Iliadis, Arthur Champagne, Chris Fox, Joe Newton Details of the detection system used at The Laboratory for Experimental Nuclear Astrophysics is described, including methods for measuring weak capture-$\gamma$-ray resonances. $\gamma\gamma$-coincidence techniques with a large solid angle NaI(Tl) annulus are described, as well as their effects on background count rates in the energy regions of interest at LENA. In order to reduce the background further, cosmic muon induced counts can be decreased with the aid of an anti-coincidence plastic scintillator shield. In order to create a compact detection system, a novel, wavelength shifting fibre method of light readout has been used. These techniques are shown to reduce background count rates significantly for cascade decays in our regions of interest, and are shown to have a significant improvement over our previous results. [Preview Abstract] |
Thursday, October 26, 2006 3:36PM - 3:48PM |
BE.00007: Q-Value Gating Techniques for Gamma-ray Background Reduction Aaron Couture, Joachim Goerres, Elizabeth Strandberg, Michael Wiescher For certain classes of experiments, the limiting background comes from beam induced reactions on the nucleus of interest rather than external sources. Techniques using a combination of high-efficiency and high-resolution detectors were developed for separating the $^{19}$F(p,$\gamma$) and $^{19}$F(p,$\alpha\gamma$) reactions--both of which are important for quiescent and explosive CNO burning. The measurement will be compared to previous attempts with particular emphasis on the advantages Q-value gating offers in an inherently high-background scenario. [Preview Abstract] |
Thursday, October 26, 2006 3:48PM - 4:00PM |
BE.00008: Sub-Coulomb alpha transfer reactions in Nuclear Astrophysics Grigory Rogachev Prohibitively small cross section of nuclear reactions at energies, relevant for nuclear astrophysics, require application of indirect techniques to deduce the reaction rates from experimental data. One such technique is sub-coulomb $\alpha$ transfer reaction ($^6$Li,d), which can be used to measure the $\alpha$ particle Asymptotic Normalization Coefficients (ANCs) of sub and near threshold resonances. ANCs, obtained in $\alpha$ transfer reaction, performed at energy below the Coulomb barrier in both exit and entrance channels are model independent. This leads to a reliable evaluation of resonant component contribution into the total rate of nuclear reaction, which involves $\alpha$ particle capture. For example, the rate of $^{13}$C($\alpha$,n) reaction, which is considered to be the main source of neutrons for s-process in AGB stars, is uncertain by $\sim$300\% at stellar temperatures due to large uncertainty, associated with the structure of 1/2$^+$ resonance at 6.356 MeV in $^{17}$O. Measurements of the ANC (Asymptotic Normalization Coefficient) of this resonance were performed, using $\alpha$-transfer reaction $^{13}$C($^{6}$Li,d)$^{17}$O at sub-coulomb energies. The uncertainty of $^{13}$C($\alpha$,n) reaction rate at Stellar temperatures was reduced to 20\%. Possibility of application of sub-coulomb $\alpha$ transfer reaction for evaluation of rates of other astrophysically important nuclear reactions will be discussed. [Preview Abstract] |
Thursday, October 26, 2006 4:00PM - 4:12PM |
BE.00009: The Notre Dame recoil separator Manoel Couder, Georg P. Berg, Joachim Goerres, Larry O. Lamm, P.J. Leblanc, Edward Stech, Michael Wiescher Studies of ($p$,$\gamma$) and ($\alpha$,$\gamma$) reactions at low energies provide crucial information to improve our interpretation of the observed isotopic abundances, to predict the energy production and the time scale of nucleosynthesis processes during the stellar evolution and explosive events. While many radiative captures measurements have been made using various setup in direct kinematics, the very small cross section at astrophysically interesting energy of these reactions and the beam induced background limit the possible range of measurements. Reversing the kinematics and using a recoil separator to reject the beam ions which did not react in the target and to guide the reaction products to a detector is a good solution. Such a device aimed at low energy ($\alpha$, $\gamma$) measurements with stable beams is under development at the University of Notre Dame. In this talk I will present the concept of this new facility that is based on proven principles in existing devices. I will discuss the challenges encountered in low energy measurements and the solutions that we are pursuing. [Preview Abstract] |
Thursday, October 26, 2006 4:12PM - 4:24PM |
BE.00010: Alpha-induced reactions in stellar burning Joachim Goerres Alpha-induced reactions play an important role in a variety of astrophysical environments. They provide the neutron sources for the main s-process which takes place in highly convective AGB stars and for the weak process during core Helium burning in massive stars. In addition, alpha induced reactions on 15O and 18Ne provide a break-out from the CNO cycle which is important for the dynamics of explosive Hydrogen burning. To illuminate experimental difficulties in determinating reaction rates results of recent and ongoing experiments will be presented and future developments at the Nuclear Structure Laboratory at Notre Dame will be discussed. [Preview Abstract] |
Thursday, October 26, 2006 4:24PM - 4:36PM |
BE.00011: Prospects for Improved Measurements of the S-Process Neutron Source Reactions Carl R. Brune The ${}^{13}{\rm C}(\alpha,n){}^{16}{\rm O}$ reaction is thought to be the main s-process neutron source, taking place in AGB starts at temperatures around $10^8$~K. The ${}^{22}{\rm Ne}(\alpha,n){}^{25}{\rm Mg}$ reaction is also thought to be an important neutron source and takes place in more massive stars at somewhat higher temperatures of $(2-3)\times 10^8$~K. Both reaction rates are uncertain at astrophysical temperatures due to the difficulty of measuring the low cross sections. In the case of ${}^{13}{\rm C}(\alpha,n){}^{16}{\rm O}$, measurements exist down to $E_{c.m.}=300$~keV but the extrapolation to the needed range of 150-200 keV is complicated by subthreshold resonances. The ${}^{22}{\rm Ne}(\alpha,n){}^{25}{\rm Mg}$ reaction rate is dominated by narrow resonances -- the possibility of as-yet-unobserved resonances near threshold leads to significant uncertainty in this reaction rate. The prospects for improved data using high-intensity beams, inverse kinematics, and background reduction techniques will be discussed. [Preview Abstract] |
Thursday, October 26, 2006 4:36PM - 4:48PM |
BE.00012: CLAIRE - A Novel Nuclear Astrophysics Accelerator Facility at the Lawrence Berkeley National Laboratory Daniela Leitner, Damon Todd, Paul Vetter, Matthaeus Leitner, Reina Maruyama, Kevin Nan Xu CLAIRE (Center for Low Energy Astrophysics and Interdisciplinary REsearch) is a proposed nuclear astrophysics accelerator facility to be built at the 88-Inch Cyclotron at LBNL. Its primary goal will be to measure cross sections relevant to stellar burning. In particular, our focus is to build a facility powerful enough to measure the $^{3}$He($^{4}$He, $\gamma )^{7}$Be cross section near the Gamov peak. In its first phase, this facility will consist of a high-power high-voltage platform (100mA $^{3}$He$^{+}$ beam at 50 keV to 300 keV), four solenoid lenses, and one 60$^{\circ}$ bend magnet for mass separation. The high current ($>$100mA) $^{3}$He$^{+}$ beam will be delivered at sub-centimeter diameter onto the cooled high density gas jet $^{4}$He target. Germanium detectors will be used to detect the $\sim $1 MeV $\gamma $ line resulting from this reaction. As a possible second phase, a short linear post accelerator section could be added to widen the applicability of the facility to cover a higher range of energies of interest for stable ion beam astrophysics cross sections in the CNO and NaNe cycle. The preliminary layout of this new experimental facility will be shown and discussed. [Preview Abstract] |
Thursday, October 26, 2006 4:48PM - 5:00PM |
BE.00013: Proposed facility for a precise measurement of S34 Paul Vetter, Daniela Leitner, Reina Maruyama, Matthaeus Leitner, Damon Todd We propose a new measurement of the cross section for the direct capture reaction $^ {3}$He($\alpha$,$\gamma$)$^{7}$Be at a total uncertainty of 1\%. Current and planned solar neutrino experiments seek to measure the total $^{7}$Be neutrino flux at this level. A direct comparison at 1\% of measured to calculated neutrino flux can constrain models of non-standard neutrino oscillation or ``invisible" energy production, and will also constrain the Standard Solar Model, acting as a gauge of the temperature and metallicity of the sun and other main sequence stars. A new accelerator, designed for low energy $<$ 300~keV, with high current ($\approx$ 100~mA) and beam purity is necessary to improve the statistical power of measurements at low energy (at or near the solar Gamow window) to control potential systematic errors. To control target error sources, a favorable design would have a tight final focus, intersecting a dense gas jet target. High resolution germanium detectors can provide good signal to background recovery in direct capture reactions. We are currently designing such an accelerator facility. The proposed facility could be used to measure several direct capture reactions in the H, He, and CNO burning cycles. We will review the limiting uncertainties (experimental and theoretical) which constrain our design, and the current status of the project. Supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. [Preview Abstract] |
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