Bulletin of the American Physical Society
2006 Division of Nuclear Physics Annual Meeting
Wednesday–Saturday, October 25–28, 2006; Nashville, Tennessee
Session CE: Nuclear Astrophysics I |
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Sponsoring Units: DNP Chair: Ernst Rehm, Argonne National Laboratory Room: Gaylord Opryland Hermitage B |
Friday, October 27, 2006 9:00AM - 9:12AM |
CE.00001: Measurement of 7Be+p elastic and inelastic scattering R.J. Livesay, D.W. Bardayan, J.C. Blackmon, K.Y. Chae, A.E. Champagne, C. Deibel, R.P. Fitzgerald, U. Greife, K.L. Jones, R.L. Kozub, Z. Ma, C.D. Nesaraja, S.D. Pain, F. Sarazin, J.F. Shriner Jr., D.W. Stracener, M.S. Smith, J.S. Smith, D.W. Visser, C. Wrede, M.S. Johnson We have measured $^{7}$Be+p elastic and inelastic scattering cross sections at the Holifield Radioactive Ion Beam Facility (HRIBF) at ORNL. Beams of isotopically pure $^{7}$Be bombarded thin (100 $\mu$g/cm$^{2}$) polypropylene targets; scattered protons were detected in an array of silicon strip detectors. Cross sections were measured at 17 bombarding energies ranging from E$_{cm}$=0.5 to 3.4 MeV. The data at each energy were normalized using $^{7}$Be+Au elastic scattering from a combined target of polypropylene and gold. [Preview Abstract] |
Friday, October 27, 2006 9:12AM - 9:24AM |
CE.00002: Study of the $\beta$-decay of $^{11}$Li at ISAC/TRIUMF C. Mattoon, F. Sarazin, C. Andreoiu, P.E. Garrett, G.F. Grinyer, C.E. Svensson, A. Andreyev, G.C. Ball, R.S. Chakrawarthy, G. Hackman, A.C. Morton, C. Pearson, M.B. Smith, R.A.E. Austin, D. Cross, D. Melconian, J. Ressler, E.S. Cunningham, J. Daoud, J. Schwarzenberg The $\beta$-decay of $^{11}$Li was investigated at ISAC/TRIUMF with the 8$\pi$ spectrometer, an array of 20 Compton-suppressed HPGe detectors. Doppler-broadened line shapes appear in the $\beta$-decay spectrum, arising from the decay of excited states of $^{10}$Be populated by $\beta$- delayed one-neutron emission. A Monte Carlo simulation for these line shapes was developed, permitting the analysis of excited states in $^{10}$Be and neutron emitting states in $^{11}$Be . This experiment improves on a previous work [F. Sarazin et al., Phys. Rev. C 70 (2004) 031302R] through greater $^{11}$Li yield and the addition of Sceptar, a plastic scintillating array in the inner volume of the 8$\pi$. Analysis of the higher quality line shapes from the new experiment should help resolve discrepancies observed between the previous work and [Y. Hirayama et al., Phys.Lett. B 611 (2005) 239] (an experiment using polarized $^{11}$Li and time-of-flight neutron detectors) and possibly lead to new insights in the $\beta$-decay of $^{11}$Li. This work is partially supported by the US Department of Energy through Grant/ Contract No. DE-FG03-93ER40789. [Preview Abstract] |
Friday, October 27, 2006 9:24AM - 9:36AM |
CE.00003: Improving the Rate of the Triple Alpha Reaction C. Tur, A. Wuosmaa, S.M. Austin, J. Lighthall, S. Marley, N. Goodman, J.J. Bos, A. Heger, S.E. Woosley, R. Hoffman The rate of the triple alpha process is known with an accuracy of about $12\%$. Variations within those errors can significantly change the size of the iron core in core-collapse supernovae and double the surface abundance of $^{12}C$ in light ABG stars. Studies being done using the code KEPLER show that changing the triple alpha rate or the rate of the $^{12}C (\alpha,\gamma)^{16}O$ reactions can significantly affect the production of the medium weight elements in core-collapse supernova progenitors. Hence, the experiment aims at reducing the uncertainty on this rate to about $6\%$ through an accurate measurement of the pair branch for the Hoyle state excited through inelastic scattering of 10.6 MeV protons from the Tandem accelerator at WMU. The pair branch is given by the ratio of the number of $e^+e^-$ pairs in the plastic scintillators in coincidence with protons scattered at 135 degrees in the lab to the total number of such protons. The gamma ray background is considerably reduced by a coincidence requirement between a thin scintillator tube and the large block of scintillator surrounding it. The experimental status will be presented. [Preview Abstract] |
Friday, October 27, 2006 9:36AM - 9:48AM |
CE.00004: A New Measurement of the E1 Component of the $^{12}$C($\alpha,\gamma$)$^{16}$O Reaction X.D. Tang, M. Notani, K.E. Rehm, I. Ahmad, J. Greene, A.A. Hecht, D. Henderson, R.V.F. Janssens, C.L. Jiang, E.F. Moore, N. Patel, R.C. Pardo, G. Savard, J.P. Schiffer, S. Sinha, M. Paul, L. Jisonna, R.E. Segel, C. Brune, A. Champagne, A. Wuosmaa Durin the past few years we have been involved in a measurement of the E1 component of the $^{12}$C($\alpha,\gamma$)$^{16}$O reaction. Using a new approach with a set of high acceptance ionization chambers, we have measured the beta-delayed alpha decay in $^{16}$N. The subthreshold 1$^-$ state, which dominates the S-factor S(E1) at astrophysical energies, produces a small interference peak in the alpha spectrum, whose strength is sensitive to S(E1). The data have been analyzed using extrapolations obtained from R-matrix theory. The results from two independent runs will be presented and compared to previous experiments. The contributions from systematic uncertainties as well as the sensitivity of S(E1) to various R-matrix parameters will be discussed. This work was supported by the US Department of Energy, Nuclear Physics Division, under contract No. W-31-109-ENG-38 and by the NSF Grant No. PHY-02-16783 (Joint Institute for Nuclear Astrophysics). [Preview Abstract] |
Friday, October 27, 2006 9:48AM - 10:00AM |
CE.00005: A Study of the $^{13}$C($\alpha $,n) Reaction Rate Through the ANC Technique Eric Johnson, Grigory Rogachev, Lagy Baby, Warren Cluff, Amy Crisp, Eric Diffenderfer, Bert Green, Trisha Hinners, Calem Hoffman, Kirby Kemper, Olexander Momotyuk, Patrick Peplowski, Akis Pipidis, Rob Reynolds, Brian Roeder, Akram Mukhamedzhanov, V. Gol'dberg, Simon Brown The $^{13}$C($\alpha $,n) reaction is the main source of neutrons for the s-process. Currently the adopted rate has an uncertainty of $\sim $300{\%}[C. Angulo et al., Nucl. Phys. A656, 3 (1999)] at the relevant stellar temperatures($\sim $10$^{8}$ K). This leads to a large uncertainty in the modeling of AGB stars, which is where the s-process occurs. Recently, we measured the ANC of the $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} ^{+}$, 6.356 MeV, near threshold state in $^{17}$O. This was done via the $\alpha $-transfer reaction $^{13}$C($^{6}$Li,d)$^{17}$O($\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} ^{+}$, 6.356) at sub-Coulomb energies. Using this information we were able to calculate the contribution of the $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} ^{+}$ state to the astrophysical S-factor. From our S-factor curve we calculated that the $^{13}$C($\alpha $,n) reaction rate is reduced by a factor of 3, also the associated uncertainty is improved to $\sim $15{\%}[E.D. Johnson et al., currently under review with PRL]. [Preview Abstract] |
Friday, October 27, 2006 10:00AM - 10:12AM |
CE.00006: Single and Double Protons from the $^{14}$O+$\alpha $ Interaction C. Fu, V.Z. Goldberg, G.V. Rogachev, G.G. Chubarian, M. McCleskey, Y. Zhai, T. Al-Abdullah, L. Trache, A. Banu, R.E. Tribble The proton production in the $^{14}$O+$\alpha $ interaction is important because it determines the onset of the high-temperature rp-process. The 2p decay is of current interest in relation with a possibility of a correlated 2p-pair decay from the excited states in $^{20}$Ne. There are evident experimental difficulties to obtain reliable information on each of the processes in question. To solve these problems we produced an $^{14}$O beam at the K500 Cyclotron at Texas A{\&}M University using MARS. A system of double photmultiplyers looking at thin plastic scintillators provided for information on the intensity of the $^{14}$O beam, beam contaminations, and a ``start'' signal for the proton identification by the time of flight method. The $^{4}$He($^{14}$O,p) and $^{4}$He($^{14}$O,2p) reactions were studied using the Thick Target Inverse Kinematics method. TOF between Si detectors and the PM system provided for the overall time resolution ($\sim $1ns), which was enough for reliable identification of single protons from $\alpha $ particles as well as for the identification for proton decay from the highly excited states in $^{17}$F. The double proton events were detected as coincidence events in a system of 16 Si detectors. Over 4000 double proton events were accumulated which showed strong correlation between energies of the protons. [Preview Abstract] |
Friday, October 27, 2006 10:12AM - 10:24AM |
CE.00007: Trojan Horse as indirect technique in nuclear astrophysics A. Mukhamedzhanov, C. Spitaleri, R.E. Tribble, M. La Cognata, S. Cherubini, V. Crucill, Chango Fu, V. Gol'dberg, L. Lamia, R.G. Pizzone, R.G. Pizzone, S. Romano, G. Tabacaru, L. Trache, A. Tumino The Trojan Horse method (THM) is a powerful indirect technique which allows one to determine the astrophysical factor for rearrangement reactions with bare nuclei (i. e. without electron screening) down to zero energy. We will present the latest results for the astrophysical factor for the resonant reaction $^{15}$N(p,$\alpha$)$^{12}$C determined using the Trojan Horse reaction $^2$H($^{15}$N,$\alpha$$^{12}$C)n at E$_{beam}=60$ MeV. The measurements have been done at Texas A\&M University in collaboration Catania National Lab- Texas A\&M University. The astrophysical $S$ factor is compared with the direct data in the same energy region. A fair agreement is found down to $80$ keV, while the low-energy behaviour of the $S$ factor suggests a smaller rate than reported in literature. [Preview Abstract] |
Friday, October 27, 2006 10:24AM - 10:36AM |
CE.00008: The $^{17}$O(p,$\alpha$)$^{14}$N reaction measured using a novel technique B.H. Moazen, J.C. Blackmon, D.W. Bardayan, K.Y. Chae, K. Chipps, C.P. Domizioli, R. Fitzgerald, U. Greife, K.L. Jones, R.L. Kozub, R.J. Livesay, C.D. Nesaraja, S.D. Pain, J.F. Shriner Jr., M.S. Smith, J.S. Thomas The $^{17}$O(p,$\alpha$)$^{14}$N reaction is important for understanding nucleosynthesis in giant stars and in novae. We developed a new approach for measuring (p,$\alpha$) reactions and applied it to measure the energy and strength of the 183 keV resonance that was recently reported to increase the $^{17}$O(p,$\alpha$)$^{14}$N reaction rate by a factor of as much as 100. A beam of $^{17}$O from the Holifield Radioactive Ion Beam Facility tandem accelerator bombarded hydrogen gas, which filled a scattering chamber at pressures up to 4 Torr. The chamber was connected to the beamline via 4 differential pumping stages. Reaction products were detected in coincidence by a large array of silicon strip detectors, and the vertex of the reaction was determined from the relative kinematics of the two products. Results will be presented as well as plans for measurements with radioactive beams. [Preview Abstract] |
Friday, October 27, 2006 10:36AM - 10:48AM |
CE.00009: Determination of the Reaction Rate for $^{17}$F(p,$\gamma )^{18}$Ne using the Neutron Transfer Reaction $^{13}$C($^{17}$O,$^{18}$O)$^{12}$C. T. Al-Abdullah, X. Chen, C.A. Gagliardi, Y.-W. Lui, G. Tabacaru, Y. Tokimoto, L. Trache, R.E. Tribble, Y. Zhai, F. Carstoiu The electron-positron annihilation during the expansion of nova envelope leads to the emission of a $\gamma $-ray line at 511 keV and a continuum below it. To estimate the production rate of these $\gamma $-rays, it is proposed to study the nuclear reactions that create and destroy the long-lived isotope $^{18}$F ($\tau $=158 min). Its abundance may be influenced by the reaction rate for $^{17}$F(p,$\gamma )^{18}$Ne. Since direct measurements have not been performed, the ANC method is applied to determine this rate at astrophysical energies. The ANCs for the 2$^{+}$ excited states at 1.98 MeV and 3.92 MeV in $^{18}$O are sought through measuring the peripheral reaction $^{13}$C($^{17}$O,$^{18}$O)$^{12}$C, and then transposed to the mirror states in $^{18}$Ne. The elastic scatterings were measured separately with $^{17}$O and $^{18}$O beams at 12 MeV/A to obtain the optical model parameters of the incoming and outgoing channels, which are used in the DWBA calculation to predict the angular distribution for the transfer reaction. Results will be presented and discussed. [Preview Abstract] |
Friday, October 27, 2006 10:48AM - 11:00AM |
CE.00010: Gamma rays from neutron scattering in $^{18}$O Sadia Choudry, Nico Orce, Viji Varadarajan, Shelly Lesher, Dipa Bandyopadhyay, Sharmista Mukhopadhyay, Steve Yates, Marcus McEllistrem A neutron scattering experiment in $^{18}$O has been concluded using both neutron and $\gamma $-ray detection measurements. The $\gamma $-ray measurements provide the relative decay intensities of many excited levels. These, branching ratios of level-decays, and known lifetimes enable us to provide or affirm previously obtained E2 and M1 decay intensities. Simple sd-space shell model tests, repeated here, provide the dominant configurations for the excited levels of $^{18}$O. Calculations for the several 2$^{+ }$excited levels show largely mixed configurations, with (d)$^{2}$ and (sd) configurations. But the lowest 3$^{+}$ level, at 5377.8 keV excitation energy, shows a 98{\%} (s$_{1/2}$d$_{5/2})$ configuration, illustrating why that level in $^{18}$Ne plays such a strong role in the astrophysically important p + $^{17}$F reaction. The energy spread in the experiment, 250 keV, was sufficient to average over several levels in the compound system, so the scattering cross sections could be well represented by statistical models (StatM). At incident energies where direct coupling alters inelastic scattering cross sections, deviations from the StatM would be evident, indicating the energy is high enough to discern the direct amplitude, and with it, provide a test of the mirror symmetry between $^{18}$O and $^{18}$Ne. [Preview Abstract] |
Friday, October 27, 2006 11:00AM - 11:12AM |
CE.00011: First experimental constraints on the interference of $\frac{3}{2}^{+}$ resonances in the $^{18}$F($p$,$\alpha$)$^{15}$O reaction K.Y. Chae, D.W. Bardayan, J.C. Blackmon, M.S. Smith, M.W. Guidry, C.D. Nesaraja, D. Gregory, R.L. Kozub, S. Paulauskas, J.F. Shriner Jr., N. Smith, M.S. Johnson, R.J. Livesay, M. Porter-Peden, Z. Ma, S.D. Pain, J.S. Thomas The $^{18}$F($p$,$\alpha$)$^{15}$O reaction plays a crucial role in understanding $\gamma$-ray emission from novae. Because of the importance of understanding the $^{18}$F + $p$ reactions, a number of studies of the A=19 isobars have been made using stable and exotic beams. The interference effects among $J^\pi$ = $\frac{3}{2}^{+}$ resonances in the $^{18}F + p$ system, however, have never been measured, but they can change the S-factor by a factor of 20 at nova energies. $R$- matrix calculations indicate that the cross sections above the $E_{c.m.}$ = 665 keV resonance are sensitive to the interference between the $E_{c.m.}$ = 8, 38, and 665 keV resonances. In order to study the interference effects, an excitation function for the $^{1}$H($^{18}$F,$\alpha$)$^{15} $O reaction has been measured in the energy range of $E_{c.m.}$ = 663-877 keV using radioactive $^{18}$F beams at the Holifield Radioactive Ion Beam Facility. By measuring the $^{18}$F($p$,$\alpha$)$^{15}$O cross section off resonance and comparing the cross section with theoretical calculations, we could provide the first experimental constraints on the interference of $\frac{3}{2}^{+}$ resonances. [Preview Abstract] |
Friday, October 27, 2006 11:12AM - 11:24AM |
CE.00012: $^{18}$F(\textit{a},p)$^{21}$Ne reaction study for astrophysical implications Hye Young Lee, M. Couder, A. Couture, J. Goerres, E. Stech, E. Strandberg, W. Tan, M. Wiescher, H.-W. Becker, C. Angulo, E. Casarejos, P. Leleux, D. Groombridge, B. Fulton, A. Laird, M. Aliotta As an alternative neutron source for the weak r-process, one of models suggested the r-process nucleosynthesis in the supernova shock passing through the He-rich shell of the pre-supernova star. In this helium rich environment, a possible neutron source for the second r-process would be the reaction sequence $^{14}$N($\alpha $,$\gamma )^{18}$F($\alpha $,p)$^{21}$Ne($\alpha $,n)$^{24}$Mg with rapid depletion of $^{14}$N. The ($\alpha $,p) reaction on $^{18}$F will be faster than $\beta ^{+}$ decay at the high densities and temperature in the shock. The $^{18}$F($\alpha $,p)$^{21}$Ne reaction and the inverse reaction $^{21}$Ne(p,$\alpha )^{18}$F have been measured in the energy range of the Gamow window. Experimental results will be presented and compared with Hauser-Feshbach calculations and the interests in nuclear physics. The astrophysical implications of the new reaction rates will be discussed. [Preview Abstract] |
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