Bulletin of the American Physical Society
2006 Division of Nuclear Physics Annual Meeting
Wednesday–Saturday, October 25–28, 2006; Nashville, Tennessee
Session HE: Nuclear Astrophysics II |
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Sponsoring Units: DNP Chair: J.C. Blackmon, Oak Ridge National Laboratory Room: Gaylord Opryland Hermitage B |
Saturday, October 28, 2006 2:00PM - 2:12PM |
HE.00001: TOF-B$\rho$ Mass Measurements at the NSCL, MSU M. Mato\v{s}, A. Estrade, M. Amthor, D. Bazin, A. Becerril, T. Elliot, D. Galaviz, A. Gade, G. Lorusso, J. Pereira, M. Portillo, A. Rogers, D. Shapira, H. Schatz, E. Smith, A. Stolz, M. Wallace The radioactive beam facilities such as the NSCL offer ideal opportunities for time-of-flight mass measurements of very exotic ions. We have recently implemented a TOF-B$\rho$ technique at the NSCL and performed a mass measurement of neutron-rich nuclides in the Fe region. Masses of neutron rich nuclei are important for r- process calculations, and for calculations of processes occurring in the crust of accreting neutron stars. At the NSCL, a primary beam $^{86}$Kr was accelerated in the K500 and K1200 coupled superconducting cyclotrons to the energy of 100MeV/u. A fast radioactive beam was then produced by fragmentation reactions in the 47 mg/cm$^{2}$ and 94 mg/cm$^{2}$ Be targets and separated in the A1900 fragment separator. For this experiment a 58 m long time-of-flight path was used starting at the extended focal plane of the A1900 and ending at the focal plane of the S800 spectrograph. Fast scintillation detectors provided a timing resolution of about $\sigma$=30 ps, the relative magnetic rigidity B$\rho$ was measured at the momentum dispersive plane of the S800 by position sensitive micro-channel plate (MCP) detectors. Details of the experimental technique will be discussed and preliminary results will be presented. [Preview Abstract] |
Saturday, October 28, 2006 2:12PM - 2:24PM |
HE.00002: On the Gamow Peak in Thermonuclear Reactions Joseph Newton, Christian Iliadis, Arthur Champagne, Alain Coc, Yannis Parpottas, Claudio Ugalde The Gamow peak is an essential tool when considering non-resonant thermonuclear reactions. It is the mechanism for describing the effective burning window of charged particle reactions, at a given temperature. It is an especially useful tool when determining thermonuclear reaction rates. The Gamow peak concept is also widely used in connection with narrow resonances [1]. If the nuclear reaction of interest is in the narrow resonance realm, then the Gamow peak concept will break down under specific conditions. Stellar rates are often dominated by the narrow resonance contributions to the reaction rates, and therefore may be significantly affected by the misuse of the Gamow peak. We investigated ten different (p,$\gamma )$ reactions, ranging from A=21 to A=35 and determined a true effective energy burning window using each reaction, at a given energy, which is often significantly different form the Gamow peak window. [1] W.A. Fowler and F. Hoyle, Astrophys. J. Suppl. 9, 201 (1964) app. C. [Preview Abstract] |
Saturday, October 28, 2006 2:24PM - 2:36PM |
HE.00003: A new $^{30}$P(p,$\gamma$)$^{31}$S reaction rate and its astrophysical implications Z. Ma, M. Guidry, D.W. Bardayan, J.C. Blackmon, W.R. Hix, M.S. Smith, R.P. Fitzgerald, D.W. Visser, K.L. Jones, J.S. Thomas, R.L. Kozub, R.L. Livesay The $^{30}$P(p,$\gamma$)$^{31}$S reaction rate plays a crucial role in the synthesis of heavier elements in ONe nova outbursts. However, this rate is very uncertain due to the lack of spectroscopic information on the $^{31}$S levels. We have measured differential cross sections for the $^{32}$S(p,d)$^{31}$S reaction and determined excitation energies for 26 states in $^{31}$S. Spins and parities were determined or constrained for strongly populated levels through a DWBA analysis. A total of 42 levels in $^{31}$S were examined. A new $^{30}$P(p,$\gamma$)$^{31}$S rate was calculated using this new resonance information. Our results indicate that the $^{30}$P(p,$\gamma$)$^{31}$S rate is reduced by up to a factor of 10 at nova temperatures compared to an estimate made with a statistical reaction model. We have performed network calculations using the new rate. Production of elements in the Si-Ca region are found to be altered by as much as 30\%. Important isotopic ratios such as $^{12}$C/$^{13}$C, $^{14}$N/$^{15}$N, $^{26}$Al/$^{27}$Al and $^{29,30}$Si/$^{28}$Si are found to agree well with observations on presolar grains of nova origin. [Preview Abstract] |
Saturday, October 28, 2006 2:36PM - 2:48PM |
HE.00004: $^{24}$Mg($\alpha $,$\gamma )^{28}$Si Resonance Parameters at Low Alpha Energies Elizabeth Strandberg, Heide Costantini, Joachim Goerres, Hye Young Lee, Edward Stech, Michael Wiescher, Aaron Couture, Kent Scheller $^{28}$Si is formed by successive alpha captures during later stages of stellar burning; for carbon burning, the relevant alpha energy range is 1.0 to 1.5MeV. Previous measurements of the $^{24}$Mg($\alpha $,$\gamma )^{28}$Si reaction observed only one resonance in this energy range, although there are several $^{28}$Si states that appear favorable for formation by this reaction. Using a high efficiency coincidence detection system, several new resonances were observed between 1.1 and 1.5MeV, and an upper limit for any lower energy resonances was obtained. Newly calculated resonance parameters and reaction rates will be discussed. [Preview Abstract] |
Saturday, October 28, 2006 2:48PM - 3:00PM |
HE.00005: Elastic scattering on p-nuclei for the improvement of the optical model potential. A. Palumbo, W. Tan, J. Goerres, M. Wiescher, D. Galaviz, G. Gyurky, Zs. Fulop, E. Somorjai, N. Ozkan, R.T. Guray P-nuclei are thought to form through photodisintegration reactions on seed r- or s- nuclei of mainly the ($\gamma $,n) ($\gamma $,$\alpha )$ and ($\gamma $,p) types. This requires high temperatures of T$_{9}$ = 2-3. A possible production site may include Type II supernovae. Abundance calculations for the p-nuclei involve an extended network of about 20,000 nuclear reactions of almost 2000 nuclei. These rates are calculated with the statistical Hauser Feshbach Model (HF-Model). Elastic scattering experimental data provides a test for the global parameterizations that are used in these statistical model calculations. Recent experiments suggest inconsistencies between the predicted and measured ($\gamma $,$\alpha )$ rates which may be due to problems with the $\alpha $-potential parameters. To explore these parameters the alpha scattering cross sections on $^{120, 124, 126, 130}$Te have been measured at energies both close below and above the Coulomb barrier at the FN tandem accelerator of the University of Notre Dame. First results are presented and discussed. [Preview Abstract] |
Saturday, October 28, 2006 3:00PM - 3:12PM |
HE.00006: Neutron Capture Measurements on Tl-isotopes at DANCE A. Couture, T.A. Bredeweg, E.-I. Esch, M. Jandel, R.C. Haight, J.M. O'Donnell, R. Reifarth, R.S. Rundberg, J.L. Ullmann, D.J. Vieira, J.M. Wouters The thallium isotopes play an important role in the s-process nucleosynthesis at the $s$-process endpoint. Furthermore, $^{204}$Tl is one of few branch point isotopes in the endpoint region. The understanding of branch point isotopes provides modeling constraints on the temperatures during which the process takes place. The production of $s$-only $^{204}$Pb is controlled entirely by $^{204}$Tl. Measurements of the capture cross-sections of the stable Tl isotopes have recently been made using the DANCE 4-$\pi$\ array at LANSCE. This provides needed resonance information in the region as well as preparing the way for measurements of as yet unmeasured capture cross-section of the unstable $^{204}$Tl. The neutron capture data for the stable isotopes as well as the plan for future measurements will be discussed. [Preview Abstract] |
Saturday, October 28, 2006 3:12PM - 3:24PM |
HE.00007: The r-process peak at $A=130$ and $N=82$ Karl-Ludwig Kratz, Peter Moller It is well-known that r-process abundances can be modeled to some reasonable accuracy based on calculated beta-decay properties such as beta-decay half-lives, beta-delayed neutron-emission probabilities, neutron separation energies, and stellar environment conditions such as temperature, neutron densities and neutron-flux duration. However, characteristic deviations between observed and calcualted abundaces occur near the magic number peaks. Recently in measurements at Isolde full-spectroscopy data were obtained on $^{130}$Cd. This permitted an optimization of Nilsson-model parameters to this region of nuclei far from stability. Beta-decay half-lives calculated with this new set of Nilsson parameters are longer than our previously published 1997 tables. We present these and other features of the new r-process calculations near the $A=130$ peak. [Preview Abstract] |
Saturday, October 28, 2006 3:24PM - 3:36PM |
HE.00008: Nuclear Fission for Nuclei in the Region $ 190 \leq A \leq 330$ Peter Moller, Karl-Ludwig Kratz At the DNP meeting in Maui one year ago we reported on initial results on fission calculations of nuclei at the end of the r-process. We now have much more complete results for nuclei in this region. Compared to our preliminary results, in which the potential energy was calculated for about 1 000 000 different shapes we now consider more than 5 000 000 different shapes. More elongated and more mass-asymmetric shapes are included in the current more comprehensive results. We compare our calculated barrier heights to available actinide experimental data. We also present calculated barrier heights for all nuclei in this region of the nuclear chart, more than 3000. We furthermore calculate beta-decay strength functions which allows us to study beta-delayed fission at the end of the r-process. First results on the termination of the r-process by fission, which is to a large extent determined by the relations between barrier heights and neutron-separation energies will also be presented. [Preview Abstract] |
Saturday, October 28, 2006 3:36PM - 3:48PM |
HE.00009: Theory of Magnetic Fields of Astronomical Bodies Krishna Kumar As part of a Unified Field Theory of the four Fundamental forces, which has recently been developed by the author and applied to Atomic Nuclei, Neutron Stars and Black Holes, a new theory of the magnetic fields of astronomical bodies has been developed. The basic ideas are very simple. (1) Each object contains four types of charges: Electric, Gravitational (related to Mass), Weak (related to isotopic spin), and Strong. (2) Each of these charges produces an electric field due to its location, and a magnetic field due to its motion including spin. Just as in the electromagnetic field theory, interaction between an electric charge and an electric field produces electric (Coulomb) interaction energy, in the Unified Field theory, the other three types of charges also produce interaction energies and contribute to the binding energies, and determine other saturation properties like radii, compressibility, and symmetry energy. These have been tested for some Atomic Nuclei, Neutron Stars, and Black Holes. Now comes the surprising part! Other charges like mass can also produce magnetic fields. The calculated magnetic fields of electrically neutral bodies come out to be too large by one to two orders of magnitude. Total calculated magnetic field varies from 16 G for the Earth to 1.8 x 10$^{14}$ G for the Neutron Star associated with the Crab Pulsar, as compared to the experimental variation from $\sim$1 G to $\sim $10$^{12}$ G. [Preview Abstract] |
Saturday, October 28, 2006 3:48PM - 4:00PM |
HE.00010: Electron screening in $d(d,p)t$ for deuterated metals: temperature dependence Francesco Raiola The electron screening effect in the $d(d,p)t$ reaction has been studied at the Ruhr-Universit\"at Bochum for most of the metals and some insulators/semiconductors by using deuterated targets [1]. The deuterated targets were produced via implantation of low-energy deuterons. As compared to measurements performed with a gaseous $D_2$ target, a large effect has been observed for all metals. In particular work has been done to investigate the high solubility for the metals of groups 3 and 4 and the lanthanides, at a sample temperature $T=200 ^{\circ}$C. The hydrogen solubility in the samples dropped to a level of few percent (compared to $T=20 ^{\circ}$C) and a large screening became thus observable. An explanation of the large effect in metals is provided by the plasma model of Debye applied to the quasi-free metallic electrons. A first evidence of the validity of Debye's model is that the deduced number of free electrons per metallic atom agrees with the calculated number from the Hall coefficient [2], for all metals investigated. A critical test of the classical Debye model is the temperature dependence $U_e \propto T^{-1/2}$. \newline [1]F.Raiola et al.: Eur. Phys. J.A19(2004)283; \newline [2]C.M.Hurd: ``The Hall effect in metals and alloys''; (Plenum Press, 1972). [Preview Abstract] |
Saturday, October 28, 2006 4:00PM - 4:12PM |
HE.00011: Variable aggregate cross sections and RMS mean free paths Stewart Brekke In a previous paper it was suggested that the area cross sections of nuclei and particles was a variable given by the formula s = (p)b$^2$ where b = [Acos 2(p)ft]$^2$ due to nuclear vibration so that s =(p)[Acos 2(p)ft]$^2$, a variable cross section using a simple oscillator. If the aggregate cross section =n(Area)(s)dx, using the variable nuclear cross section would = n(Area)(p)[Acos2 (p)ft]$^2$ dx. If the maximum value for cos=1, aggregate variable cross section = n(Area)(p)A$^2$. RMS cos$^2$ = (1/2), so that the aggregate variable nuclear cross section has an average value= .5n(Area)pA$^2$. The mean free path also uses the area cross section so that l =1/n(s). Substituting for s the variable nuclear mean free path = 1/n(p)[Acos2(p)ft. If cos max = 1, the nuclear maximum free path = 1/n(p)A$^2$.RMS average mean nuclear free path = 2/n(p)A$^2$. b = the impact parameter, A = the amplitude of nuclear motion and Acos2(p)ft is the nuclear oscillator. In all cases Acos2(p)ft is greater than the nuclear radius. [Preview Abstract] |
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