Bulletin of the American Physical Society
2007 APS April Meeting
Volume 52, Number 3
Saturday–Tuesday, April 14–17, 2007; Jacksonville, Florida
Session X15: Minisymposium on Nuclear Astrophysics with Transfer Reactions I |
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Sponsoring Units: DNP Chair: Carl Brune, Ohio University Room: Hyatt Regency Jacksonville Riverfront City Terrace 11 |
Tuesday, April 17, 2007 10:45AM - 11:21AM |
X15.00001: Transfer reactions: challenges for the future Invited Speaker: Transfer reactions offer a unique tool to study spectroscopic of nuclei. It is extremely important to devise a method to extract structure information from transfer cross sections, reliable even when moving toward the driplines. The transfer amplitude is typically surface peaked and sensitive to the overlap function of the nucleus of interest in the surface region. As there is a non negligible contribution to the cross section from the interior of the nucleus under study, spectroscopic factors can usually be extracted. For low energies, the transfer cross sections may only be sensitive to the tail of the overlap function, and then only an Asymptotic Normalization Coefficients (ANC) should be extracted. The standard theory to analyze transfer reactions is still the same for decades, namely the Distorted Wave Born Approximation. Several recent studies, revisiting the topic, have unveiled problems but also indicate possible solutions. For example higher order effects often need to be considered, either from breakup, from inelastic excitations or from couplings to other reaction channels. In this talk I will review the present status and underline the importance of performing, in parallel to a transfer measurement, an experiment to determine the ANC independently. [Preview Abstract] |
Tuesday, April 17, 2007 11:21AM - 11:33AM |
X15.00002: Charge-exchange reactions: a tool for extracting weak rates for stellar evolution Remco G.T. Zegers A variety of charge-exchange (CE) reactions are used to extract Gamow-Teller (GT) strengths in nuclei of relevance for astrophysics, in particular late stellar evolution. An important feature of CE reactions at bombarding energies above 100 MeV/n is that, once calibrated using beta decay, GT strengths over a wide excitation-energy range can be extracted model-independently. At the NSCL, the (t,3He) reaction on stable targets is used to extract strengths in the beta+ direction. Moreover, experimental techniques to extract GT strength away from the valley of stability (using the (7Li,7Be) and (p,n) reactions) are being developed. The presentation will focus on the experimental program at the NSCL and the associated theoretical efforts concerning the CE reactions and GT strength extraction. [Preview Abstract] |
Tuesday, April 17, 2007 11:33AM - 11:45AM |
X15.00003: The $^{15}$O($\alpha$,$\gamma$)$^{19}$Ne trigger for X-ray bursts Wanpeng Tan, Jacob Fisker, Joachim G{\"o}rres, Manoel Couder, Michael Wiescher Neutron stars in close binary stars systems often accrete matter from their companion star. Thermonuclear ignition of the accreted material in the atmosphere of the neutron star leads to a thermonuclear explosion which is observed as an X-ray burst occurring periodically between hours and days depending on the accretion rate. The burst conditions are characterized by a sensitive interplay between fuel supply and depletion by nuclear burning. This balance depends critically on the ignition through the nuclear trigger reaction $^{15}$O($\alpha$,$\gamma$)$^{19}$Ne that regulates the flow between the $\beta$-limited hot CNO cycle and the rapid proton capture process. The reaction rate of $^{15}$O($\alpha$,$\gamma$)$^{19}$Ne was previously not known experimentally and the theoretical estimates carried significant uncertainties. Here we report on the first successful measurement of the critical nuclear parameters for the determination of the rate of the $^{15}$O($\alpha$,$\gamma$)$^{19}$Ne reaction and we analyze the impact of these experimental results on the burst pattern and burst periodicity for a range of accretion rates. [Preview Abstract] |
Tuesday, April 17, 2007 11:45AM - 11:57AM |
X15.00004: Exploring the $\alpha$p process with transfer reactions at RCNP Shawn O'Brien, G.P.A. Berg, J. G\"orres, P. LeBlanc, M. Wiescher, A. Matic, A.M. van den Berg, M. Harakeh, H.J. W\"ortche, T. Adachi, Y. Fujita, Y. Shimbara, H. Fujita, K. Fujita, K. Hatanaka, Y. Kitamura, Y. Sakemi, Y. Shimizu, Y. Tameshige, A. Tamii, M. Yosoi, H. Schatz, T. Wakasa Several experiments have been performed at RCNP that exploit the high resolution and 0$^\circ$ capabilities of the Grand Raiden spectrometer and the WS beamline, which can deliver a fully dispersion matched beam to target. $^{22}$Mg and $^{26}$Si have been studied with (p,t) and ($^{4}$He,$^{6}$He) reactions on thin $^{24}$Mg and $^{28}$Si foils. These transfer reactions will help us to experimentally measure the energies of potential resonance states in these nuclei above the proton and alpha thresholds. This information is vital to understanding the nucleosynthesis occurring during explosive hydrogen burning, which, for example, is thought to occur during type-I X-ray bursts. The experimental technique will be discussed, and the experimental results will be presented. [Preview Abstract] |
Tuesday, April 17, 2007 11:57AM - 12:09PM |
X15.00005: Experimental evidence of a natural parity state in $^{26}Mg$ at astrophysically relevant energies via the $^{22}Ne(^{6}Li,d)^{26}Mg$ reaction. C. Ugalde, A. Champagne, S. Daigle, C. Iliadis, J. Newton, E. Osenbaugh, J. Clark, C. Deibel, A. Parikh, P. Parker, C. Wrede The $^{22}Ne(\alpha,n)^{25}Mg$ reaction has been regarded as the main neutron source for the s-process in core He-burning massive stars and of relevance in He-shell burning in AGB stars. Its present rate is one of the most important sources of uncertainty in the nucleosynthesis of heavy elements. We have studied natural parity states in $^{26}$Mg via the $^{22}Ne(^{6}Li,d)^{26}Mg$ direct process with Yale University's Enge split-pole spectrograph. Our method significantly improves the energy resolution of previous experiments and as a result we report the observation of a natural parity state in $^{26}Mg$ at an energy where the stellar rate of the $^{22}Ne(\alpha,\gamma)^{26}Mg$ reaction may be reduced. This fact gives place to an increase in the production of weak component s-process neutrons via the $^{22}Ne(\alpha,n)^{25}Mg$ reaction. Possible spin-parity assignments are suggested as well. [Preview Abstract] |
Tuesday, April 17, 2007 12:09PM - 12:21PM |
X15.00006: Cross sections for reactions in explosive H burning from transfer reactions at 10 MeV/u L. Trache, T. Al-Abdullah, A. Banu, C. Fu, C.A. Gagliardi, A.M. Mukhamedzhanov, G. Tabacaru, R.E. Tribble, Y. Zhai, F. Carstoiu, V. Burjan A review will be given of the methods for nuclear astrophysics from transfer reactions with stable and radioactive beams at TAMU. We'll list the proton transfer reactions used to determine the cross sections for radiative p-capture reactions for H burning. Then we'll show how one can use neutron transfer and charge symmetry to obtain information for mirror nuclei. From experiments we extract structure information (spectroscopic factors or ANCs) by comparing with DWBA calculations. Recognizing that in all these indirect methods it is of capital importance to have reliable DWBA calculations, we made extensive elastic scattering studies with p-shell nuclei projectiles around 10 MeV/u to find a global procedure to determine optical model potentials using semi-microscopic double folding models with various effective NN-interactions. This is particularly important for reactions induced by rare nuclei beams. We'll show how using the JLM interaction we had very good success in predicting/describing elastic scattering for a large number of RNBs: $^{7}$Be, $^{8}$B, $^{11}$C, $^{12}$N, $^{13}$N, etc. We intend to extend our studies to sd-shell nuclei, using similar methods. [Preview Abstract] |
Tuesday, April 17, 2007 12:21PM - 12:33PM |
X15.00007: Unbound States of Neutron-Rich Oxygen Isotopes: Investigation into the N=16 Shell Gap C. Hoffman, S.L. Tabor, T. Baumann, W.A. Peters, H. Scheit, A. Schiller, M. Thoennessen, N. Frank, P.A. DeYoung, J. Hinnefeld The energy of the first excited state in $^{24}$O and the ground state decay energy of $^{25}$O have been measured for the first time. Due to the lack of observation of an excited state in $^{24}$O using $\gamma$-ray spectroscopy, as well as the known unbound nature of $^{25}$O, techniques involving neutron spectroscopy had to be used. $^{25}$O ($^{24}$O$^{*}$) was populated via proton (proton-neutron) removal from a $^{26}$F beam. Break-up neutrons were detected by the Modular Neutron Array (MoNA) located at 0$^{\circ}$ relative to the beam direction. Charged fragments of desired rigidity were selected by a large-gap dipole (Sweeper) magnet and their properties were measured in the focal plane. Complete 4-vector reconstruction for neutrons and fragments at their break-up point revealed resonant energies of 750(50) keV and 840(30) keV for n-$^{23}$O and n-$^{24}$O coincidences respectively. Using the known neutron separation energy (S$_{\rm n}$) of $^{24}$O, its first excited state is calculated to be 4.45(40) MeV. These results are to be compared with current shell model predictions to aid in understanding of the N=16 shell gap. [Preview Abstract] |
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