Bulletin of the American Physical Society
3rd Joint Meeting of the APS Division of Nuclear Physics and the Physical Society of Japan
Volume 54, Number 10
Tuesday–Saturday, October 13–17, 2009; Waikoloa, Hawaii
Session KH: Mini-Symposium on Direct Reactions Involving Unstable Nuclei I |
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Chair: Susumu Shimoura, CNS, University of Tokyo Room: Kings 3 |
Saturday, October 17, 2009 9:00AM - 9:30AM |
KH.00001: Overview of reaction theories to probe exotic nuclear structure Invited Speaker: Reactions are the key connection between nuclear structure and experiments. Determining the detailed structure of short-lived exotic nuclei is best done by an accurate application of reaction theory. Only with a good reaction theory can we probe the spectroscopic properties of those nuclei. Collective and single-particle reactions have traditionally been first analyzed by one-step DWBA methods. Such simple methods allow spectroscopic information to be extracted as the ration between experimental cross sections and unit theoretical predictions. However, especially when studying states close to threshold, it is vital to calculate couplings to the continuum, as well as other higher-order contributions. I therefore discuss modern methods that include a discretized continuum, as well as dynamic core excitations. There is now no longer a simple ratio between spectroscopic nuclear properties and observed cross sections, but more accurate and realistic measurements are thereby obtained. Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
Saturday, October 17, 2009 9:30AM - 9:45AM |
KH.00002: Structure of $^{12}$Be studied via the $^{11}$Be(d,p) reaction Rituparna Kanungo Understanding the evolution of the unusual characteristics of unstable nuclei is of great interest in recent times. The breakdown of the N=8 shell closure in $^{12}$Be with its neighbouring isotope $^{11}$Be being a one-neutron halo makes it one of the important sites for investigating the role of binding energy and pairing in the distribution of intruder $s$-wave strength. We will report the first measurement of the $^{11}$Be(d,p) reaction performed at ISACII, TRIUMF that provides a highly selective way to understand the s-wave occupancies in the ground and excited states of $^{12}$Be. Interesting new observations will be presented. [Preview Abstract] |
Saturday, October 17, 2009 9:45AM - 10:00AM |
KH.00003: Study of the $^{12}$B(d,p)$^{13}$B reaction with the HELIOS spectrometer H.Y. Lee, J.P. Schiffer, A.H. Wuosmaa The $^{12}$B(d,p)$^{13}$B reaction has been studied in inverse kinematics at ATLAS at an energy of 6.25 MeV/u with the HELIOS spectrometer to study the positive-parity orbitals for the neutron-rich $^{13}$B nucleus. Two states previously suggested to have positive-parity at 3.48 and 3.68 MeV were resolved and their proton angular distributions measured. The $^{11}$B(d,p)$^{12}$B reaction was also studied as a reference standard. The angular distribution for the 3.48 MeV state is consistent with an l=0 transition as expected from shell-model calculations which suggest a 1/2$^{+ }$state that is primarily an s$_{1/2}$ neutron coupled to the 1$^{+}$ ground state of $^{12}$B. The 3.68-MeV angular distribution is dominantly l=2 (with a slight l=0 admixture), consistent with the shell-model expectation of a 3/2$^{+ }$state, although the spectroscopic factor relative to that for the 3.48-MeV state is much smaller than the shell-model prediction. [Preview Abstract] |
Saturday, October 17, 2009 10:00AM - 10:15AM |
KH.00004: Missing mass spectroscopy on the proton-unbound $^{12}$O nucleus and the breakdown of the $Z$~=~8 shell closure D. Suzuki, H. Iwasaki, D. Beaumel We will report on the first observation of an excited state in the proton-unbound nucleus $^{12}$O with $Z$~=~8. Recent studies have shown that the shell closure at $N$~=~8 disappears far from stability. A further interest then arises in mirror nuclei, addressing an open question about a persistence or a disappearance of the proton magicity at $Z$~=~8. Level properties of the low-lying states of $^{12}_{~8}$O$_{4}$ should provide an invaluable information since studies on the low-lying intruder states in the mirror partner $^{12}_{~4}$Be$_{8}$ have highlighted the breakdown of the neutron shell closure. However, no definite observation has been made for excited states of $^{12}$O, which lies beyond the proton-drip line. In this study, we applied the missing mass method to the $^{14}$O($p$,$t$)$^{12}$O reaction at 50 MeV/u. The experiment was performed at the GANIL-SPEG facility. The secondary $^{14}$O beam was produced in the SISSI device, and impinged on a 1-mm-thick solid hydrogen target. Recoiling particles were detected by an array of the MUST2 telescopes, each of which was composed of a double-sided silicon strip detector and a CsI calorimeter. A large active area of 100$\times$100 mm$^2$ with a high granularity achieved efficient measurements as well as a good momentum resolution. Based on the experimental result, we will discuss the breakdown of the $Z$~=~8 shell closure in $^{12}$O. [Preview Abstract] |
Saturday, October 17, 2009 10:15AM - 10:30AM |
KH.00005: Determination of the radiative neutron capture rate on $^{14}$C via indirect methods Matthew McCleskey, A.M. Mukhamedzhanov, R.E. Tribble, E. Simmons, A. Spiridon, A. Banu, B. Roeder, V. Goldberg, L. Trache, X.F. Chen, Y.-W. Lui $^{14}$C(n,$\gamma )^{15}$C is being used as a test case in the development of an indirect method to determine neutron capture cross sections on neutron-rich unstable nuclei at astrophysical energies. Our approach combines information about the peripheral component of the reaction (ANC) with information from the interior contribution (spectroscopic factor). The ANC for $^{15}$C has been determined using HI neutron transfer with a 12 MeV/nucleon $^{14}$C beam on a $^{13}$C thin foil target. The spectroscopic factor will be determined using $^{14}$C(d,p) in forward kinematics with a incident deuteron energy of about 60 MeV. Both experiments were done with the high-resolution MDM spectrometer of Texas A{\&}M University. [Preview Abstract] |
Saturday, October 17, 2009 10:30AM - 10:45AM |
KH.00006: Developing a surrogate for neutron capture reactions with rare isotope beams J.A. Cizewski, P.D. O'Malley, W.A. Peters, R. Hatarik, J. Escher Neutron capture reactions on unstable nuclei have important implications for nuclear astrophysics and applications of nuclear science, e.g., nuclear energy and national security. Given the limited capabilities to measure such reactions directly (because of intense background from targets with half-lives shorter than 100 days), it is important to determine if a surrogate reaction is effective and, if so, develop the techniques for these reactions with beams of rare isotopes. The neutron-transfer reaction, (d,p), in which the final nucleus is populated at excitations above the neutron separation energy and the gamma-ray de-excitation is measured, is a promising candidate. Not only does this reaction transfer relatively little angular momentum, in inverse kinematics the reaction protons are preferentially emitted at back angles in the laboratory. This talk will summarize the efforts to benchmark the (d,p$\gamma)$ reaction as a surrogate for neutron capture and the experimental techniques that are being developed to measure the (d,p$\gamma)$ reaction with beams of rare isotopes. [Preview Abstract] |
Saturday, October 17, 2009 10:45AM - 11:00AM |
KH.00007: Determining Cross Sections for Low-Energy Neutron Capture Reactions via the Surrogate method - Recent Progress Jutta E. Escher, Frank S. Dietrich, Nicholas D. Scielzo Many reactions of interest to nuclear energy and astrophysical applications cannot be measured directly since they involve short-lived or highly radioactive target nuclei. The Surrogate reaction method is an indirect approach for determining compound-nuclear reaction cross sections via a combination of theory and a transfer-reaction or inelastic-scattering experiment. Past applications of the method have demonstrated that it can provide useful cross section estimates for neutron-induced fission of actinides. Most analyses of fission data carried out so far have made approximations that are expected to break down for situations relevant to extracting (n,$\gamma$) cross sections from Surrogate measurements. This presentation focuses on the prospects for employing the Surrogate method to obtain cross sections for neutron capture on unstable nuclei. A brief outline of the approach will be given and recent progress made in moving beyond currently-employed approximations, such as the Weisskopf-Ewing and Ratio approximations, will be discussed. An application of the newly-developed tools to data taken recently by STARS/LiBerACE collaboration for the gadolinium region will be presented. [Preview Abstract] |
Saturday, October 17, 2009 11:00AM - 11:15AM |
KH.00008: Complex-Scaled CDCC method for nuclear breakup reactions Masaaki Takashina, Takayuki Myo, Yuma Kikuchi, Yoshiharu Hirabayashi, Kiyoshi Kato Nuclear breakup process is very important for light unstable nuclei (typically halo nuclei) induced reactions because of their weak-binding nature. The continuum-discretized coupled-channel (CDCC) method is known to be one of the powerful method to describe the nuclear breakup reaction. Indeed, CDCC has been applied to a number of analyses for the breakup reactions of both the stable and unstable nuclei, and the successful results have been obtained. In the present study, we propose complex-scaled CDCC (CS-CDCC) method, in which only the internal coordinate and momentum of the projectile are complex-scaled. The expected advantages of CS-CDCC are (1) in spite of the discretization, we can obtain the continuous S matrix elements without the smoothing function, because the continuum level density is correctly obtained, (2) in the framework of the complex scaling method, three-body scattering state can be solved properly, (3) the resonance state is strictly separated from the continuum states, and this fact is more advantageous for investigation of reaction mechanism than the ordinary CDCC method. We apply CS-CDCC to the $d \to p+n$ breakup reaction on a $^{58}$Ni target at $E_d$=80 MeV to confirm the availability of CS-CDCC. We also plan to apply it to breakup reactions of light unstable nuclei. [Preview Abstract] |
Saturday, October 17, 2009 11:15AM - 11:30AM |
KH.00009: Core Excitation in Few-Body Reaction Theory Neil Summers, Filomena Nunes, Ian Thompson, Steven Pain Direct reactions theories involving one-neutron halo nuclei such as $^{11}$Be, typically treat the nucleus as a two-body projectile where the valence neutron and core degrees are freedom are explicitly included in the reaction. This allows inclusion of the two- body breakup continuum in the reaction calculation. One such model is the Continuum Discretized Coupled Channels (CDCC) method, where the breakup continuum is expanded into partial waves and discretized in energy. Recent advances in this method, called XCDCC (X for eXtended), allow for the treatment of excited states for the $^{10}$Be core. This goes beyond the single particle model that reaction theories typically employ, and allows for coupled-channel descriptions of the projectile. By including excited states of the core the properties of the projectile can be more accurately described in the reaction, such as the B(E1) strength crucial for Coulomb excitation and breakup. We will present the results of extracting the B(E1) strength of $^{11}$Be from the Coulomb excitation experiments performed at GANIL, MSU and RIKEN. [Preview Abstract] |
Saturday, October 17, 2009 11:30AM - 11:45AM |
KH.00010: Proposed experiment for the observation of the isovector spin monopole resonance via the exothermic charge-exchange reaction using the SHARAQ spectrometer Shumpei Noji, Hideyuki Sakai We are developing the exothermic charge-exchange reaction induced by the $\beta$-unstable beam, $({}^{12}\mathrm{N},{}^{12}\mathrm{C})$, as a new probe for the study of spin-isospin modes in nuclei. Good features of this reaction include the large mass difference of ${}^{12}\mathrm{N}$ and ${}^{12}\mathrm{C}$, the spin-isospin selection of $\Delta S = \Delta T = 1$, and the surface-sensitivity due to the strong absorption. They are suited for the study of spin-isospin modes such as the isovector spin monopole resonance (IVSMR). We have proposed, at the RI Beam Factory (RIBF) at RIKEN, a measurement of the ${}^{90}\mathrm{Zr}({}^{12}\mathrm{N},{}^{12}\mathrm{C})$ reaction at $200 A \, \mathrm{MeV}$ at $0$ degrees to observe the IVSMR in ${}^{90}\mathrm{Nb}$. The ${}^{12}\mathrm{N}$ beam is produced via the projectile fragmentation of the ${}^{14}\mathrm{N}$ primary beam at $250 A \, \mathrm{MeV}$ and separated in the BigRIPS, and transported through the dispersion-matched beam line to the reaction target of ${}^{90}\mathrm{Zr}$. The reaction product of ${}^{12}\mathrm{C}$ is momentum analyzed by the newly constructed SHARAQ spectrometer. We performed commissioning experiments in March and May, 2009, to study the production of the ${}^{12}\mathrm{N}$ secondary beam and the ion optical properties of the beam line and the SHARAQ spectrometer. We report the results from the commissioning experiments together with the present situation of this project. [Preview Abstract] |
Saturday, October 17, 2009 11:45AM - 12:00PM |
KH.00011: Constructing formula for total reaction cross sections without adjustable energy-dependent parameters Akihisa Kohama, Kei Iida, Kazuhiro Oyamatsu We review our formula for a proton-nucleus total reaction cross section, $\sigma_{\rm R}$, constructed in the black-sphere approximation, in which a nucleus is viewed as a ``black'' sphere of radius ``$a$''. In this formula, the cross section, $\pi a^2$, is expressed as a function of the mass and neutron excess of the target nucleus and the kinetic energy of incident proton, $T_p$, in a way free from any adjustable $T_p$-dependent parameter. We deduce the dependence of $\sigma_{\rm R}$ on $T_p$ from a simple argument involving the proton ``optical'' depth within the framework of the black-sphere approximation of nuclei. We find that, for stable nuclei, this formula remarkably well reproduces the empirical $T_p$ dependence of $\sigma_{\rm R}$ at $T_p=100$--1000 MeV without introducing any adjustable energy-dependent parameter. We show that, in this formula, the energy dependence of $a$ is determined by that of nucleon-nucleon total cross sections, while the target-mass-number dependence of $a$ is sensitive to the surface thickness of the target. In the future experiments of neutron-rich unstable nuclei, we could expect that the neutron-excess dependence of $a$ would play an important role in deducing the density dependence of nuclear symmetry energy. [Preview Abstract] |
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