Session E6: Nuclear Astrophysics

Nuclear Pasta

For decades it has been theorized that just below nuclear saturation density matter undergoes a series of phase transitions. These phases, which are expected to exist in core-collapse supernovae and neutron stars, involve a range of exotic nuclear shapes collectively known as nuclear pasta. Recently, Jose Pons and collaborators suggested that ``the maximum period of isolated X-ray pulsars may be the first observational evidence for an amorphous inner crust, ..., possibly owing to the existence of a nuclear pasta phase.'' In this talk we present results of semi-classical molecular dynamics simulations of nuclear pasta and discuss how each phase might contribute to neutron star crust properties.   

Dissipation in multi-component compact stars

We proposed a novel mechanism for the saturation of unstable oscillation modes in multi-component compact stars, which is based on the periodic conversion between different phases, i.e. the movement of their interfaces, induced by pressure oscillation in the star. The case of r-modes in a hybrid star with a sharp interface between a quark matter core and a nuclear matter crust is studied in detail and we find that this mechanism can lead to low saturation amplitudes, and thereby it could present the dominant damping mechanism in hybrid stars. We study the dissipation due to hadron-quark burning in a hybrid star using a steady-state approximation and find that in this case the dissipation entirely vanishes in the subthermal regime, but becomes finite and very strong once the oscillation amplitude reaches a critical value. This strong dissipation saturates unstable r-modes just above the critical value and as a result leads to a simple analytic prediction for the saturation amplitude. We find that the r-mode saturation amplitude can be as low as about 10$^{-4}$ for conditions present in typical observed pulsars.   

The Compact Accelerator System for Performing Astrophysical Research Underground - CASPAR

An accelerator laboratory (CASPAR) to be installed at the Sanford Underground Research Facility (SURF) is being constructed by a collaboration lead by South Dakota School of Mines and Technology. The study of alpha induced reactions of astrophysical interest in a quasi-background free environment is the goal of the laboratory. Specifically, neutron producing reactions for the s-process will be investigated. This process is responsible for the nucleosynthesis of half of the the elements heavier than iron. An outline of CASPAR, its timeline and scientific goals will be presented.   

Experimental techniques to use the (d,n) reaction for spectroscopy of low-lying proton-resonances

Studies of rp-process nucleosynthesis in stellar explosions show that establishing the lowest $l=0$ and $l=1$ resonances is the most important step to determine reaction rates in the astrophysical $rp$--process path. At the RESOLUT facility, we have used the $(d,n)$ reaction to populate the lowest $p$-- resonances in $^{26}$Si, and demonstrated the usefulness of this approach to populate the resonances of astrophysical interest [1]. In order to establish the $(d,n)$ reaction as a standard technique for the spectroscopy of astrophysical resonances, we have developed a compact setup of low-energy Neutron-detectors, ResoNEUT and tested it with the stable beam reaction $\mathrm{^{12}C(d,n)^{13}N}$ in inverse kinematics. Most recently, the detectors were included in a study of the radioactive beam reaction $\mathrm{^{17}F(d,n)^{18}Ne}$ in inverse kinematics. Performance data from these experiments will be presented. \\[4pt]{[1]} P.N. Peplowski {\it et al.} Phys.Rev.{\bf C 79}, 032801 (2009)   

Measurement of the $^{25}$Al(d,n)$^{26}$Si(p) reaction at RESOLUT: Spectroscopy of $l=0$ and $l=1$ resonances

Studies of rp-process nucleosynthesis in stellar explosions show that establishing the lowest $l=0$ and $l=1$ resonances is the most important step to determine reaction rates in the astrophysical $rp$--process path. In an experiment performed at the RESOLUT radioactive beam facility of Florida State University, we have studied the $^{25}$Al(d,n)$^{26}$Si reaction in inverse kinematics to establish the spectrum of the lowest $l=0$ and $l=1$ resonances. Recent results include neutron coincidences from the newly developed neutron detector array RESONEUT.   

The thermonuclear reaction rate of $^{17}$O($p$,$\gamma$)$^{18}$F---a low-energy, high beam current study at LENA

Classical novae are thought to be the dominant source of $^{17}$O in our Galaxy. These energetic events produce $^{18}$F that, as it decays to $^{18}$O, drives the ejection of nuclear ``ash'' into the interstellar medium. The importance of the non-resonant component of the $^{17}$O($p$,$\gamma$)$^{18}$F reaction is well established, and numerous studies have been performed to analyze this reaction. However, the temperature regime relevant to explosive hydrogen burning during classical novae corresponds to very low proton bombarding energies. At these low energies, the Coulomb barrier suppresses the reaction yield in the laboratory, and environmental backgrounds dominate the detected signal making it difficult to differentiate the direct capture $\gamma$-cascade from background. At the Laboratory for Experimental Nuclear Astrophysics (LENA), our electron cyclotron resonance (ECR) ion source produces intense, low-energy protons ($\approx$ 2.0 mA at the target), and these high currents boost the thermonuclear reaction yield. The LENA facility also has a coincidence detector setup that reduces environmental background contributions. Improved $^{17}$O($p$,$\gamma$)$^{18}$F direct capture reaction rates are currently being determined, and our progress will be reported.   

Lifetime Measurement of the 6.79 MeV State in 15O to Help Constrain the 14N(p,gamma)15O Reaction Rate

The $^{14}$N(p,$\gamma$)$^{15}$O reaction is the slowest reaction in the CNO cycle. The rate of this reaction is an important input into calculating the ages of globular cluster stars, determining the primordial core composition of our Sun and affects the amount of He ash produced in H burning shells in red giant stars and hence the nucleosynthesis of heavier elements. The largest remaining uncertainty in calculating the reaction rate is the lifetime of the 6.79 MeV excited state of $^{15}$O. We report an upper limit of 1.84 fs on this lifetime. In addition we measured the lifetime of the 6.86 MeV state of $^{15}$O to be 13.3$^{+0.8}_{?1.2}$ fs.   

\mbox{Precision Angular Distribution Data for the \boldmath${}^{16}\mathrm{O}(\gamma,\alpha){}^{12}\mathrm{C}$} Reaction in the Region of the \boldmath$1^-$ Resonance at 9.6~MeV

The HI$\gamma$S Optical Time Projection Chamber has been used to measure angular distribtions for the ${}^{16}\mathrm{O}(\gamma,\alpha){}^{12}\mathrm{C}$ reaction at beam energies of 9.4, 9.5, and 9.8~MeV\@. Intense, nearly-mono\-energetic $\gamma$-ray beams produced at the HI$\gamma$S facility were used with a \mbox{N$_2$O} gas target, and the outgoing $\alpha$ particles were detected using an optical time projection chamber. High statistics runs were made and full angular distributions were obtained at all three beam energies. The data are being analyzed in an effort to resolve previous discrepancies between the relative $E1$-$E2$ phase extracted from ${}^{12}\mathrm{C}(\alpha,\gamma){}^{16}\mathrm{O}$ data~[1] and those predicted from elastic $\alpha$-particle scattering on ${}^{12}\mathrm{C}$~[2]\@.\\[4pt] [1] Assun\c{c}\~{a}o~\emph{et al.}, PRC \textbf{73}, 055801 (2006) \\[0pt] [2] Tischhauser~\emph{et al.}, PRC \textbf{79}, 055803 (2009)