Session E15: Nuclear Astrophysics I: Reactions

Nuclear input for the p-process

To properly describe the astrophysical p-process, both the stellar environment and the complete nuclear input are needed. The stellar environment most favored to-date is the O/Ne layer of a 25 solar mass type II supernova. The nuclear input requires reaction rates for about 20,000 reaction involving nearly 2,000 nuclei. A vast majority of the reactions is calculated using Hauser-Feshbach statistical models and only a small fraction has been verified experimentally. Here, impact of various parameters of the HF models on the prediction of the production of the p-nuclei is presented.   

Measurement of $^{19}$Ne(d,n)$^{20}$Na(p) reaction at RESOLUT

The $^{19}$Ne(p,$\gamma$)$^{20}$Na reaction is believed to be a link between the hot CNO cycle and the rp-process. States near the proton threshold in $^{20}$Na play a critical role in determining the reaction rate. Most notably, a known state at 450 keV above the proton threshold has yet to be firmly assigned a spin and parity. Using a radioactive $^{19}$Ne beam produced at the RESOLUT radioactive beam facility at FSU we have studied the $^{19}$Ne(d,n)$^{20}$Na(p) reaction to identify the spin and parity of near proton threshold states in $^{20}$Na.   

Inelastic Scattering of Alphas on $^{24}Mg$ as a Surrogate for Stellar Carbon Burning

Inelastic excitation of $^{24}Mg$ is used as a surrogate for the $^{12}C+^{12}C$ reaction at stellar energies. The branching ratio for $^{12}C+^{12}C\rightarrow^{20}Ne+\alpha$ and $^{12}C+^{12}C\rightarrow^{23}Na+p$ is determined by the ratio of decays via the alpha and proton decay channels of the excited $^{24}Mg$. An experiment was conducted at the Texas A\&M Cyclotron Institute in November of 2014 using the STARLiTeR detector array and the K150 (88'') Cyclotron. The experiment used a 40 MeV alpha beam and a thin $^{24}Mg$ target. The scattered alpha and the ejected alpha or proton were detected using silicon detectors while gammas from the often excited daughters were detected using an array of germanium ``clover'' detectors. This work was supported in part by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344; Texas A\&M under DOE Office of Nuclear Physics grant DE-FG02-93ER40773 and NNSA grants DE-FG52-09NA29467 and DE-NA0000979.   

Discovery of $^{34g,m}$Cl($p,\gamma$)$^{35}$Ar resonances

Sulfur isotopic ratios have potential to aid in the classification of presolar grains. Limited knowledge of the $^{34g,m}$Cl($p,\gamma$)$^{35}$Ar reaction rates leads to uncertainties in the production of $^{34}$S in oxygen-neon classical nova models. To determine these reaction rates, we have indirectly measured $^{34g,m}$Cl($p,\gamma$)$^{35}$Ar resonance energies up to 800 keV above the $^{35}$Ar proton threshold. $^{35}$Ar excited states were populated using the $^{36}$Ar($d,t$)$^{35}$Ar reaction and reaction products were momentum analyzed by a high resolution quadrupole-dipole-dipole-dipole (Q3D) magnetic spectrograph. Seventeen new $^{35}$Ar levels have been discovered and uncertainties on previously known levels have been substantially reduced. Experimental level densities were then compared to those calculated using the WBMB Hamiltonian within the $sd-pf$ model space, indicating that most of the expected resonances have been observed.   

Results from the NSCL TOF Mass Measurement of Neutron-rich Isotopes of Argon through Iron

A recent experiment at the National Superconducting Cyclotron Laboratory has resulted in an extension of the nuclear mass surface for neutron-rich isotopes of argon through iron. The time-of-flight technique was employed to experimentally determine the masses of several nuclei for the first time. Results include the identification of argon as the lowest even-$Z$ element exhibiting the $N=28$ shell closure, as well the uncovering of a relatively small odd-even mass stagger between $^{56}$Ti and $^{56}$Sc. The latter result yields strong Urca cooling when implemented in a state-of-the-art accreted neutron star crust reaction network. A strong $^{56}$Ti--$^{56}$Sc Urca neutrino cooling layer makes shallow neutrino cooling in the crust of accreting neutron stars a strong and robust effect due to the copious production of $A=56$ material in thermonuclear burning processes that occur at the accreted neutron star surface.   

Study of the $^{12}$C($\alpha $,$\Upsilon )^{16}$O reaction via cross section measurements of $^{12}$C($^{6}$Li,d)$^{16}$O and $^{12}$C($^{7}$Li,t)$^{16}$O

The $^{12}$C($\alpha $,$\Upsilon )^{16}$O reaction is crucial for the understanding of helium burning in massive stars, but the cross section for this reaction at low energy is far too small for direct measurement using presently available techniques. Despite many experimental studies in the last four decades, the low-energy cross section of the $^{12}$C($\alpha $,$\Upsilon )^{16}$O reaction remains highly uncertain. To address this problem, a new determination of the $^{12}$C($\alpha $,$\Upsilon )^{16}$O reaction cross-section has been performed via a measurement of the transfer reactions $^{12}$C($^{6}$Li,d)$^{16}$O and $^{12}$C($^{7}$Li,t)$^{16}$O at the Edwards Accelerator Laboratory at Ohio University, Athens. The differential cross-section of the reactions has been measured for the 0$^{+}$ (6.05 MeV), 3$^{-}$ (6.13 MeV), 2$^{+}$ (6.92 MeV), and 1$^{-}$ (7.12 MeV) states of $^{16}$O. Those measurements have been done by detecting the charged particles, deuterons and tritons. The time of flight technique was used to separate the different particles resulting from various possible reactions.   

Underground Nuclear Astrophysics -- from LUNA to CASPAR

It is in the nature of astrophysics that many of the processes and objects are physically inaccessible. Thus, it is important that those aspects that can be studied in the laboratory are well understood. Nuclear reactions are such quantities that can be partly measured in the laboratory. These reactions influence the nucleosynthesis of the elements in the Big Bang as well as in all objects formed thereafter, and control the associated energy generation and evolution of stars. Since 20 years LUNA (Laboratory for Underground Nuclear Astrophysics) has been measuring cross sections relevant for hydrogen burning in the Gran Sasso Laboratory and demonstrated the research potential of an underground accelerator facility. Unfortunately, the number of reactions is limited by the energy range accessible with the 400 kV LUNA accelerator. The CASPAR (Compact Accelerator System for Performing Astrophysical Research) Collaboration will implement a high intensity 1 MV accelerator at the Sanford Underground Research Facility (SURF) and overcome the current limitation at LUNA. This project will primarily focus on the neutron sources for the so-called s-process, e.g. $^{13}C(\alpha,n)^{16}O$ and $^{22}Ne(\alpha,n)^{25}Mg$, and lead to unprecedented measurements compared to previous studies.   

Advancing Underground Nuclear Astrophysics with CASPAR

The advancement of experimental nuclear astrophysics techniques and the requirement of astrophysical network models for further nuclear data over greater energy ranges, has led to the requirement for the better understanding of nuclear reactions in stellar burning regimes. For those reactions of importance to stellar burning processes and elemental production through stellar nucleosynthesis, the energy range of astrophysical interest is always problematic to probe. As reaction measurements approach the burning window of interest, the rapid drop off in cross-section hampers laboratory investigation. The natural background suppression of underground accelerator facilities enables the extension of current experimental data to lower energies. An example of such reactions of interest are those thought to be sources of neutrons for the s-process, the major production mechanism for elements above the iron peak. The reactions $^{13}$C($\alpha $,n)$^{16}$O and $^{22}$Ne($\alpha $,n)$^{25}$Mg are the proposed initial focus of the new nuclear astrophysics accelerator laboratory (CASPAR) currently under construction at the Sanford Underground Research Facility, Lead, SD.