Session HC: Nuclear Astrophysics I

Studies of nuclear reactions relevant to stellar or Big-Bang Nucleosynthesis using ICF plasmas at OMEGA

The $^{\mathrm{3}}$He$+^{\mathrm{3}}$He, T$+^{\mathrm{3}}$He, and p$+$D reactions directly relevant to Stellar or Big-Bang Nucleosynthesis (BBN) have been studied at the OMEGA laser facility using high-temperature low-density `exploding pusher' implosions. The advantage of using these plasmas is that they better mimic astrophysical systems than cold-target accelerator experiments. Measured proton spectra from the $^{\mathrm{3}}$He$^{\mathrm{3}}$He reaction are used to constrain nuclear R-matrix modeling. The resulting T$+^{\mathrm{3}}$He gamma-ray data rule out an anomalously-high $^{\mathrm{6}}$Li production during the Big Bang as an explanation to the high observed values in metal poor first generation stars. The proton spectrum from the T$+^{\mathrm{3}}$He reaction is also being used to constrain the R-matrix model. Recent experiments have probed the p$+$D reaction for the first time in a plasma; this reaction is relevant to energy production in protostars, brown dwarfs and at higher CM energies to BBN. This work was partially supported by the US DOE, NLUF, LLE, and GA.   

Measurements of $^7Li+d \rightarrow n+\alpha+\alpha$ and $^7Be+d \rightarrow p+\alpha+\alpha$ nuclear reactions and their implication in the Standard Big Bang Nucleosynthesis (SBBN)

Current models of SBBN predict 3-4 times more $^7Li$ than observed. The nuclear reaction $^7Be+d\rightarrow p+\alpha+\alpha$ at energies relevant to SBBN, could destroy a fraction of mass-7 nuclei. We investigate the $^7Be+d$ reaction at SBBN energies using a radioactive $^7Be$ beam and deuterium gas target inside ANASEN (Array for Nuclear Astrophysics Studies with Exotic Nuclei). ANASEN is an active target detector system which tracks the charged particles using a position sensitive proportional counter and 24-SX3 and 4-QQQ position sensitive Silicon detectors, all backed up by CsI detectors. The experiment measures a continuous excitation function by slowing down the beam in the target gas down to zero energy. Our set-up provides a high detection efficiency for all relevant reaction channels. We also performed an experiment for the mirror nuclear reaction $^7Li+d \rightarrow n+\alpha+\alpha$ with ANASEN in solid target mode using $CD_2$ target and a neutron detectors wall. The results of the experiment along with details of ANASEN and plans for the $^7Be+d$ experiment using ANASEN in gas target mode will be presented.   

Neutron Interactions With $^{7}$Be and the Primordial $^{7}$Li Problem

We study the interaction of neutrons with $^{7}$Be to estimate the direct destruction of $^{7}$Be during BBN; i.e. the predicted primordial $^{7}$Li. We plan to use a $^{7}$Be target (15 GBq) prepared by electro-deposition at PSI. The intense neutron flux of up to 5x10$^{10}$ n/sec/cm$^{2}$ are produced with proton beams and a high power liquid-lithium target (LiLiT) from the SARAF (phase I) facility in Israel. The outgoing particles will be measured using CR-39 plates that were tested to be insensitive to the large neutron flux and were calibrated with protons and alpha-particles from the TUNL. In a separate stage implanted $^{7}$Be target will be prepared at the ISOLDE facility of CERN. The results of the calibration of the CR-39 plates and the test experiment at SARAF with $^{10}$B target as well as a very low activity $^{7}$Be test target prepared at PSI, will be presented.   

Time of Flight measurement of stopping cross sections for low energy light ions in H2, He, N2, and Ne gas

The majority of available data for the stopping cross section of light ions in light gases is concentrated in the medium and high energy regimes, with little or no data available at energies below 25 keV/u. This energy regime applies to the temperature range of many stellar cores, where fusion reactions between light nuclei are common. Knowledge of the stopping cross section for light ions which interact in this environment is crucial to the accurate modeling of stellar nucleosynthesis. The current work uses time-of-flight techniques to directly measure the stopping cross section of H2, He, N2 and Ne gas for H and He ions with energies between 15-22 keV. The gas target is isolated using differential pumping, bypassing the need for entrance and exit foils.   

Enhanced Low-temperature Triple-alpha and Helium-accreting White Dwarfs

The triple-alpha reaction is of critical importance to a variety of astrophysical phenomena. Despite this relevance, the non-resonant contribution to the reaction rate at temperatures below $10^8\,\mathrm{K}$ remains uncertain, with calculations by different groups spanning over 20 orders of magnitude around $10^7\,\mathrm{K}$ Recently, Nguyen et al. (2012) showed that their calculation of the reaction rate, although enhanced at low temperatures compared to the standard NACRE rate, remains consistent with post-main-sequence evolution and the well-observed red giant branch. Nevertheless, there are other astrophysical scenarios where an enhancement of the triple-alpha rate at low temperatures may have observable consequences. One example is AM CVn systems, in which a white dwarf accretes helium-rich material from a low-mass companion in a tight binary. As the white dwarf accretes, runaway helium burning may ignite at the base of the envelope, resulting in a ``helium nova.'' Using the MESA stellar evolution code, we find that for the most energetic outbursts the new triple-alpha rate increases both the time delay and mass of the helium envelope at ignition by a factor of two or more, which may affect the observable frequency and energetics of these explosive events in future surveys.   

Including higher energy data in the $R$-matrix extrapolation of $^{12}$C$(\alpha,\gamma)^{16}$O

The phenomenological $R$-matrix technique has proved to be very successful in describing the cross sections of interest to nuclear astrophysics. One of the key reactions is $^{12}$C$(\alpha,\gamma)^{16}$O, which has frequently been analyzed using $R$-matrix but usually over a limited energy range. This talk will present an analysis that, for the first time, extends above the proton and $\alpha_1$ separation energies taking advantage of a large amount of additional data. The analysis uses the new publicly released JINA $R$-matrix code AZURE2. The traditional reaction channels of $^{12}$C$(\alpha,\gamma)^{16}$O, $^{12}$C$(\alpha,\alpha_0)^{12}$, and $^{16}$N$(\beta\alpha)^{12}$C are included but are now accompanied by the higher energy reactions. By explicitly including higher energy levels, the uncertainty in the extrapolation of the cross section is significantly reduced. This is accomplished by more stringent constraints on interference combination and background poles by the additional higher energy data and by considering new information about subthresold states from transfer reactions. The result is the most comprehensive $R$-matrix analysis of the $^{12}$C$(\alpha,\gamma)^{16}$O reaction to date.   

CASPAR - Nuclear Astrophysics Underground

The work of the LUNA Collaboration at the Laboratori Nationali del Gran Sasso demonstrated the research potential of an underground accelerator for the field of nuclear astrophysics. Several key reactions could be studied at LUNA, some directly at the Gamow peak for solar hydrogen burning. 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. The installation of the accelerator in the recently rehabilitated underground cavity at SURF started in Summer 2015 and first beam should be delivered by the end of the year. This project will primarily focus on the neutron sources for the 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. A detailed overview of the science goals of CASPAR will be presented.   

Prospects of Optical Single Atom Detection for Nuclear Astrophysics

We will discuss the prospects of optically detecting single atoms captured in a cryogenic thin film of a noble gas such as neon. This proposed detection scheme, when coupled with a recoil separator, could be used to measure rare nuclear reactions relevant for nuclear astrophysics. In particular, we will focus on the $^{22}$Ne($\alpha$,$n$)$^{25}$Mg reaction, which is an important source of neutrons for the $s$-process. Noble gas solids are an attractive medium because they are optically transparent and provide efficient, pure, stable, \& chemically inert confinement for a wide variety of atomic and molecular species. Atoms embedded inside of noble gas solids have a fluorescence spectrum that is often significantly shifted from its absorption spectrum. This makes possible the detection of individual fluorescence photons against a background of intense excitation light, which can be suppressed using the appropriate optical filters. We will report on our efforts to optically detect single Yb atoms in solid Ne. Yb is an ideal candidate for initial studies because it emits a strong green fluorescence when excited by blue light and it has an atomic structure that very closely resembles that of Mg.