Session S11: Nuclear Astrophysics

$s$-wave scattering lengths for $^7$Be+p system from $R$-matrix analysis

The astrophysical S factor near zero energy for the $^7$Be+p radiative capture reaction depends on the $s$-wave scattering lengths. The $s$-wave scattering lengths can be determined via $R$-matrix analysis of the scattering data. We report the measurement of elastic and inelastic scattering cross section for the $^7$Be+p system in the center-of-mass energy range of 0.474-2.740 MeV. The radioactive $^7$Be beam produced at Oak Ridge National Laboratory's Holifield Radioactive Ion Beam Facility was accelerated and bombarded a 100-$\mu$g/cm$^2$ polypropylene target. The scattered particles were detected in a segmented silicon setector array. The $R$-matrix code AZURE2 has been used for the simultaneous fitting of the elastic and inelastic scattering data from this measurement and other available data. The $s$-wave scattering lengths were determined using the best fit $R$-matrix parameters and will be presented. The preliminary results from the $R$-matrix analysis of the data will be presented and the level structure of $^8$B will be discussed. The prospects for a new measurement of $^7$Be+p elastic scattering at the DRAGON facility of TRIUMF using gas target will also be discussed.   

Cross Section Measurements of the $^{12}$C($\alpha, \gamma$)$^{16}$O Reaction at $E_\text{c.m.}$ = $3.7$, $4.0$, and $4.2$ MeV

The $^{12}$C($\alpha, \gamma$)$^{16}$O reaction is one of the most important nuclear reactions in astrophysics, as it determines the C/O ratio at the end of the helium burning in red giant stars. This ratio has significant effects for the subsequent stellar evolution and supernova explosions. We have used the DRAGON recoil separator for the measurements of the $^{12}$C($\alpha, \gamma$)$^{16}$O reaction at the higher energies of $E_\text{c.m.} = 3.7, 4.0$, and $4.2$ MeV. The measurements will constrain global R-Matrix fits by providing information on high energy levels, aiding the extrapolation to helium burning energies. The experiment was performed in inverse kinematics where a $^{12}$C beam was impinged on windowless He gas target surrounded by 30 BGO detectors which detect the $\gamma$-rays. The $^{16}$O recoils were detected by a double-sided silicon strip detector located at the end of the DRAGON separator. The array of BGO detectors is able to separate transitions to various $^{16}$O final states.   

Absolute normalization of the ${}^{13}{\rm C}(\alpha,n){}^{16}{\rm O}$ cross section and resonance strengths below $E_\alpha=2$ MeV

The ${}^{13}{\rm C}(\alpha,n){}^{16}{\rm O}$ reaction is important as a neutron source for $s$-process nucleosynthesis and as a background in detectors used for reactor anti-neutrino and geoneutrino detection. The time-reversed process ${}^{16}{\rm O}(n,\alpha){}^{13}{\rm C}$ is likewise important in neutron applications. In all of these scenarios, it is desirable to know the absolute cross section as well as possible. We have revisited previous measurements of this reaction that were performed using a polyethylene moderator and ${}^3{\rm He}$-filled proportional counters for neutron detection [C.R. Brune {\em et al.}, Phys. Rev. C {\bf 45}, 1382 (1992); {\em ibid} {\bf 48}, 3119 (1993)]. Subsequent to these publications, the efficiency of the neutron detector was accurately characterized using a combination of Monte Carlo simulations and calibrated neutron sources. The Monte Carlo simulations include the effects of the reaction's kinematics and angular distribution on the efficiency. We have conducted a re-analysis of the above measurements using this improved detector efficiency. These reanalyzed data will be compared with existing measurements below $E_\alpha=2$~MeV.   

Mass Measurements of Rare-Earth Nuclei Near N = 100

The recent observation of gravitational wave event GW170817 has confirmed that neutron star mergers are a site of heavy-element production from rapid neutron capture nucleosynthesis ($r$ process). As we learn more about the nature of the $r$ process, the importance of accurate nuclear data of neutron-rich isotopes far from stability will become paramount. In order to constrain calculations which model the formation of the rare-earth peak at late stages in the $r$ process, more nuclear data in the region is needed. Many of these neutron-rich isotopes are readily available at the CARIBU facility of Argonne National Laboratory where the Canadian Penning Trap mass spectrometer (CPT) is housed. A phase-imaging mass measurement technique (PI-ICR) has dramatically increased the experimental sensitivity of the CPT allowing for several new mass measurements in the rare-earth region. The experimental results as well as the astrophysical implications of these measurements will be discussed.   

A new experimental technique for measuring (p,n) reactions relevant to the neutrino-p process in the ReA3 facility

Proton rich neutrino driven winds in core-collapse supernovae can be a suitable environment for the formation of elements up to Z$\sim$50 via the so called neutrino-p ($\nu$p-)process. The strength of $\nu$p-process depends on key (n,p) reactions like the $^{56}$Ni(n,p)$^{56}$Co and $^{64}$Ge(n,p)$^{64}$Ga for which no experimental data exists. With the current state of the art any direct measurement of (n,p) reactions on neutron deficient nuclei is extremely challenging. For this purpose, a new experimental technique has been developed in the ReA3 facility at National Superconducting Cyclotron Laboratory for the study of astrophysical important (n,p) reactions via measuring the time reverse (p,n) reactions. In this presentation, a description of the technique and results from the first proof-of-principle run will be shown.   

Neutrino scattering in supernovae and spin correlations of a unitary gas

Core collapse supernova simulations can be sensitive to neutrino interactions near the neutrinosphere. This is the surface of last scattering. We model the neutrinosphere region as a warm unitary gas of neutrons. A unitary gas is a low density system of particles with large scattering lengths. We calculate modifications to neutrino scattering cross sections because of the universal spin and density correlations of a unitary gas. These correlations can be studied in laboratory cold atom experiments. We find significant reductions in cross sections, compared to free space interactions, even at relatively low densities. These reductions could reduce the delay time from core bounce to successful explosion in multidimensional supernova simulations.   

Equation of state of low-density supernova matter with multiple nuclei in the mean field approximation

The Lattimer-Swesty (LS) approach[1], often used in the construction of equations of state (EOS) for astrophysical applications, considers only a mixture of nucleons, alpha particles and electrons in the dissociated region (where conditions are such that heavy nuclei cannot persist). Here, we develop a framework which supplements the excluded-volume technique of LS with a mean-field treatment of attractive interactions and, furthermore, incorporates other light nuclei such as deuterons, tritons, and Helium-3 which are present and affect the thermal characteristics of the EOS as was demonstrated in the virial approach adopted by Horowitz and Schwenk[2] and Arcones et al.[3]. The nuclear statistical equilibrium method, commonly used to account for these nuclei, is applicable in the case when interactions between the various constituents can be ignored and hence it is not valid at the densities of relevance to this region. We show numerical results to demonstrate the extent to which abundances of the various nuclei and the EOS are affected and, where possible, comparisons with the virial approach are made. [1] J. M. Lattimer and D. Swesty, Nucl. Phys. A535 (1991) 331. [2] C. J. Horowitz and A. Schwenk, Nucl. Phys. A776 (2006) 55. [3] A. Arcones, et al., Phys.Rev.C 78 (2010) 015806.   

A Phenomenological Equation of State for Hot and Dense Homogeneous Nucleonic Matter

The equation of state (EOS) of matter is a central microphysical input required for numerical simulations of core-collapse supernovae and neutron star mergers. These simulations probe a wide range of baryon densities ($\mathrm{n_B}$), temperatures ($\mathrm{T}$) and electron fractions ($\mathrm{Y_e}$). We construct new set of EOSs for homogeneous nucleonic matter including protons and neutrons. These EOSs independently implement quantification of uncertainties in different regimes. For non-degenerate nuclear matter, we employ the virial expansion taking inputs from nucleon-nucleon scattering phase shift. For zero temperature neutron matter, we employ quantum Monte Carlo up to saturation densities and Neutron star EOS above saturation densities. For zero temperature nuclear matter, we employ Skyrme models parametrized by experimentally-measured nuclear data. For hot matter near the saturation density, we describe finite temperature corrections with a Skyrme model designed to fit results obtained from chiral effective theory. We enforce causality at high density for the full combined EOS. Our final EOSs are compared with several other models at representative densities and temperatures.   

Beta equilibrium in neutron star merger conditions

Conventionally, neutrino-transparent nuclear matter is said to be in beta equilibrium if the electron and proton chemical potentials add up to the neutron chemical potential. We find that at temperatures above 1 MeV, which are reached in neutron star mergers and supernovae, the traditional criterion of beta equilibrium needs to be modified by adding an isospin chemical potential of order 10 MeV. This modification of beta equilibrium alters the direct and modified Urca rates for densities above and below the direct Urca threshold density, which has implications for the bulk viscosity of neutron star mergers.