Session DD: Nuclear Astrophysics: r-process and Nucleosynthesis

The sensitivity of r-process nucleosynthesis to beta-delayed neutron emission probabilities

In the classic picture of rapid neutron capture, or r-process, nucleosynthesis, the heaviest elements are formed far from stability in conditions of $(n,\gamma)$-$(\gamma,n)$ equilibrium. When equilibrium fails, neutron captures, photodissociations, and beta decays all compete as material moves back toward stability. The beta decays of the very neutron-rich nuclei created in the r-process are often followed by the emission of one or more neutrons. This beta-delayed neutron emission plays a key role in setting the final abundance pattern during the decay back to stability. Here we describe how beta-delayed neutron emission probabilities determine the availability of neutrons for capture at late times in the r-process, and discuss the importance of individual $P_{n}$ values in fixing the details of the r-process abundance pattern. We will point out the beta-delayed neutron emission probabilities that most strongly influence the r-process abundance pattern in a range of possible astrophysical scenarios.   

Correlated Nuclear Uncertainties in Astrophysical Processes

With increasing sophistication of stellar models the role of accurate nuclear physics input has become more important, leading to sensitivity studies of stellar abundances to nuclear reaction rates. While full nuclear uncertainties are available from evaluated nuclear data libraries, there is a need for similarly detailed studies in astrophysics. Including multiple neutron capture rates simultaneously, we show the effect of propagating systematic nuclear uncertainties from different theoretical models to final abundances in the i-process. To consider statistically correlated uncertainties, we similarly perform a full nuclear physics uncertainty study within a given Hauser- Feshbach model and demonstrate the role of correlations on the final stellar abundance uncertainties.   

Nuclear masses near N=82 and their effects on r-process abundances

We have performed for the first time a complete r-process mass sensitivity study in the N=82 region. We take into account how a change in a single nuclear mass propagates to influence important quantities of neighboring nuclei, including Q-values, neutron capture rates, photo-dissociation rates, beta-decay rates and the probability to emit neutrons. We identify key nuclei in the study whose mass has a substantial impact on final r-process abundances. We show that these nuclei are within reach of future radioactive ion beam measurements.   

Dynamics of Nuclear Pasta Phase Transitions

Knowing how matter organizes itself as its density increases from low densities to beyond nuclear saturation density has been a long standing problem in nuclear physics. It has been speculated that between these two limits matter undergoes a series of phase transitions that involve a range of exotic nuclear shapes. These shapes are collectively known as nuclear pasta. In this work we use large semi-classical molecular dynamics simulations to explore the dynamics of the phase transitions between different pasta shapes. We, then, use topological quantities known as Minkowski functionals to characterize the pasta shapes and study the equilibration of the system.   

A Density Functional Equation of State for Supernova Simulations with 3-body forces and Quark Gluon Plasma

We present an updated and improved equation of state (which we call the NDL~EoS) for use in neutron-star structure and core-collapse supernova simulations. This EoS is begins with a framework originally developed by Bowers \& Wilson, but there are numerous changes. Among them are: (1) a reformulation in the context of density functional theory; (2) the possibility of the formation of material with a net proton excess ($Y_e > 0.5$); (3) an improved treatment of the nuclear statistical equilibrium and the transition to heavy nuclei as the density approaches nuclear matter density; (4) an improved treatment of the effects of pions in the regime above nuclear matter density including the incorporation of all the known mesonic and baryonic states at high temperature; (5) the effects of 3-body nuclear forces at high densities; and (6) the possibility of a first-order or crossover transition to a QCD chiral symmetry restoration and deconfinement phase at densities above nuclear matter density. This paper details the physics of, and constraints on, this new EoS and describes its implementation in numerical simulations. We show comparisons of this EoS with other equations of state commonly used in supernova collapse simulations.   

Enhanced stability beyond the neutron drip-line near the third peak of r-process nucleosynthesis in the deformed relativistic Hartree-Bogoliubov theory

We have investigated the shell structure of nuclei in the region of the $r$-process path beyond $N=126$. Employing the framework of the relativistic Hartree-Bogoliubov theory in deformed space, ground-state properties of nuclei with $Z=58-68$ in the highly neutron-rich region beyond $N=126$ have been explored. It is shown that in approaching the $r$-process path above $N=126$, nuclei in several isotopic chains ($Z > 60$) exhibit enhanced stability. This shifts the expected drip line significantly farther into the neutron-rich region. A large number of nuclides near the $r$-process path are shown to exhibit a coexistence of well-deformed prolate and oblate shapes in the ground state. Consequences of the enhanced stability and the shape coexistence on the $r$-process nucleosynthesis will be discussed.   

$\beta$-decay of very neutron-rich Pd and Ag nuclei

The astrophysical origin of about half of the elements heavier than iron have been attributed to the rapid neutron capture process. The modeling of such a process requires not only the correct astrophysical conditions but also reliable nuclear physics. The properties of neutron-rich nuclei in the region just below the $N=82$ shell closure are of particular interest as they are responsible for the $A=130$ peak in the solar abundance pattern. An experiment to investigate half-lives and $\beta$-delayed neutron emission branching ratios of very neutron-rich Pd and Ag isotopes was performed at the GSI projectile FRagment Separator (FRS). The FRS was used to separate products from in-flight fission of a 900~MeV/u $^{238}$U beam. Ions of interest were then implanted in the Silicon IMplantation detector and Beta Absorber (SIMBA) array. The high pixelation of the implantation detectors allowed for time-position correlation of the order of several seconds between implants and decays. Neutrons emitted during the decay were detected by the BEta deLayEd Neutron detector (BELEN) which surrounded the SIMBA array. Resulting analysis of half-lives and neutron emission branching ratios including a time-dependent background will be presented.   

$\beta$-$\gamma$ and $\beta$-neutron-$\gamma$ emission in mass A=137 Decay Chain Studied with the Modular Total Absorption Spectrometer (MTAS)

The Modular Total Absorption Spectrometer (MTAS) is a detector made up of 19 separate hexagon modules of NaI which results in over a ton of NaI in the MTAS detector. MTAS was designed to capture as much of the electromagnetic energy release in $\beta$-decays as possible. MTAS was constructed at the Holifield Radioactive Ion Beam Facility and measured over 20 decay products of $^{238}$U fission products in its inaugural measurement campaign in January 2012. The measurements were focused on nuclei identified as important for decay heat analysis of the nuclear fuel cycle. Silicon detectors placed at the center of MTAS to provide $\beta$ triggers, make for extremely clean signals in MTAS. Preliminary results on the average electromagnetic energy release in the $\beta$ decay of $^{137}$Xe and $^{137}$I isotopes will be presented. These isotopes are among the priority 1 cases listed by the NEA. The $^{137}$I also has a $beta$-neutron decay branch that is detected in MTAS. Neutron detection in a large NaI detector will also be discussed.   

Excited state lifetimes in 190Tc and 109Ru via the fast-timing method

The evolution of nuclear structure across isotopic and isobaric chains are of great interest to nuclear structure and for structure applications to nuclear astrophysics, specifically the r-process. The neutron-rich region around A=110 is characterized by rapidly evolving structure, which is currently not completely understood. As such, we have investigated the A=109 $\beta$-decay chain at the Univ. of Jyvaskyla IGISOL facility. $^{109}$Mo was populated via proton-induced fission of $^{238}$U, which $\beta$- decays to $^{109}$Tc and subsequently $^{109}$Ru. Lifetimes and gamma spectroscopy were measured with a multi-detector array consisting of of 2 HPGe, 2 LaBr and 1 plastic scintillator. $\beta$-$\gamma$-$\gamma$ triple coincidences were used to construct/check both level schemes, and measure lifetimes by the fast-timing method. New gamma ray transitions and picosecond range lifetimes will be presented for $^{109}$Tc and $^{109}$Ru.