Session RM: Nuclear Astrophysics: Supernovae

Nucleosynthesis of $^{60}$Fe and constraints on the nuclear level density and photon strength function.

$^{60}$Fe is created in massive stars prior to core collapse supernova. The signature $\gamma$-rays from $\beta$-decay of this isotope indicate ongoing nucleosynthesis in the Galaxy among other interesting astrophysical processes. In order to understand these observations a complete understanding of the creation, destruction and nuclear properties of $^{60}$Fe in the astrophysical environment are required. Due to the short half-life of $^{59}$Fe a direct capture reaction experiment to determine the cross section of $^{59}$Fe(n,$\gamma$)$^{60}$Fe has been a challenge and remains the most uncertain link in the reaction chain to date. Using the $\beta$-decay of a $^{60}$Mn radioactive beam to populate high energy states in the $^{60}$Fe nucleus, an indirect constraint for this reaction was made using the $\beta$-Oslo Method. Results from this analysis were used as input in TALYS and the new constraint on $^{59}$Fe(n,$\gamma$)$^{60}$Fe will be presented.   

Constraining electron capture rates in core-collapse supernovae for nuclei near N{\$}$=${\$}50.

Electron captures on medium-heavy nuclei play an important role in the late stages of the evolution of core-collapse supernovae, just prior to the explosion. In particular, nuclei around N$=$50, just above $^{\mathrm{78}}$Ni, have been identified as especially important for the deleptonization of the core. The astrophysical simulations require accurate electron-capture rates. One has to largely rely on theoretical models, which must be benchmarked and guided by experimental data. This work describes a broad effort to improve the electron capture-rates for nuclear astrophysical simulations, focusing on nuclei near N$=$50. This includes (t,$^{\mathrm{3}}$He) charge-exchange experiments for extracting Gamow-Teller strengths, the comparison with theoretical models used for calculating electron-capture rates for the astrophysical simulations, and results from sensitivity studies by using 1D core-collapse simulations.   

The Effects of Neutrino Oscillations on Core-Collapse Supernova Explosions

At the present time even the most sophisticated, multi-dimensional simulations of core-collapse supernovae do not (self-consistently) include neutrino flavor transformation. This physics is missing despite the importance of neutrinos in the core-collapse explosion paradigm. Because of this dependence, any flavor transformation that occurs in the region between the proto-neutron star and the shock could result in major effects upon the dynamics of the explosion. We present the first hydrodynamic core-collapse supernova simulation which simultaneously includes flavor transformation of the free-streaming neutrinos in the neutrino transport. These oscillation calculations are dynamically updated and evolve self-consistently alongside the hydrodynamics. Using a $M=20\;{\rm M_{\odot}}$ progenitor, we find that while the oscillations can have an effect on the hydrodynamics, flavor transformation alone does not lead to a successful explosion of this progenitor.   

Effects of the nuclear equation of state on the outcome of core-collapse supernovae

Massive stars end their lives when their core collapses under gravity, resulting in either a core-collapse supernova (successful explosion) or a black hole (failed explosion). Despite many efforts, it is not yet fully understood which massive stars will successfully explode in a core-collapse supernova and which ones will collapse to a black hole. Here, we investigate the impact of the nuclear equation of state (eos) on the outcome of core collapse (successful or failed explosion) and the subsequent nucleosynthesis. We model the explosion in spherical symmetry using the effective push method together with general-relativistic hydrodynamics and neutrino transport. We use several supernova eos(s)and study the variation in explosion properties and nucleosynthesis yields for stars with different zams mass. We find that the eos significantly impacts the outcome of our simulations.   

The Design, Validation, and Future Plans for a New Neutron Detector at Ohio University

Though ($\alpha$,n) reaction cross sections play a key role in nuclear astrophysics and applications, many are poorly constrained by nuclear experiments and have significant uncertainties in theoretical predictions. Improving this situation will be done in part using a newly developed neutron long counter, HeBGB, at the Ohio University Edwards Accelerator Lab. The detector was designed using MCNP6 to have near constant efficiency in the neutron energy range relevant for core-collapse supernovae and special nuclear materials. Efficiency validation measurements have been performed with HeBGB, which utilize well-characterized reactions with constrained cross sections and known neutron energies. The first measurement planned for HeBGB is $^{27}$Al($\alpha$,n) near threshold, which dominates the astrophysical rate and has disagreement between theoretical predictions and the only prior measurement in this energy regime. In preparation, various aluminum targets have been tested for purity using RBS, PIXE, and PIGE nuclear reaction analysis techniques. We find that store bought aluminum foils offer higher purity than traditional foil suppliers. In addition to these results, an update will be provided on the validation measurements of the HeBGB long counter.   

Reaction rates that limit $^{44}$Ti from core-collapse supernovae as dense matter constraint: Shocking results

Recent observational advances have enabled high resolution mapping of $^{44}$Ti in core-collapse supernova (CCSN) remnants. Comparisons between observations and 3D models provide stringent constraints on the CCSN mechanism. However, recent work has identified several uncertain nuclear reaction rates that influence $^{44}$Ti and $^{56}$Ni production in model calculations of shock-driven nucleosynthesis. We evolved 15M$_{\odot}$, 18M$_{\odot}$, 22M$_{\odot}$ and 25M$_{\odot}$ stars from ZAMS to CCSN in MESA (Modules for Experiments in Stellar Astrophysics) and investigated previously identified sensitivities of $^{44}$Ti and $^{56}$Ni production in CCSN to varied reaction rates. I will present our final results of this sensitivity study. I will also briefly discuss ongoing experimental work motivated by this study namely a direct cross section measurement of $^{39}$K(p,$\gamma$)$^{40}$Ca.   

Crustal Heating During the Epoch of Crust Replacement

Neutron stars in X-ray binaries may completely replace their crusts with accreted matter. If the composition of the material freezing out of the ocean changes then the structure of the reaction layers in the crust should change, resulting in fluctuations in crustal heating. Recent work has calculated the composition and heat sources in steady state accreted neutron star crusts and uses them to study the evolution of the heating in partially replaced 'hybrid crusts.' In addition, we report calculations of the heating in hybrid crusts formed during the replacement of cold catalyzed crusts by accreted matter and make suggestions for resolving crust replacement observationally.   

Cross Section Measurements of $(p,\gamma)$ Reactions in A=100-110 region relevant to the p-process

How to accurately model and predict the observed abundances of the 35 stable p-nuclei remains an open question in the field of nuclear astrophysics. Recent sensitivity studies with regard to reaction network models predicting p-nuclei abundances have identified several radiative capture reactions whose uncertainties have the largest impact on the network model. In order to constrain those uncertainties, the $^{102}$Pd$(p,\gamma)^{103}$Ag,$^{108}$Cd$(p,\gamma)^{109}$In, and $^{110}$Cd$(p,\gamma)^{111}$In reaction cross sections were measured at the University of Notre Dame Nuclear Science Laboratory. The measurements were performed at lab energies $E_{p} = 3 - 8 $ MeV using the HECTOR detector and $\gamma$-summing technique. Our results are compared to various theoretical models from the Talys 1.9 and NON-SMOKER reaction codes as well with previous measurements. The theoretical model that best fits the experimental data is used to calculate the inverse $(\gamma,p),(\gamma,n)$ reaction rates. Discrepancies with the new reaction rates compared to older theoretical calculations which may have an impact on the reaction network are discussed. This work is supported by the NSF under grants: PHY-1614442, PHY-1713857 (NSL) and PHY-1430152 (JINA-CEE).   

Searching for ($\gamma$,$\alpha$)/($\gamma$,n) branching points in the $\gamma$-process path around A=100

In order to model the $\gamma$-process it is important to determine the branching points along isotopic chains. The $^{90}$Zr($\alpha$,$\gamma$)$^{94}$Mo, $^{102}$Pd($\alpha$,$\gamma$)$^{106}$Cd, and $^{108,110}$Cd($\alpha$,$\gamma$)$^{112,114}$Sn cross sections were measured to determine if $^{94}$Mo, $^{106}$Cd, and $^{112,114}$Sn were branching points in the $\gamma$-process reaction flow. The reactions were measured at the University of Notre Dame using the High EfficienCy TOtal absorption spectrometeR (HECTOR). The $^{90}$Zr($\alpha$,$\gamma$)$^{94}$Mo and $^{108}$Cd($\alpha$,$\gamma$)$^{112}$Sn measurements extended the range of previously measured cross sections down to lower energies and the $^{102}$Pd($\alpha$,$\gamma$)$^{106}$Cd and $^{110}$Cd($\alpha$,$\gamma$)$^{114}$Sn reactions were measured for the first time. The measurements were compared to theoretical models from Talys 1.9. The ($\gamma$,$\alpha$) and ($\gamma$,n) rates were then calculated using the best fit model and compared in order to investigate the relative intensity between the reaction pathways. It was found that in all cases the ($\gamma$,$\alpha$) reaction pathway begins to dominate within the temperature range of 1.5-3.5 GK.   

\textbf{Photoneutron reaction cross section measurements on }$^{\mathrm{\mathbf{94}}}$\textbf{Mo and }$^{\mathrm{\mathbf{90}}}$\textbf{Zr relevant to the }\textbf{\textit{p}}\textbf{-process nucleosynthesis}

The photodisintegration cross sections for the $^{\mathrm{94}}$Mo($\gamma $,n) and $^{\mathrm{90}}$Zr($\gamma $,n) reactions have been experimentally investigated with quasi-monochromatic photon beams at the High Intensity $\gamma $-ray Source (HI$\gamma $S) facility of the Triangle Universities Nuclear Laboratory (TUNL). The energy dependence of the photoneutron reaction cross sections was measured with high precision from the respective neutron emission thresholds up to 13.5 MeV. These measurements contribute to a broader investigation of nuclear reactions relevant to the understanding of the $p$-process nucleosynthesis. The results are compared with the predictions of Hauser-Feshbach statistical model calculations using two different models for the dipole $\gamma $-ray strength function. The resulting $^{\mathrm{94}}$Mo($\gamma $,n) and $^{\mathrm{90}}$Zr($\gamma $,n) photoneutron stellar reaction rates as a function of temperature in the typical range of interest for the $p$-process nucleosynthesis show how sensitive the photoneutron stellar reaction rate can be to the experimental data in the vicinity of the neutron threshold.