Session K6: Mini-Symposium on Nuclear Physics: Sensitive Input for Understanding Nucleosynthesis II

Understanding the sensitivity of core-collapse supernovae to weak interaction rates

In the past decade the treatment of electroweak interactions in core-collapse supernovae (CCSNe) simulations has improved significantly. As an example, we now understand the critical role electron capture plays in destabilizing the stellar core to collapse and its impact on the whole thermodynamic composition/configuration during the in-fall epoch. Electron capture rates are therefore crucial nuclear physics inputs to CCSNe simulations. These rates are estimated on the basis of theoretical nuclear structure calculations which are evaluated against results from beta-decay and charge-exchange experiments. By implementing detailed weak interaction rates into neutrino transport calculations utilized by the CCSNe code GR1D, we are now able to test in great detail the sensitivity of the late evolution of supernovae to electron-capture rates. This effort will help identify which experiments (including those in the future at FRIB) are most important for improving nuclear theory and, by extension, the astrophysical models. In this presentation I will briefly discuss these recent efforts, their connection to experiment, and a few preliminary results.   

Informing Neutron-Capture Rates through (d,p) Reactions on Neutron-Rich Tin Isotopes

Level energies and spectroscopic information for neutron-rich nuclei provide important input for r-process nucleosynthesis calculations; specifically, the location and strength of single-neutron $\ell=1$ states when calculating neutron-capture rates. Surman and collaborators have performed sensitivity studies to show that varying neutron-capture rates can significantly alter final r-process abundances. However, there are many nuclei important to the r-process that cannot be studied. Extending studies to more neutron-rich nuclei will help constrain the nuclear shell-model in extrapolating to nuclei even further from stability. The (d,p) reaction has been measured with radioactive ion beams of $^{126}$Sn and $^{128}$Sn to complete the set of (d,p) studies on even mass tin isotopes from doubly-magic $^{132}$ to stable $^{124}$Sn.   

Towards an In-Beam Measurement of the Neutron Lifetime to 1 Second

A precise value for the neutron lifetime is required for consistency tests of the Standard Model and is an essential parameter in the theory of Big Bang Nucleosynthesis. A new measurement of the neutron lifetime using the in-beam method is planned at the National Institute of Standards and Technology Center for Neutron Research. The systematic effects associated with the in-beam method are markedly different than those found in storage experiments utilizing ultracold neutrons. Experimental improvements, specifically recent advances in the determination of absolute neutron fluence, should permit an overall uncertainty of 1 second on the neutron lifetime. The dependence of the primordial mass fraction on the neutron lifetime, technical improvements of the in-beam technique, and the path toward improving the precision of the new measurement will be discussed.   

A Novel Approach to Study of Neutron Producing Reactions for Nuclear Astrophysics

Neutron producing reactions such as $^{13}$C($\alpha$,n)$^{16}$O which serve as dominate neutron sources for the s-process, incur experimental challenges due to the difficulties in detection of neutrons. Measurements of such reactions at low energies usually involve the use of 3He counters or n/$\gamma$-convertors but these methods do not provide neutron spectroscopy. Neutron Time-of-Flight (n-ToF) provides spectroscopy but requires ether beam pulsing or a fast recoil trigger. The University of Michigan Deuterated Scintillator Array appears to be well suited for such measurements either above or below ground. The array has been shown to provide n/$\gamma$ discrimination, low background, and can yield neutron spectroscopic information without the use of n-ToF relying instead on matrix inversion techniques for spectrum unfolding. Methods such as MLEM, CGRN, and Artificial Neural Networks permit extraction of discrete neutron energy groups imposed on a continuous background. Preliminary measurements of the $^{13}$C($\alpha$,n)$^{16}$Oreaction conducted at the 10 MV FN tandem and the new 5U high intensity accelerator at the University of Notre Dame will be shown. This work is supported by NSF grants PHY 0969456.   

Neutron Transport and Systematic Studies with the UCN$\tau$ Experiment at LANL

The neutron lifetime $\tau_{n}$ is important for understanding charged-current interactions and is a key input for understanding nucleosynthesis. The $\tau_{n}$ uncertainty dominates the uncertainty in the fractional $^{4}$He abundance $Y_{p}$, and comparing $Y_{p}$ with astrophysical measurements constrains the effective number of neutrino degrees of freedom which is sensitive to beyond standard model physics. Recent inconsistencies in the $\tau_{n}$ global data have motivated new experiments and the study of systematic effects including those related to the dynamics and loss of trapped ultracold neutrons (UCN). We present the progress of the UCN$\tau$ experiment at the Los Alamos Neutron Science Center which uses a NdFeB Halbach array to magnetically and gravitationally confine UCN with minimal loss aside from neutron $\beta$ decay. The UCN can be emptied from the trap and counted in order to deduce the storage time or detected using \textit{in situ} UCN detection. We present an overview of the apparatus and discuss systematic studies used to develop a next generation measurement of $\tau_{n}$. We discuss recent neutron guide transmission studies to increase the number of trapped UCN and present recent results with a vanadium foil activation technique for UCN detection.   

Time-of-flight Mass Measurement of Neturon-rich Nuclei

Nuclear masses can be used to identify changes in nuclear structure and are necessary for accurate modeling of extreme astrophysical environments. Beyond the limit of known masses, theoretical predictions are relied upon, however these predictions often disagree. The TOF-B$\rho $ method provides a way to measure the masses of nuclei far from the valley of beta-stability with sufficient precision to map general features in nuclear structure and substantially reduce nuclear physics uncertainty in astrophysics simulations. We recently performed a TOF-B$\rho $ mass measurement at the NSCL where significant progress has been made on the neutron-rich side of stability in the Sulfur to Zinc region. Preliminary data and details of the analysis procedure will be discussed.   

First experiments performed with the JENSA gas jet target system

With the Jet Experiments in Nuclear Structure and Astrophysics (JENSA) gas jet target, we have the unique opportunity to study reactions with pure gas targets. One reaction of interest is $^{20}$Ne(p,t)$^{18}$Ne. $^{18}$Ne can decay via electron capture to the potential astrophysical observable $^{18}$F, and states in $^{18}$Ne affect the rate of some other astrophysically important reactions, such as $^{17}$F(p,$\gamma$)$^{18}$Ne . We present some of the first transfer reaction results using the JENSA gas jet target system performed at Oak Ridge National Laboratory, with a focus on the $^{20}$Ne(p,t)$^{18}$Ne transfer reaction performed with a 37MeV proton beam.   

Determining the resonance strength of the $^{56}$Ni \textit{rp}-process waiting point through (d,n) with VANDLE and MoNA-LISA

The rapid proton capture (\textit{rp}) process of explosive nucleosynthesis is believed to be the driver of $X$-ray bursts and creates nuclei up to around mass 110. Whereas much of this process burns in an equilibrium determined by half-lives and masses, the waiting point at $^{56}$Ni is unique. At this point the process reaches its peak luminosity and the synthesis of almost all heavier nuclei pass through the $^{56}$Ni(p,$\gamma )^{57}$Cu reaction. Since the gamma-decay width dominates the relevant resonance in $^{57}$Cu, a measurement of its proton partial width can be used to extract the proton-capture resonance strength. An experiment to do this was performed at the NSCL using the Versatile Array of Neutron Detectors at Low Energy (VANDLE) along with the MoNA-LISA neutron detector arrays; and was the commissioning experiment for VANDLE with a transfer reaction. The events in the digitizing electronics of VANDLE were event-matched to the MoNA-LISA-Sweeper data acquisition system.   

Particle-gamma measurements for nuclear astrophysics

Transfer reactions in inverse kinematics are one of the few probes available to study in detail the single-particle structure of neutron-rich nuclei involved in r-process nucleosynthesis. Measurement of de-excitation gamma rays in coincidence with the charged reaction products can aid significantly in resolving the states populated. In addition, the measurement of gamma rays can provide constraints on numerous other properties, such as spin-parities, branching ratios and lifetimes of levels, and is critical to the surrogate technique for determining statistical neutron-capture cross sections. The Gammasphere ORRUBA Dual Detectors for Experimental Structure Studies (GODDESS) is a system designed for such measurements with beams from the ATLAS facility at Argonne National Laboratory. Details of the system and the first planned measurements will be presented.