Session G17: The Nucleosynthesis of Proton-Rich Isotopes

Live

The proton-rich side of the valley of stability hosts a variety of astrophysical processes, powering thermonuclear explosions and contributing to cosmic nucleosynthesis. Understanding these processes is key to answering fundamental questions about our universe, such as "where were the elements made?" and "how does matter behave at ultra high densities?". I will briefly review the status of nuclear astrophysics involving proton-rich nuclides, focusing on proton-rich nucleosynthesis involving neutron star formation and accreting neutron star systems.   

Live

The dual-accelerator, Laboratory for Experimental Nuclear Astrophysics (LENA) was built to allow direct measurements of astrophysically-interesting cross sections and has been used to study nucleosynthesis and stellar evolution in a variety of scenarios. A unique feature of LENA has been its high-current, 230-kV ECR accelerator, which when combined with a $\gamma$-ray coincidence spectrometer, permits measurements at sensitivities that rival what can be achieved at existing underground facilities. We are in the process of designing and building a new facility that replaces the current one, LENA II. LENA II will also feature two accelerators: An upgraded ECR accelerator and a 2-MV Singletron accelerator. The ECR upgrade is intended to produce H beams with DC currents of up to 20 mA and with the ability to produce pulsed beams that will allow for further reductions in detected room and cosmic-ray backgrounds. The Singletron accelerator (currently under construction at High Voltage - Europa) is an entirely new design, combining high beam intensities with ns pulsing. This talk will describe the capabilities of LENA II and give an example of experiments that will be made possible by this new facility.   

Testing the Importance of Nuclear Reactions in X-ray Bursts Using MESA

Type I x-ray bursts are thought to occur on accreting neutron stars via the (r-p) process, a sequence of rapid proton captures and $\beta+$ decays that proceed up to the A = 100 region. Understanding these reactions is key to understanding the explosion mechanism, but many involve unstable nuclei that are difficult to produce for use in experiments. Thus, sensitivity studies are a useful steering mechanism to guide the experimental community and optimize the application of resources. We have begun such a study, based on a model of an accreting neutron star using Modules for Experiments with Stellar Astrophysics, which incorporates a nuclear reaction network of 305 species and 3000 reactions, including (\textit{n-$\gamma$}), (\textit{n,p}), (\textit{n-$\alpha$}), (\textit{p-$\gamma$}), (\textit{$\alpha$-p}), (\textit{$\alpha$-$\gamma$}), weak reactions, and a few important complex reactions. A series of models is calculated in which each reaction is varied by a factor to test its effect on important features of the model, such as observables and abundances. From this the most impactful reactions are selected in order to identify the most useful and important reactions to the X-ray burst environment and the (r-p) process. This talk will discuss the process and preliminary results   

Neutron Spectroscopy of Low-Lying Resonances in $^{26}$Si via ($^{3}$He,n)

Spectroscopic information of low-lying resonant states in the proton-rich isotope $^{26}$Si is needed in order to understand various astrophysical scenarios. Final abundances of $^{26}$Al in novae are strongly dependent on the $^{25}$Al(p,$\gamma$)$^{26}$Si reaction-rate. The $^{25}$Al(p,$\gamma$)$^{26}$Si reaction also has implications in the rp-process occurring in novae and X-ray bursts. Additional information is needed to provide spin-parity assignments and to reduce the uncertainties in the partial widths of several low-lying states above the proton threshold in $^{26}$Si. A spectroscopy study of the low-lying resonance states in $^{26}$Si will be done at John D. Fox lab using neutron/gamma coincidence with the recently developed CATRiNA neutron detector and the FSU Clover gamma detector array at the sensitivity levels needed to constrain the astrophysical reaction rates. States in $^{26}$Si are populated via the $^{24}$Mg($^{3}$He,n)$^{26}$Si reaction at beam energies of 10 MeV. The CATRiNA array consists of 16 deuterated-benzene liquid scintillator detectors and uses the time-of-flight technique alongside pulse-height information is to extract neutron energies. In this work, we povide neutron information obtained with the CATRiNA detector.   

Not Participating

Sensitivity studies have identified the $^{17}$F($\alpha,p$)$^{20}$Ne reaction as being among the most important reactions affecting the X-ray burst light curve in some cases, but this reaction rate has been based on statistical model predictions with large uncertainties. We used existing information and an $R$ matrix analysis of $^{20}$Ne$+p$ reaction data to determine the $^{17}$F($\alpha,p$)$^{20}$Ne reaction rate at X-ray burst temperatures. We characterized the uncertainties in the reaction rate arising from limitations in existing data. Experimental efforts are aimed at providing new data to reduce the uncertainty in the reaction rate arising from the uncertain properties of resonant states in $^{21}$Na. We will present results from a recent measurement of the $^{20}$Ne($p,\alpha$)$^{17}$F reaction performed at the Fox Laboratory at FSU. A proton beam bombarded a $^{20}$Ne gas cell, and positron annihilation was measured to determine the $^{20}$Ne($p,\alpha$)$^{17}$F cross section by the activation technique. Results will be compared to earlier measurements. We will also discuss future plans for measurements performed in inverse kinematics using beams of $^{20}$Ne and the importance they have in constraining the $^{17}$F($\alpha,p$)$^{20}$Ne reaction rate.   

Not Participating

A key open question in the field of nuclear astrophysics relates to the production of heavy elements throughout our Galaxy. In particular, the origins of 35 n-deficient nuclides in between Se and Hg, that cannot be formed by neutron capture processes remain obscure. At present the production sites for p-nuclei are believed to be Type-II supernova, however a lack of experimental information on reaction rates makes any comparison with astrophysical observations extremely difficult. Sensitivity studies have been performed to identify reactions which significantly affect the production of p-nuclei, one of which is $^{\mathrm{83}}$Rb(p,$\gamma )$ $^{\mathrm{84}}$Sr. Due to the energies involved in the p-process and the need for an intense beam of $^{\mathrm{83}}$Rb there is currently no experimental information on this reaction rate, or indeed any p-process rate involving a radioactive reactant. Using the newly commissioned recoil mass spectrometer EMMA in combination with the $\gamma $-ray spectrometer TIGRESS we have performed a direct measurement of the $^{\mathrm{83}}$Rb(p,$\gamma )^{\mathrm{84}}$Sr reaction. This represents the first direct measurement of a supernova reaction using a radioactive beam in the Gamow energy window of p process burning. This talk will discuss the results of the measurement and its implications for the production of p nuclei.