Session FG: Mini-Symposium: Electrodisintegration Constraints on Astrophysical Radiative Capture Rates

Electrodisintegration of $^{16}$O: Measurement and Astrophysical implication

Radiative capture reactions play an essential role in stellar nucleosynthesis, but for some of them, the precise determination of their reaction rates at astrophysical energies proved to be extremely challenging. The most prominent example is the $^{12}$C$(\alpha,\gamma)^{16}$O reaction, for which even after five decades of experimental effort the uncertainty of the reaction rate at stellar energies still did not reach the goal of $\sim$10\%. By using the state-of-the-art gas jet target and the new generation of energy-recovery linear accelerators (ERLs) to achieve high luminosity, the measurement of the electrodisintegration of $^{16}$O close to threshold can be utilized to determine the $^{12}$C$(\alpha,\gamma)^{16}$O reaction rate with significantly increased precision. We present the formalism, which relates real- and virtual-photon-disintegration reactions and discuss some aspects of designing an optimal experiment. After the new ERLs come online, the presented approach needs to be validated experimentally, but if successful, the same procedure can be used to improve the precision of other astrophysically-important radiative capture reactions.   

Impact of $^{16}$O($e,e'\alpha$)$^{12}$C and $^{16}$O($\gamma,\alpha$)$^{12}$C measurements on the $^{12}$C($\alpha,\gamma$)$^{16}$O astrophysical reaction rate

The $^{12}$C($\alpha,\gamma$)$^{16}$O reaction, an important component of stellar helium burning, plays a key role in nuclear astrophysics. Providing a reliable estimate for the energy dependence of this reaction at stellar helium burning temperatures has been a major goal for the field. In this work, we study the role of potential new measurements of the inverse reactions, $^{16}$O($e,e'\alpha$)$^{12}$C and $^{16}$O($\gamma,\alpha$)$^{12}$C, in reducing the overall uncertainty. A multilevel $R$-matrix analysis is used to make extrapolations of the astrophysical S factor for this reaction to the stellar energy of 300 keV. The statistical precision of the $S$-factor extrapolation is determined by performing multiple fits to existing $E1$ and $E2$ ground state capture data, including the impact of possible future measurements of the $^{16}$O($e,e'\alpha$)$^{12}$C and $^{16}$O($\gamma,\alpha$)$^{12}$C reactions. In particular, we consider a proposed MIT experiment that will make use of a high-intensity low-energy electron beam that impinges on a windowless oxygen gas target and a proposed Jefferson Lab experiment that will make use of bremsstrahlung and a bubble chamber in order to measure the total cross section for the inverse reaction.   

Extrapolating the $^{12}$C$(\alpha,\gamma)^{16}$O cross section to astrophysical energies using phenomenological $R$-matrix

The $^{12}$C$(\alpha,\gamma)^{16}$O reaction plays a lead role in the energy production and nucleosynthesis in many astrophysical environments. At the representative energy of 0.3~MeV, the cross section is estimated to be only 2$\times$10$^{-17}$ barns. Compare this to the lowest energy measurements at about 1~MeV, where the cross section is about 2$\times$10$^{-12}$ barns (similar to Higgs boson production), and it is easy to see why it is such a struggle to measure this reaction directly. The underlying nuclear structure of $^{16}$O produces broad resonances in the $^{12}$C$(\alpha,\gamma)^{16}$O cross section. As it happens, the region of interest lies right in an off-resonance area where interference dominates. This is the main reason why extrapolating the cross section below the region of experimental data is so challenging. In this talk, I will discuss the underlying reaction components from which we can gain further insight into additional measurements that can be made to better constrain the model and thus improve the extrapolation. As it turns out, this can be achieved not only by pushing measurements to lower energy, but also through targeted measurements at higher energies. An emphasis will be placed on upcoming inverse $^{16}$O$(\gamma,\alpha)^{12}$C measurements.   

Studies of the $^{16}\mathrm O(\gamma^*,\alpha)^{12}\mathrm C$ reaction for astrophysical relevance at MAGIX/MESA

MAGIX is a versatile fixed-target experiment and will be operated at the new electron accelerator MESA (Mainz Energy-Recovering Superconducting Accelerator) in Mainz. The accelerator will deliver (un)polarized electron beams with currents up to $1\,\mathrm{mA}$ at $105\,\mathrm{MeV}$. Using its internal gas-target, MAGIX will reach a luminosity of $\mathcal{O}$$({10}^{35}\,\mathrm{cm}^{-2}\mathrm{s}^{-1})$. This allows to study processes with very low cross section at small momentum transfer in a rich physical program. \\[2ex] The nucleosynthesis process $^{12}\mathrm C(\alpha,\gamma)^{16}\mathrm O$ has a high astrophysical relevance. At MAGIX, an experiment is planned to determine the S-Factor of this reaction by measuring the inverse reaction $^{16}\mathrm O(\gamma^*,\alpha)^{12}\mathrm C$. Therefore electrons will be scattered inelastically on oxygen atoms, the scattered electrons and the produced $\alpha$-particles are detected in coincidence. The cross section will be determined as a function of the outgoing center of mass energy of the carbon-$\alpha$-system for the calculation of the S-factor. In this talk the experimental setup and the results of the current simulations are discussed. Furthermore, the accessible parameter range at MAGIX is specified.   

Design and Operation of a Windowless Gas Target Internal to a Solenoidal Magnet for Use with a Megawatt Electron Beam

A windowless gas target has been designed, assembled, and tested, which is driven by the DarkLight experiment to search for a new force mediator beyond the standard model. The target is an essential component of the experiment that runs with the 100 MeV scale megawatt electron beam at an Energy Recovery Linac (ERL). After the target system was operated at a commissioning run in 2016 at Low Energy Recirculator Facility (LERF) at Jefferson Lab, it was further improved and calibrated at MIT Bates in 2017. The windowless gas target was verified to maintain sufficiently high density and desired pressure gradient as required for experiments in an ERL environment including the DarkLight.   

Precision absolute polarimeter development for the 3He$++$ ion beam at 5.0-6.0 MeV energy.

There is opportunity for precision measurements of the absolute $^{\mathrm{3}}$He$^{\mathrm{++}}$ polarization at beam energies 5.0-6.0 MeV after the EBIS Linac. The analyzing power for the elastic scattering of spin-1/2 particles ($^{\mathrm{3}}$He) on spin-0 particles ($^{\mathrm{4}}$He) can reach the maximum theoretical value A$_{\mathrm{N}}=$1 at some point (E$_{\mathrm{beam}}$, $\theta _{\mathrm{CM}})$. The main effort of this R@D will be development of precision absolute polarimeter for the measurements of the $^{\mathrm{3}}$He$^{\mathrm{++}}$ beam polarization produced in the EBIS as a reference for the further polarization measurements along accelerator chain. The polarimeter vacuum system is integrated in the spin-rotator transport line. The $^{\mathrm{3}}$He$^{\mathrm{++}}$ ion beam will enter the scattering chamber through the thin window to minimize beam energy losses. The scattering chamber is filled with $^{\mathrm{4}}$He gas at \textasciitilde 5 torr pressure. The silicon strip detectors will be used for energy and TOF measurements of the scattered $^{\mathrm{3}}$He and recoil $^{\mathrm{4}}$He nuclei (in coincidence) for the identification of the scattering kinematics.   

A single fluid bubble chamber for measuring radiative capture reactions.

Radiative capture reactions play a critical role in nucleosynthesis. Direct studies by detecting the outgoing $\gamma $-rays are often performed at facilities deep underground to reduce the cosmic ray background. We have developed a new method to measure the time-inverse photo-dissociation reactions using a single-fluid bubble chamber. The large range of the $\gamma $-radiation allows for thicker targets, increasing the luminosity by several orders of magnitude, in addition to a factor of 10-100 gain in luminosity from the reciprocity theorem. We will describe the operational principle of the bubble chamber and discuss first results of test measurements at JLAB where we have measured the cross section of the photodisintegration process $^{\mathrm{19}}$F($\gamma $,$\alpha )^{\mathrm{15}}$N by bombarding a superheated fluid of C$_{\mathrm{3}}$F$_{\mathrm{8}}$ with Brem $\gamma $ rays reaching cross sections of the time-reversed $^{\mathrm{15}}$N($\alpha $,$\gamma )^{\mathrm{19}}$F reaction of about 80 picobarn. Results of the $^{\mathrm{14}}$N($\gamma $,p)$^{\mathrm{13}}$C reaction will also be presented.   

Gas-Jet-, Cluster-Jet- and Droplet-Targets: Multi-purpose Tools for Nuclear Physics

Many experiments in nuclear physics and astrophysics demand for windowless jet targets delivering well-defined target beams of highest purity from various gases. Depending on the concrete experimental design, these requirements can be fulfilled by different target types. One possibility is a pure gas-jet target, providing supersonic jet streams with target thicknesses of, e.g., $10^{18}\,$atoms/cm$^2$ directly behind the nozzle. A further development of such gas targets are cluster-jet-targets. By using fine Laval-type nozzles in combination with cryogenic gases, the production of nm-sized clusters is possible, leading to well-defined cluster-jet beams with high thicknesses up to $10^{15}\,$atoms/cm$^2$ in a distance of even more than $2\,$m behind the nozzle. Therefore, such targets are well suited for $4\pi$ experiments. Even larger thicknesses can be achieved with more macroscopic objects like $\mu$m-sized, liquid or frozen droplets generated with a droplet target. High demands on all of these target types by recent experiments led to a boost with respect to new technological developments, resulting in an enormous improvement in performance. The properties of these target types, some prominent examples, and recent achievements will be presented and discussed.