Feasibility Studies for Measuring the Astrophysical S-Factor of the Reaction ¹²C(α,γ)¹⁶O via Electro-Disintegration at MAGIX

Abstract

The nucleosynthesis process ¹²C(α,γ)¹⁶O is of atmost importance for nuclear astrophysics. The S-factor of this reaction is a major parameter for determining the abundance ratio of oxygen to carbon in stellar nucleosynthesis and plays a crucial role in understanding the pre-supernova evolution. The current relative error of more than 17% in the S-factor for energies in the Gamow-peak does not allow a prediction of the abundance of higher mass elements produced in the pre-supernova evolution of stars. Since the cross section of the reaction ¹²C(α,γ)¹⁶O is low, a high-precision low-energy experiment with a high luminosity is required to measure the S-factor. One such experiment that aims at measuring this key and challenging quantity is MAGIX. MAGIX is a versatile fixed-target experiment, which will be built at the new electron accelerator MESA in Mainz, Germany. The accelerator will deliver polarized and unpolarized electron beams to MAGIX with currents up to 1mA at 105MeV. Using its internal gas jet target, MAGIX will reach a luminosity of O(10³⁵ cm⁻²s⁻¹ ). MAGIX will be equipped with two magnetic spectrometers, that allow to measure the kinematic parameters of the scattered electrons with a momentum resolution of δp/p<10⁻⁴ and an angular resolution of ∆Θ<0.05°. To detect heavy charged particles, MAGIX will employ a silicon strip detector array. This will allow studying processes with very low cross sections at small momentum transfer within a rich physics program. At MAGIX, a measurement is planned in which the S-factor will be determined by investigating the electro-disintegration reaction ¹⁶O(e,e'α)¹²C. Electrons will be scattered inelastically on oxygen and MAGIX will detect the scattered electrons and the produced α-particles in coincidence. With this measurement, it will be possible to determine the S-factor of the reaction ¹²C(α,γ)¹⁶O as a function of the center of mass energy of the ¹²C-α system. The simulations predict that MAGIX will be able to measure the S-factor for center of mass energies above 1MeV with a precision competitive to the existing data. To improve these results, MAGIX will be operated in addition with a Zero Degree Tagger to detect the scattered electrons in the range θₑ < 0.5°. Given the promising simulation results, MAGIX expects to make important contributions to the measurement of the S-factor of the reaction ¹²C(α,γ)¹⁶O down to 0.5MeV, exploring a new territory in nuclear astrophysics.