The fusion reaction of carbon and helium to oxygen is the key to understanding the evolution of stars and the relative abundances of both elements. The reaction rate of 12C(α,γ)16O has to be known with an uncertainty of lower than 10% at a center-of-mass energy of 300 keV during Helium burning conditions. So far, experiments have studied the reaction down to about 1 MeV, and the necessary extrapolation to lower energies is difficult and causes large uncertainties.
To access the experimentally challenging energy regime, the inverse reaction was studied: The Coulomb dissociation of 16O into 4He and 12C was measured at the R3B setup in a first campaign (experiment s454) within FAIR Phase-0 at GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt. The goal is to improve the accuracy of the experimental data and to reach lower center-of-mass energies of about 800 keV.
The experiment required beam intensities of one billion 16O ions per second at an energy of 500 AMeV. The rare case of Coulomb breakup into 12C and 4He posed another challenge: particles with the same magnetic rigidity as the primary beam had to be detected, which are not separated in position by the superconducting magnet GLAD. Radical changes of the R3B setup were necessary: All detectors had slits to allow the passage of the unreacted 16O ions, while 4He and 12C would hit the detectors' active areas. These detectors were developed and built based on organic scintillators to track and identify the reaction products with sufficient precision.
The novel detector setup was fully operational and well tested before the beamtime. During the run, all charges of interest were identified in the online analysis (Z = 8 for oxygen, Z = 6 for carbon, and Z = 2 for helium). The tracking of the particles during data analysis will allow the determination of their kinematic properties.
Unfortunately, after an unexpected long setup phase of two days, the accelerator could not reach the conditions desired by the experiment. The extraction method was changed to knock-out extraction to obtain a focused beam with acceptable intensities. However, the extraction method caused a micro-structure of the spills resulting in high deadtimes of the data acquisition system. Overall, the statistics on tape is about a factor 20 lower than expected, which is especially problematic for reactions with very small cross sections at low center-of-mass energies.
Nevertheless, many events at higher energies were recorded which will allow the validation of the method and the comparison against results of previous direct reaction measurements. The first analysis of the accelerator performance states that the beam requirements are technically achievable by the facility. Further investigations will be carried out during the engineering run at the end of 2019.
Michael Heil, Kathrin Göbel and René Reifarth
for the s454 experiment (spokesperson Michael Heil) and the R3B collaboration