Improving the rate of the triple alpha reaction
The rate of the triple alpha process, which plays a central role in the production of 12C in stars, is known with an accuracy of about 12%. Variations within the +/- 12% errors in this rate can cause significant changes in the determination of the mass of the iron core in core-collapse supernovae (type II) and the composition of the material later ejected in the interstellar medium, as well as a factor of two change in the surface abundance of 12C in light ABG stars. The present triple alpha experiment aims therefore at reducing the uncertainty on the knowledge of this rate to about 6% by measuring more accurately than has been done in the past the pair branch for the 7.654 MeV state in 12C. This state is excited by inelastic proton scattering, taking advantage of a strong resonance at an excitation energy of 10.6 MeV and a scattering angle of 135 degrees in the lab. The protons are produced using the Tandem accelerator at Western Michigan University. A reduction in the gamma ray background is achieved by a coincidence requirement between a thin scintillator tube and the large block of plastic scintillator surrounding it. The pair branch is then given by the ratio of the number of electron-positron pairs detected in the plastic scintillators in coincidence with the protons scattered at 135 degrees to the total number of such scattered protons. The experimental status will be presented.