出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：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
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 triple alpha experiment presented here is a collaborative effort between theWestern Michigan
University (WMU) and the National Superconducting Cyclotron Laboratory (NSCL) and aims 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 ( 12C(p; p0)12C(7:654MeV) ), taking advantage of a strong
resonance at a bombarding energy of about 10.6 MeV and a scattering angle of 135 degrees in
the laboratory. The decay pairs are detected in an almost 4p scintillator system surrounding the
target. The protons are produced using the Tandem accelerator at Western Michigan University
and detected by silicon detectors at 135 degrees in the lab. The pair branch is given by the ratio
of the number of electron-positron pairs detected in the plastic scintillators in coincidence with
the protons in the 7.654 MeV silicon spectra to the number of those protons. A reduction in the
gamma ray background, mainly due to the cascade gamma decay of the 7.654 MeV state through
the 4.44 MeV 2+ state, is achieved by a coincidence requirement between a thin scintillator tube
and the large block of plastic scintillator surrounding it. The measurement is expected to take
place in the summer of 2006.
