Nuclear Physics Seminar

Most of elements heavier than Fe in the Universe are produced by a series of neutron capture reaction and ??-decay in stars. The s-process, which occurs under moderate neutron environments (~107-10 neutrons/cm3) such as in He burning of massive stars, is responsible for producing almost half of the heavy elements. Neutrons for the s-process environment is believed to be supplied by two dominant reactions, one of which is 22Ne(?,n)25Mg reaction. This reaction in massive stars is dominated by a few resonance reactions. Nevertheless, there remain large uncertainties about contribution of the reaction to the s-process nucleosynthesis because the reaction cross sections are too small for direct measurements due to Coulomb barrier (E? = 400-900 keV in the lab system). In the first half of this seminar, I will present our experiment to determine these resonance strengths with a cyclotron accelerator at Texas A&M University. The experiment was performed by an indirect approach using 6Li(22Ne,25Mg+n)d ?-transfer reaction, in which resonance properties such as neutron decay branching ratios of produced 26Mg were studied by measuring deuterons, ?-ray, and 26Mg in coincidence using large arrays of Si and Ge, and a magnetic spectrometer. Our results showed neutron production from 22Ne(?,n)25Mg reaction can be about 10 times lower than past measurements. The effect of our measurements on the s-process nucleosynthesis will be discussed. In the second half of this seminar, I will present our experiments to determine neutron capture cross sections of radioactive nuclei using the Surrogate Reaction method [1]. Neutron capture reactions for the s-process involve relatively long-lived nuclei neighboring stability in the nuclear chart. Therefore, the Surrogate Reaction, which creates the same compound nuclei as the neutron capture reaction using a stable beam and target, can be a useful approach. On the other hand, the r- process, which produces the other half