Polarized neutron capture intoC13: Evidence for a secondary doorway state effect

Abstract

Angular distributions for the $^{12}\mathrm{C}(\mathrm{n},{\ensuremath{\gamma}}_{0})^{13}\mathrm{C}$ reaction have been measured with neutrons corresponding to excitation energies of 16.0, 19.2, 20.1, 20.8, and 21.8 MeV. Legendre polynomial coefficients have been determined to second order (${a}_{3}$ is small). Polarized neutron measurements were obtained at excitation energies of 16.0, 18.6, 19.2, 20.1, 21.0, 21.8, and 22.3 MeV. A structure seen in the coefficient of the Legendre polynomial ${P}_{2}$ for the $^{12}\mathrm{C}(\mathrm{p},{\ensuremath{\gamma}}_{0})^{13}\mathrm{N}$ reaction is also seen in the present data. A secondary doorway state at 20.5 MeV with an apparent width of 0.5 MeV is suggested by previously measured $^{11}\mathrm{B}(\mathrm{d},{\ensuremath{\gamma}}_{0})^{13}\mathrm{C}$ data. The presence of this state provides an explanation for the energy dependence of the cross section, the ${a}_{2}$ coefficient, and the analyzing power coefficient ${b}_{2}$ in the $^{12}\mathrm{C}(\mathrm{n},{\ensuremath{\gamma}}_{0})^{13}\mathrm{C}$ data. The same model is also able to describe the energy dependence of the cross section and the ${a}_{2}$ coefficient observed in the $^{12}\mathrm{C}(\mathrm{p},{\ensuremath{\gamma}}_{0})^{13}\mathrm{N}$ reaction in the region of 16 to 22 MeV.