Forward-to-backward asymmetry of the (<i>γ</i>,n) reaction in the energy range 20<i>–</i>30 MeV

Abstract

The forward-to-backward asymmetry of neutrons emitted in the (\ensuremath{\gamma},n) reactions on $^{\mathrm{nat}}\mathrm{Pb}$ and $^{\mathrm{nat}}\mathrm{Cd}$ targets was measured for photons in the range of 20--30 MeV, where the isovector quadrupole giant resonance is expected to lie. The asymmetry was observed to increase from small values (\ensuremath{\approxeq}0.2) to large ones (\ensuremath{\approxeq}0.6 and 0.8) for $^{\mathrm{nat}}\mathrm{Cd}$ and $^{\mathrm{nat}}\mathrm{Pb}$, respectively. This phenomenon is interpreted as the interference between E1 and E2 amplitudes. From an analysis of the asymmetry the excitation energies of the E2 isovector resonances were estimated to be 23.5\ifmmode\pm\else\textpm\fi{}1.5 MeV and 26.5\ifmmode\pm\else\textpm\fi{}1.5 MeV for the Pb and Cd nuclei, respectively. The E2 isovector resonances are found to be considerably wider than the E1 resonances.