Neutron decay from the giant resonance via theB10(e,e'n) reaction

Abstract

The cross sections and angular correlations for neutron decay into various states in the residual nucleus following the $^{10}\mathrm{B}$($e,{e}^{'}n$) reaction have been measured over the excitation energy range of 18--33 MeV at an effective momentum transfer of 0.56 fm${}^{\ensuremath{-}1}$. In the giant resonance, neutron emission leads to the population of two higher excited states in addition to the ground-state transition: 6.97 MeV $7/{2}^{\ensuremath{-}}({n}_{5}$) and 11.70 MeV $7/{2}^{\ensuremath{-}}+12.06$ MeV $3/{2}^{\ensuremath{-}}({n}_{6,7}$). This is the first observation of the neutron population of these states. The angular correlations for ${n}_{0}$ show a strong forward-backward asymmetry, which suggests interference from a transition with the opposite parity to $E1$. The angular correlations for ${n}_{5}$ and ${n}_{6,7}$ have a peak shift of about ${50}^{\ifmmode^\circ\else\textdegree\fi{}}$ at lower excitation energy and recover above about 24 and 25 MeV for ${n}_{5}$ and ${n}_{6,7}$, respectively. Their patterns are considerably different from that for ${n}_{0}$. The angular correlations for each transition were fitted with a Legendre polynomial. The longitudinal-transverse interference coefficient ${C}_{2}/{A}_{0}$ is negligible for all populations. For ${n}_{0}$ decay, all Legendre coefficients ${b}_{i}$ are positive, but ${b}_{2}$ and ${b}_{3}$ for the ${n}_{5}$ and ${n}_{6,7}$ decays are negative at lower excitation energy, and the latter causes a shift of the forward peak. The negative values may come from the signs of the phase differences of $\mathrm{cos}{\ensuremath{\delta}}_{21}$ and $\mathrm{cos}{\ensuremath{\delta}}_{20}$. The $^{10}\mathrm{B}$($e,{e}^{'}n$) cross section measured up to ${E}_{x}~32$ MeV agrees well with that of $^{10}\mathrm{B}$($\ensuremath{\gamma},n$), except for a peak at 23 MeV of the giant resonance. In comparison with shell-model calculations, the partial cross section for ${n}_{0}$ is sizable up to higher excitation energy, and predicted large partial cross sections populating the 6.97 MeV $7/{2}^{\ensuremath{-}}$ and 11.70 MeV $7/{2}^{\ensuremath{-}}+12.06$ MeV $3/{2}^{\ensuremath{-}}$ states in the giant resonance were not observed.