Lifetimes of negative parity states inC11

Abstract

The lifetime limits ${\ensuremath{\tau}}_{m}<34, <12, <11, <8, \mathrm{and} <23$ fs for the negative parity states in $^{11}\mathrm{C}$, at 2000, 4319, 4804, 6478, and 8425 keV, respectively, were determined with the Doppler shift attenuation method through the $^{10}\mathrm{B}$($p$,$\ensuremath{\gamma}$)$^{11}\mathrm{C}$ reaction. The analog of the $^{11}\mathrm{C}$ 6478 keV state lies at 6743 keV in $^{11}\mathrm{B}$ and was measured to have a lifetime ${\ensuremath{\tau}}_{m}=15\ifmmode\pm\else\textpm\fi{}6$ fs using the $^{7}\mathrm{Li}$($\ensuremath{\alpha}$,$\ensuremath{\gamma}$)$^{11}\mathrm{B}$ reaction. Experimentally obtained correction factors for the nuclear and electronic stopping powers were used in the Doppler shift attenuation analysis. In the calculations the Monte-Carlo method was employed. A new $\ensuremath{\gamma}$-ray decay scheme obtained for the $^{10}\mathrm{B}$($p$,$\ensuremath{\gamma}$)$^{11}\mathrm{C}$ reaction at ${E}_{p}=1110$ keV together with the angular distribution data illustrate that the properties of the resonant state reported to exist at ${E}_{x}=9.73$ MeV in $^{11}\mathrm{C}$ are strongly influenced by background processes and that the resonance structure is ambiguous. In the light of the new lifetime values, the transition strengths are discussed in terms of shell model calculations.NUCLEAR REACTIONS $^{10}\mathrm{B}$($p$,$\ensuremath{\gamma}$), $E=1.11$ MeV; $^{7}\mathrm{Li}$($\ensuremath{\alpha}$,$\ensuremath{\gamma}$), $E=0.96$ MeV; measured ${E}_{\ensuremath{\gamma}}$, ${I}_{\ensuremath{\gamma}}(\ensuremath{\theta})$, Doppler-shift attenuation. $^{11}\mathrm{C}$ levels deduced $\ensuremath{\gamma}$-ray branching ratios, ${\ensuremath{\tau}}_{m}$. $^{11}\mathrm{B}$ level deduced ${\ensuremath{\tau}}_{m}$. Ge (Li) detector, enriched targets.