Lifetime measurements of excited states in Cr55

Abstract

Excited states in $^{55}\mathrm{Cr}$ have been populated via the fusion-evaporation reaction $^{48}\mathrm{Ca}(^{11}\mathrm{B},p3n){\phantom{\rule{0.16em}{0ex}}}^{55}\mathrm{Cr}$ at a beam energy of 32 MeV. The Cologne plunger device surrounded by a $\ensuremath{\gamma}$-ray detector array was employed to determine lifetimes with the recoil-distance Doppler-shift method. $\ensuremath{\gamma}$ rays were observed by one Ge EUROBALL cluster detector and five HPGe detectors. $\ensuremath{\gamma}\ensuremath{\gamma}$-coincidence data were analyzed with help from the differential decay-curve method, and precise lifetimes for the first excited states were extracted from the $5/{2}^{\ensuremath{-}}\ensuremath{\rightarrow}3/{2}^{\ensuremath{-}}$ and the $9/{2}^{\ensuremath{-}}\ensuremath{\rightarrow}5/{2}^{\ensuremath{-}}$ transitions. Reduced transition strengths $B(\ensuremath{\sigma}\ensuremath{\lambda})$ were determined and compared to shell-model calculations employing four interactions KB3G, FPD6, GXPF1A, and GXPF1Br. The calculations were also performed for the $N=31$ isotonic chain from $^{53}\mathrm{Ti}$ to $^{61}\mathrm{Zn}$. Inspection of the wave functions as well as particle-plus-rotor model calculations allow for a detailed understanding of the excited states and the reduced transition strength in $^{55}\mathrm{Cr}$. The interactions GXPF1A and GXPF1Br reproduce well the experimental findings in $^{55}\mathrm{Cr}$ and in other isotones.