Precise determination of Λ12C level structure by γ-ray spectroscopy

Abstract

The level structure of the ${}^{12}_{\Lambda }{\rm C}$ hypernucleus was precisely determined by means of $\gamma$-ray spectroscopy. We identified four $\gamma$-ray transitions via the ${}^{12}{\rm C}(\pi ^{+ },K^{+ }\gamma)$ reaction using a germanium detector array, Hyperball2. The spacing of the ground-state doublet $(2^{-},1^{-}_{1})$ was measured to be $161.5 \pm 0.3{\text {(stat)}}\pm 0.3(\hbox {syst})$keV from the direct $M1$ transition. Excitation energies of the $1^{-}_{2}$ and $1^{-}_{3}$ states were measured to be $2832\pm 3\pm 4$keV and $6050\pm 8\pm 7$keV, respectively. The level energies obtained provide definitive references for the reaction spectroscopy of $\Lambda$ hypernuclei.