Gamma-ray Spectroscopy of 19ΛF

Abstract

A study of ΛN interaction is considered as the first step to extend the nucleon-nucleon world to the general baryon-baryon world. Due to short lifetime of Λ, spectroscopies of Λ hypernuclei have been the most powerful tool for the ΛN interaction study. Especially, excited energy levels of s-shell and p-shell Λ hypernuclei were precisely measured by gamma-ray spectroscopies, and it provided crucial information of the spin-dependent ΛN interaction. However, the experiments have been limited to light hypernuclei, so far. We performed a gamma-ray spectroscopy of 19ΛF, which is the first experiment of sd-shell hypernuclei. The experiment were carried out at K1.8 beamline in J-PARC hadron hall facility from May to June in 2015. 19ΛF hypernuclei were produced via (K-, pi-) reaction with the beam momentum 1.8 GeV/c and liquid CF4 target. The produced 19ΛF hypernuclei were idendified using Superconducting Kaon Spectrometer (SKS) and K1.8 beam line spectrometer. At the same time, gamma rays from the 19ΛF were detected by using a new germanium detector system (Hyperball-J). Four gamma rays from 19ΛF are observed at 315 keV, 895 keV, 1266 keV, and 953 keV. From the gamma rays, the energy spacing between the ground state spin doublet of 19ΛF is determined, and it is well represented by shell-model calculation estimated from ΛN spin-dependent interaction in p-shell hypernuclei. We expect the measured excitation energies of 19ΛF will provide important information of the spin-dependent interaction, especially the radial dependence of spin-spin interaction. The experiment will be also a guide for future experiments of heavier Λ hypernuclei.