GAMMASPHERE+FMA: A journey beyond the proton drip-line

The majority of experiments performed during the 2-year long stay of GAMMASPHERE at the Argonne National Laboratory aimed to study proton-rich nuclei far from the line of stability at and beyond the proton drip-line. A high reaction channel selectivity was required to assign in-beam γ-ray transitions to weakly populated exotic nuclei in the presence of background from strong reaction channels. In many of the experiments this was achieved by using the Argonne fragment mass analyzer to separate heavy-ion fusion-evaporation reaction products from scattered beam and disperse them according to their mass-over-charge-state ratio. For medium mass and heavy α and proton emitters the Recoil-Decay Tagging method was implemented. In-beam γ-ray transitions were observed in several proton emitters between Z=50 and Z=82. Among others, rotational bands were assigned to ¹⁴¹Ho and ¹³¹Eu. A quadrupole deformation of β= 0.25(4) was deduced for the ground state in ¹⁴¹Ho from the extracted dynamic moment of inertia. Based on observed band crossings and signature splittings the 7/2^-[523] and 1/2⁺[411] configurations were proposed for the ground state and the isomeric state, respectively. A comparison with particle-rotor calculations indicates, however, that ¹⁴¹Ho may have significant hexadecapole deformation and could be triaxial.