Discovery of the new isotope 251Lr: Impact of the hexacontetrapole deformation on single-proton orbital energies near the Z=100 deformed shell gap

The products of the 203,205Tl(50Ti, 2n) fusion-evaporation reactions were studied using the recently commissioned Argonne Gas-Filled Analyzer at Argonne National Laboratory. Two alpha-decay activities with energies of 9210(19) and 9246(19) keV and half-lives of 42+42-14 and 24.4+7.0 -4.5 ms were observed which were followed by the known alpha decays of 247Md and 243Es. They are interpreted as originating from the 1/2-[521] and 7/2-[514] single-proton Nilsson states in the hitherto unknown isotope 251Lr. From the measured Q alpha values the 1/2- level was placed 117(27) keV above the 7/2- level in 251Lr in contrast to 255Lr where the 1/2- level is the lowest. Also, the alpha decay of 253Lr was studied in more detail and a new alpha line at 8660(20) keV was found and a new half-life value of 2.46(32) s for an isomeric state in 253Lr was measured. The 251,253,255Lr Q alpha values were compared with predictions of various mass models. The relative energies of the 1/2-[521] and 7/2-[514] single-proton Nilsson states in 251,253,255Lr isotopes were compared with results of the cranking shell model with pairing treated using the particle-number-conserving method. The level separation and, in particular, the level order change between 251Lr and 255Lr was reproduced only when the hexacontetrapole deformation epsilon 6 was included in the calculations.