Discovery of the new isotope <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>Lr</mml:mi><mml:mprescripts/><mml:none/><mml:mn>251</mml:mn></mml:mmultiscripts></mml:math>: Impact of the hexacontetrapole deformation on single-proton orbital energies near the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Z</mml:mi><mml:mo>=</mml:mo><mml:mn>100</mml:mn></mml:mrow></mml:math> deformed shell gap
T. H. Huang, D. Seweryniak, B. B. Back, P. C. Bender, M. P. Carpenter, P. Chowdhury, R. M. Clark, P. Copp, Xiao-Tao He, R.-D. Herzberg, D. E. M. Hoff, H. Jayatissa, T. L. Khoo, F. G. Kondev, G. Morgan, C. Morse, A. Korichi, T. Lauritsen, C. Müller-Gatermann, D. H. Potterveld, W. Reviol, A. M. Rogers, S. Saha, G. Savard, Kanishka Sharma, S. Stolze, S. Waniganeththi, G. L. Wilson, J. Wu, Yang-Yang Xu, S. Zhu
Abstract
The products of the $^{203,205}\mathrm{Tl}(^{50}\mathrm{Ti},2n)$ fusion-evaporation reactions were studied using the recently commissioned Argonne Gas-Filled Analyzer at Argonne National Laboratory. Two $\ensuremath{\alpha}$-decay activities with energies of 9210(19) and 9246(19) keV and half-lives of ${42}_{\ensuremath{-}14}^{+42}$ and $24.{4}_{\ensuremath{-}4.5}^{+7.0}$ ms were observed which were followed by the known $\ensuremath{\alpha}$ decays of $^{247}\mathrm{Md}$ and $^{243}\mathrm{Es}$. They are interpreted as originating from the $1/{2}^{\ensuremath{-}}[521]$ and $7/{2}^{\ensuremath{-}}[514]$ single-proton Nilsson states in the hitherto unknown isotope $^{251}\mathrm{Lr}$. From the measured ${Q}_{\ensuremath{\alpha}}$ values the $1/{2}^{\ensuremath{-}}$ level was placed 117(27) keV above the $7/{2}^{\ensuremath{-}}$ level in $^{251}\mathrm{Lr}$ in contrast to $^{255}\mathrm{Lr}$ where the $1/{2}^{\ensuremath{-}}$ level is the lowest. Also, the $\ensuremath{\alpha}$ decay of $^{253}\mathrm{Lr}$ was studied in more detail and a new $\ensuremath{\alpha}$ line at 8660(20) keV was found and a new half-life value of 2.46(32) s for an isomeric state in $^{253}\mathrm{Lr}$ was measured. The $^{251,253,255}\mathrm{Lr}\phantom{\rule{4pt}{0ex}}{Q}_{\ensuremath{\alpha}}$ values were compared with predictions of various mass models. The relative energies of the $1/{2}^{\ensuremath{-}}[521]$ and $7/{2}^{\ensuremath{-}}[514]$ single-proton Nilsson states in $^{251,253,255}\mathrm{Lr}$ 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 $^{251}\mathrm{Lr}$ and $^{255}\mathrm{Lr}$ was reproduced only when the hexacontetrapole deformation ${\ensuremath{\varepsilon}}_{6}$ was included in the calculations.