Cascade of high- <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>K</mml:mi> </mml:math> isomers in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mmultiscripts> <mml:mi>No</mml:mi> <mml:mprescripts/> <mml:mn>102</mml:mn> <mml:mn>255</mml:mn> </mml:mmultiscripts> <mml:mn>153</mml:mn> </mml:msub> </mml:math>

Abstract

This study of isomeric states in $^{255}\mathrm{No}$ was performed with the GABRIELA detector array at the focal plane of the SHELS recoil separator. The $^{208}\mathrm{Pb}(^{48}\mathrm{Ca},$ ${xn)}^{256\text{--}x}\mathrm{No}$ fusion-evaporation reaction was used to produce nobelium isotopes with a beam energy optimized for the one-neutron and two-neutron evaporation channels. These nobelium isotopes were studied by decay spectroscopy leading to the identification of isomeric states in $^{255}\mathrm{No}$ by means of the calorimetric method. In order to isolate the radioactive decays of $^{255}\mathrm{No}$ from the more numerous decays of $^{254}\mathrm{No}$, the characteristic signal of the internal decay of the daughter $^{251}\mathrm{Fm}$ was used as a tag. Under these conditions, the decay of four isomeric states was observed. They are interpreted as high-$K$ structures decaying in cascade. The first isomeric state is assigned to a $\ensuremath{\nu}[725]$ ${\frac{11}{2}}^{\ensuremath{-}}$ neutron one quasiparticle (qp) configuration, in good agreement with the trend of this state in the neighboring isotones. The second and the third ones were attributed to three-qp $21/{2}^{+}$ and $27/{2}^{+}$ states and are interpreted as resulting from the coupling of the same $\ensuremath{\nu}[725]$ ${\frac{11}{2}}^{\ensuremath{-}}$ one-qp configuration with the ${\ensuremath{\pi}}^{2}{{[624]{\frac{9}{2}}^{+}\ensuremath{\bigotimes}[521]{\frac{1}{2}}^{\ensuremath{-}}}}_{{5}^{\ensuremath{-}}}$ and with the ${\ensuremath{\pi}}^{2}{{[624]{\frac{9}{2}}^{+}\ensuremath{\bigotimes}[514]{\frac{7}{2}}^{\ensuremath{-}}}}_{{8}^{\ensuremath{-}}}$ two-qp configurations, respectively. According to its excitation energy E*$\ensuremath{\ge}2.5$ MeV, the fourth isomeric state should have a five-qp structure, which requires more statistics to be detailed.

Topics & Concepts