Deformation increase of high-spin core-excited isomers in the astatine nuclei

Abstract

Quadrupole moments of six high-spin isomers in the At isotopes have been measured with the level-mixing-spectroscopy method:'At(16 ), At( -+ ), ' At(19+, 15 ), "At( -',-+ ). The results show that level mixing spectroscopy is a promising technique to determine quadrupole moments of isomers that are dificult to measure by other in-beam hyperfine interaction methods. A large increase of the quadrupole moment is observed if neutrons are excited across or removed from the X= 126 shell closure. This behavior is explained in terms of an enhanced core softness for fewer core neutrons; the aligned valence particles, moving in equatorial orbits, then easily polarize the core towards oblate deformation.

Nuclear magnetic and quadrupole moments for nuclear structure research on exotic nuclei

“…The effective charges calculated in [37] are for valence particles in [doubly magic ± 1] nuclei. Measurements of quadrupole moments of isomeric and ground states in the more neutrondeficient isotopes in this mass region have revealed a systematic increase of the quadrupole moments with decreasing neutron number [41][42][43][44][45][46]. This increase is a signature for the increase of the nuclear deformation, attributed to an increasing quadrupole collectivity induced by interactions between the proton particles and holes in the neutron shell.…”

“…Typical values for the core deformations vary between |β core 2 | = 0.01 up to |β core 2 | = 0.05 for all high-spin isomers in the Z = 82-88 nuclei down to N = 116. Also for neutron core-excited isomeric states in At [43] and Fr [44], the core deformation is not larger than β core 2 = −0.055, suggesting that neutron excitations across the N = 126 shell gap do not induce an extra deformation.…”

“…We can deduce this deformation from the measured quadrupole moments, assuming strong coupling with K = I (that means the nuclear spin along the oblate symmetry-axis). Following [37,43] we have calculated the core deformation β core 2 using expression (2.9). The intrinsic quadrupole moment of the core is obtained from the spectroscopic core quadrupole moment…”

“…Here, we present the comparison of calculated and experimental moments of high-spin isomers in the trans-lead region, including neutron core-excited states (table 3 and figure 9). The data are taken from [1,43,44]. The single particle quadrupole moments that are used in the calculations were obtained using (2.7), with a neutron dependent proton effective charge e emp π (N ) from table 2 and a constant neutron effective charge e emp ν = 0.95e.…”

“…The LEMS technique has been applied until now for the in-beam study of fusionevaporation isomers. Isomeric states in the neutron-deficient Pb region [42][43][44][45]34] as well as K isomers [200] have been investigated successfully.…”