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Collectivity in nuclei in the vicinity of the N = Z line may be enhanced by neutron-proton
interactions occupying similar orbits near the Fermi level. Therefore, information on single-particle
energies and residual interactions with respect to the 100 Sn core are extremely important. The region
of 100 Sn has been extensively investigated due to unusual B(E2;0 + →2 + ) values observed for light Sn
isotopes. But the direct study of the properties of 2 + state neutron-deficient Sn-isotopes is
challenging due to presence of long-lived isomers up to the range of nanoseconds.
In the proposed experiment we propose to probe the collectivity in the neighbouring light Sb
nuclei. The dominant feature of low-energy levels of the odd-mass Sb nuclides with 54<N<82 is the
dramatic monopole shift of the d 5/2 and g 7/2 levels in which the g7/2 level moves from a position 852
keV above the d 5/2 in 111 Sb to a position 963 keV below the d 5/2 level in 133 Sb [1]. The monopole
effect how spherical single-particle energies are shifted as protons or neutrons occupy certain orbits
is postulated in ref. [2]: it arises as a consequence of spin-orbit interaction that diminishes as N/Z
ration increases.
Another peculiarity for odd-A Sb isotopes is the presence of two 9/2+ states. One of 9/2+ 1 is
interpreted as the result of the coupling a d 5/2 proton to the 2+ state of adjacent even-even Sn core.
As follow from this simple approach for a pure particle-vibration coupled state B(E2;9/2+→ 5/2+)
= B(E2;2+→0+), see the Eq. (6-467) of volume II of Bohr-Mottelson. Indeed, the full shell-model
calculations using the CD-Bonn interaction [3] performed by Chong Qi [4], confirm the trend.
However for heavier Sb (A>113) due to the monopole shift the proton is moved to g7/2 and as a
result, the mixing between d5/2 and g7/2 orbitals which leads to the significant drop in the
calculated B(E2; 9/2+→g.s.) transition strength. The energy of 9/2+ 1 is relatively insensitive to the
neutron number and remains in close proximity to 2 + level in the underlying Sn core. The second
9/2+ 2 might be due the promotion of a g 9/2 proton into a higher orbital, which gives
2p1hconfiguration. Both 9/2+ states, which have different intrinsic configuration, are closely spaced
and, therefore, may be mixed with each other. The situation is similar to one observed near Z = 28
shell [5].
We propose to study lifetime of the low-lying states in 105,107,109 Sb, by using a 1n and 2n
knock-out reaction from 106,108,110 Sb, respectively. The 106,108,110 Sb fragments will be produced from
the fragmentation of a primary 124 Xe beam at 345 MeV/A on a Be target. The reaction fragments
will be separated and identified by he BigRIPS separator. The fragments of interest will impinge on
a 9 Be target surrounded by a high-purity germanium array (MINIBALL). The final reaction
products will be identified by the ZeroDegree spectrometer.
[1] J.Shergur, D.J.Dean, D.Seweryniak Phys. Rev. C. 71 064323 (2005)
[2] J. P. Schiffer et al., Phys. Rev. Lett. 92 (2004)
[3] C. Qi, Z.X. Xu Phys. Rev. C 86, 044323 (2012)
[4] Qi Chong, private comm.
[5] I. Stefanescu et al. Phys. Rev. Lett. 100, 112502 (2008)
