$Z=50$ Shell Gap near ${}^{100}\mathrm{Sn}$ from Intermediate-Energy Coulomb Excitations in Even-Mass ${}^{106-112}\mathrm{Sn}$ Isotopes

Rare isotope beams of neutron-deficient ${}^{106,108,110}\mathrm{Sn}$ from the fragmentation of ${}^{124}\mathrm{Xe}$ were employed in an intermediate-energy Coulomb excitation experiment. The measured $B(E2,0_1^{+}\to 2_1^{+})$ values for ${}^{108}\mathrm{Sn}$ and ${}^{110}\mathrm{Sn}$ and the results obtained for the ${}^{106}\mathrm{Sn}$ show that the transition strengths for these nuclei are larger than predicted by current state-of-the-art shell-model calculations. This discrepancy might be explained by contributions of the protons from within the $Z=50$ shell to the structure of low-energy excited states in this region.

Figure 1

$\gamma$-ray spectra measured by the 90° ring of SeGA for the ${}^{108-112}\mathrm{Sn}$ projectiles (top) and the corresponding Au target Coulomb excitations (bottom) within the 45 mrad scattering angle in the laboratory reference frame.

Figure 2

$\gamma$-ray spectra measured with SeGA for the ${}^{106-112}\mathrm{Sn}$ projectile within the scattering angle limited by the hardware S800 spectrograph acceptance.

Figure 3

Experimental data on $B(E2,0_1^{+}\to 2_1^{+})$ in a Sn isotopic chain from the current results for ${}^{106-112}\mathrm{Sn}$ and from Refs. 4, 5, 6. The dotted line shows the predictions of the large-scale shell-model calculations of Ref. 6 performed with ${}^{90}\mathrm{Zr}$ core. For ${}^{108-112}\mathrm{Sn}$ the error bars represent statistical errors; the corresponding systematic errors are marked by arrows. The ${}^{106}\mathrm{Sn}$ value is tentative, as indicated in the text.

Figure 4

Experimental data on $B(E2,0_1^{+}\to 2_1^{+})$ in a Ni isotopic chain [4] compared to $pf$ shell-model calculations obtained with the GPFX1 Hamiltonian [23]. The lines are labeled by the maximum number of nucleons $t$ allowed to be excited out of the $f_{7/2}$ orbit with the full-space results indicated by “full $pf$”.

Figure 5

Schematic representation of proton $2p2h$ excitations across the $Z=50$ shell gap in ${}^{102}\mathrm{Sn}$. The occupation of the same proton and neutron orbitals above the $Z=N=50$ shell leads to $\alpha$-like correlations between the valence nucleons.