Structural Surprises Above Doubly-magic 100Sn
The exotic isotope 100Sn has an equal number of protons and neutrons (Z=N=50) and is believed to be a “doubly-magic” nucleus (i.e. the particle number 50 is a shell closure for both neutrons and protons). The properties of nuclei near a doubly magic core are described quite well by the nuclear shell model, initially proposed in 1950s by the Nobel Laureates Maria Goeppert-Mayer and Hans Jensen. While the modern shell model, rooted in a mutual interaction between nucleons, describes nuclei near stability fairly accurately, its predictive power for short-lived unstable systems, such as 100Sn and its neighbors is still an open question. Hence, the experimental studies of 101Sn, with one neutron outside the 100Sn core, are providing clear tests for modern nuclear theory.
In an experiment performed at the Holifield Radioactive Ion Beam Facility (HRIBF) the low-lying states in 101Sn were populated via the alpha decay chain: 109Xe → 105Te → 101Sn. Sophisticated system of semiconductor detectors was used to capture the energetic 109Xe ions and measure the subsequent alpha decays. An array of very large germanium detectors was used to detect gamma rays correlated with the alpha decay. After careful analysis of terabytes of data, strong and unexpected population of the 1st excited state in 101Sn (N=51, Z=50) was found following the alpha decay of 105Te, see Fig. 1. It has been concluded, that the spins of the ground-state (J=7/2) and 1st excited-state (J=5/2) in 101Sn are reversed with respect to the scenario postulated for the heavier Sn isotopes. This new result contradicted the traditional shell model ordering of the levels. The observed inversion has been confirmed by the state-of-the-art calculations (see Fig. 1). The explanation is given in terms of pairing interactions between neutrons that strongly depend on particle’s orbit. The microscopic theoretical approach used to interpret the experimental data derives the properties of the interaction between nucleons in a nucleus from the interaction between free nucleons. The new data from HRIBF strongly support the validity of this theoretical approach which elegantly explains the switch of nuclear states.
Fig. 1. The experimental alpha and gamma energy spectra observed for the 109Xe → 105Te → 101Sn decay. The strongest α -transition at 4710 keV energy from 105Te populates the excited state at 172 keV in 101Sn (left panel). The shell model analysis (right panel) of odd-mass tin isotopes accounts for the 7/2+-5/2+ level sequence in 101Sn and explains the reverse order of levels in heavier tin isotopes.