A Study of the Structure of Light Tin Isotopes via Single-Neutron Knockout Reactions

The region around 100 Sn [100Sn] is important because of the close proximity to the N=Z=50 magic numbers, the rp process, and the proton drip-line. Alpha decay measurements show a reversal in the spin-parity assignments of the ground and first excited states in 101 Sn [101Sn] compared to 105 Te [105Te]. However, the lightest odd- mass tin isotope with a firm spin-parity assignment is 109 Sn [109Sn]. The d 5/2 [d5/2] and g 7/2 [g7/2] single-particle states above N=50 are near degenerate, evidenced by the excitation energy of the first excited state in 101 Sn at only 172 keV. The correct ordering of these single-particle states and the degree of neutron configuration mixing has been the subject of debate.

Spectroscopic studies have been performed close to 100 Sn [100Sn], utilizing the S800 and CAESAR at the NSCL. These studies make use of a single neutron knockout reaction on beams of 108 Sn [108Sn] and 106 Sn [106Sn]. The momentum distributions of the resulting residues reflect the l-value [l-value] of the removed neutron. Additionally, γ-rays [gamma-rays] were measured in coincidence with the momentum distributions allowing for the separation of the knockout channel where the residue is left in an excited state from the channel to the ground state. The odd-mass residue can then be characterized in terms of a hole in the d- or g- orbital with reference to the even-mass nucleus. The relative population of final states in the odd-mass residue are indicative of the mixing in the ground state of 108,106 Sn [108,106Sn].

Comparing the momentum distributions with reaction calculations shows that both 105 Sn [105Sn]and 107 Sn [107Sn] have a J π [J pi] = 5/2 + ground state and a J π [J pi]= 7/2 + first excited state at 200 keV and 151 keV respectively. The exclusive cross sections for one-neutron knockout from 106 Sn [106Sn] and 108 Sn [108Sn] show that the ground state are dominated by the d 5/2 [d5/2] single-particle state.