Systematics of the (t, p) reaction in Yb and Hf isotopes near the N=108 "subshell closure"

Angular distributions of protons from the 170, 174, 176 Yb(t, p) 172, 176, 178 Yb and 178,180 Hf(t, p) 180, 182 Hf reactions have been studied using beams of 15 MeV tritons from the McMaster University tandem Van de Graaff accelerator. The reaction products were analyzed with a magnetic spectrograph and detected with photographic emulsions. Levels up to ~2.5 MeV excitation were studied in each nuclide, with a typical overall resolution of ~15 keV (full-width half-maximum). Measurements were also made with targets of natural Yb and natural Hf, to improve the accuracy of relative strengths in each chain of isotopes, and with a target of 172 Yb to facilitate the normalization of previously published data to the present results. One of the most notable features of the data is the large amount of L = 0 strength to excited I π = 0 + states for all cases where the residual nucleus has a neutron number N ≤ 108. The populations of these states are not explained by either the pairing rotational or vibrational models. The SU(3) limit of the interacting boson model yields qualitative but not quantitative agreement. For two of the residual nuclides studied, 178 Yb and 182 Hf, there was little or no nuclear structure information previously available. The (t, p) experiments have located a number of excited states in each of these cases, and also give the first measurements of two-neutron separation energies; S(n) = 12 333 ± 6 keV for 178 Yb and 12 413 ± 6 keV for 182 Hf.