Neutron Resonance Spectroscopy. XI. The Separated Isotopes of Yb

Abstract

Neutron time-of-flight resonance spectroscopy results, using the Nevis synchrocyclotron, for the separated Yb isotopes (170, 171, 172, 173, 174, 176) are given. Transmission and self-indication measurements were made for several sample thicknesses of each isotope. Resonance parameters, Γn0 (or gΓn0), are given to ∼1.8 keV for 171 and 173, and to ∼10 keV for 172, 174, and 176. Levels in 170 were those seen in the natural element, but not in the 171-176 isotopes. Many resonance Γγ and J values were also obtained for 171 and 173, and a few Γγ values for 172 and 174. The 10S04 values are 2.25± 1.0, 1.86± 0.16, 1.68± 0.20, 1.60 ± 0.28, 1.62± 0.21, and 2.29± 0.32 for 170, 171, 172, 173, 174, and 176, respectively. 〈Γγ〉=76.5 meV (37 levels), 72.3 meV (3 levels), and 73.8 meV (33 levels) for 171, 172, and 173, respectively. A shape fit to the asymmetric level in 174 at 342.7 eV gave R′=(7.9±0.5) fm. The increasing σ below ∼100 eV for 174 and natural Yb, and the known thermal 174 partial cross sections, were fitted assuming a bound 174 level at E0=−25 eV, Γn0=160 meV. Comparison of the (gΓn0)12 distributions with Porter-Thomas theory and the nearest-neighbor energy spacings with the Wigner theory gave best agreement for 172, as did other statistical orthogonal ensemble (O.E.) tests. There were missing weak s levels for 171 and 173 and extra p levels for 176. The 174 and 176 results were also compatible with O.E. theory but provided poorer test cases than 172.