Neutron Resonance Spectroscopy. IX. The Separated Isotopes of Samarium and Europium

Neutron transmission and self-indication measurements have been made for two of the separated isotopes of Sm (A=152 and 154) and the separated isotopes of Eu (A=151 and 153) at the Nevis synchrocyclotron. The measurements used a 200-m path for transmission measurements and a 40-m path for self-indication measurements, covering an energy range from approximately 1 eV to greater than 5 keV. Since these isotopes are at a region of a peak in the $l=0$ strength function, mainly s resonances are involved. The results for the even-even Sm isotopes show that essentially complete s populations were obtained for the first 70 levels in Sm¹⁵² and 27 levels in Sm¹⁵⁴. Values of Γₙ (or gΓₙ for Eu) were obtained for the observed resonances. Values of Γ_γ, obtained for 9, 3, 45, and 46 levels of Sm¹⁵², Sm¹⁵⁴, Eu¹⁵¹, and Eu¹⁵³, respectively, give 〈Γ_γ〉 values of 65, 79, 90.0, and 94.8 meV, respectively, with most of the individual values within 20% of 〈Γ_γ〉. The s strength function is 10⁴S₀=2.2±0.4, 1.8±0.5, 3.2±0.5, and 2.3±0.4 for Sm¹⁵²(n=90), Sm¹⁵⁴(n=33), Eu¹⁵¹(n=63), and Eu¹⁵³(n=68), where n is the number of levels in the sample. Somewhat smaller S₀ values result for the Sm isotopes if a larger energy interval is used. Values of the mean s level spacing for these isotopes of 51.8±{}1.5, 115±{}8, {~}0.7, and {~}1.1 eV, respectively, were found. Good fits are obtained with the Porter-Thomas theory for the reduced-neutron-width distributions and the Wigner formula for the nearest-neighbor level separation. Other statistical tests applied to Sm¹⁵² and Sm¹⁵⁴, respectively, are: (a) the Dyson-Mehta Δ statistic, which is particularly sensitive to long-range order in the level spacings, Δ=0.40 and 0.38 versus ΔDM=0.42±0.11 and 0.32±0.11; (b) correlation coefficients of adjacent nearest-neighbor spacings are -0.26±{}0.10 and -0.32±{}0.18 versus -0.27 predicted for an orthogonal ensemble.