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

Abstract

Neutron transmission and self-indication measurements have been made for two of the separated isotopes of Sm ($A=152\mathrm{} \mathrm{and} 154$) and the separated isotopes of Eu ($A=151\mathrm{} \mathrm{and} 153$) 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 $l=0\mathrm{}$ 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 ${\mathrm{Sm}}^{152}$ and 27 levels in ${\mathrm{Sm}}^{154}$. Values of ${\Gamma }_n$ (or $g{\Gamma }_n$ for Eu) were obtained for the observed resonances. Values of ${\Gamma }_{\gamma }$, obtained for 9, 3, 45, and 46 levels of ${\mathrm{Sm}}^{152}$, ${\mathrm{Sm}}^{154}$, ${\mathrm{Eu}}^{151}$, and ${\mathrm{Eu}}^{153}$, respectively, give $〈{\Gamma }_{\gamma }〉$ values of 65, 79, 90.0, and 94.8 meV, respectively, with most of the individual values within 20% of $〈{\Gamma }_{\gamma }〉$. The $s$ strength function is $10{}_{}{}^4{}_{}{}^{}S_0^{}{}_{}{}^{}=2.2\mathrm{}\pm 0.4, 1.8\pm 0.5, 3.2\pm 0.5, \mathrm{and} 2.3\pm 0.4$ for ${\mathrm{Sm}}^{152}\mathrm{}(n=90\mathrm{})\mathrm{}$, ${\mathrm{Sm}}^{154}\mathrm{}(n=33\mathrm{})\mathrm{}$, ${\mathrm{Eu}}^{151}\mathrm{}(n=63\mathrm{})\mathrm{}$, and ${\mathrm{Eu}}^{153}\mathrm{}(n=68\mathrm{})\mathrm{}$, where $n$ is the number of levels in the sample. Somewhat smaller $S_0$ 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 ${\mathrm{Sm}}^{152}$ and ${\mathrm{Sm}}^{154}$, respectively, are: (a) the Dyson-Mehta $\Delta$ statistic, which is particularly sensitive to long-range order in the level spacings, $\Delta =0.40\mathrm{} \mathrm{and} 0.38$ versus ${\Delta }_{\mathrm{DM}}=0.42\mathrm{}\pm 0.11 \mathrm{and} 0.32\pm 0.11$; (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.