Spin measurements forSm147+nresonances: Further evidence for nonstatistical effects

Abstract

We have determined the spins $J$ of resonances in the $^{147}\mathrm{Sm}$($n,\ensuremath{\gamma}$) reaction by measuring multiplicities of $\ensuremath{\gamma}$-ray cascades following neutron capture. Using this technique, we were able to determine $J$ values for all but 14 of the 141 known resonances below ${E}_{n}=1$ keV, including 41 firm $J$ assignments for resonances whose spins previously were either unknown or tentative. These new spin assignments, together with previously determined resonance parameters, allowed us to extract level spacings (${D}_{0,3}=11.76\ifmmode\pm\else\textpm\fi{}0.93$ and ${D}_{0,4}=11.21\ifmmode\pm\else\textpm\fi{}0.85$ eV) and neutron strength functions (${10}^{4}{S}_{0,3}=4.70\ifmmode\pm\else\textpm\fi{}0.91$ and ${10}^{4}{S}_{0,4}=4.93\ifmmode\pm\else\textpm\fi{}0.92$) for $J=3$ and 4 resonances, respectively. Furthermore, cumulative numbers of resonances and cumulative reduced neutron widths as functions of resonance energy indicate that very few resonances of either spin have been missed below ${E}_{n}=700$ eV. This conclusion is strengthened by the facts that, over this energy range, Wigner distributions calculated using these ${D}_{0}$ values agree with the measured nearest-neighbor level spacings to within the experimental uncertainties, and that the ${\ensuremath{\Delta}}_{3}$ values calculated from the data also agree with the expected values. Because a nonstatistical effect recently was reported near ${E}_{n}=350$ eV from an analysis of $^{147}\mathrm{Sm}$($n,\ensuremath{\alpha}$) data, we divided the data into two regions; $0<{E}_{n}<350$ eV and $350<{E}_{n}<700$ eV. Using neutron widths from a previous measurement (corrected for new unresolved doublets identified in this work) and published techniques for correcting for missed resonances and for testing whether data are consistent with a Porter-Thomas distribution, we found that the ${\ensuremath{\Gamma}}_{n}^{0}$ distribution for resonances below 350 eV is consistent with the expected Porter-Thomas distribution. However, we found that ${\ensuremath{\Gamma}}_{n}^{0}$ data in the $350<{E}_{n}<700$ eV region are inconsistent with a Porter-Thomas distribution, but in good agreement with a ${\ensuremath{\chi}}^{2}$ distribution having $\ensuremath{\nu}\ensuremath{\geqslant}2$ We discuss possible explanations for these observed nonstatistical effects and their possible relation to similar effects previously observed in other nuclides.