Spin measurements for {sup 147}Sm+n resonances: Further evidence for nonstatistical effects
- Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (United States)
We have determined the spins J of resonances in the {sup 147}Sm(n,{gamma}) reaction by measuring multiplicities of {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{sub 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{sub 0,3}=11.76{+-}0.93 and D{sub 0,4}=11.21{+-}0.85 eV) and neutron strength functions (10{sup 4}S{sub 0,3}=4.70{+-}0.91 and 10{sup 4}S{sub 0,4}=4.93{+-}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{sub n}=700 eV. This conclusion is strengthened by the facts that, over this energy range, Wigner distributions calculated using these D{sub 0} values agree with the measured nearest-neighbor level spacings to within the experimental uncertainties, and that the {delta}{sub 3} values calculated from the data also agree with the expected values. Because a nonstatistical effect recently was reported near E{sub n}=350 eV from an analysis of {sup 147}Sm(n,{alpha}) data, we divided the data into two regions; 0<E{sub n}<350 eV and 350<E{sub 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 {gamma}{sub n}{sup 0} distribution for resonances below 350 eV is consistent with the expected Porter-Thomas distribution. However, we found that {gamma}{sub n}{sup 0} data in the 350<E{sub n}<700 eV region are inconsistent with a Porter-Thomas distribution, but in good agreement with a {chi}{sup 2} distribution having {nu}{>=}2 We discuss possible explanations for these observed nonstatistical effects and their possible relation to similar effects previously observed in other nuclides.
- OSTI ID:
- 21062010
- Journal Information:
- Physical Review. C, Nuclear Physics, Vol. 76, Issue 2; Other Information: DOI: 10.1103/PhysRevC.76.025804; (c) 2007 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA); ISSN 0556-2813
- Country of Publication:
- United States
- Language:
- English
Similar Records
Non-Statistical Effects in Neutron Capture
Neutron resonance spectroscopy. XII. The separated isotopes of W