Threshold Effects in Neutron Elastic Scattering

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Abstract
In an attempt to observe the Wigner-cusp phenomenon in neutron elastic scattering near inelastic thresholds, we have measured the differential elastic and total cross sections of Li, Fe, Zr, Ba, and Ce near the threshold energy for exciting the first-excited states of the dominant isotopes of these elements. The energy region investigated extended approximately 300 keV on either side of threshold. Successful analysis of observed cusp phenomena can provide information on the spin and parity of excited states. In addition, the presence or absence of below-threshold effects in regions of neutron energy and atomic number where "energy-averaged" Wigner-cusp theory applies in principle can give an indication of the interaction mechanism, i.e., direct interaction or compound nucleus formation. The theory of Wigner cusps and "energy-averaged" Wigner cusps is supplemented in the present work by a consideration of slightly more general angular-momentum situations and by explicit cusp expressions for differential-elastic cross sections. Cusp predictions are given in terms of the coefficients for expansion of the cross sections into a series of Legendre polynomials. This form facilitates comparison of theory and experiment. Unusual features were observed at the inelastic thresholds in the elastic and total cross sections of Li and Fe. Here one expects to notice a below, as well as above, threshold effect characteristic of Wigner cusps. In these cases, however, the interpretation of the experimental results is not unambiguous—in Li because the predicted cusp effect is too small to be observable in the present experiment and in Fe because the resonance structure of the cross sections overshadows the expected cusp effect. A drop, commencing at inelastic thresholds, is observed in the elastic cross sections of Ce, Ba, and Zr. Analysis of this effect in terms of the theory of "energy-averaged" Wigner-cusps yields good agreement between theory and experiment.