Fast neutron elastic scattering and polarization

• Main Author: Begum, Amena
• Published: University of Edinburgh 1979
• Subjects: 539.7
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.641488

Fast neutron elastic scattering has been studied for 2.9,16.1 and 14.2MeV energy from medium and heavy nuclei. Angular distributions of polarization and cross-section have been obtained for 2.9 MeV neutrons scattered from Fe, W, Tl, Pb, Bi and U. Simultaneous measurements of angular dependence of polarization and cross-section has also been done for 16.1 MeV neutrons from Fe, Cu, I, W, Bg, Pb and C by the pulsed beam time of flight technique. The angular ranges covered are from 20° to 160° at intervals of .1k. The experimental distributions have been corrected for the attenuation of the incident neutron flux in the scatterer, neutron multiple scattering and for the finite source-sample-detector geometry using a Monte Carlo computer progamme. Correction for relevant inelastic scattering has been done wherever possible with available experimental inelastic crosssections. The measurements are compared with the optical model calculations based on 'global-fit' parameters as well as parameters reported previously for particular nuclei. These parameters are found not to be very successful in fitting both differential cross-section and polarization for either 2.9 MeV or 16.1 Nev scattering. Much better fits to both cross-section and polarization are obtained by individual analyses for most of the nuclei. The calculations for 2.9MeV neutrons are combined with Hauser-Feshbach compound elastic calculations done with and without the Mol auer level width fluctuation correction. It has been observed that while for U both the calculations provide a similar fit to the expermental data, for W, T1, Pb and Bi calculations including the level width fluctuations correction provide a better fit. For the 16.1 Mev case the fit to the polarization with the optimum parameter set for heavier nuclei are poorer than for the lighter ones, with a marked discrepancy in the magnitude of polarization around 200 for most of the nuclei. The parameter sets which give the best fits to the data at 2.9 MeV and 16.1MeV are presented. A large spin-orbit potential is observed for the heavier nuclei at 2.9MeV while for the 16.1MeV case there is evidence for ? a volume imaginary term. °14.2MeV neutron double scattering measurements on Cu and Pb for 20° and 34 employing the associated particle time of flight technique resulted in a similarly large magnitude of polarization as observed for the present 16.1Mev scattering and also reported earlier. Optical model calculated polarization for 14.2MeV, based on global parameters shows a small magnitude around 20° as in the case of 16.1MeV neutrons. Attempts to modify conventional optical model parameters to account for such a discrepancy are made. It is observed that increasing the radius or the diffuseness of the spin-orbit term reproduces the large magnitude of polarization around 20° without affecting the polarization for the rest of the angles or the cross-section.