Experimental studies of fast neutron elastic scattering in nuclei

The Studsvik neutron time-of-flight spectrometer has been completely rebuilt to facilitate elastic and inelastic scattering studies in the 20 MeV region. The beam pulsing equipment, the gas target system, the neutron detectors and the shadow bar system have been totally redesigned. Using a flight path of 4 m, scattering measurements can now be performed at 20 MeV with a total energy resolution of 0.5 MeV under significantly improved background conditions. A routine method for accurate neutron energy measurements using t.o.f. technique has also been developed. The applicability of the method has been investigated by measuring the energy of different neutron groups in the range 1 - 20 MeV. The uncertainties are estimated to be 0.1 - 0.3 % over the energy range mentioned. The equipment described has been used to measure differential fast neutron elastic scattering angular distributions at 21.6 MeV for the natural elements Mg, Al, Si, S, Ca, Cr, Fe, Co, Ni, Y, Ce, Pbr (radiogenic lead) and Bi. The experimental data have been analysed in terms of a standard phenomenological spherical optical model and also by using three local microscopic optical potentials. Reasonably good agreement with the measurements is obtained for all the models studied. Moreover, elastic scattering of 1.5 to 4.0 MeV neutrons from Pbr and Bi has been measured using the previous t.o.f. facility, which had sufficient resolution for this energy region. The data were analysed in terms of a phenomenological spherical optical model The energy dependence of the angle integrated shape elastic cross sections calculated for Pbr and Bi show only minor differences, whereas the compound elastic cross section is considerably larger for Bi. (author)