Total Cross Sections of Heavy Nuclei for Fast Neutrons

Abstract

The total neutron cross sections of twenty-three heavy elements from iron to bismuth have been determined as a function of neutron energy from about 0.05 Mev to 3.2 Mev. Every element whose major isotope (>50 percent abundant) has a closed neutron shell of 50, 82, or 126 neutrons is included among the elements investigated. Below 1.4 Mev, neutrons having an energy spread of 20 kev were employed to take measurements at 20-kev intervals on strontium, yttrium, barium, lanthanum, cerium, and praseodymium. Niobium and molybdenum were also investigated in this region using energy intervals of 70 kev between measurements. These measurements and earlier low energy measurements on fifteen other heavy elements were extended to 3.2 Mev, using an energy spread of 20 kev and intervals between points of about 200 kev. The most unusual feature of the results is a very broad maximum which is clearly observed in the total cross sections of thirteen of the elements studied. This maximum appears to move to higher neutron energies with increasing mass number. The position and shape of the maximum seems to be independent of all other nuclear details, such as the nuclear spin or the binding energy of the added neutron. When averaged over resonances, the total cross sections of the closed-shell nuclei do not appear to be different from the total cross sections of other nuclei of about the same masses. There is some indication that the spacing of resonances in the total cross sections of the closed-shell nuclei is greater than observed in the cross sections of other nuclei.