Determination of the Upper Limits of the Fission Cross-sections of Lead and Bismuth for Li-D Neutrons by a Chemical Method

Abstract

THEORETICALLY, fission of elements of atomic number 83 and less is not excluded. The fission thresholds for the compound nuclei formed in neutron capture by lead and bismuth have been estimated as 9.3 MeV. for Bi²¹⁰, 10.0 for Pb²⁰⁷, 10.4 for Pb²⁰⁸ and 10.7 for Pb^{209 1}. If the neutron-binding energy is 5.4 MeV. for initial nuclei with an even, and 6.4 MeV. with an odd, number of neutrons, 3.9, 4.6, 4.0 and 5.3 MeV. have to be supplied as kinetic neutron energy to Bi²⁰⁹, Pb²⁰⁶, Pb²⁰⁷ and Pb²⁰⁸ to reach the presumed fission thresholds. We have searched for the fission of lead and bismuth with fast neutrons from the Li-D reaction by looking for any radio-iodine formed. In chemical methods for the measurement of fission-rates, very much larger amounts of material can be used than in fission chambers. On the other hand, chemical methods in a hypothetical process are based on the admittedly uncertain assumption that the element selected would appear as a fission product. We have chosen iodine as it is in the middle of one of the groups of fission products from uranium, and several of its isotopes are produced abundantly with this element. The ease of extraction of iodine was the reason why Libby² used it in an experiment to set a limit to the spontaneous fission-rate in uranium.