A study has been made of the characteristics of the fission induced in bismuth and lead by irradiation with the high energy deuterons using radiochemical methods for the determination of yields of the fission products. The fission yield curve resulting from 190-Mev deuteron fission of bismuth was found to be a single symmetrical peak with a maximum fission yield of 5.0 percent at a mass number of approximately 100. The ratio of neutrons to protons was observed to be nearly constant for all nuclides formed in relatively good yield. The excitation function of the ${\mathrm{Mo}}^{99}$ yield in two lead samples enriched, respectively, in ${\mathrm{Pb}}^{204}$ and ${\mathrm{Pb}}^{208}$ showed that the fission cross section is larger and decreases less rapidly as the deuteron energy decreases in the lead sample which has a higher proportion of the light isotopes. A mechanism has been postulated for the nuclear reaction which involves prior neutron evaporation from the initial highly excited nucleus followed by fission with unchanged charge distribution.

• Received 1 June 1949

DOI:https://doi.org/10.1103/PhysRev.76.628