The NA50 experiment at the CERN SPS has been equipped with a Cerenkov quartz detector to measure the charge of projectilelike fragments emitted in interactions of the lead beam of $158A\hspace{0.5em}\mathrm{GeV}$ with a 12 mm thick lead target. A clear fission peak has been observed in the light output distribution of the fragment detector and the measured number of fission events per incident Pb ion is $(1.26\mathrm{}\pm 0.16)\times {10}^{-2}.$ The information provided by the NA50 zero-degree calorimeter has allowed us to check that fission occurs in extremely peripheral collisions. To provide the first information about the fission mechanism, the expected yield of electromagnetic fission events in our experimental conditions has been computed: it turns out to be about $40\%$ smaller than the observed one. The approximations necessarily made in our calculation as well as the contribution due to fission induced by nuclear interaction could account for such a difference.

• Received 22 July 1998

DOI:https://doi.org/10.1103/PhysRevC.59.876