Observation of fission in Pb-Pb interactions at 158A GeV

The NA50 experiment at the CERN SPS has been equipped with a Cerenkov quart z detector to measure the charge of projectilelike fragments emitted in int eractions of the lead beam of 158A GeV with a 12 mm thick lead target. A cl ear 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 +/- 0.16) x 10(-2). The information provided by the NA50 zero -degree calorimeter has allowed us to check that fission occurs in extremel y peripheral collisions. To provide the first information about the fission mechanism, the expected yield of electromagnetic fission events in our exp erimental conditions has been computed: it turns out to be about 40% smalle r than the observed one. The approximations necessarily made in our calcula tion as well as the contribution due to fission induced by nuclear interact ion could account for such a difference. [S0556-2813(99)05501-6].