We report the first observation of the doubly-magic nucleus 100Sn. Thi
s isotope was produced by nuclear fragmentation of 124Xe projectiles a
t 1095 A.MeV using the heavy-ion synchrotron SIS at GSI, Darmstadt. Th
e projectile fragments were separated in flight with the projectile-fr
agment separator FRS and identified by measuring event by event the ma
gnetic rigidity, the time of flight and the energy deposition.