A spontaneous fission activity with about a 14 msec lifetime was discovered in 1961 by Polikanov et al. 1 following the bombardment of U and Pu targets with 16O and 22Ne ions in an attempt to synthesize element 104. This activity was produced in some other reactions, 2 , 3 but the isotopic assignment could not be done by chemical or mass separation methods due to the short lifetime and the reduced cross-section (lower than 1 µb). Only later on, by using the method of crossed reactions, was it established that the isomer just discovered belongs to 242Am, and its properties have been more accurately measured. 4 6