The source of "background" fission events in experiments on superheavy elements

In experiments for the synthesis of superheavy elements at the velocity filter SHIP, GSI, we observed fission events, which could not be attributed to decay chains of superheavy isotopes from fusion reactions. Usually, the observation of spontaneous fission is a crucial first step for the detection of decay chains. In order to avoid random correlations and misidentifications of superheavy isotopes, it is therefore essential to know the features and cross- sections of fission events not originating from decay chains of superheavy nuclei. The special properties of the velocity filter allowed us to identify and study the " background" fission events as decay products of heavy target- like nuclides populated in nucleon transfer reactions. Here, we will discuss the results obtained in collisions of Ca-48(20) + Cm-248(96), Cr-54(24) + Cm-248(96) and Ni-64(28)+U-238(92), which were applied for the synthesis of elements Z = 116 and 120, respectively.