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The superheavy element with atomic number Z=117 was produced as an evaporation residue in the (48)Ca+(249)Bk fusion reaction at the gas-filled recoil separator TASCA at GSI Darmstadt, Germany. The radioactive decay of evaporation residues and their α-decay products was studied using a detection setup that allowed measuring decays of single atomic nuclei with half-lives between sub-μs and a few days. Two decay chains comprising seven α decays and a spontaneous fission each were identified and are assigned to the isotope (294)117 and its decay products. A hitherto unknown α-decay branch in (270)Db (Z = 105) was observed, which populated the new isotope (266)Lr (Z = 103). The identification of the long-lived (T(1/2) = 1.0(-0.4)(+1.9) h) α-emitter (270)Db marks an important step towards the observation of even more long-lived nuclei of superheavy elements located on an "island of stability."

The reaction 54 Cr + 248 Cm was investigated at the velocity filter SHIP at GSI, Darmstadt, with the intention to study production and decay properties of isotopes of element 120. Three correlated signals were measured, which occurred within a period of 279 ms. The heights of the signals correspond with the expectations for a decay sequence starting with an isotope of element 120. However, a complete decay chain cannot be established, since a signal from the implantation of the evaporation residue cannot be identified unambiguously. Measured properties of the event chain are discussed in detail. The result is compared with theoretical predictions. Previously measured decay properties of even element super-heavy nuclei were compiled in order to find arguments for an assignment from the systematics of experimental data. In the course of this review, a few tentatively assigned data could be corrected. New interpretations are given for results which could not be assigned definitely in previous studies. The discussion revealed that the cross-section for production of element 120 could be high enough so that a successful experiment seems possible with presently available techniques. However, a continuation of the experiment at SHIP for a necessary confirmation of the results obtained in a relatively short irradiation of five weeks is not possible at GSI presently. Therefore, we decided to publish the results of the measurement and of the review as they exist now. In the summary and outlook section we also present concepts for the continuation of research in the field of super-heavy nuclei. 1 Introduction, theoretical background, and status of experiments Scientific attempts to synthesize new elements beyond uranium started in the middle of the 1930s, when the

Shell-correction energies determine the stability and the fission barriers of Super-Heavy Nuclei (SHN), the latter being a main factor responsible for their production yield. Although recent experiments performed at FLNR in Dubna (see review articles [1,2]) have confirmed the existence of an island of SHN, the site and strength of highest stability is still uncertain. Predicted Q  values as shown in Fig. 1a reveal the ambiguity. The macroscopic-micro-scopic (MM) models [3,4] predict a closed proton shell at Z = 114 and thus increasing Q  values beyond. The chiral mean-field model (CMF) [5] and the semi-empirical model (SE) [6] predict subshells or shells at 120 and 126, respectively, resulting in less steep or even decreasing Q  values. Experimental data, also shown in Fig. 1a and known up to Z = 116 do not give preference to a specific model. However, decisive information could be obtained from the -decay properties of elements 118 and 120. The following discussion supports our search experiment for element 120. The study also reveals an important uncertainty related to the prediction of cross-sections of the synthesis of SHN. Shell-correction energiesModel dependent experimental shell-correction energies can be deduced from measured nuclear masses by subtraction of theoretically determined liquid-drop masses. In our case, however, absolute experimental nuclear masses are not known, but relative values can be determined for nuclei within an -decay chain using the experimental Q  values. Normalizing these relative masses to the theoretical ones at the end of the decay chain, which is closer to the region of known masses, where relatively good agreement was established [7], results in a reliable approximation of masses up to the heaviest nuclei of the -decay chain. In one case, where a relatively long decay chain starting at 291 Lv and ending at 267 Rf is known, we have performed such an estimate of shell-correction energies. Interestingly, this is the decay chain which would be populated in a 3n channel of the reaction 54 Cr + 248 Cm. In Figs. 1b and 1c, Rf to 279 Ds were normalized by a least squares fit to the theoretical masses. In order to distinguish between the so determined masses, shell-correction energies and fission barriers from the theoretical ones, we denote those 'experimental data' in the following.

A set of measurements with the CAPRICE ion source at the GSI test bench has been carried out to investigate its behavior in terms of intensity and shape of the extracted beam when the microwaves generating the plasma sweep in a narrow range of frequency (+/-40 MHz) around the klystron center frequency (14.5 GHz). Remarkable variations have been observed depending on the source and the beamline operating parameters, confirming that a frequency dependent electromagnetic distribution is preserved even in the presence of plasma inside the source. Moreover, these observations confirm that the frequency tuning is a powerful method to optimize the electron cyclotron resonance ion source performances. A description of the experimental setup and of the obtained results is given in the following.