Z. Phys. A - Atoms and Nuclei 315, 145-158 (1984) Zeitschrift Atoms for PhysikA and Nuclei 9 Springer-Verlag 1984 Evidence for Element 109 from One Correlated Decay Sequence Following the Fusion of SSFe with 2~ G. Miinzenberg, W. Reisdorf, S. Hofmann, Y.K. Agarwal*, F.P. Hel3berger, K. Poppensieker, J.R.H. Schneider, W.F.W. Schneider, K.-H. Schmidt, H.-J. Sch6tt, and P. Armbruster Gesellschaft fiir Schwerionenforschung mbH, Darmstadt, Federal Republic of Germany C.-C. Sahm and D. Vermeulen Technische Hochschule, Darmstadt, Federal Republic of Germany Received September 5, 1983 An experiment to synthesize element 109 is presented. Decay patterns characteristic of complete fusion products were searched for in an irradiation of 2~ targets with SaFe projectiles at specific incident energies of 4.95, 5.05, and 5.15 MeV/u. A total dose of 7 X 1017 particles was obtained. The experimental method involves in-flight separation of forward peaked reaction products with a static-field velocity filter, their passage through a time-of-flight device and their final implantation into position sensitive solid state detectors to measure their kinetic energy, approximate mass and their time and position of incidence. The subsequent decay of the narrowly localised reaction products by cascades of alpha particles and/or spontaneous fission is also registered in terms of the energies and times of all the emitted particles. One outstanding decay sequence that started with the emission of two alpha particles within subsequent time intervals of 5 ms and 22 ms and ended with spontaneous fission after 13 s was found at 5.15 MeV/u. The first alpha particle had a kinetic energy of (11.10_+0.04) MeV. A detailed analysis of all the alternative interpretations of this observation, such as a purely random correlation of signals, the decay of a product from a transfer reaction or of any of the various energetically possible evaporation residues, shows that the isotope with mass 266 of element 109, i.e. the one neutron evaporation channel after complete fusion, is the statistically most significant assignment. The outlook for new element synthesis is also briefly discussed. 1. Introduction Attempts to extend the Periodic Table beyond ele- ment 107 failed until now, as the limits, where the attractive nuclear forces no longer can overcome the disruptive Coulomb forces in nuclei, are almost reached. Connected with this, a severe difficulty in efforts to synthesize even heavier elements arises be- cause the heavy ion fusion reactions used result in the formation of excited compound nuclei which will predominantly undergo fission rather than deexcit- ing by emission of light particles such as neutrons, * Permanent Address: Tata Institute of Fundamental Research, Bombay, India i.e. the 'survival' probabilities are exceedingly small. The statistical theory of compound nuclear deexci- tation [1] describes fairly well this survival prob- ability and allows us in particular to understand its known decrease with the multiplicity of the emitted neutrons and hence with the excitation energy of the compound system. One method, firstly suggested by Oganessian et al. [-2], to limit the excitation energy in the initial com- pound system, is to produce heavy elements in fu- sion reactions using targets of Pb or Bi and pro- jectile beams such as 5~ or S4Cr at incident en- ergies close to or even below the classic fusion bar-