Experimental Studies of the Synthesis and the Survival Probability of Transactinides

Abstract: In 1999 scientists in Dubna announced the successful synthesis of the nucleus 283112 by the reaction 48Ca+ 238U, with a cross-section of 5.0 pb. The isotope was found to decay by spontaneous fission (t1/2=81 s). Current nuclear theory, supported by the experimental data on 277112, 284112 and 285112, predicts significantly lower production cross-sections and α-emission as the predominant decay mode for isotopes of element 112. The reported properties of 283112 challenge theoretical predictions and raise questions on the subsequent syntheses of the nuclei 287114 and 291116. In order to confirm this discovery, the reaction was repeated under the same conditions, and events similar to the reported ones were searched for. During our first investigation, in 2001, we failed to observe any events to meet the criteria of the 1999 observation. Our second experiment is presented here. The current situation concerning isotope 283112 is discussed.The survival probability of fissionable heavy nuclei is described by the ratio n/f. The ratio can be directly evaluated from measurements of the neutrons emitted before and after fission from the excited compound nucleus and the fission fragments, respectively. An investigation on the survival probabilities of 264Rf and 263Rf against fission is described. The isotopes were produced by the reactions 26Mg+238U and 25Mg+238U and the angular distributions of the emitted neutrons were measured. The average Γn/Γf ratios for the evaporation chains of the two isotopes were examined, and an attempt to evaluate the absolute ratio Γn/Γf of the first-chance fission of 264Rf was made. The results of a similar earlier study on the isotopes 258No and 257No are also presented.