Prepared for Nobel Symposium on the Chemistry and Physics of Heavy and Super Heavy Elements held at Bäckaskog Castle in Sweden May 29 – June 3, 2016.
For half a century, various theoretical models have predicted the existence of an area of enhanced stability of heaviest (superheavy) nuclei (SHN). A group of nuclei in the vicinity of the predicted doubly magic spherical nucleus 298114184 were named superheavy. Just after the appearance of the first theoretical predictions of this area of SHN, numerous attempts were undertaken to synthesize them artificially; however, all the efforts were in vain. Later on, it became clear that in order to synthesize SHN the sensitivity of experiments should be increased by two to three orders of magnitude.
In 1998, experiments aimed at the synthesis of SHN were initiated in FLNR, JINR. In these experiments, we employed the Dubna gas filled recoil separator (DGFRS) that allows the collection of the products of complete-fusion reactions in a detection system and the separation of these from the beam of bombarding ions, elastically scattered nuclei and products of incomplete fusion. The detection system includes proportional chambers used to measure the time of flight (TOF) of particles and several semiconductor detectors; the latter are separated in position-sensitive strips.
During 15 years of experiments with the DGFRS, six new superheavy elements located on the shore of the "Island of stability" and having atomic numbers 113 through 118 were synthesized for the first time. Radioactive properties of more than 50 new heaviest nuclides were explored in the experiments carried out in collaboration with the laboratories of the USA in Livermore (LLNL), Oak Ridge (ORNL), Knoxville (UT), Nashville (VU) and with Russian centers RIAR (Dimitrovgrad) and RIEPh (Sarov).
Radioactive properties of the synthesized nuclides demonstrate a substantial increase of nuclear stability with increase of neutron number and closer approach to the predicted spherical shells Z=114-126 and N=184. New observations establish a consistent pattern of nuclear properties in the area of the heaviest nuclides. They demonstrate the decisive role of nuclear shells and provide experimental proof of the existence of the predicted "Island of stability" of superheavy elements.