We present results of the second experiment on the chemical identification of element 112. Similar to the first test in 2000, we aimed at the production of the spontaneously fissioning 283112 nuclei with T1/2≈3min. A natU3O8 (with some Nd) target, 2mg of U/cm2 thick, was bombarded with 233-MeV 48Ca ions (the energy in the middle of the target layer). The nuclei recoiling from the target were thermalized in flowing helium. The target chamber was connected through a 25m long capillary to detectors of α-particles and fission fragments. All the equipment and detectors were kept at ambient temperature. According to the test experiments, of all the heavy elements produced in the bombardment, only Hg, Rn and At could be transported to the detectors. The first detecting device was similar to that used earlier – an assembly of 8 pairs of PIPS detectors coated with gold. Here one would observe the decay of element 112 atoms if they like Hg adsorbed on gold. The atoms which were not retained and freely passed through the PIPS detectors entered a new, flow-through ionization chamber, 5000 cm3 in volume, optimized for detecting fission fragments. The PIPS detectors and the ionization chamber were placed inside a large assembly of 3He – filled neutron counters to detect prompt neutrons from the fission events. In 22.5 days, a beam dose of 2.8×1018 ions was accumulated. More than 95 of the simultaneously produced α-active 185Hg (T1/2=49 s) were found deposited already on the first pair of PIPS detectors; meanwhile, all the PIPSs did not detect any fission event. In the ionization chamber, eight fission events were observed in coincidence with neutron counts while the expected background was insignificant. Hence, the spontaneous fissions of the volatile activity can be conclusively attributed to the decay of element 112 produced in the fusion reaction 48Ca+ 238U, and formerly observed in Dubna physical experiments. Evaluation of the experimental data in terms of the adsorption enthalpies indicates much weaker interaction of element 112 with Au than that of Hg. One can conclude that in the given chemical environment, element 112 behaves like Rn rather than like Hg. The formation cross section of 283112 estimated from the data amounts to several pb. The experiments were carried out at the Flerov Laboratory of Nuclear Reactions at JINR in November–December 2001.
© 2003 Oldenbourg Wissenschaftsverlag GmbH