Identification of element 107 by? correlation chains

Abstract

The production of nuclei at the upper end of the periodic table in heavy ion fusion reactions is limited by the high fissility of the compound system. Formation cross-sections decrease rapidly with increasing excitation energy. With targets near the double magic 2~and neutron rich metal ions like 5OTi or 54Cr as projectiles, fusion close to the barrier yields weakly excited compound systems with excitation energies of less than 25 MeV. The asymmetry of the target-projectile combination is still sufficient to expect no dynamic effects on the barrier. Consequently, the irradiation of 2O9Bi with 54Cr as proposed by Oganessian et al /1/ is up to our present knowledge the most appropriate reaction to synthesize element 107 by heavy ion fusion.With the velocity filter SHIP /2/ for separation and surface barrier detectors for identification /3/ of the fusion products we can investigate the predominant decay modes, spontaneous fission and decay over the entire expected halflife range down to us with low background from other reactions.Up to now in experiments to produce element 107 only spontaneous fission was observed. Oganessian et al. /I/ found two spontaneous fission activities in bombardments of 2O9Bi with 54Cr. One with a halflife of (1-2) ms, which was assigned to the 2n channel. The other activity with a halflife of 5 s was explained by an unobserved decay of 261107 leading to 257105 which was regarded to undergo spontaneous fission with the corresponding halflife.The aim of our experiment was to produce element 107 in the same reaction and to identify the isotopes unambiguously by correlated a decay chains to known transitions. We also investigated the reaction 5OTi on 209Bi to find the unknown daughter decays of element 105 and Lr. Experimental SetupThe S4Cr beam from the UNILAC linear accelerator had an average intensity of 6 x 1011 particles/s.The ion source was operated with isotopically enriched material. 209Bi targets with an average thickness of 660 ug/cm 2 had been evaporated on a 30 ug/cm 2 carbon foil and covered with a 30 2 ~g/cm carbon film for radiative cooling. They were mounted on a rotating wheel to stand the high beam intensities /4/. Target thickness was monitored by Rutherford scattering. After in-flight separation by SHIP, the evaporation residues passed two large area detectors for time-of-flight measurement and were implanted into a bench of 7 position sensitive surface barrier detectors. The point of the identification method is that all decays belonging to a decay chain which starts from any incident evaporation residue have to occur at its position of implantation within a window given by the detector resolution. Correlation times are limited by the rates of random events within this position window. The detectors are cooled to 268 K and have an energy resolution of 27 keY FWHM, the position resolution is 0.3 mm FWHM.The evaporation residues are separated with efficiencies of about 20%. The response of the surface barrier detector for ~ decays is 50%, as the evaporation residues are impl...

Excitation function for the production ofBh262(Z=107) in the odd-Z-projectile reactionFolden1, Nelson2, Düllmann3 et al. 2006Phys. Rev. C

“…Although 246 Cf was detected in the Cf fractions, many assumptions were necessary to assert that it is produced as a result of the decay of 262 Bh. These results were not confirmed, and the first definitive evidence for the production of bohrium was reported by Münzenberg et al [10,11]. They reported that 262 Bh decays from the ground state by emission of alpha groups of several different energies and a half-life of 102 ± 26 ms based on the observed distribution of lifetimes.…”

“…These considerations suggest that a measurement of the complete 209 Bi( 54 Cr, n) 262 Bh excitation function should be performed. However, it should be noted that the experiments reported in [11] were primarily designed to confirm the discovery of bohrium [10] and were later judged to be convincing [43]. The decay properties measured for the nuclides produced are certainly valid and independent of the measurement of the detailed excitation function.…”

Synthesis of superheavy elements by cold fusion

“…Similar is the difference between the even elements from 104 to 110 when the projectile is changed from 50 Ti to 54 Cr to 58 Fe and to 62,64 Ni. In addition, the combination of odd elements projectiles 51 V, 55 Mn, 59 Co, and 65 Cu with targets of 208 Pb for synthesis of the odd elements 105, 107, 109, and 111 results in similar cross-sections as in reactions with even element projectiles and 209 Bi targets (see Table 1). This comparison shows that the presence of an odd proton, independent whether it belongs to the projectile or to the target nucleus, reduces the cross-section by an amount which is similar as the increase of the fissility (probability for re-separation) when the projectile is changed from Z = 22 to 24 etc.…”

“…The reaction used was 54 Cr + 209 Bi → 263 Bh * . Five decay chains of 262 Bh were observed [65]. The next lighter isotope, 261 Bh, was synthesized at higher beam energy [66].…”

“…4), it seems more likely that 260 272 Rg was achieved in an experiment at LBNL [54]. However, for the synthesis, the reaction 65 Cu + 208 Pb → 273 Rg * used.…”