Search

A search for superheavy elements was made in bombardments of $^{248}\mathrm{Cm}$ with $^{48}\mathrm{Ca}$ ions performed at projectile energies close to the interaction barrier in order to keep the excitation energy of the compound nucleus $Z=116$, $A=296$ as low as possible. No evidence for superheavy nuclei was obtained in a half-life region from 1 \ensuremath{\mu}s to 10 yr with a production cross section greater than ${10}^{\ensuremath{-}34}$ to ${10}^{\ensuremath{-}35}$ ${\mathrm{cm}}^{2}$.

A gas-filled on-line recoil separator at the Berkeley SuperHILAC is described. The separator consists of a magnetic dipole and a quadrupole doublet. The system is filled with He at a pressure of about 1 Torr (∼ 130 Pa). It separates particles according to their average magnetic rigidity. This allows a separation of evaporation residues, scattered target atoms and beam projectiles in a nuclear reaction. The separation time is in the order of 10 −6 s; the transmission is about 50% for evaporation residues, less than 10 −3 for scattered target-like recoil ions and transfer products, and about 10 −15 for full energy beam projectiles. Experimental data over a wide range of reactions are given and it is shown that the average charge of the recoils is determined by the atomic shell structure of the moving ionized atom in the gas.

Attempts were made to identify new elements in heavy ion fusion reactions. After an effective separation by the velocity filter SHIP the ions passed a pair of thin secondary electron detectors and were finally implanted into position sensitive Si-detectors. With these techniques the odd elements 105, 107, and 109 have been investigated in the years 1981 and 1982. The even elements 106 and 108 have been studied in the year 1984, and in 1985 attempts were made to produce element 110. So far in most experiments 208Pb and 209Bi targets have been irradiated with beams of 50Ti, 54Cr, 58Fe, and 64Ni. In a recent experiment (April/May, 1986) the decay of the ee nucleus 264108 was measured. This isotope was produced in the reaction 58Fe+207Pb with a cross section of (3.2 -2·6 +6·1 ) pb. The nucleus 264108 decays by α emission with a half-life of (76 -35 +365 ) μs. A deduced α energy is (10.8 -0·3 +0·1 ) MeV. Also 260106 is decaying by a emission with a branching of 50%. The decay sequence 264108(α) → 260106(α) → 256104(sf) demonstrates that the heavier element isotopes 264108 and 260106 are more stable against fission than the lighter element isotope 256104. The mass excesses of 264108 and 260106 are (120.0±0.3) MeV and (106.58+0.10) MeV, respectively. The values are obtained by α chain summation. The investigated isotopes are situated near the center of a shell stabilized region of deformed nuclei. The fission barriers for the discovered isotopes are mainly due to shell correction energies. Values as large as 5 MeV have been deduced from the data.

A search for superheavy elements which are expected to occur around the predicted nuclear shell closures at atomic number 114 and neutron number 184 was made in bombardments of 248 Cm with 48 Ca ions. We have carried out this search at energies close to the Coulomb barrier to keep the excitation energy of the compound nucleus Z = 116, A = 296 as low as possible. The experiments were performed at the accelerators SUPERHILAC (Lawrence Berkeley Laboratory (LBL)) and UNILAC (GSI) and used a variety of improved physical and chemical techniques for the isolation and detection of superheavy elements to increase the sensitivity relative to earlier experiments. The small-angle separator system (SASSY) at LBL and the separator for heavy-ion reaction products (SHIP) at GSI were used for shortlived nuclides, and several radiochemical techniques were applied for longer half-lives. Although a broad range of half-lives, 10 −6 to 10 8 s, and excitation energies, 16 to 40 MeV, has been examined, no evidence for the formation of superheavy elements with cross sections greater than 10 −34 to 10 −35 cm 2 was found.

Neutron deficient isotopes of polonium and bismuth have been produced in the reactions $^{56}\mathrm{Fe}$ + $^{\mathrm{nat}}\mathrm{Ce}$ and $^{56}\mathrm{Fe}$ + $^{141}\mathrm{Pr}$, separated from the primary beam by our gas-filled on-line mass separator. The half-lives have been determined by measuring the lifetimes of individual nuclei. The new isotope $^{192}\mathrm{Po}$ (${T}_{\frac{1}{2}}=34\ifmmode\pm\else\textpm\fi{}3$ ms, ${E}_{\ensuremath{\alpha}}=7.17$ MeV) and a short-lived isomeric state in $^{191}\mathrm{Bi}$ (${T}_{\frac{1}{2}}=150\ifmmode\pm\else\textpm\fi{}15$ ms, ${E}_{\ensuremath{\alpha}}=6.86$ MeV) have been identified.