First Direct Measurements of Superheavy-Element Mass Numbers.

An experiment was performed at Lawrence Berkeley National Laboratory's 88-in. Cyclotron to determine the mass number of a superheavy element. The measurement resulted in the observation of two α-decay chains, produced via the ^{243}Am(^{48}Ca,xn)^{291-x}Mc reaction, that were separated by mass-to-charge ratio (A/q) and identified by the combined BGS+FIONA apparatus. One event occurred at A/q=284 and was assigned to ^{284}Nh (Z=113), the α-decay daughter of ^{288}Mc (Z=115), while the second occurred at A/q=288 and was assigned to ^{288}Mc. This experiment represents the first direct measurements of the mass numbers of superheavy elements, confirming previous (indirect) mass-number assignments.

Decay and Fission Hindrance of Two- and Four-Quasiparticle K Isomers in

Two isomers decaying by electromagnetic transitions with half-lives of 4.7(1.1) and 247(73) µs have been discovered in the heavy ^{254}Rf nucleus. The observation of the shorter-lived isomer was made possible by a novel application of a digital data acquisition system. The isomers were interpreted as the K^{π}=8^{-}, ν^{2}(7/2^{+}[624],9/2^{-}[734]) two-quasineutron and the K^{π}=16^{+}, 8^{-}ν^{2}(7/2^{+}[624],9/2^{-}[734])⊗8^{-}π^{2}(7/2^{-}[514],9/2^{+}[624]) four-quasiparticle configurations, respectively. Surprisingly, the lifetime of the two-quasiparticle isomer is more than 4 orders of magnitude shorter than what has been observed for analogous isomers in the lighter N=150 isotones. The four-quasiparticle isomer is longer lived than the ^{254}Rf ground state that decays exclusively by spontaneous fission with a half-life of 23.2(1.1) µs. The absence of sizable fission branches from either of the isomers implies unprecedented fission hindrance relative to the ground state.

48Ca+249Bk fusion reaction leading to element Z = 117: long-lived α-decaying 270Db and discovery of 266Lr.

The superheavy element with atomic number Z=117 was produced as an evaporation residue in the (48)Ca+(249)Bk fusion reaction at the gas-filled recoil separator TASCA at GSI Darmstadt, Germany. The radioactive decay of evaporation residues and their α-decay products was studied using a detection setup that allowed measuring decays of single atomic nuclei with half-lives between sub-µs and a few days. Two decay chains comprising seven α decays and a spontaneous fission each were identified and are assigned to the isotope (294)117 and its decay products. A hitherto unknown α-decay branch in (270)Db (Z = 105) was observed, which populated the new isotope (266)Lr (Z = 103). The identification of the long-lived (T(1/2) = 1.0(-0.4)(+1.9) h) α-emitter (270)Db marks an important step towards the observation of even more long-lived nuclei of superheavy elements located on an "island of stability."

Spectroscopy of element 115 decay chains.

A high-resolution α, x-ray, and γ-ray coincidence spectroscopy experiment was conducted at the GSI Helmholtzzentrum für Schwerionenforschung. Thirty correlated α-decay chains were detected following the fusion-evaporation reaction 48Ca + 243Am. The observations are consistent with previous assignments of similar decay chains to originate from element Z=115. For the first time, precise spectroscopy allows the derivation of excitation schemes of isotopes along the decay chains starting with elements Z>112. Comprehensive Monte Carlo simulations accompany the data analysis. Nuclear structure models provide a first level interpretation.

Production and decay of element 114: high cross sections and the new nucleus 277Hs.

The fusion-evaporation reaction 244Pu(48Ca,3-4n){288,289}114 was studied at the new gas-filled recoil separator TASCA. Thirteen correlated decay chains were observed and assigned to the production and decay of {288,289}114. At a compound nucleus excitation energy of E{*}=39.8-43.9 MeV, the 4n evaporation channel cross section was 9.8{-3.1}{+3.9} pb. At E^{*}=36.1-39.5 MeV, that of the 3n evaporation channel was 8.0{-4.5}{+7.4} pb. In one of the 3n evaporation channel decay chains, a previously unobserved α branch in 281Ds was observed (probability to be of random origin from background: 0.1%). This α decay populated the new nucleus 277Hs, which decayed by spontaneous fission after a lifetime of 4.5 ms.

New superheavy element isotopes: ²4²Pu(48Ca,5n) ²85114.

The new, neutron-deficient, superheavy element isotope ²85114 was produced in 48Ca irradiations of ²4²Pu targets at a center-of-target beam energy of 256 MeV (E*=50 MeV). The α decay of ²85114was followed by the sequential α decay of four daughter nuclides, 281Cn, 277Ds, 273Hs, and 269Sg. 265Rf was observed to decay by spontaneous fission. The measured α-decay Q values were compared with those from a macroscopic-microscopic nuclear mass model to give insight into superheavy element shell effects. The²4²Pu (48Ca,5n²)²85114 cross section was 0.6(-0.5)+0.9 pb.

Independent verification of element 114 Production in the ;{48}Ca + ;{242}Pu reaction.

Independent verification of the production of element 114 in the reaction of 244-MeV ;{48}Ca with ;{242}Pu is presented. Two chains of time- and position-correlated decays have been assigned to ;{286}114 and ;{287}114. The observed decay modes, half-lives, and decay energies agree with published results. The measured cross sections at a center-of-target energy of 244 MeV for the ;{242}Pu(;{48}Ca,3-4n);{287,286}114 reactions were 1.4_{-1.2};{+3.2} pb each, which are lower than the reported values.

Lightest isotope of Bh produced via the 209Bi(52Cr,n)260Bh reaction.

The lightest isotope of Bh was produced in the new 209Bi(52Cr,n)260Bh reaction at the Lawrence Berkeley National Laboratory's 88-Inch Cyclotron. Positive identification was made by observation of eight correlated alpha particle decay chains in the focal plane detector of the Berkeley Gas-Filled Separator. 260Bh decays with a 35(-9)(+19) ms half-life by alpha particle emission mainly by a group at 10.16 MeV. The measured cross section of 59(-20)(+29) pb is compared to model predictions. The influence of the N=152 and Z=108 shells on alpha decay properties is discussed.

Development of an odd-Z-projectile reaction for heavy element synthesis: 208Pb(64Ni,n)271Ds and 208Pb(65Cu,n)(272)111.

Seven 271Ds decay chains were identified in the bombardment of 208Pb targets with 311.5 and 314.3 MeV 64Ni projectiles using the Berkeley Gas-filled Separator. These data, combined with previous results, provide an excitation function for this reaction. From these results, an optimum energy of 321 MeV was estimated for the production of (272)111 in the new reaction 208Pb(65Cu,n). One decay chain was observed, resulting in a cross section of 1.7(+3.9)(-1.4) pb. This experiment confirms the discovery of element 111 by the Darmstadt Group who used the 209Bi(64Ni,n)(272)111 reaction.

Chemical investigation of hassium (element 108).

The periodic table provides a classification of the chemical properties of the elements. But for the heaviest elements, the transactinides, this role of the periodic table reaches its limits because increasingly strong relativistic effects on the valence electron shells can induce deviations from known trends in chemical properties. In the case of the first two transactinides, elements 104 and 105, relativistic effects do indeed influence their chemical properties, whereas elements 106 and 107 both behave as expected from their position within the periodic table. Here we report the chemical separation and characterization of only seven detected atoms of element 108 (hassium, Hs), which were generated as isotopes (269)Hs (refs 8, 9) and (270)Hs (ref. 10) in the fusion reaction between (26)Mg and (248)Cm. The hassium atoms are immediately oxidized to a highly volatile oxide, presumably HsO(4), for which we determine an enthalpy of adsorption on our detector surface that is comparable to the adsorption enthalpy determined under identical conditions for the osmium oxide OsO(4). These results provide evidence that the chemical properties of hassium and its lighter homologue osmium are similar, thus confirming that hassium exhibits properties as expected from its position in group 8 of the periodic table.