Electron-capture delayed fission was observed in 37-s Es-244 produced via the Np-237(C-12,5n)Es-244 reaction at 81 MeV (on target) with a production cross section of 0.31+/-0.12 mub. The kinetic energies of coincident fission fragments were measured with our rotating wheel detection system and the average pre-neutron-emission total kinetic energy of the fragments was found to be 186+/-19 MeV. The mass-yield distribution of the fission fragments is predominantly asymmetric. Based on the ratio of the number of fission events to the measured number of alpha decays from the electron-capture daughter Cf-244 (100% alpha branch), the probability of delayed fission was determined to be (1.2+/-0.4) X 10(-4). This value for the delayed fission probability fits the experimentally observed trend of increasing delayed fission probability with increasing Q value for electron capture.

The published claim for the discovery of element 118 is retracted.

The Cryo-Thermochromatographic Separator (CTS) was designed and constructed for rapid, continuous on-line separation and simultaneous detection of highly volatile compounds of short-lived α-decaying isotopes of osmium and hassium (Hs, Z=108). A flowing carrier gas containing the volatile species is passed through a channel formed by two facing rows of 32 α-particle detectors, cooled to form a temperature gradient extending from 247K at the channel entrance down to 176K at the exit. The volatile species adsorb onto the SiO2-coated detector surfaces at a characteristic deposition temperature and are identified by their observed α-decay energies. The CTS was tested on-line with OsO4 prepared from 169–173Os isotopes produced in 118,120Sn(56Fe, 3,4,5n) reactions. An adsorption enthalpy for OsO4 of −40.2±1.5kJ/mol on SiO2 was deduced by comparing the measured deposition distribution with Monte Carlo simulations. This successful result demonstrates that such a system provides a viable means for the first chemical study of hassium—which is expected to be a homologue of osmium—by determining whether it forms a similarly volatile oxide.

The Cryo-Thermochromatographic Separator (CTS): A new detection and separation system for highly volatile osmium and hassium (element 108) tetroxides U. W. Kirbach, H. Nitsche, C. M. Folden III, T. N. Ginter, K. E. Gregorich, D. C. Hoffman, D. M. Lee, V. Ninov, J. P. Omtvedt 1 , J. B. Patin, N. K. Seward, D. A. Strellis, R. Sudowe, P. A. Wilk, and P. M. Zielinski We implemented a new concept for heavy element chemistry research using an ion separator to separate the desired products from the beam, transfer products and other undesirable by-products prior to chemical studies. First, a Recoil product Transfer Chamber (RTC) was designed and attached to the Berkeley Gas-filled Separator (BGS) to collect and transfer the recoiling products to the chemical separation system. The RTC consists of a wire-grid-supported thin mylar foil (≤) 200 µg/cm 2 that separates the BGS detector chamber, at 1.3 mbar pressure, from the chemistry system at different pressures ranging from 480 mbar to 2000 mbar. The overall transport efficiency ranged between 30% and 15%, compared to the activity measured in the focal plane detector of the BGS. The CTS was designed as a separation and α-decay detection system for the highly volatile tetroxides of osmium and hassium, element 108. The CTS, shown in figure 1, consists of two rows of 32-α detectors arranged along a negative temperature gradient. The tetroxides adsorb on the surface of one of the silicone photodiodes at a certain deposition temperature, and the nuclide is then identified by the α-decay. To test the CTS with the expected hassium homologue osmium, different α-active osmium isotopes were produced using the nuclear reactions 118 Sn( 56 Fe, 4,5n) Os and 120 Sn( 56 Fe, 4,5n) 172,171 Os. After pre- separation in the BGS, a mixture of 90% helium and 10% oxygen was used to transport the osmium to a quartz tube heated to 1225 K, where OsO 4 was formed. The negative temperature gradient in the CTS ranged from 248 K to 173 K. Using a flow rate of 500 mL/min, most of the osmium activity was adsorbed at a temperature of about 203 K. From the measured α-activity distribution, an adsorption enthalpy of 40±1 kJ/mol for OsO 4 on the detector surface was calculated using Monte Carlo simulations. The results show that the CTS is working properly and can be used for experiments studying the chemical properties of hassium. front view side view activity from RTC or other target chamber; He/O 2 transport gas oxidation at 1200 K in a quartz tube - two rows of 32 PIN diodes, gradient cooled by LN 2 from ambient temper- ature to 120 K - deposition of HsO 4 on the silicon α - detector surface Fig. 1: Schematic and working principle of the Cryo-Thermographic Separator (CTS) and detector that was used to identify volatile osmium tetroxide, a homologue of hassium (Hs, element 108). This experimental set-up will be used for the first chemical identification of hassium. University of Oslo, Norway

Electron-capture delayed fission was observed in 26-min Es-248 and 7.7-min Es-246, which were produced at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron via Cf-249(p,2n) and Cf-249(p,4n) reactions at proton energies of 18 and 37 MeV on target, respectively. The delayed fission probabilities were determined to be (3.5+/-1.8) x 10(-6) for Es-248 and (3.7(-3.0)(+8.5)) x 10(-5) for Es-246. These values were determined in separate experiments using our automated rotating wheel online collection and measurement system. Collection and measurement times were optimized for the isotope being measured. The measured delayed fission probabilities are consistent with the experimentally observed trend of increasing delayed fission probability with increasing Q value for electron capture.

Subsecond 224 Pa (T 1/2 = 0.85 s) was produced via the 209 Bi(18 O,3n)224 Pa reaction at the 88 inch cyclotron at the Lawrence Berkeley National Laboratory. After production it was transported via a gas-jet system to the centrifuge system SISAK 3. Following on-line extraction with trioctylamine/scintillation solutions from 1M lactic acid, 224 Pa was detected applying on-line α-liquid scintillation counting. Unambiguous identification was achieved using time-correlated α-α-decay chain analysis. This constitutes the first chemical on-line separation and detection of a subsecond α-decaying nuclide, 0.85-s 224 Pa with the fast extraction system SISAK 3.

The Heavy Element Volatility Instrument (HEVI), an on-line isothermal gas chromatography system, has been used to separate the volatile bromide compounds of the group 4 elements Zr and Hf and the transactinide Rf according to their volatilities, and to provide data on the gas phase chemical properties of very short-lived isotopes in amounts as low as a few atoms. For these studies261Rf was produced via the248Cm(18O, 5n) reaction.165-167Hf was produced via the reactionnatEu(19F, xn), and85Zr was produced via thenatCu(28Si, 3p3n) reaction. The half-life for261Rf was measured to be 75±7 seconds.A Monte Carlo code was used to deduce the enthalpy of adsorption (ΔHa) from the observed volatility and parameters of the chromatography system. The resulting adsorption enthalpies for the Zr, Hf, and Rf tetrabromides are: -108±5 kJ · mol-1, -113±5 kJ · mol-1 and -87±7 kJ · mol-1, respectively.Volatilities of the group 4 bromides support the conclusion from previous results for the group 4 chlorides that Rf deviates from the trend expected by simple extrapolation of the properties of its lighter homologs in the periodic table. The group 4 bromides are also observed to be less volatile than their respective chlorides, as predicted by relativistic calculations.

The excitation function for the U-233(He-3,4n)Pu-232 reaction was measured. The maximum production cross section is 6.2+/-1.5 mu b at 32.5+/-2.2 MeV. The plutonium fraction was chemically separated from interfering activities. The Pu-232 half-life was determined to be 33.1+/-0.8 min using the interference-free data obtained from alpha-alpha correlations of daughter activities in equilibrium with the Pu-232 decay.

The excitation function for the ${}^{233}\mathrm{U}{(}^{3}\mathrm{He}{,4n)}^{232}\mathrm{Pu}$ reaction was measured. The maximum production cross section is $6.2\ifmmode\pm\else\textpm\fi{}1.5\ensuremath{\mu}\mathrm{b}$ at $32.5\ifmmode\pm\else\textpm\fi{}2.2\mathrm{MeV}.$ The plutonium fraction was chemically separated from interfering activities. The ${}^{232}\mathrm{Pu}$ half-life was determined to be $33.1\ifmmode\pm\else\textpm\fi{}0.8\mathrm{min}$ using the interference-free data obtained from \ensuremath{\alpha}-\ensuremath{\alpha} correlations of daughter activities in equilibrium with the ${}^{232}\mathrm{Pu}$ decay.

The arrangement of the chemical elements in the periodic table highlights resemblances in chemical properties, which reflect the elements’ electronic structure. For the heaviest elements, however, deviations in the periodicity of chemical properties are expected 1 , 2 , 3 : electrons in orbitals with a high probability density near the nucleus are accelerated by the large nuclear charges to relativistic velocities, which increase their binding energies and cause orbital contraction. This leads to more efficient screening of the nuclear charge and corresponding destabilization of the outer d and f orbitals: it is these changes that can give rise to unexpected chemical properties. The synthesis of increasingly heavy elements 4 , 5 , 6 , now including that of elements 114, 116 and 118, allows the investigation of this effect, provided sufficiently long-lived isotopes for chemical characterization are available 7 . In the case of elements 104 and 105, for example, relativistic effects interrupt characteristic trends in the chemical properties of the elements constituting the corresponding columns of the periodic table 8 , whereas element 106 behaves in accordance with the expected periodicity 9 , 10 , 11 , 12 . Here we report the chemical separation and characterization of six atoms of element 107 (bohrium, Bh), in the form of its oxychloride. We find that this compound is less volatile than the oxychlorides of the lighter elements of group VII, thus confirming relativistic calculations 13 that predict the behaviour of bohrium, like that of element 106, to coincide with that expected on the basis of its position in the periodic table.

New neutron rich isotopes 267107Bh and 266107Bh were produced in bombardments of a 249Bk target with 117-MeV and 123-MeV 22Ne ions at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron. Identification was made by observation of correlated alpha-particle decays between the Bh isotopes and their Db and Lr daughters using a rotating wheel system. 267Bh was produced with a cross section of approximately 70 pb and decays with a 17(+14)(-6) s half life by emission of alpha particles with an average energy of 8.83+/-0.03 MeV. One atom of 266Bh was observed, decaying within 1 s by emission of a 9.29-MeV alpha particle.

A new sample changer system has been designed to rapidly transport activity from a heavy element production target chamber at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron to a fission, α-particle, X-ray, and γ-ray detection site. This changer was specifically designed to help study the nuclear structure of isotopes involved in electron-capture-delayed fission decay with half-lives greater than 30s. The system integrates a rotating wheel and a cycling piston to transport samples from the end of a gas transport capillary line to the detection site. The overall efficiency of the sample changer system was determined by running off-line experiments to determine the activity collection site efficiency and the detector efficiencies. Under the most favorable conditions, we calculated a relative He–KCl gas transport and collection efficiency of around 70%. Efficiencies were calculated by comparing the yield of 221Fr (half-life=4.8 min) at the sample changer collection site to the activity collected directly on a piece of glass filter paper.