Summary Two experiments aiming at the chemical investigation of element 112 produced in the heavy ion induced nuclear fusion reaction of 48Ca with 238U were performed at the Gesellschaft für Schwerionenforschung (GSI), Darmstadt, Germany. Both experiments were designed to determine the adsorption enthalpy of element 112 on a gold surface using a thermochromatography setup. The temperature range covered in the thermochromatography experiments allowed the adsorption of Hg at about 35 °C and of Rn at about -180 °C. Reports from the Flerov Laboratory for Nuclear Reactions (FLNR), Dubna, Russia claim production of a 5-min spontaneous fission (SF) activity assigned to 283112 for the 238U(48Ca,3n) 283112 reaction. Hence, Experiment I was designed to detect spontaneously fissioning (SF) isotopes of element 112 with half-lives (t 1/2) longer than about 20 s. 11 high-energy events were detected. 7 events exhibit a deposition pattern resembling a chromatographic peak in the vicinity of Rn deposition. However, the energy of the events observed in Experiment I was lower than expected for a SF-decay of 283112. Therefore, these events could not be unambiguously attributed to the decay of 283112. In contradiction with earlier publications newer reports from FLNR Dubna claim that 283112 decays by α-particle emission (E α = 9.5 MeV) with t 1/2 = 4 s followed by a SF-decay of 279Ds (t 1/2 = 0.2 s). Therefore, Experiment II was designed to be sensitive to both claimed decay properties of 283112. However, during this experiment neither short α-SF correlations nor SF coincidences were detected. The conclusion is that 283112 was not unambiguously detected, neither in Experiment I nor in Experiment II.

R. Eichler, W. Brüchle, R. Buda, S. Bürger, R. Dressler, Ch.E. Düllmann, J. Dvorak, K. Eberhardt, B. Eichler, C.M. Folden III, H.W. Gäggeler, K.E. Gregorich, F. Haenssler, H. Hummrich, E. Jäger, J.V. Kratz, B. Kuczewski, D. Liebe, D. Nayak, H. Nitsche, D. Piguet, Z. Qin, U. Rieth, M. Schädel, B. Schausten, E. Schimpf, A. Semchenkov, S. Soverna, R. Sudowe, N. Trautmann, P. Thörle, A. Türler, B. Wierczinski, N. Wiehl, P.A. Wilk, G. Wirth, A.B. Yakushev, A. von Zweidorf Paul Scherrer Institut, CH-5232 Villigen, Switzerland 1

SummaryHassium, element 108, was produced in the fusion reaction between26Mg and248Cm. The hassium recoils were oxidizedin-situto a highly volatile oxide, presumably HsO4, and were transported in a mixture of He and O2to a deposition and detection system. The latter consisted of 16 silicon PIN-photodiodes facing a layer of NaOH, which served, in the presence of a certain partial pressure of water in the transport gas, as reactive surface for the deposition of the volatile tetroxides. Six correlated α-decay chains of Hs were detected in the first 5 detectors centred around detection position 3. In analogy to OsO4, which forms Na2[OsO4(OH)2], an osmate(VIII), with aqueous NaOH, HsO4presumably was deposited as Na2[HsO4(OH)2], a hassate(VIII).

After the discovery of the neutron by Chadwick in 1932, the group led by Fermi in Rome began bombarding different elements with neutrons to study the radioactivity induced through (n,γ) reactions. The decay characteristics of the radioactivity in neutron-irradiated uranium was interpreted to indicate that some of the products were probably transuranium elements. Similar conclusions were reached by other research groups; for example in 1937 Meitner, Hahn and Strassman in Berlin proposed the following reaction/decay series (omitting mass numbers)

A. von Zweidorf1, R. Angert1, W. Brüchle1, S. Bürger2, K. Eberhardt2, R. Eichler1,3, H. Hummrich2, E. Jäger1, R. Jera2, H.-O. Kling2, J. V. Kratz2, U. Krille2, B. Kuczewski2, G. Langrock2, G. Lehr2, M. Mendel2, A. Nähler2, A. Peil2, V. Pershina1, U. Rieth2, M. Schädel1, B. Schausten1, E. Schimpf1, H.-J. Schött1, E. Stiel1, P. Thörle2, N. Trautmann2, K. Tsukada4, N. Wiehl2, G. Wirth1 1Gesellschaft für Schwerionenforschung, Darmstadt, 2Institut für Kernchemie, Johannes Gutenberg-Universität Mainz, 3now at PSI, Villigen, 4JAERI, Tokai In October/November 2002, after an intensive optimization of many experimental parameters, the CALLISTO-project finally led to a hassium chemistry experiment [1] at the UNILAC. Since it has been predicted, that hassium forms a volatile tetroxide [2], which was recently confirmed by C. Düllmann et al. [3], we decided to investigate this compound of hassium. For the very volatile OsO4, it is known that it dissolves in strongly basic alkali hydroxide solutions thereby forming red, diamagnetic osmates(VIII) of stoichiometry [OsO4(OH)2]. In previous beamtimes, in which OsO4 was produced in-situ directly behind the target [4], we demonstrated, that the volatile OsO4 deposits effectively on NaOH surfaces from humid He gas. This behaviour was used to design a continuously working system for the formation, transport, deposition and detection of OsO4 and HsO4 [1]. This system, which uses 4 computercontrolled valves and 4 detection arrays, each with 4 alphadetectors [5], combines the advantages of a continuously operating system and adds the possibility to change the deposition material, a thin layer of NaOH on a plate of stainless steel, on a regular basis without interrupting the experiment. Changing the deposition material is necessary, as the deposition efficiency decreases with time [1].

Summary 27-s 263Db was produced in the 249Bk ( 18O, 4n) reaction at 93 MeV. The activity was transported by a He/KCl-jet to the laboratory where it was collected for 15 min and then subjected to a chemical separation specific for group-4 elements. The activity was dissolved in 0.5 M unbuffered α-HiB and eluted from a cation-exchange column. The effluent was made 9 M in HCl and group-4 tetrachlorides were extracted into TBP/Cyclohexane which was evaporated to dryness on a Ta disc. The Ta discs were assayed for α and SF activity. A SF activity with a half life on the order of 20 min was observed and assigned to the nuclide 263Rf. It is formed by electron-capture decay of 263Db with a decay branch of 3+4 -1%.

A. von Zweidorf1, R. Angert1, W. Brüchle1, S. Bürger2, K. Eberhardt2, R. Eichler1,3, H. Hummrich2, E. Jäger1, R. Jera2, H.-O. Kling2, J. V. Kratz2, U. Krille2, B. Kuczewski2, G. Langrock2, G. Lehr2, M. Mendel2, A. Nähler2, A. Peil2, V. Pershina1, U. Rieth2, M. Schädel1, B. Schausten1, E. Schimpf1, H.-J. Schött1, E. Stiel1, P. Thörle2, K. Tsukada4, N. Wiehl2, G. Wirth1 1Gesellschaft für Schwerionenforschung, Darmstadt, 2Institut für Kernchemie, Johannes Gutenberg-Universität Mainz, 3now at PSI, Villigen, 4JAERI, Tokai It has been predicted [1] and confirmed [2], that hassium forms a volatile tetroxide. The first attempt to study a chemical reaction of this compound, and to learn more about the chemical behaviour of Hs, is described in a companion contribution [3]. For this purpose, CALLISTO [4] has been developed in recent years. This system, which is in principle gas chemistry using reactive surfaces, was completely redesigned to integrate all the results of the many preliminary experiments. The volatile tetroxide is formed in-situ in the target chamber, where the recoils are stopped in the jet gas (a mixture of 1 l/min He and 0.1 l/min O2)[5]. At the exit of the target chamber, the gas flow passes a quartz glass tube containing a quartz wool plug, both heated to 500 °C in order to complete the oxidation of osmium and hassium to the tetroxide. The gas is transported via a 13 m long PTFE capillary to the detection system. Because water seems to influence the deposition of OsO4 on the NaOH surface [6], it was necessary to introduce it into the chemical system. As H2O cannot be added before the He passes the target chamber (it prohibits an accurate beam current measurement), it was added after the target chamber and before the detection system. For that reason, a special moisturizing unit was designed for CALLISTO. It consists of a large, thermostated moisturizer [7], which continuously adds at a defined temperature (30 °C) water to a second helium jet (0.1 l/min with the option to use 0.05 2 l/min). This humidified helium passes through a declusterizer at 200 °C to evaporate all remaining water aerosols. Thereafter, the humidity of the gas was monitored with a dewpoint transmitter (~20 g H2O per kg gas). This moisturized helium is added to the jet gas from the target chamber, containing only a few ppm of water and resulting in a humidity of about 2 g H2O per kg gas in the final gas jet. This jet is then distributed through a system of 4 computer-controlled valves to 4 detection arrays (Fig. 1). During an experiment, the He gas is flowing through 3 detection arrays, whereas 1 detection array is cut off from the gas flow; it is in a “service mode” to change the deposition material. We used a thin layer of NaOH as a deposition material, which was prepared by coating plates of stainless steel with 1M ethanolic NaOH and by drying these plates. Every 60 minutes, the valves were automatically switched and the deposition plate of the detection array, being in the “service mode”, was manually changed, cleaned and recoated. Thus, a continuously working detection and deposition system was realized. The surface of the deposition material looses reactivity with time [6]. One possible explanation is, that NaOH is partially neutralized by CO2, which is an impurity of the used gases and probably formed by a reaction of the carbon beam dump with the oxygen of the jet gas, too. The Na2CO3 has a decreased reactivity and shows only a yield of about 50%, compared to NaOH. After the volatile oxides (OsO4, HsO4) are deposited, their αdecay and spontaneous fission can be detected with the detection arrays, each consisting of four (10x10) mm2 large PINdiodes facing the deposition material (Fig. 2).

Summary Gas-phase ion chemistry in a Penning trap has been explored for future experiments on the chemical properties of the heaviest elements. The Mainz Cluster Trap, a Penning trap experiment devoted to metal cluster research, has served as a model apparatus for SHIPTRAP which is being installed behind the velocity filter SHIP (Separator for Heavy Ion reaction Products) at the Gesellschaft für Schwerionenforschung (GSI) at Darmstadt. The reactions of stored Ru + and Os + with oxygen have been studied and the reaction products MO + and MO 2 + (M = Ru, Os) have been observed. The corresponding rate constants have been measured and the results are discussed with respect to future studies of similar ion-molecule reactions of the element 108 (hassium) in SHIPTRAP.

A. von Zw eidorf, R. Angert, W . Brüchle, S. Bürger, K. Eberhardt, R. Eichler, H . H um m rich, E. Jäger, R. Jera, H .-O. Kling, J. V. Kratz, U . Krille, B. Kuczew ski, G. Langrock, G. Lehr, M . M endel, A. N ähler, A. Peil, V. Pershina, U . Rieth, M . Schädel, B. Schausten, E. Schim pf, H .-J. Schött, E. Stiel, P. Thörle, N . Trautm ann, K. Tsukada, N . W iehl, G. W irth Gesellschaft für Schw erionenforschung, Darm stadt, Institut für Kernchem ie, Johannes Gutenberg-Universität M ainz, now at PSI, V illigen, JAERI, Tokai