The recoil transfer chamber—An interface to connect the physical preseparator TASCA with chemistry and counting setups

Nuclear Instruments and Methods in Physics Research A 638 (2011) 157–164

Contents lists available at ScienceDirect

Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima

The recoil transfer chamber—An interface to connect the physical preseparator TASCA with chemistry and counting setups b a,b,c ¨ ¨ J. Even a,n, J. Ballof a, W. Bruchle , R.A. Buda a, Ch.E. Dullmann , K. Eberhardt a, A. Gorshkov d, a a b b a ¨ , J. Khuyagbaatar , J.V. Kratz , J. Krier b, D. Liebe a, M. Mendel a, D. Nayak f, E. Gromm , D. Hild , E. Jager b ¨ , K. Opel e, J.P. Omtvedt e, P. Reichert a, J. Runke a, A. Sabelnikov e, F. Samadani e, M. Schadel b a b b,d,e a ¨ , P. Thorle-Pospiech , A. Toyoshima g, B. Schausten , N. Scheid , E. Schimpf , A. Semchenkov d,1 ¨ , V. Vicente Vilas a, N. Wiehl a, T. Wunderlich a, A. Yakushev d,2 A. Turler a

Institut f¨ ur Kernchemie, Johannes Gutenberg-Universit¨ at Mainz, 55099 Mainz, Germany GSI Helmholtzzentrum f¨ ur Schwerionenforschung GmbH, 64291 Darmstadt, Germany c Helmholtz-Institut Mainz, 55099 Mainz, Germany d Technische Universit¨ at M¨ unchen, 85748 Garching, Germany e University of Oslo, N0315 Oslo, Norway f Saha Institute of Nuclear Physics, Kolkata 700 064, India g Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan b

a r t i c l e i n f o

abstract

Article history: Received 29 June 2010 Received in revised form 14 February 2011 Accepted 14 February 2011 Available online 23 February 2011

Performing experiments with transactinide elements demands highly sensitive detection methods due to the extremely low production rates (one-atom-at-a-time conditions). Preseparation with a physical recoil separator is a powerful method to significantly reduce the background in experiments with sufficiently long-lived isotopes (t1/2 Z 0.5 s). In the last years, the new gas-filled TransActinide Separator and Chemistry Apparatus (TASCA) was installed and successfully commissioned at GSI. Here, we report on the design and performance of a Recoil Transfer Chamber (RTC) for TASCA—an interface to connect various chemistry and counting setups with the separator. Nuclear reaction products recoiling out of the target are separated according to their magnetic rigidity within TASCA, and the wanted products are guided to the focal plane of TASCA. In the focal plane, they pass a thin Mylar window that separates the  1 mbar atmosphere in TASCA from the RTC kept at  1 bar. The ions are stopped in the RTC and transported by a continuous gas flow from the RTC to the ancillary setup. In this paper, we report on measurements of the transportation yields under various conditions and on the first chemistry experiments at TASCA—an electrochemistry experiment with osmium and an ion exchange experiment with the transactinide element rutherfordium. & 2011 Elsevier B.V. All rights reserved.

Keywords: Gas jet Gas-filled separator TASCA Physical preseparation Recoil transfer chamber Transactinides

1. Introduction An exciting field in nuclear physics and chemistry is the research on SuperHeavy Elements (SHE), here defined as elements with Z Z104. Until now, observations of the elements up to Z ¼118 are reported [1,2]. It is expected from theory that a so-called island of stability in the region Z  114  126 and N ¼184 exists, where nuclei with significant nuclear stability are

n ¨ Kernchemie, Johannes GutenbergCorresponding author at: Institut fur ¨ Mainz, Fritz-Straßmann-Weg 2, D-55128 Mainz, Germany, Universitat Tel.: + 49 6131 39 25878, fax: + 49 6131 39 25253 E-mail address: [email protected] (J. Even). 1 Now at: Paul Scherrer Institut, 5232 Villigen PSI, Switzerland. 2 ¨ Schwerionenforschung GmbH, 64291 Now at: GSI Helmholtzzentrum fur Darmstadt, Germany.

0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.02.053

expected. This strongly motivates physics experiments in this region. Besides purely nuclear physical aspects, the study of the structure of the electronic shell and the resulting chemical behavior of these elements is extremely fascinating. The influence of relativistic effects, which increases roughly Z2 is expected to be dominant in the chemistry of these elements. Until now, the chemical behavior of the elements up to Z¼ 108 and of element Z¼112 and Z ¼114 has been studied [3–6]. SHEs are synthesized as EVaporation Residues (EVR) of heavyion induced nuclear fusion reactions. The big challenges in chemical studies of SHEs are the low production rates (of the order of pb  nb) and the short half-lives (with a few exceptions less than one minute). The necessity to identify single atoms in experiments with these elements requires lowest possible background conditions. One way to

158

J. Even et al. / Nuclear Instruments and Methods in Physics Research A 638 (2011) 157–164

isolate SHE from all the lighter elements produced in the nuclear reactions is by spatial separation in a physical separator [7]. Behind a separator, it is possible to perform efficient chemistry experiments under strongly reduced background conditions. Therefore, the new gas-filled separator called the TransActinide Separator and Chemistry Apparatus (TASCA) with a classical DQQ (D ¼dipole, Q¼quadrupole magnet) magnetic configuration was ¨ Schwerionenconstructed at the GSI Helmholtzzentrum fur forschung, Darmstadt (GSI) [8,9]. TASCA is used as a physical preseparator [10] for nuclear chemistry and nuclear physics studies. This means, the ions are separated by their physical properties – in this case their magnetic rigidity – before they are guided to the detection or chemistry devices. An important factor in SHE chemistry experiments is a fast transport of the EVRs from the target to the chemistry devices. An aerosol particle gas jet is a well-established technique to transport isotopes with half-lives of a few seconds [11,12]. In this technique, the EVRs are thermalized in a gas-filled chamber. The chamber is flushed with a carrier gas seeded with aerosol particles. Nonvolatile fusion products attach onto these particles and are transported with the transport gas to the collection site. A drawback of this technique is that in a recoil chamber placed directly behind the target, unwanted by-products of the nuclear reaction, the decay of which interferes with the unambiguous identification of the nuclides of interest are not suppressed. One further problem is the plasma created by the intense projectile beam behind the target, which destroys the aerosol particles. This reduces the gas jet yield dramatically at high beam intensities [13]. For selected chemical systems, these drawbacks can be circumvented by working without aerosol particles and instead adding reactive gas to the carrier gas to produce volatile compounds – like OsO4 or HsO4 [14] – which can be transported in the gas stream. Also volatile elements like Rn, Hg, Cn and element 114 [5,6] can be transported in a pure, e.g., helium stream without adding any reactive gas or clusters. However, only elements that are either volatile in their elemental state or form simple inorganic compounds of high thermal stability and volatility, can be studied with this technique. It is not possible to introduce organic compounds directly into the recoil chamber and to transport the fusion products as, e.g., organometallic compounds, as these are easily destroyed by the plasma behind the target [15]. All these problems can be overcome by using a recoil separator as a preseparator. The group at the Berkeley Gas-filled Separator (BGS) was the first who developed a so-called Recoil Transfer Chamber (RTC) for coupling a physical preseparator and a chemistry setup connected by a gas jet [16]. The first chemical studies with a transactinide element behind a gas-filled separator are reported in Ref. [17]. In the last years, the combination of a physical preseparator and a gas jet system was also established at the GAs-filled Recoil Ion Separator (GARIS) [13], at RIKEN in Wako, Japan, at the Dubna Gas-Filled Recoil Separator (DGFRS) at the Flerov Laboratory of Nuclear Reactions in Dubna, Russia [18], and in the present work. This paper presents the design of a RTC for TASCA together with initial results of the tests with this device. It also gives an overview of the possibilities how TASCA can be connected to various setups by a gas jet system.

Fig. 1. Schematic drawing of TASCA. The beam from the UNILAC passes the differential pumping section of TASCA and impinges on the target wheel. The targets used in TASCA are three banana-shaped target segments mounted on a rotating target wheel. The target wheel rotation is synchronized to the beam macrostructure of the UNILAC beam. TASCA can be filled with a variety of gases, at pressures of  0.3  2.0 mbar. The evaporation residues leaving the target interact with the gas inside TASCA, thereby attaining an average charge state. Inside the dipole magnet of TASCA, the fusion products are separated from the primary beam and the transfer products according to their magnetic rigidity. The separated products are focused by the two quadrupole magnets into the focal plane. For chemistry experiments, a Recoil Transfer Chamber (RTC) is installed right behind the focal plane as an interface between the preseparator and the chemistry setups. The degrader foils can be inserted in front of the RTC to slow down the evaporation residues if necessary.

transfer products according to their magnetic rigidity [19] Br ¼ ðmvÞ=ðqave eÞ

ð1Þ

where Br is the magnetic rigidity [Tm], m the mass of the ion [kg], v the velocity of the ion [m/s], qave the average charge state and e the elementary charge [C]. The separated products are focused by the two quadrupole magnets into the focal plane. TASCA can be operated in two different ion-optical modes—the high transmission mode (HTM, magnetic configuration¼DQhQv, where the indices refer to vertical (v) and horizontal (h) focusing) and the small image mode (SIM, DQvQh) [20]. In the focal plane, various detector systems can be installed like the focal plane detector for the HTM [21] or TASISpec [22] for the SIM. The design of TASCA was optimized for the study of 48Ca-induced fusion reactions with actinide targets. TASCA’s efficiency for this nuclear reaction type is currently unsurpassed as was demonstrated in a recent experiment of production of element 114 [23]. For chemistry experiments, a RTC is installed right behind the focal plane as an interface between the preseparator and the chemistry setups. Three different types of RTC – one for the SIM and two for the HTM  were built for TASCA. Here we will present in detail the design and performance of the HTM-RTCs, while the SIM-RTC will be described elsewhere [24].

3. Design of the HTM-RTCs at TASCA 2. TASCA—TransActinide Separator and Chemistry Apparatus TASCA, see Fig. 1, is set up at the X-branch at the UNIversal Linear ACcelerator (UNILAC) at GSI. TASCA consists of one dipole and two quadrupole magnets. It is typically filled with  1 mbar He or He/H2 mixture. Inside the dipole magnet of TASCA, the fusion products are separated from the primary beam and the

There are certain requirements to the design of a RTC. The RTC window must withstand a pressure difference of up to 2000 mbar, it has to provide a low leaking rate, and it has to be thin enough for the EVRs to pass through it. This should ideally include low energetic ones from asymmetric reactions that lead to relatively long-lived isotopes of elements 104 108. The RTC volume should

J. Even et al. / Nuclear Instruments and Methods in Physics Research A 638 (2011) 157–164

be as small as possible to allow for a short flushing time, which is especially important in studies of short-lived nuclei. On the other hand, the RTC must be large enough to collect a majority of the separated EVRs and deep enough to thermalize all recoiling reaction products of interest to ensure that they do not implant into the rear part. The RTC window design was chosen based on results of Monte Carlo simulations, which suggest that in the model reaction 238 U(48Ca,3n)283112, about 60% of the EVRs illuminate an area of 4  14 cm2 in the focal plane. The window size is limited by the size of the standard Stainless Steel Conflats flange with a nominal diameter of 150 mm at the end of TASCA, on which the window has to be mounted. The maximum window size that can be accommodated by this flange has been chosen for the HTM and is 140  40 mm2. As a window material, Mylar foil was chosen. The thickness of the foil depends on the kinetic energy of the EVRs and is tailored to the respective range of the EVRs in Mylar. Because there is an enormous pressure on the large HTM window, two 1.0 mm wide supporting bars were implemented in the flange. They carry a grid with a honeycomb structure with 0.3 mm wide spokes and a hole pitch of 2.9 mm. This grid was made from hard Stainless Steel by laser cutting, its geometrical transparency is 80%. To minimize losses due to non-parallel trajectories of the EVRs, the grid has a thickness of only 1 mm. Supported by this grid, Mylar foils with thicknesses larger than 1.3 mm are able to withstand the pressure difference between TASCA and the RTC. For thinner windows, a 20 mm thick Ni mesh made by electro-etching with square 0.3  0.3 mm2 holes and 20 mm wide spokes can be placed on top of the metal grid. It has a transparency of around 90%. Two different HTM-RTCs were built—a shallow RTC with a minimum depth of 1.5 cm and a deeper RTC with a minimum depth of 3.0 cm. The depths of both RTCs can be increased by adding spacers (see Fig. 2A). The shallow one was constructed for evaporation residues with a low recoil energy as produced in asymmetric nuclear fusion reactions like 244Pu(22Ne,5n)261Rf [25]. The deeper RTC has three openings on the right, three on the left-hand side, one at the bottom, one at the top and one in its rear cover plate. The openings at the bottom and the top are used to connect a venting valve and a pressure gage. The gas inlets/outlets on the right and left-hand sides of the thin RTC are funnel shaped. The central hole in the RTC rear cover plate is in some cases used as the gas outlet. The shallow RTC is similar. Instead of three openings on the right and three openings on the left side, it has only one funnelshaped opening on the right as well as one on the left side. The deeper RTC was tested in two different gas flow modes— using the three openings on the right side as inlets and the ones on the left side as outlets (see the yellow arrows in Fig. 2B), and using all six funnel-shaped openings as inlets and the central opening in the RTC cover plate as outlet (see the black arrows in Fig. 2B). The flushing time of the RTC depends on the flow rate, depth of the RTC, and capillary length. The minimum flushing time calculated for a flow of 4 L/min at a pressure of 1 bar inside the RTC and a 10 m long capillary with an inner diameter of 2 mm is: 2.2 s for a 1.7 cm deep HTM-RTC and 3.6 s for a 3 cm deep RTC. Due to turbulences, the real flushing time is a few seconds longer. The HTM-RTCs are therefore the optimum choice for isotopes with half-lives of at least a few seconds. For experiments with more short-lived isotopes, the SIM-RTC will ideally be used.

4. Standalone tests of the deeper RTC The efficiency of the deeper HTM-RTC was measured in a series of experiments with suitable lead and osmium isotopes produced

159

Fig. 2. (A) Exploded drawing of the deep RTC. (1) Flange with the Mylar window; (2) the RTC with its in and outlets; (3) spacer, which can be added if needed to increase the depth of the RTC; (4) rear cover plate with outlet. (B) Photograph of the deeper RTC installed at TASCA. The yellow and the black arrows show two different gas flow modes. The yellow arrows show the mode with the three inlets on the left side and the three outlets on the right-hand side. The black arrows show the mode with the six inlets from the side and the outlet through the rear cover. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

in the reactions, 152Gd(40Ar,xn)192 xPb and natCe(40Ar,xn)175 177Os, respectively. In test experiments with Pb isotopes, an ARTESIA target [26] wheel with three 152Gd2O3 (enriched to 34.8% 152Gd) targets with average thicknesses of 308 mg/cm2 Gd, (electrodeposited on 2 mm thick Ti backings) was irradiated with 40Ar. The beam energy in the center of the target material was 182 MeV [27]. TASCA was set to center products with a Br of 1.65 T m in the focal plane. The pressure inside TASCA was kept at 1 mbar He. The RTC window was a 3.3 mm thick Mylar foil supported by the honeycomb structured grid. In front of the RTC window, various retractable Mylar degrader foils of thickness between 1.3 and 3.3 mm were installed. The degrader foils are used to slow down the EVRs to prevent their implantation into the back plate of the RTC. Depending on the measurement, one, two, or no degrader foils were placed in front of the RTC window. At a helium gas flow rate of 2.8 L/min, a pressure of 1200 mbar in the RTC, and a KCl oven temperature of 660 1C, various RTC settings were tested. To determine the gas jet yields, a 15 mm thick aluminum catcher foil was placed directly behind the RTC window and was used to collect the preseparated activity for a certain time and was then measured off-line with a g-ray detector (GeLi detector with an efficiency of 36.3%). The g-lines at 216 keV (191Tl, the daughter nuclide of 191Pb), at 365 keV (193mPb), at 384 keV (195mPb), and at 423 keV (192Tl, the daughter nuclide of 192 Pb) were evaluated. The measured yields were normalized to

160

J. Even et al. / Nuclear Instruments and Methods in Physics Research A 638 (2011) 157–164

the beam integral. The beam intensity was measured by an induction coil in front of the target. Afterwards, the Pb isotopes transported by a KCl gas jet through a polyethylene (PE) capillary (with an inner diameter of 2 mm and a length of 10 m or 5 m) were collected on glass fiber filters, which were subsequently measured off-line with the same g-detector as used to measure the aluminum catcher foil. All measurements were also normalized to the beam integral and compared to values measured with the aluminum catcher foils. The collection time, the time between the end of bombardment and start of counting, as well as the measuring time were kept the same for all measurements. Fig. 3 shows a typical g-ray spectrum of a sample collected on a glass fiber filter. Measured yields for the experiments with Pb are given in Table 1. The maximum yield was achieved with a degrader foil of 3.3 mm thickness. This is in agreement with SRIM [27] calculations, which predict a range for Pb recoil nuclei of 11 mm up to 20 mm in helium after passing a 3.3 mm thick Mylar degrader foil and a 3.3 mm thick Mylar RTC window. When a thinner window or a thinner degrader foil is used, a fraction of the recoil nuclei implants into the rear cover plate of the RTC and is lost. This is also in agreement with the SRIM calculations. There are no significant yield differences if the RTC configuration is changed from six inlets on the sides and one outlet at the rear cover, to three inlets on one side and three outlets at the opposite side. The gas jet yield as a function of He flow rate was also measured with 175–177Os, produced in the reaction natCe(40Ar,xn)175–177Os. The beam energy in the middle of the target was 196 MeV [27].

Three arc shaped Ce2O3 targets with an average thickness of 360 mg/cm2 Ce on 2 mm thick Ti backings were mounted on an ARTESIA target wheel. TASCA was filled with 1.0 mbar He and was set to 1.66 T m. The RTC depth was 4 cm and the thickness of the RTC window 5.8 mm. At the beginning of the experiment, a 15 mm thick aluminum catcher foil was placed directly behind the RTC window. It was used to collect the preseparated activity for a certain time and was then measured off-line with a g-ray detector (HPGe detector with an efficiency of 80%). The 125 keV g-lines of 175,177Os were analyzed and normalized to the beam integral. Afterwards, the gas jet was connected to the RTC. The KCl oven temperature was kept constant at 650 1C and the pressure inside the RTC was 1400 mbar. The RTC had six gas inlets and one gas outlet at the back plate. The KCl aerosol particles were transported by He gas flow through a 10 m long PE capillary and collected on glass fiber filters. The samples were measured at the detector, normalized to the beam integral, and compared with the measurements of the aluminum foil. As it was possible to collect the Os activity on glass fiber filters, the Os has to be transported attached on the KCl clusters. Apparently, the presence of trace amounts of O2 in the carrier gas is not sufficient for the formation of the volatile OsO4. The jet yield for the Os transport as function of the He flow rate is shown in Fig. 4. The yield increases with the gas flow rate up to 4.8 L/min. With a gas flow rate in excess of 3 L/min, the yield remains more or less stable at 80 78%. These results are comparable with the results reported in Ref. [13].

Fig. 3. g-ray spectrum of the products of the reaction 152Gd(40Ar,xn)192  xPb. The figure shows a spectrum of a sample collected on a direct catch. The He flow rate was 1.5 L/min, the pressure inside the RTC was 1200 mbar.

Fig. 4. Measured gas jet yield as a function of the gas flow rate. These data were measured in the reaction natCe(40Ar,xn)175–177Os. The depth of the RTC was 4 cm and the pressure inside the RTC was 1400 mbar.

Table 1 Gas jet yields for various configurations of the deeper RTC using the 152Gd(40Ar,xn) 192  xPb reaction at 182 MeV in the middle of the target. The RTC window was a 3.3 mm thick Mylar foil. RTC depth (mm)

Thickness of the Mylar degrader foil (mm)

RTC configuration

Capillary

Yield (%)

30 30 30

0 1.3 3.3

Six inlets, one outlet in the middle Six inlets, one outlet in the middle Six inlets, one outlet in the middle

10 m PE, 2 mm ID 10 m PE, 2 mm ID 10 m PE, 2 mm ID

3 71 12 71 64 72

40 40

1.3 3.3

Six inlets, one outlet in the middle Six inlets, one outlet in the middle

10 m PE, 2 mm ID 10 m PE, 2 mm ID

407 3 65 75

30 30

3.3 3.3

Three inlets, three outlets Three inlets, three outlets

10 m PE, 2 mm ID 5.5 m PE, 1.5 mm ID

67 73 73 73

J. Even et al. / Nuclear Instruments and Methods in Physics Research A 638 (2011) 157–164

4.1. Electrodeposition of carrier-free osmium—without and with TASCA With stable gas jet yields of the order of 80%, it is possible to conduct reliable chemistry experiments behind TASCA. The first chemistry experiment at TASCA, which is described later, was an electrodeposition experiment with Os as a test experiment for electrochemical studies of hassium (Hs, Z¼108), which is a heavier homolog of Os. For the deposition of tracer amounts of metal ions on various metal electrodes, the Nernst equation is not valid any more. In this case, the deposition potential depends strongly on the metal–metal interaction of the deposited species with the electrode material [28]. If the interaction between the electrode metal and the deposited species is stronger than the interaction between the atoms of the deposited metal itself, the deposition potential is shifted to higher values than the Nernst equation would predict. This phenomenon is called underpotential deposition [28]. In this experiment, the underpotential deposition of Os was studied. Underpotential deposition has been shown by Hummrich et al. [29] to be a suitable method for studying the electrochemical behavior of various metals. It was shown that for such investigations, the nuclide should have a half life of at least 10 s. Thus, 270Hs (T1/2 23 s [30,31]) appears to be a good candidate for electrochemical experiments with a transactinide element.

4.1.1. Electrodeposition of carrier-free osmium without preseparation In an initial experiment, which was performed without preseparation, the short-lived Os isotopes were produced in the reaction natCe(40Ar,xn). An 40Ar beam was accelerated to 280 MeV and impinged on a 20 mm thick Be foil used to separate the target chamber from the beamline vacuum. The stationary Ce2O3 target was electrodeposited on a 15 mm thick Be backing and had a thickness of 590 mg/cm2 Ce. The beam energy in the middle of the target was 186 MeV. The reaction products were stopped directly behind the target in a 96.7 mm deep recoil chamber kept at a pressure of 1.05 bar and transported via a He/KCl jet with a flow rate of 2.0 L/min to a direct catch (DC) apparatus and to the Automated Liquid Online Heavy element Apparatus (ALOHA) [32]. Inside the direct catch apparatus the aerosol clusters were collected on glass fiber filters.

Fig. 5. Schematic of the Ta disk inside the ALOHA. The disk contains four cavities with small holes in the center. The disk can be rotated in 901 steps. The KCl clusters from the gas jet are collected in the cavity in position 1. In position 2, the cavity is flushed by a syringe pump. The KCl and with it the activity is solved and flushed through the perforation into the chemistry device. In position 3, the cavity is cleaned with acetone and in position 4 the cavity is dried in a nitrogen stream.

161

The main part of the ALOHA is a tantalum disk with a diameter of 6.0 cm (see Fig. 5). Four cavities with volumes of 4.2 mL with a small hole in the middle of each cavity are machined into the disk. The disk is rotated by a stepping motor in 901 steps. At the first of the four positions, the gas jet is forced through a nozzle of 0.3 mm diameter into a chamber evacuated to around 5 mbar. The aerosol particles impact into the bottom of the cavity and are deposited there. After a preset time, the disk is rotated by 901, which places the cavity with the deposited KCl clusters into the second position, where the clusters are dissolved. A syringe pump flushes electrolyte solution into the cavity, the KCl and with it the Os activity is dissolved in the electrolyte solution. The activity is transported with the solution stream through the small hole in the bottom of the cavity into the chemistry apparatus. In the third position, the cavity is cleaned with acetone, and in the fourth position, it is dried by an intense nitrogen stream. To monitor the gas jet yield, the aerosol clusters were collected on glass fiber filters, which were measured by g-spectroscopy. No g-lines of Os isotopes were visible in the spectra due to the high background originating from projectile-like transfer products (see Fig. 6A). Furthermore the fusion products were collected in the ALOHA, after 2 min collection time, the sample was dissolved and flushed into the electrolytic cell [29] with 1 mL 0.1 M HCl delivered by a syringe pump. After running the electrolysis for 2 min at a potential of  1000 mV vs. Ag/AgCl, the Pd electrodes were measured with a g-detector. No Os g-lines could be identified. This clearly demonstrates the need for a physical preseparation.

4.1.2. Electrodeposition of carrier-free osmium behind TASCA To overcome these background problems, the chemistry setup and the direct catch apparatus were connected to TASCA, which served as a physical preseparator. A wheel with three arc shaped Ce2O3 targets with an average thickness of 466 mg/cm2 of Ce electrodeposited on 2 mm thick Ti foils was irradiated with 40Ar at a beam energy of 196 MeV in the center of the target. The dipole magnet of TASCA was set to 1.66 T  m. TASCA was filled with 1 mbar He. A 3.3 mm thick Mylar degrader foil was installed in front of the RTC. The RTC was separated from TASCA by a 3.3 mm thick Mylar window. The pressure inside the RTC was at 1100 mbar. The total depth of the RTC was 37 mm. The RTC features six gas inlets and one gas outlet at the back plate. The KCl aerosol particles were transported with the He gas flow through a 10 m long PE capillary with an inner diameter of 2 mm from the HTM-RTC to the chemistry laboratory. In order to monitor the jet yield, the aerosol particles were collected on glass fiber filters. Behind TASCA, 175Os–177Os could be clearly identified and were seen as the main products in the g-spectrum (see Fig. 6B). The transfer products were strongly suppressed. For electrochemical studies, the KCl aerosol particles were transported to the ALOHA [32]. After 2 min collection time, the sample was dissolved and flushed into the electrolytic cell [29] with 1 mL 0.1 M HCl delivered by a syringe pump. After running the electrolysis for 2 min, the electrodes were measured with a g-detector. The electrolysis was repeated at various potentials vs. an Ag/AgCl reference electrode at a temperature around 70 1C. Measurements with Pd and Ni electrodes were performed. The data were analyzed according to Ref. [33] and a typical result is shown in Fig. 7. The E50%-values, i.e., the potential at which a deposition yield of 50% was observed, were + 123 mV for the deposition on Pd and + 39 mV for the deposition on Ni. Uncertainties are estimated to be 750 mV. This indicates that the adsorption of Os on Pd is significantly stronger than the adsorption of Os on Ni.

162

J. Even et al. / Nuclear Instruments and Methods in Physics Research A 638 (2011) 157–164

4.2. First transactinide chemistry behind TASCA

Fig. 6. g-ray spectra of the samples of the products of the reaction natCe(40Ar,xn) collected on glass fiber filters. Spectrum A is the sample taken without preseparation. Only projectile-like transfer products could be identified. Spectrum B shows the sample taken behind TASCA. Herein 175Os, 177Os, and 176Re, the daughter nuclide of 176Os were identified.

Fig. 7. Deposition yield of Os on Pd at  70 1C from 0.1 M HCl solution as a function of the electrode potential. The critical potential Ecrit is the potential at which the deposition yield starts to increase. The E50% value is the potential at which 50% of the Os is deposited.

The final phase of the TASCA commissioning program included a series of experiments with 260Rf, 261a,bRf, and 262Rf produced in the 244Pu(22Ne,xn) reaction [25]. In one of the experiments, the RTC gas jet system was used to couple TASCA with the ROtating wheel Multidetector Analyzer ROMA to study long-lived 261a,bRf. These experiments are described in detail in Ref. [25]. The TASCA transmission for 261Rf to the TASCA focal plane was estimated by Monte Carlo simulations to 10.5%. The transparency of the RTC window was 80% and the transport yield of Rf to ROMA was estimated to be 60%. From the measurements with ROMA, the cross-sections for 261aRf and 261bRf were preliminarily determined to be 4.4 and 1.8 nb, respectively. Another experiment served as a proof-of-principle chemistry experiment that coupling of TASCA with a chemistry setup allows chemistry experiments with a transactinide element, 261aRf, which has a half life of 68 s [34]. It is well known that Rf forms anionic fluoride complexes of the type [RfF6]2  [35]. The Automated Rapid Chemistry Apparatus (ARCA) [36] was used to study the formation of fluoride complexes of Rf in dilute HF solutions by anion-exchange chromatography. The 244PuO2 targets (isotopic composition: 97.9% 244Pu; 1.3% 242 Pu; 0.7% 240Pu; o0:1% other) were irradiated with a 22Ne beam with a laboratory frame energy of 116 MeV in the middle of the target and an average intensity of 0.8 mAparticle. The targets were deposited on 2.2 mm Ti backings and had an average thickness of 469 mg/cm2 244Pu. TASCA was set to 2.0 T m and operated in the HTM filled with 0.4 mbar He. After passing the Mylar window of 1.2 mm thickness, EVRs were thermalized in He at 1200 mbar in the shallow RTC. The total depth of the RTC was 17 mm. The gas jet entered the RTC from the left and the right and exited it through the center of the cover. The 261aRf was transported to ARCA by a He–KCl jet through a 10 m long PE capillary with an inner diameter of 2 mm at a gas flow rate of 2.9 L/min. To monitor the gas jet yield during the experiment and to compare the ARCA efficiency with ROMA, an 227Ac emanation source was connected to the RTC. He, with a flow rate of 20 mL/min, was passed through the source and transported the volatile decay product 219Rn (T1/2 ¼3.96 s) to the RTC, where it decayed to nonvolatile 211Bi (T1/2 ¼2.14 min). The yield of the decay product 211 Bi was first measured with the ROMA [25] and afterwards with ARCA. The 211Bi yield in ARCA, including collection, dissolution in 7  10  4 M HF solution, and evaporation on a Ta disk, was 50% of that in ROMA. For anion-exchange chromatography in ARCA, the column magazines were filled with the resin MCI GEL CA08Y from Mitsubishi Chemical Corporation, particle size 2275 mm, which was transferred into the hydroxide form as described in Ref. [37]. KCl clusters were collected in ARCA for 90 s. During sample collection, a column was preconditioned for 65 s with the HF solution. After the collection, products were dissolved in 200 mL of 7  10  4 M HF solution and were subsequently fed onto the anion-exchange column at a flow rate of 1.0 mL/min. This effluent of the column was collected on a Ta disk as fraction 1. The fraction of the Rf, which was adsorbed on the resin, was eluted with 250 mL of 5 M HNO3 and was collected on a second Ta disk. Both fractions were evaporated to dryness by infrared light and a hot helium stream. These two Ta disks were then subjected to aspectroscopy. Counting of the first and the second fraction started 60 and 65 s after the end of the collection interval, respectively. After 18 h experiment duration, the concentration of the HF solution was changed to 1  10  3 M HF and the experiments were continued for another 25 h. The summed a-spectrum is shown in Fig. 8. In total, seven a-events were detected in the energy range of 7950 up to

J. Even et al. / Nuclear Instruments and Methods in Physics Research A 638 (2011) 157–164

163

that TASCA is a powerful and versatile preseparator for studies of the chemical behavior of short-lived isotopes, including the transactinides.

Acknowledgements

Fig. 8. Summed a-particle spectrum obtained during the ARCA experiment. A total of seven events in the energy range of 7950–8300 keV were obtained during the experiment and are attributed to 261aRf and 257No. Three events were measured during the first part of the experiment, when 7  10  4 M HF was used as extraction solution; the other four events were detected during the second part when 1  10  3 M HF was used. All events were in the HNO3 fraction. To monitor the gas jet yield an 227Ac was connected to the gas jet and the sprectra were compared with the ROMA measurement.

8300 keV, which we attribute to 261aRf (T1/2 ¼68 73 s, Ea ¼8.30 MeV) and its daughter 257No (T1/2 ¼ 24.570.5 s, Ea ¼8.22 MeV (83%) and 8.32 MeV (17%)) [34] based on the measured a-decay energies and lifetimes. For the data analysis, an energy window from 7950 up to 8350 keV was chosen. From the analysis of a background spectrum acquired over a time of 481 h, 0.48 events from background were expected in this energy range in the spectrum shown in Fig. 8. As a time window, a maximum time of 160 s after start of the measurement was chosen. All of them were observed in the second (HNO3) fraction. Two of these events were detected during the experiments with 7  10  4 M HF, the other five events were detected while 1  10  3 M HF was used. As no events were observed in the first (HF) fractions, a lower limit for the %ads value can be extracted. The %ads value is defined as the percentage of events obtained in the second fraction. According to Ref. [38], the %ads values of Z67.1% in 7  10  4 M HF and Z74.5% in 1  10  3 M HF (68.3% confidence level) were evaluated. From a comparison with the number of 261aRf/257No events in ROMA, we have to conclude that the chemical yield in ARCA was low, on the order of 30% only. This is in line with earlier observations [39,40] indicating possibly some sorption of transactinides from HF solutions on the slider made from Kel-F in ARCA. Nevertheless, the experiment with 261aRf clearly demonstrated that chemistry experiments with transactinides preseparated in TASCA are feasible.

5. Summary With the HTM-RTCs, efficient interfaces for the coupling of TASCA with chemical setups are realized. The different RTCs were successfully tested with Pb and Os isotopes and also with the transactinide element Rf. Gas jet yields from the RTC to the chemistry laboratory of the order of 80 78% were achieved in the test experiment with osmium at flow rates higher than 3 L/min (see Fig. 4). The first chemistry experiment behind TASCA with Os as well as the first transactinide chemistry experiment with Rf showed

We would like to thank the mechanical workshop at the ¨ Kernchemie at Mainz University for the construction Institut fur of the RTCs and the GSI target laboratory for the production of the target backing foils. We also thank Hiromitsu Haba for donating us 1.2 mm thick Mylar foil. The authors express their gratitude to the ion source and accelerator staff at GSI for providing stable beams during these experiments. Financial support by the BMBF under contract numbers 06MZ223I and 06MT247I and by the GSI Helmholtz center for heavy-ion research under contract numbers ¨ is gratefully acknowledged. The Oslo group MZJVKR and MT/TUR gratefully acknowledges financial support from the Norwegian Research Council under project number 148994/V30. References [1] Yu.Ts. Oganessian, J. Phys. G: Nucl. Part. Phys. 34 (2007) R165. [2] Yu.Ts. Oganessian, F.Sh. Abdullin, P.D. Bailey, D.E. Benker, M.E. Bennett, S.N. Dmitriev, J.G. Ezold, J.H. Hamilton, R.A. Henderson, M.G. Itkis, Yu.V. Lobanov, A.N. Mezentsev, K.J. Moody, S.L. Nelson, A.N. Polyakov, C.E. Porter, A.V. Ramayya, F.D. Riley, J.B. Roberto, M.A. Ryabinin, K.P. Rykaczewski, R.N. Sagaidak, D.A. Shaughnessy, I.V. Shirokovsky, M.A. Stoyer, V.G. Subbotin, R. Sudowe, A.M. Sukhov, Yu.S. Tsyganov, V.K. Utyonkov, A.A. Voinov, G.K. Vostokin, P.A. Wilk, Phys. Rev. Lett. 104 (2010) 142502. ¨ [3] M. Schadel (Ed.), The Chemistry of the Superheavy Elements, Kluwer Academic Publishers, Dordrecht, The Netherlands, , 2003. ¨ [4] M. Schadel, Angew. Chem. Int. Ed. 45 (2006) 368. [5] R. Eichler, N.V. Aksenov, A.V. Belozerov, G.A. Bozhikov, V.I. Chepigin, ¨ S.N. Dmitriev, R. Dressler, H.W. Gaggeler, V.A. Gorshkov, F. Haenssler, M.G. Itkis, A. Laube, V.Ya. Lebedev, O.N. Malyshev, Yu.Ts. Oganessian, O.V. Petrushkin, D. Piguet, P. Rasmussen, S.V. Shishkin, A.V. Shutov, A.I. Svirikhin, E.E. Tereshatov, G.K. Vostokin, M. Wegrzecki, A.V. Yeremin, Nature 447 (2007) 72. [6] R. Eichler, N.V Aksenov, Yu.V. Albin, A.V. Belozerov, G.A. Bozhikov, ¨ V.I. Chepigin, S.N. Dmitriev, R. Dressler, H.W. Gaggeler, V.A. Gorshkov, R.A. Henderson, A.M. Johnsen, J.M. Kenneally, V.Ya. Lebedev, O.N. Malyshev, K.J. Moody, Yu.Ts. Oganessian, O.V. Petrushkin, D. Piguet, A.G. Popeko, P. Rasmussen, A. Serov, D.A. Shaughnessy, S.V. Shishkin, A.V. Shutov, M.A. Stoyer, N.J. Stoyer, A.I. Svirikhin, E.E. Tereshatov, G.K. Vostokin, M. Wegrzecki, P.A. Wilk, D. Wittwer, A.V. Yeremin, Radiochim. Acta 98 (2010) 133. ¨ [7] Ch.E. Dullmann, Nucl. Instr. and Meth. B 266 (2008) 4123. ¨ [8] M. Schadel, Eur. Phys. J. D 45 (2007) 67. ¨ [9] M. Schadel, D. Ackermann, L.-L. Andersson, J. Ballof, M. Block, R.A. Buda, ¨ ¨ W. Bruchle, I. Dragojevic, Ch.E. Dullmann, J. Dvorak, K. Eberhardt, J. Even, J.M. Gates, J. Gerl, A. Gorshkov, P. Golubev, R. Graeger, K.E. Gregorich, ¨ E. Gromm, W. Hartmann, F.P. Heßberger, D. Hild, R. Hoischen, A. Hubner, ¨ E. Jager, J. Khuyagbaatar, B. Kindler, I. Kojouharov, J.V. Kratz, J. Krier, N. Kurz, S. Lahiri, D. Liebe, B. Lommel, M. Maiti, M. Mendel, E. Merchan, H. Nitsche, D. Nayak, J. Nilssen, J.P. Omtvedt, K. Opel, P. Reichert, D. Rudolph, A. Sabelnikov, F. Samadani, H. Schaffner, B. Schausten, R. Schuber, E. Schimpf, ¨ A. Semchenkov, L. Stavsetra, J. Steiner, J. Szerypo, P. Thorle-Pospiech, ¨ A. Toyoshima, A. Turler, J. Uusitalo, N. Wiehl, H.-J. Wollersheim, T. Wunderlich, A. Yakushev, GSI Scientific Report 2008, GSI Report 2009  1, 138  139, GSI ¨ Schwerionenforschung GmbH, 64291 Darmstadt, Helmholtzzentrum fur Germany /http://www.gsi.de/informationen/wti/library/scientificreport2008S and /http://www.gsi.de/tascaS. ¨ [10] Ch.E. Dullmann, C.M. Folden III, K.E. Gregorich, D.C. Hoffman, D. Leitner, G.K. Pang, R. Sudowe, P.M. Zielinski, H. Nitsche, Nucl. Instr. and Meth. A 551 (2005) 528. ¨ [11] H. Wollnik, H.G. Wilhelm, G. Robig, H. Jungclas, Nucl. Instr. and Meth. 127 (1975) 539. [12] H. Wollnik, Nucl. Instr. and Meth. 139 (1976) 311. [13] H. Haba, D. Kaji, H. Kikunaga, T. Akiyama, N. Sato, K. Morimoto, A. Yoneda, K. Morita, T. Takabe, A. Shinohara, J. Nucl. Radiochem. Sci. 8 (2007) 55. ¨ ¨ [14] Ch.E. Dullmann, W. Bruchle, R. Dressler, K. Eberhardt, B. Eichler, R. Eichler, ¨ ¨ H.W. Gaggeler, T.N. Ginter, F. Glaus, K.E. Gregorich, D.C. Hoffman, E. Jager, D.T. Jost, U.W. Kirbach, D.M. Lee, H. Nitsche, J.B. Patin, V. Pershina, D. Piguet, ¨ ¨ Z. Qin, M. Schadel, B. Schausten, E. Schimpf, H.-J. Schott, S. Soverna, ¨ ¨ R. Sudowe, P. Thorle, S.N. Timokhin, N. Trautmann, A. Turler, A. Vahle, G. Wirth, A.B. Yakushev, P.M. Zielinski, Nature 418 (2002) 859. ¨ [15] Ch.E. Dullmann, Eur. Phys. J. D 45 (2007) 75.

164

J. Even et al. / Nuclear Instruments and Methods in Physics Research A 638 (2011) 157–164

[16] U.W. Kirbach, C.M. Folden III., T.N. Ginter, K.E. Gregorich, D.M. Lee, V. Ninov, ¨ J.P. Omtvedt, J.B. Patina, N.K. Seward, D.A. Strellis, R. Sudowe, A. Turler, P.A. Wilk, P.M. Zielinskia, D.C. Hoffman, H. Nitsche, Nucl. Instr. and Meth. A 484 (2002) 587. [17] J.P. Omtvedt, J. Alstad, H. Breivik, J.E. Dyve, K. Eberhardt, C.M. Folden III, T. Ginter, K.E. Gregorich, E.A. Hult, M. Johansson, U.W. Kirbach, D.M. Lee, ¨ M. Mendel, A. Nahler, V. Ninov, L.A. Omtvedt, J.B. Patin, G. Skarnemark, L. Stavsetra, R. Sudowe, N. Wiehl, B. Wierczinski, P.A Wilk, P.M. Zielinski, J.V. Kratz, N. Trautmann, H. Nitsche, D.C. Hoffman, J. Nucl. Radiochem. Sci. 3 (2002) 121. [18] D. Wittwer, F.Sh. Abdullin, N.V. Aksenov, Yu.V. Albin, G.A. Bozhikov, ¨ S.N. Dmitriev, R. Dressler, R. Eichler, H.W. Gaggeler, R.A. Henderson, S. Hubener, J.M. Kenneally, V.Ya. Lebedev, Yu.V. Lobanov, K.J. Moody, Yu.Ts. Oganessian, O.V. Petrushkin, A.N. Polyakov, D. Piguet, P. Rasmussen b, R.N. Sagaidak c, A. Serov a,b, I.V. Shirokovsky, D.A. Shaughnessy, S.V. Shishkin, A.M. Sukhov, M.A. Stoyer, N.J. Stoyer, E.E. Tereshatov, Yu.S. Tsyganov, V.K. Utyonkov, G.K. Vostokin, M. Wegrzecki, P.A. Wilk, Nucl. Instr. and Meth. B 268 (2009) 28. [19] A. Ghiorso, S. Yashita, M.E. Leino, L. Frank, J. Kalnins, P. Armbruster, J.-P. Dufour, P.K. Lemmertz, Nucl. Instrum. Methods Phys. Res. A. 269 (1988) 192. ¨ ¨ ¨ ¨ [20] A. Semchenkov, W. Bruchle, E. Jager, E. Schimpf, M. Schadel, C. Muhle, F. Klos, ¨ A. Turler, A. Yakushev, A. Belov, T. Belyakova, M. Kaparkova, V. Kukhtin, E. Lamzin, S. Sytchevsky, Nucl. Instr. and Meth. B 266 (2008) 4153. ¨ [21] A. Gorshkov, J.M. Gates, R. Graeger, A. Turler, A. Yakushev, D. Ackermann, W. ¨ ¨ Bruchle, Ch.E. Dullmann, H.G. Essel, E. J¨ager, J. Khuyagbaatar, I. Kojouharov, J. Krier, N. Kurz, M. Sch¨adel, B. Schausten, E. Schimpf, J. Dvorak, L.-L. Andersson, D. Rudolph, M. Wegrzecki, GSI Scientific Report 2009, GSI Report 2010-1, 179, GSI ¨ Schwerionenforschung GmbH, 64291 Darmstadt, GerHelmholtzzentrum fur many /http://www.gsi.de/informationen/wti/library/scientificreport2009S. [22] L.-L. Andersson, D. Rudolph, P. Golubev, R.-D. Herzberg, R. Hoischen, ¨ E. Mercha´n, D. Ackermann, Ch.E. Dullmann, K. Eberhardt, J. Even, J. Gerl, ¨ F.P. Heßberger, E. Jager, J. Khuyagbaatar, I. Kojouharov, J.V. Kratz, J. Krier, ¨ N. Kurz, W. Prokopowicz, M. Schadel, H. Schaffner, B. Schausten, E. Schimpf, ¨ ¨ A. Semchenkov, A. Turler, H.-J. Wollersheim, A. Yakushev, P. Thorle-Pospiech, ¨ W. Hartmann, A. Hubner, B. Lommel, B. Kindler, J. Steiner, Nucl. Instr. and Meth. A 622 (2010) 164. ¨ ¨ ¨ [23] Ch.E. Dullmann, M. Schadel, A. Yakushev, A. Turler, K. Eberhardt, J.V. Kratz, ¨ D. Ackermann, L.-L. Andersson, M. Block, W. Bruchle, J. Dvorak, H.G. Essel, P.A. Ellison, J. Even, J.M. Gates, A. Gorshkov, R. Graeger, K.E. Gregorich, ¨ ¨ W. Hartmann, R.-D. Herzberg, F.P. Heßberger, D. Hild, A. Hubner, E. Jager, J. Khuyagbaatar, B. Kindler, J. Krier, N. Kurz, S. Lahiri, D. Liebe, B. Lommel, M. Maiti, H. Nitsche, J.P. Omtvedt, E. Parr, D. Rudolph, J. Runke, B. Schausten, ¨ E. Schimpf, A. Semchenkov, J. Steiner, P. Thorle-Pospiech, J. Uusitalo, M. Wegrzecki, N. Wiehl, Phys. Rev. Lett. 104 (2010) 252701. [24] J.-P. Omtvedt et al., Nucl. Instr. and Meth. A, in preparation. ¨ ¨ [25] A Gorshkov, Ch. E. Dullmann, M. Schadel, J. Khuyagbaatar, A. Yakushev, ¨ K. Eberhardt, J. Even, D. Ackermann, W. Bruchle, J. Ballof, M. Block, ¨ I. Dragojevic´, J.M. Gates, R. Graeger, H. Nitsche, E. Jager, J.V. Kratz, J. Krier,

[26]

[27] [28] [29]

[30]

[31]

[32]

[33] [34] [35]

[36] [37]

[38] [39]

[40]

D. Liebe, J.P. Omtvedt, D. Rudolph, A. Sabelnikov, F. Samadani, B. Schausten, ¨ ¨ E. Schimpf, A. Semchenkov, L. Stavsetra, P. Thorle, A. Toyoshima, A. Turler, J. Uusitalo, Phys. Rev. C, in preparation. ¨ ¨ K. Eberhardt, W. Bruchle, Ch.E. Dullmann, K.E. Gregorich, W. Hartmann, ¨ ¨ A. Hubner, E. Jager, B. Kindler, J.V. Kratz, D. Liebe, B. Lommel, H.-J. Maier, ¨ M. Schadel, B. Schausten, E. Schimpf, A. Semchenkov, J. Steiner, J. Szerypo, ¨ ¨ P. Thorle, A. Turler, A. Yakushev, Nucl. Instr. and Meth. A 590 (2008) 134. J.F. Ziegler, Nucl. Instr. and Meth. B 219–220 (2004) 1027. B. Eichler, J.V. Kratz, Radiochim. Acta 88 (2000) 475. ¨ H. Hummrich, N.L. Banik, M. Breckheimer, W. Bruchle, R. Buda, F. Feist, ¨ ¨ E. Jager, J.V Kratz, B. Kuczewski, D. Liebe, L Niewisch, M. Schadel, B. Schausten, E. Schimpf, N. Wiehl, Radiochim. Acta 96 (2008) 73. ¨ ¨ J. Dvorak, W. Bruchle, M. Chelnokov, R. Dressler, Ch.E. Dullmann, ¨ ¨ K. Eberhardt, V. Gorshkov, E. Jager, R. Krucken, A. Kuznetsov, Y. Nagame, ¨ F. Nebel, Z. Novackova, Z. Qin, M. Schadel, B. Schausten, E. Schimpf, ¨ ¨ A. Semchenkov, P. Thorle, A. Turler, M. Wegrzecki, B. Wierczinski, A. Yakushev, A. Yeremin, Phys. Rev. Lett. 97 (2006) 242501. ¨ ¨ J. Dvorak, W. Bruchle, M. Chelnokov, Ch.E. Dullmann, Z. Dvorakova, ¨ ¨ K. Eberhardt, E. Jager, R. Krucken, A. Kuznetsov, Y. Nagame, F. Nebel, ¨ K. Nishio, R. Perego, Z. Qin, M. Schadel, B. Schausten, E. Schimpf, ¨ ¨ R. Schuber, A. Semchenkov, P. Thorle, A. Turler, M. Wegrzecki, B. Wierczinski, A. Yakushev, A. Yeremin, Phys. Rev. Lett. 100 (2008) 132503. ¨ ¨ A. Kronenberg, J.V. Kratz, U. Krille, G. Lehr, U. Rieth, W. Bruchle, M. Schadel, E. ¨ Kernchemie, Universitat ¨ Mainz (2001), Schimpf, Annual Report, Institut fur A4. /http://www.kernchemie.uni-mainz.de/downloads/jb2001S. F. Joliot, J. Chem. Phys. 27 (1930) 119. ¨ ¨ Ch.E. Dullmann, A. Turler, Phys. Rev. C 77 (2008) 064320. H. Haba, K. Tsukada, M. Asai, A. Toyoshima, K. Akiyama, I. Nishinaka, M. Hirata, T. Yaita, S. Ichikawa, Y. Nagame, K. Yasuda, Y. Miyamoto, T. Kaneko, S. Goto, S. Ono, T. Hirai, H. Kudo, M. Shigekawa, A. Shinohara, Y. Oura, H. Nakahara, K. Sueki, H. Kikunaga, N. Kinoshita, N. Tsuruga, ¨ ¨ A. Yokoyama, M. Sakama, S. Enomoto, M. Schadel, W. Bruchle, J.V. Kratz, J. Am. Chem. Soc. 126 (2004) 5219. ¨ ¨ ¨ M. Schadel, W. Bruchle, J.V. Kratz, E. Jager, U.W. Scherer, E. Schimpf, H.P. Zimmermann, Radiochim. Acta 48 (1989) 171. A. Toyoshima, H. Haba, K. Tsukada, M. Asai, K. Akiyama, S. Goto, Y. Ishii, I. Nishinaka, T.K. Sato, Y. Nagame, W. Sato, Y. Tani, H. Hasegawa, K. Matsuo, D. Saika, Y. Kitamoto, A. Shinohara, M. Ito, J. Saito, H. Kudo, A. Yokoyama, ¨ ¨ M. Sakama, K. Sueki, Y Oura, H. Nakahara, M. Schadel, W. Bruchle, J.V. Kratz, Radiochim. Acta 96 (2008) 125. ¨ W. Bruchle, Radiochim. Acta 91 (2003) 71. ¨ ¨ ¨ ¨ R. Gunther, W. Paulus, A. Posledni, J.V. Kratz, M.Schadel, W. Bruchle, E.Jager, E. Schimpf, B. Schaustn, D. Schumann, R.Binder, Jahresbericht 1994 Institut ¨ Kernchemie, Universitat Mainz, 1995, pp. 2. fur C.D. Kacher, K.E. Gregorich, D.M. Lee, Y. Watanabe, B. Kadkhodayan, B. Wierczinski, M.R. Lane, E.R. Sylwester, D.A. Keeney, M. Hendricks, N.J. Stoyer, J. Yang, M. Hsu, D.C. Hoffman, A. Bilewicz, Radiochim. Acta 75 (1996) 127.