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Indication for a Volatile Element 114 Radiochim. Acta 98, 133–139 (2010) / DOI 10.1524/ract.2010.1705 © by Oldenbourg Wissenschaftsverlag, München Indication for a volatile element 114 By R. Eichler1,2,∗,N.V.Aksenov3, Yu. V. Albin3, A. V. Belozerov3 ,G.A.Bozhikov3, V. I. Chepigin3 , S. N. Dmitriev3, R. Dressler1,H.W.Gäggeler1 ,2, V. A. Gorshkov3, R. A. Henderson4, A. M. Johnsen4,J.M.Kenneally4 , V. Ya. Lebedev3, O. N. Malyshev3, K. J. Moody4, Yu. Ts. Oganessian3,O.V.Petrushkin3, D. Piguet1,A.G.Popeko3, P. Rasmussen1, A. Serov1,2, D. A. Shaughnessy4, S. V. Shishkin3, A. V. Shutov3,M.A.Stoyer4,N.J.Stoyer4, A. I. Svirikhin3, E. E. Tereshatov3, G. K. Vostokin3, M. Wegrzecki5 , P. A. Wilk4, D. Wittwer2 and A. V. Yeremin3 1 Labor für Radio- und Umweltchemie, Paul Scherrer Institut, 5232 Villigen, Switzerland 2 Departement für Chemie und Biochemie, Universität Bern, 3012 Bern, Switzerland 3 Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, 141980 Dubna, Russia 4 Lawrence Livermore National Laboratory, Livermore, CA 94551, USA 5 Insitute of Electron Technology, 02-668 Warsaw, Poland (Received June 30, 2009; accepted in revised form November 2, 2009) Transactinides / Element 114 / Adsorption / into group 14 together with carbon, silicon, germanium, Thermochromatography tin, and lead. The enhancement of the metallic character with increasing atomic number Z is an obvious trend ob- served along the main groups 13–17 of the periodic table. Summary. Recently, the chemical investigation of element Thus, a metallic character can be expected for element 112 revealed a highly volatile, noble metallic behaviour, as 114 [9, 10]. However, relativistic calculations of the elec- expected for the last group 12 member of the periodic table. tronic structures of transactinides suggest contraction of The observed volatility and chemical inertness were ascribed the spherical s-andp / -electron orbitals [11–13], thus to the growing influence of relativistic effects on the chemical 1 2 increasing the chemical stability of the elemental atomic properties of the heaviest elements with increasing nuclear state for element 114 with an electronic ground state con- charge. Here, we report for the first time on gas phase chem- 14 10 2 2 ical experiments aiming at a determination of element 114 figuration of Rn: 5 f 6d 7s 7p1/2 [11–13]. Therefore, properties. This element was investigated using its isotopes a high volatility and chemical inertness were postulated 287114 and 288114 produced in the nuclear fusion reactions of for this element [14–19]. Modern relativistic calculation 48Ca with 242Pu and 244Pu, respectively. Identification of three methods predict atomic properties for element 114 indi- atoms of element 114 in thermochromatography experiments cating a higher chemical inertness compared to that of and their deposition pattern on a gold surface indicates that the lighter group 14 metal lead, but still with pronounced this element is at least as volatile as simultaneously investi- metallic properties [15–21]. In this work, we describe gated elements Hg, At, and element 112. This behaviour is experiments which have been conducted to assess these rather unexpected for a typical metal of group 14. chemical properties of element 114 based on the meas- urement of its adsorption properties on a gold surface. In 1. Introduction general, the thermochromatographic method allows for the most efficient determination of the deposition pattern of The enhanced nuclear stability around the closed nuclear single atoms of a volatile species on a chromatographic shells predicted by microscopic theory [1–3] motivated an surface, where a stationary negative temperature gradient extensive experimental quest for elements with Z ∼ 114. is applied. The characteristic interaction potential between However, searches in nature (for review see [4]) and among the single atomic species and the surface can be quanti- the products of fusion reactions or nuclear transfer [5–7] fied from this deposition behaviour [22]. This method is were not successful before 1999. Finally, element 114 was suitable to investigate transactinide elements produced in first ever detected at the Flerov Laboratory for Nuclear nuclear reactions on a one-atom-at-a-time basis and was Reactions (FLNR), Dubna, Russia in 2004, where the for- used already for chemical investigations of transactinide mation of the isotopes 286−289114 was observed in the nu- elements Rf [23], Db [24], Sg [25], Hs [26], and element clear fusion reactions of 48Ca with 242,244Pu (for review 112 [27, 28]. see [8]). The observed longer half-lives of up to several seconds indicate an enhanced nuclear stability of these heaviest nuclei. As an important consequence, chemical in- vestigations of element 114 appeared to be feasible. The 2. Experimental systematic order of the periodic table places element 114 The experiments were performed with the insitu volatiliza- tion and on-line detection technique [29] in combination *Author for correspondence (E-mail: [email protected]). with the cryo-online detector (COLD) [27, 28, 30, 31]. The 134 R. Eichler et al. products of the heavy ion induced nuclear fusion reaction re- coil out of the target, are stopped in a carrier gas, and are subsequently transported through a perfluoroalkoxy (PFA) Teflon capillary into a hot oven (850 ◦C). The oven contains a quartz wool filter and a small piece of tantalum metal, serving as a trap for aerosol particles, water, oxygen, and other reactive species. Only volatile and chemically inert products pass through this separation step and are further transported to the thermochromatographic detection system COLD described elsewhere [27, 28, 31]. Here, species are deposited along the established temperature gradient accord- Fig. 1. Schematic section of the open COLD channel. The detec- tors considered to be reached by the particles emitted due to the ing to their adsorption interaction with the detector sur- alpha and SF decay of an atom adsorbed on a gold covered top de- faces. COLD was equipped with 32 silicon detector pairs, tector D of a detector pair mounted in the detector array COLD. of which one side was covered by a thin gold layer whereas Note, that the gold coverage (light grey) is slightly larger than the the opposite side was passivated by a thin layer of SiO .It active detector surface (inside the dashed-line square). Detector dimen- 2 h = . was assumed that single atoms of element 114 adsorbed on sions in the channel were: Gas slit thickness 1 52 mm, Detector size: x = 11.30 mm, y = 11.60 mm, Gold coverage: xmax = 10.6mm, the Au covered detectors due to the generally stronger in- ymax = 10.6 mm, Active detector surface: xa = 9.7mm, ya = 9.7mm. teraction observed for almost all elements in the periodic Hence, 83.74% of the gold surface covers the active detector surface. table with gold compared to SiO2. Moreover, the gold cov- ered side is at about 5 K lower temperature compared to the uncovered side due to the cooling arrangement of the channel. The event-by-event measurement of α-decays and 233–251 MeV was calculated as explained above. Due to the spontaneous fission (SF) decays in COLD was analysed as longer half-lives, the transport efficiencies are 15% (288114) outlined in Fig. 1. This procedure should provide an on- and 56% (289114), respectively. The temperature range cov- line identification of single atoms. The same setup was al- ered within COLD in experiment (b) was established be- ready applied to investigate chemical properties of element tween +8 ◦Cand−170 ◦C. 112 [27, 28]. In both experiments, 185Hg was produced simultaneously In the first experiment (Experiment a), performed in 2007 due to a neodymium (50 µgcm−2, natural isotopic composi- at the U-400 cyclotron at FLNR Dubna, element 114 was tion) admixture to the target in the nuclear fusion reaction produced in the nuclear fusion reaction 242Pu(48Ca, 3n)287114 with 48Ca. 219Rn and isotopes of astatine were also perma- 242 (T1/2 ∼ 0.5s,α)[8]. A stationary grid-supported PuO2 tar- nently produced in multi-nucleon transfer reactions of the get (1.4mgcm−2; the target material was facing the beam) 48Ca beam with 242,244Pu and transported to the COLD. The deposited on a thin Ti foil backing (0.7mgcm−2) was irra- simultaneous measurements of 185Hg turned out to be very diated during 16 d with an overall beam dose of 3.8 × 1018 important to determine the retention capability of the de- 48Ca particles at a projectile energy in the target material of tector gold surface. After typically one week, the detector 228–246 MeV. This energy was calculated from the beam surfaces started to be less able to retain 185Hg quantita- energy delivered by the accelerator of 273 ± 3MeV,anden- tively. At this point it was always decided to replace the ergy losses within an entrance window of 1.81 mg/cm2 Ti detectors and/or to clean their surfaces. To achieve this and a 5 mm thick slit of He/Ar (50%/50%) between the goal, a washing procedure was applied using methanol, vacuum window and the target using the SRIM code [32]. acetone, and diethylether. The origin of the contamination The target grid transmission was 80%. The gas flow rate in remained unclear, though minor impurities in the Ar car- the experiments was 1.5l/min. Because the measured mean rier gas seem the most likely cause. A water content of overall transport time of volatile reaction products was 2.2s, 0.1 ppm corresponding to a dew point of water of −95 ◦C the 287114 isotope is expected to be transported though the was obtained in the carrier gas by the drying compo- 8-m-long PFA Teflon capillary (1.56 mm inner diameter) nents installed in the gas loop consisiting of a 950 ◦CTa to the COLD thermochromatography detector with an effi- getter and two drying tubes filled with Siccapent.
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