Chemical studies of elements with Z ⩾ 104 in gas phase

Accepted Manuscript Chemical studies of elements with Z ≥ 104 in gas phase

Andreas Türler, Robert Eichler, Alexander Yakushev

PII: DOI: Reference:

S0375-9474(15)00219-5 http://dx.doi.org/10.1016/j.nuclphysa.2015.09.012 NUPHA 20434

To appear in:

Nuclear Physics A

Received date: Revised date: Accepted date:

1 May 2015 18 September 2015 23 September 2015

Please cite this article in press as: A. Türler et al., Chemical studies of elements with Z ≥ 104 in gas phase, Nucl. Phys. A (2015), http://dx.doi.org/10.1016/j.nuclphysa.2015.09.012

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Chemical studiees of elem ments with w Zı1104 in gas phasee Andrreas Türlerra,b,c *, Robert Eichlerra,b, Alexan nder Yakushevd,e a Pauul Scherrer Instittut, CH-5232 Villligen PSI, Switzerrland Universitty of Bern, Deparrtment of Chemisttry and Biochemiistry, Freiestr. 3, CH-3012 Bern, Switzerland S c University of o Bern, Albert Eiinstein Center forr Fundamental Physics, P Sidlerstr.5 5, CH-3012 Bernn, Switzerland d GSI Helmhholtzzentrum für Schwerionenforsschung GmbH, Pllanckstrasse 1, D-64291 Darmstaddt, Germany e H Helmholtz-Institu ut Mainz, D-550999 Mainz, Germanny b

Abstraact Chemiical investigatio ons of superheeavy elements in the gas-phaase, i.e. elemen nts with Z≥1044, allow assessing the influennce of relativisstic effects on their t chemical properties. Furrthermore, for some s superheav vy elements annd their compoounds quite unnique gas-phasee chemical prooperties were predicted. Thee experimental verification of these properrties yields suppporting evidencce for a firm asssignment of thhe atomic numbber. Prominent examples are thhe high volatillity observed foor HsO4 or the very weak inteeraction of Cn with gold surfa faces. The uniqu ue properties of o HsO4 were eexploited to disccover the doubly-magic even--even nucleus 2770Hs and the neew isotope 271Hs. H The combinaation of kinem matic pre-separaation and gas-pphase chemistrry allowed gaiining access too a new classs of relatively fragile S through Mt. A not yet resollved issue conccerns the interacction of compoounds, the carboonyl complexess of elements Sg Fl witth gold surfacees. While com mpeting experim ments agree onn the fact that Fl is a volatiile element, thhere are discreppancies concern ning its adsorpttion on gold surrfaces with resppect to its daugh hter Cn. The ellucidation of theese and other questions amouunt to the fasccination that gaas-phase chemiical investigatioons exert on cuurrent researchh at the me limits of chemistry today. extrem

© 20111 Published by b Elsevier Lttd. Selection and a peer-revieew under respoonsibility of Desheng D Dashh Wu Keyworrds: Gas-phase chhemistry; superheeavy elements; reelativistic effects

*C Corresponding autthor. Tel.: +41-566-310-2401; fax: +41-56-310-44355. E-m mail address: [email protected].

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1. Introduction Gas-phase chemical techniques proved instrumental in transactinide chemistry research despite the refractory nature of the early transactinide elements. The first chemical investigation of rutherfordium (Rf, Z=104) in the 1960ies was conducted in form of frontal isothermal gas chromatography in a chlorinating atmosphere [1]. At the present time, while conducting experiments with superheavy elements (SHE) copernicium (Cn, Z=112) [2, 3] and flerovium (Fl, Z=114) [4, 5], gas-phase separation procedures play an important role in elucidating the influence of relativistic effects on their chemical properties. Production of transactinide elements at accelerators for chemical investigations usually implies a thermalization of the primary products in a gas, a so-called recoil chamber [6], since in nuclear fusion reactions the reaction products are imparted with the momentum of the beam and ejected from the target. It is rather straightforward to connect such a recoil chamber to a gas chromatographic separation system and to condense the separated volatile species for α-particle spectroscopy and spontaneous-fission detection. Atoms of the transactinide elements are produced at extremely low rates: atoms per minute for Rf and Db, down to atoms per day for elements Sg through Fl. They are produced among much larger amounts of “background” radioactivities, which hinder the detection and identification of decay of the transactinide atoms of interest. For these reasons, there is a recognized need for a physical pre-separation of the transactinide element atoms before chemical separation. Thus, a recoil chamber can be coupled to a kinematic pre-separator instead of a focal plane detector array [7-10]. In order to attain the required short separation times rather high gas flow rates have to be used, which negatively affect chemical separation factors and gas chromatographic resolution. Furthermore, there exist a rather limited number of inorganic volatile species that are suitable for gas chromatographic investigations. One has to keep in mind that the retention temperature regime in quartz chromatography columns is limited to maximum temperatures of about 1000 °C, while on the low temperature side, condensation of water vapors and/or CO2 present experimental challenges concerning the purity of the employed carrier gases. In addition, due to the short half-lives of transactinide nuclides, the kinetics of the formation of chemical compounds must be fast. Gas chromatographic methods involving a carrier gas limit the accessible half-lives to longer than about one second. An alternative is provided by vacuum chromatographic methods, where under molecular flow conditions, atoms or molecules can in principle be isolated within about 100 milliseconds [11]. Currently, two variants of gas chromatography are applied in transactinide chemistry experiments: thermochromatography (TC) and isothermal chromatography (IC). Sometimes also combinations of the two have been used. The basic principles of TC and IC are explained in Figure 1. In TC [12], a carrier gas is flowing through a chromatography column, to which a negative longitudinal temperature gradient has been applied. Open or filled columns can be employed. Species, that are volatile at the starting point, are transported downstream of the column by the carrier gas flow. Due to the decreasing temperature in the column, the time the species spend in the adsorbed state increases exponentially. Different species form distinct deposition peaks, depending on their adsorption enthalpy (−ΔHads) on the column surface and are thus separated from each other. A characteristic quantity is the deposition temperature (Ta), which depends on various experimental parameters. In recent years, TC detectors proved enormously successful. The carrier gas containing volatile atoms or molecules is flowing through a narrow channel formed by a series of planar silicon diodes. These detectors are referred to in the community under the acronyms CTS (Cryo-Thermochromatographic Separator) [13], Cryo OnLine Detector (COLD) [14], or Cryo-Online-Multidetector for Physics and Chemistry of Transactinides (COMPACT) [14]. Along this channel a longitudinal negative temperature gradient is established. Due to the close proximity of the silicon diodes facing each other, the probability to register a complete decay chain consisting of a series of α-particle decays is very high. Furthermore, spontaneous-fission (SF) decays are mostly registered as coincident signals of the two fragments. Due to the required operating conditions of silicon diodes, only temperatures from +30°C to about −180°C can be realized. The temperature range could be increased in the future by using diamond detectors; however, these are

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currenntly still too small s in size to t allow consttruction of a full f TC chann nel. It should be b noted, that in TC all innformation about the half-llife of the deeposited nucllide is lost, which w is a seerious disadvaantage, especially for nucllides decayingg by SF, sincce SF is a noon-specific deecay mode off many actinidde and des. However,, TC experimeents with transsactinides deccaying by SF have h an unsurppassed transaactinide nuclid sensittivity (provideed that the chhromatographiic separation from actinidees is sufficiennt), since all species s are evventually adso orbed in the column c and thhe decay of eaach nuclide iss registered. Thus, T the posittion of each ddecay in the column c contribbutes chemicaal informationn about the in nvestigated speecies. A photoograph of an open TC channnel is provideed in Figure 2. 2 The silicon detectors disp played in Figu ure 2 actually consist c of grooups of 4 detectors on a siingle chip to minimize inaactive detectorr surface and have been coovered with a thin layer off gold.

Fig. 1. Left-hand side paanels: temperaturre profiles employyed in isothermall chromatographyy and thermochrom matography; righht-hand anels: integral chrromatogram and deposition d peak reesulting from isothermal chromatoography and therm mochromatography, side pan respecttively [14].

Fig. 2. Open thermochro omatographic chaannel consisting of o silicon detectorrs for α-particle- and SF-fragmentt spectroscopy. Along A i applied. the chaannel a longitudinnal negative temperature gradient is

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In IC, a carrier gas is flowing through a chromatography column of constant, isothermal temperature. Open or filled columns can be employed. Depending on the temperature and on −ΔHads of the species on the column surface, the species travel slower through the length of the column than the carrier gas. The retention time can be determined either by injecting a short pulse of the species into the carrier gas and measuring the time at which it emerges through the exit of the column or by continuously introducing a short-lived nuclide into the column and detecting the fraction of nuclides that have not decayed at the exit of the column [14]. A characteristic quantity is the temperature at which half of the introduced nuclides are detected at the exit (T50%). In this case, the retention time in the column is equal to the half-life of the introduced nuclide. The half-life of the nuclide is thus used as an internal clock of the system. The T50% temperature depends on various experimental parameters. It can be shown that for similar gas flow rates and column dimensions Ta ≈ T50%. By varying the isothermal temperature, an integral chromatogram is obtained. The yield of the species at the exit of the column changes within a short interval of isothermal temperatures from zero to maximum yield. On-line IC is ideally suited to rapidly and continuously separate short-lived radionuclides in the form of volatile species from less volatile ones. Less volatile species are retained much longer and the radionuclides eventually decay inside the column. A schematic of the On-Line Gas chromatography Apparatus (OLGA) is provided in Figure 3. Reaction products recoiling from the target are thermalized in a recoil chamber and rapidly transported through a thin capillary to the chromatography setup with the aid of an aerosol gas-jet transport system. With typical He flow rates of 1 to 2 l/min and inner diameters of the capillaries of 1.5 to 2 mm, transport times of less than 10 s were easily achieved over distances of 10 m. This way, the chromatography system and also the detection equipment could be set up in an accessible, fully equipped chemistry laboratory close to the shielded irradiation vault. The aerosol particles carrying the reaction products are collected on quartz wool inside a reaction oven. Reactive gases are introduced to form volatile species, which are transported downstream by the carrier gas flow to an adjoining heated isothermal section of the column, where the chromatographic separation takes place. Since volatile species rapidly emerge at the exit of the column, they can be condensed and assayed with nuclear spectroscopic methods. This is done by turbulent mixing of the gas stream exiting the chromatography column (containing the volatile species under investigation) with a second gas stream containing aerosol particles. Volatile species will condense on the surface of these aerosol particles and can be transported to a counting device, i.e. a rotating wheel system or a tape system, where they are collected by impaction and subsequently assayed by α-particle and SFspectroscopy. A disadvantage of IC concerns the determination of −ΔHads on the column surface of transactinide nuclei. In order to determine the T50% temperature, a measurement sufficiently above and below this temperature is required. Since for transactinide elements this temperature is a priori unknown, several measurements at different isothermal temperatures must be performed, which means that long measurements are required below the T50% temperature that demonstrate that the transactinide compound is retained long enough that most of the nuclei decayed in the column. Such an approach is very beam time consuming. Furthermore, it must be demonstrated that the experiment was performing as expected and the non-observation of transactinide nuclei was not due to a malfunctioning of the apparatus. By applying microscopic models that allow the simulation of the transit of an atom or molecule through an open column [15] −ΔHads on the column surface can be deduced using a Frenkel-type approach connecting the average adsorption time to the phonon vibration frequency (1/τ0) via a Boltzmann factor containing –ΔHads (τ0 is the period of oscillations of the molecule in the absorbed state perpendicular to the surface). This thermochemical quantity is characteristic for a certain atom− or molecule to surface interaction and not dependent on experimental conditions, such as gas flow rate or temperature gradient. So far, mostly inorganic compounds have been synthesized and separated such as halides and oxyhalides. This class of compounds, mostly in form of chlorides and bromides or oxychlorides and oxybromides, respectively, proved to be ideal for the 6d elements of groups 4 to 7. Volatile compounds of

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SHE in groups 4 and 5, Rf and Db, formed in halogen or/and oxygen containing gas media, were investigated during the past 30 years. For the group 6 and 7 elements (only Tc and Re), also the oxide/hydroxide molecules have been synthesized. For elements of group 8, the tetroxide is the species of choice, since this molecule is very volatile. Very promising for gas-phase chemical studies appear carbonyl complexes of group 6 to 9 elements. First experiments with homologs revealed a rapid formation of the compound and a similarly volatile behavior as observed for group 8 oxides. The first experimentally observed transactinide carbonyl complex was Sg(CO)6 and its volatility with respect to its lighter homologs was investigated in a TC experiment [16].

Fig. 3. The On-line Gas chromatography Apparatus (OLGA) coupled to the ROtating Multidetector Apparatus (ROMA) [17].

The elements Cn and Fl, having closed- and quasi-closed electron shell configurations, are expected to be rather inert noble metal-like in their elemental state, due to the expected strong relativistic stabilization of the filled 7s and 7p1/2 valence orbitals. Early theoretical studies went as far as attributing them even a noble-gas like behavior [18]. First experiments with Cn and Fl could confirm a high volatility in the atomic state and only a rather weak interaction with surfaces like gold or Teflon®. Nevertheless, the concurrently investigated noble-gas Rn showed a significantly higher volatility and a very weak surface interaction that can be explained by van der Waals forces only. A first experiment to study element 113 (Uut) was conducted and the results have been published. However, the experimental findings are not yet at a stage to conclusively claim first chemical identification of this element. Currently accessible, but not yet studied experimentally, is ununpentium (Uup, Z=115), while the longest-lived known isotopes of livermorium (Lv, Z=116) and beyond are too short for chemical investigations with current methods. Similar chemical arguments as for Cn and Fl hold for the expected properties of Lv, while ununoctium (Uuo, Z=118) might exhibit properties similar to Rn [19]. The odd-Z elements with atomic numbers 113 and 115 are expected to be less reactive and more volatile compared to their lighter homologs Tl and Bi. However, an accurate prediction of chemical properties, especially in comparison to the behavior of their even-Z neighbors is difficult. Chemical instrumentation and studies of elements with Zı104 in gas phase have extensively been reviewed in the recent past [19, 20]. In this contribution we will concentrate on recent developments and accomplishments. For the sake of completeness we will provide a brief overview for elements for which no recent new results were reported.

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2. Rutherfordium (Rf, element 104) Due to the high sublimation enthalpies of group-4 elements, gas chromatographic separations of the atoms are not feasible at temperatures below 1000°C. Under halogenating conditions, however, group-4 elements form mono-molecular pure halides such as tetra fluorides, chlorides, bromides and iodides. The volatility decreases according to MCl4 > MBr4 > MI4 > MF4 with M= Zr and Hf. Evidently, chlorides and bromides are clearly the best choices for gas chemical studies. Iodides have the disadvantage of a poor thermal stability and fluorides are least volatile. The first chemical study of Rf [1] was part of the discovery claim of this element by scientists from Dubna, Russia. For production of Rf isotopes the nuclear reaction 242Pu(22Ne, xn)259,260Rf (n=4,5) was employed. These pioneering studies using isothermal frontal gas chromatography showed that in a chlorinating atmosphere Rf forms a highly volatile molecule. Detection of Rf nuclides relied on the registration of latent fission tracks left in mica solid state detectors. Further on TC experiments were conducted and, much later, isothermal chromatography experiments with direct identification of the separated nuclides by their α-particle decay chains. In a recent review [19], all available data on experiments with group-4 chlorides and bromides was subjected to a standardized reanalysis procedure. The newly evaluated adsorption enthalpies on quartz surfaces (intentionally or unintentionally modified by the used reactive agents) resulted in the values summarized in Table 1. Table 1 Comparison of published and evaluated −ΔHads-values measured for group-4 tetrachlorides and tetrabromides in various experiments. Table adopted from [19] Ref.

Year

Carrier gas/ aerosol particle material

ZrCl4

HfCl4

RfCl4

ZrCl4

HfCl4

RfCl4

FC

[21]

1969

N2

84

84

84-96

84

84

100-109a

TC

[22]

1971

N2

-

155c

104c

-

146b

110b

-

105

c

c

c

-c

d

TC

[23]

1991

Ar

d

-

-

[24]

1992

He/KCl

-

≤70

-

≤75

≤85

IC (HEVI)

[25]

1996

He/MoO3

74±5

96±5

77±5

79±5

103±5

82±5

IC (OLGA III)

[26]

1998

He/C

-

110c

92c

97e

103

87

ZrBr4

HfBr4

RfBr4

ZrBr4

HfBr4

RfBr4

-

82c

63c

-

86f

68f

[23]

1991

Ar

d

IC (OLGA II)

[24]

1992

He/KCl

-

125

IC (HEVI)

[27]

2000

He/KBr

91±6

113±5g

data from Ref. [28] see Figure 35 in [19] c data from Ref. [29], evaluation not possible due to missing experimental details d data from Ref. [30] e unpublished data by Türler et al. f see also Ref. [31] g data from Ref. [32] b

83

c

IC (OLGA II)

TC

a

−ΔHads evaluated (kJ⋅mol-1)

−ΔHads published (kJ⋅mol-1)

Technique

≤80

105

d

89±5g

-

130

111

95±6

117±5

93±5

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3. Dubnium (Db, element 105) Dubnium is expected to be a firm member of group 5 of the Periodic Table, positioned below the refractory elements niobium and tantalum. Group-5 elements are most stable in their maximum oxidation state +5 and, therefore, form pentahalides. Most volatile are the pentafluorides, followed by the pentachlorides and the pentabromides. Besides the pure halides, also the oxyhalides (MOX3) are stable in the gas phase. They should be less volatile compared to the pure halides. Therefore, gas chemical investigations of Db focused on its halides and oxyhalides. Due to the high tendency of group-5 elements to react with trace amounts of oxygen or water vapor, investigations of the pure pentahalides proved to be experimentally challenging, since formation of oxyhalides was always thermodynamically favored. Early chemical investigations of single atoms of Db were restricted to rapid gas-phase chemical investigations due to the relatively short 261Db (T1/2=1.8 s) that was accessible in the 22Ne+243Am reaction and were conducted in Dubna, Russia, employing similar techniques as for investigations of Rf. Only when sufficient amounts of the very rare and short-lived target material 249Bk (T1/2=320 d) became available, the longer-lived 34-s 262Db was synthesized in the reaction 249Bk(18O, 5n). In these experiments the separated Db nuclides were directly identified by registering characteristic α-particle decay chains. In Table 2 the evaluated −ΔHads-values measured for group-5 halides and oxyhalides of Nb, Ta, and Db in various experiments are summarized [19]. In cases where the assignment to either the pure pentahalide or the oxytrihalide was uncertain, the obtained −ΔHads-value was listed in a separate column between the two species. In the very first TC experiments with group-5 chlorides on glass and/or mica surfaces [33] similar −ΔHads-values were measured for Nb and Db-chlorides, but the assignment to either the pentachloride or the oxytrichloride could not be made. In isothermal experiments with Nb- and Dbchlorides [34] two species of different volatility were observed and attributed to MCl5 and MOCl3 (M = Nb, Db). While DbOCl3 is less volatile than NbOCl3, only an upper limit could be established for −ΔHads(DbCl5), which allowed no conclusions about the relative volatility of NbCl5 and DbCl5. Table 2 Comparison of −ΔHads-values evaluated for group-5 halides and oxyhalides in various experiments. Table adopted from [19]. Technique

TC

Ref. [35]

TC

a

Year

Carrier gas/ aerosol particle material

−ΔHads evaluated (kJ⋅mol−1) NbCl5

NbOCl3

1973

N2

85

1991

Ar

95a

TaOCl3

DbCl5

DbOCl3 88

a

118 a

IC (HEVI)

[23, 36]

1993

He/MoO3

75±5 b

IC (OLGA III)

[34]

1996

He/C

80±1

99±1

NbBr5

NbOBr3

TC

[35]

1973

He

87a

157±12 b

TC

[23]

1991

Ar

83a

TaBr5

TaOBr3

76±10 b

≤97

117±3

DbBr5

DbOBr3

82a 108a c

IC (OLGA II)

[37]

1992

He/KCl

93±4

IC (OLGA III)

[38]

2012

He/KBr

89±5

evaluated data [29]. 13 reanalyzed data with τ0 = 2⋅10− s [36]. c 13 data from Gäggeler et al. [37] reanalyzed with τ0 = 2⋅10− s [34]. b

TaCl5

a

101±4 155±5

c

103±5

121±11c 71±5

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4. Seaborgium (Sg, element 106) Seaborgium is expected to be a member of group 6 of the Periodic Table and thus homologues to Cr, Mo, and W. In the elemental state all group-6 elements are extremely refractory. While both Mo and W are chemically very similar, there is not much similarity with Cr. There exist a number of volatile inorganic Mo and W compounds that are suitable for gas chromatographic investigations. Mo and W form volatile halides, oxyhalides, oxide hydroxides, and also carbonyls. In contrast to the pure halides, the oxyhalides of group-6 elements are more stable and show a similarly high volatility. For the 6+ oxidation state two stoichiometric types MOX4 and MO2X2 (M = Mo, W; X = F, Cl) exist. By analogy to Mo and W the oxides and oxide hydroxides of Sg are expected to be moderately volatile, whereas the heavy actinides and the transactinides Rf and Db do not form volatile oxides and oxide hydroxides. For this reason this class of compounds should be very selective with regard to a gas chromatographic isolation of Sg from the plethora of by-products of the nuclear formation reaction. A characteristic feature of d-group elements is their ability to form complexes with π-acceptor type ligands such as CO. All group-6 elements Cr, Mo, and W form very volatile and stable hexacarbonyls and constitute the only complete family of stable carbonyls. 4.1. Oxychlorides of seaborgium The first chemical identification of Sg as volatile oxychloride in TC experiments was reported by the Dubna group in a preliminary report by Timokhin et al. [39] in 1993 and again in 1996 [40]. Later, Yakushev et al. [40] reported ancillary experiments with Mo and W nuclides and a further experiment of the same type with Sg. A full paper giving a detailed account of all Sg TC experiments was published by Zvara et al. [41] In these experiments the nuclide 263Sg (T1/2=0.9 s) was produced in the reaction 249 Cf(18O, 4n). Reaction products were thermalized behind the target set-up in a rapidly flowing stream of argon gas and flushed to the adjoining TC column. Volatile oxychlorides were synthesized by adding air saturated with SOCl2 as reactive agent. The formed oxychloride species migrated downstream the fused silica chromatography column, to which a longitudinal, negative temperature gradient was applied, and finally deposited according to their volatility. In contrast to earlier experiments, no mica plates were inserted, but the fused silica column itself served as SF track detector. The deposition of Sg was registered after completion of the experiment by searching for latent SF tracks left by the SF decay of 263 Sg. Based on the results of ancillary experiments with short-lived W nuclides, it was concluded that in a first, fast step volatile MO2Cl2 (M = W, Sg) molecules were formed and in a second, slower step the deposited MO2Cl2 was converted to more volatile MOCl4. Therefore, the Sg deposition peak was attributed to the compound SgO2Cl2, whereas the 176W deposition peak was attributed to WOCl4. Due to the occurrence of two different species as well as due to the large differences in half-life no information about the relative volatility of MO2Cl2 (M = Mo, W, Sg) or MOCl4 (M = Mo, W, Sg) was obtained within group 6. As most of the earlier TC experiments with transactinide elements, the Sg experiments have not unanimously been accepted as the first positive identification of Sg after chemical separation. Criticism was voiced by Kratz [42, 43] concerning mostly the magnitude of a SF-branch in 263Sg but also the assignment of the volatile species. An extended discussion can also be found in [19]. For on-line gaschromatographic studies of Sg as volatile SgO2Cl2 compound the OLGA III system in conjunction with the ROMA detection system and the Sg synthesis reaction 22Ne + 248Cm was employed (see Figure 3) [44, 45]. Typically, the reaction products recoiling from the target were transported to the OLGA III set-up with the aid of a C-aerosol gas-jet. As chlorinating agents Cl2 saturated with SOCl2 and traces of O2 were introduced to the reaction oven, where the formation of oxychlorides occurred. Volatile species travelled through the colder, isothermal section of the column and were reattached to KCl aerosol particles while exiting into the recluster chamber. This second aerosol gas-jet transported the separated radioactivities to the ROMA detection system. The registered spectra were dominated by α-particle lines

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originnating from isotopes of Poo and Bi. Prresumably, thhese nuclides were producced in multinuucleon transffer reactions from fr Pb impurrities in the Cm m target. These elements were w not or onnly partly retaiined in the chhromatographiic column. Exxcept for 211Poom and 212Pom, all very shoort-lived Po acctivities were due to longerr-lived Bi and d/or Pb precurrsors. In two runs r at isotherrmal temperattures betweenn 300 ºC and 400 4 ºC ten decay chains attributed to 265Sg were registered, r whhich were suummarized inn one data pooint of α α ⎯→ → 261Rfa ⎯ ⎯→ → 257No c consissted of a com mplete chain 265Sga ⎯ 350±550 ºC. One of the decay chains α 253 ⎯ ⎯→ → Fm, whiich unambiguuously demonnstrated that Sg S was chem mically isolateed (see [46] for f the denotation of isomeric states in 265Sg and 261Rf). R At 250 ºC C additional thhree decay chains were obsserved. n only the cchromatographhic transport through the ccolumn Due tto the compliccated detectioon technique not but allso the decay and detectionn of Sg nucleii was modeledd with a Montte Carlo proceedure. This way w the most probable num mber of initiaally produced 265Sg nuclei and the chem mical yield co ould be determ mined, 9 +−25 kJ·mol−1 (68 % ( whichh allowed exttracting a proobability density distributioon of −ΔHads(SgO 2Cl2) = 98 −1 error interval). For 168WO2Cl2 −Δ ΔHads(WO2Cll2) = 96 ± 1 kJ·mol k was deduced, d wherreas for 104MooO2Cl2 −1 −ΔHadds(MoO2Cl2) = 90 ± 3 kJ·m mol resulted, in good agreeement with theoretical preddictions. Howeever, it shouldd be noted thaat due to the very v limited nu umber of obseerved events it i was not posssible to e.g. provide p 95% oor 99% error intervals, andd there remainns an about 15% chance thhat SgO2Cl2 iss more volatille than MoO2Cl2 comparedd to an 85% chhance that it is i less volatilee as reflected by b the deduced −ΔHads valuue. The observved yield curv ves for 104MoO O2Cl2, 168WO2Cl2, and 265SggO2Cl2 are dissplayed in Figure 5.

Fig. 5. Relative yield off 104MoO2Cl2, 168W WO2Cl2, and 265SggO2Cl2 as a functiion of isothermal temperature in thhe chromatographhy columnn.

4.2. Oxides/hydrox O ides of seaborrgium Hüübener et al. [447] investigatted the behaviior of group 6 oxide hydrooxides includinng Sg in the system s O2-H2O(g)/SiO2(s).. It is well knnown that in an a O2/H2O atm mosphere the solid MO3 (M M = Mo, W) are in equiliibrium with gaaseous MO2(O OH)2. Under the t assumptionn that in one-atom-at-the-tiime experimennts the gaseoous MO2(OH)2 undergo a diissociative adssorption proceess, the processs can be described as follow ws: MO2(O OH)2(g) ' MO O3(ads) +H2O(gg)

(M = Moo,W, Sg)

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Thhe gas chrom matographic investigation i of Sg oxidee hydroxides in quartz-g glass columnss with He/O2/H2O as carrier gas muust be highlyy characterisstic since neeither actinides nor the lighter transaactinides shou uld form volaatile species that would obscure o the unequivocal u id dentification of Sg. Prepaaratory experiiments involvving TC andd high-tempeerature on-linne IC were carried out, which demonnstrated that Mo- and W ooxide hydroxiides are not ttransported by y simple reversible adsorpttion of MO2((OH)2 (M = Mo, M W) but can c be best described d by a microscopiic description of the dissociative adsorpption process. The relativee yields of 104Mo M and 168W oxide hydroxxides as a funnction of isothhermal tempeerature are shoown in Figure 6 [47, 48]. Inn the actual exxperiment with h Sg the synth hesis reaction 22Ne + 248 Cm m was employeed. Reaction products p recoiling from thee target were stopped s in Hee seeded with MoO3 aerosool particles an nd transporteed to the HIT TGAS set-up [49]. At the entrance to the t chromatoggraphy colum mn moist O2 was w added to the gas-jet. The T temperatuure of the quaartz chromatography colum mn was 1325 K in the reacction zone andd 1300 K in thhe isothermall zone. Retenttion times of about 8 to 9 s were mined from measurements m with short-lived Mo and W nuclides at a isothermal temperatures above determ 1270 K. By conddensing the separated vo olatile speciees directly onn metal foils mounted on o the mference of th he rotating whheel of the RO OMA detectionn system, the time-consumiing reclusterinng step circum could be avoided, however, h at thhe expense off reduced detection efficienccy for α-partiicles. The searrch for d chains revealed two candidate eveents, which must m be attribuuted to the seqquence genetiically linked decay α 265 261 ⎯→ Sgb ⎯ Rfb (see [46] forr the denotatioon of isomericc states in 265Sg S and 261Rf). Since Sg apppeared to be volatile underr the conditionns of the expeeriment, it shoowed the typiccal behavior of o a group 6 ellement was transported d presumably as Sg oxide hydroxide. h and w

Fig. 6. Relative yields inn isothermal gas chromatography c of 104Mo ({) andd 168W (z) oxide hydroxides in quuartz columns usinng mponent. Sg was observed at an isothermal temperaature of 1300 K. The T solid lines arre the humid oxygen as reactivve carrier gas com o a Monte Carlo model based on a microscopic desscription of the dissociative adsorp ption process[50]] with result of 0 −1 0 −1 ΔH diss.aads(MoO2(OH)2) = −54 kJ⋅mol annd ΔH diss.ads(WO2(OH)2) = −56 kJ⋅mol . The dasheed line representss a hypothetical yield y curve assuming a that gro oup-6 oxide hydrooxides are transpoorted by simple reeversible adsorptiion with ΔHa0 = −220 − kJ⋅mol−1 [477]. ©2001 Oldenbourg Wisssenschaftsverlagg GmbH.

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4.3. Seaborgium S caarbonyl compllexes The uuse of kinemattic pre-separattors in combinnation with reecoil transfer chambers opeened the prosppective to genntle formationn of volatile SH HE compound ds, which couuld not be form med under harrsh conditions inside a recooil chamber behind b a target or at a verry high temperature. A neew compoundd class of SH HE was introdduced recentlyy with the synnthesis of Sg hexacarbonyll, the first carb bonyl compleex of a transacctinide elemeent [16].

Fig. 7. Spectra of fission n products collectted on a charcoal filter, transported by a N2 gas (lefft panel) or by a N2/CO gas mixturre (right G panel) [51]. ©2014 de Gruyter.

Thhe in-situ prod duction of voolatile carbon nyl complexess with single atoms was started with fission produucts at the TR RIGA Mainz research r reactor (Germanyy). A continuo ous synthesis of transitionn metal carbonnyl complexees with short-llived isotopess combined with w a rapid traansport [51, 52] 5 was observved. A 249 Cf target was plaaced in a recoil chamber neear the reactorr core. The tarrget chamber was w flushed with w N2 m Volatiile compoundds could be raapidly transpoorted by the gas stream att room or a N2/CO gas mixture. tempeerature to a ch harcoal filter, which w was monitored with an HPGe Ȗ-ddetector. Not only o the elemeentally volatiile fission pro oducts I, Se, and Xe, butt also the reffractory elements Mo, Tc,, Ru, and Rhh were transpported if CO gas g was addedd. Obviously,, the transitionn metals Mo, Tc, Ru, and Rh R reacted with w the CO annd formed vollatile carbonyll complexes (F Figure 7). Thhe studies of caarbonyl compplex formation n were continuued at the University of Berrn (Switzerlannd) and at thee Institute of Modern M Physsics in Lanzhoou (China) with w fission products from a 252Cf sourcee [53]. Carboonyl complexees of the refraactory elements Mo, Tc, Ruu, and Rh werre investigated d by the IC method. m Mo annd Ru are knnown to form mononuclearr carbonyl com mplexes Mo(C CO)6 and Ru(CO)5, respecctively. Howeever, no binarry, stable, moononuclear carrbonyls are known kn for Tc and Rh. Thuus for metals of o odd groupps, Tc and Rhh, no ambiguoous chemical formula f couldd be assigned.. A real challeenge when stuudying carbonnyls of fissioon products are precursorr effects, whhich complicaate the unam mbiguous distiinction betweeen the chemiically separateed and detected species. Inn many cases it remains un nclear, if a deetected elemeent was indeedd transported as a carbonyl complex, or if i transport off the ȕ−-decay precursor occcurred, whichh decayed to itts daughter in the charcoal filter [52-54]. The most reliiable adsorption enthalpy value v is that of Mo(CO)6 onn a quartz surfface. The prod duction of Moo carbonyl and d its adsorptioon was studiedd at the GA research reeactor and with w 252Cf sourrces by IC inn a quartz column, and in addition by TC in TRIG COM MPACT at the Gas-filled Reecoil Ion Sepaarator (GARIS S) in RIKEN (Japan). The adsorption ennthalpy valuess determined in different IC I experimentts agree well [51-53], how wever the valuue obtained inn a TC ® experiment was diffferent (see beelow) [54]. Th he adsorption oof carbonyl coomplexes on Teflon T was sstudied

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for M Mo, Ru, W, and d Os. It was found fo that the adsorption onn Teflon® is abbout 2 kJ⋅moll-1 weaker com mpared to the adsorption on n quartz [53].

Fig. 8. Schematic drawinng of experimentts at TASCA [51]]. Three chemicall devices were used: 1) TC with COMPACT, C 2) IC C in a c 3) therm mal decompositionn in a quartz tube. ©2014 de Gruyyter. quartz column,

Affter the successsful experimeents with fission products, the t next logical step was thhe in-situ prodduction of carrbonyl compleexes with prodducts of nuclear fusion reacctions. Havingg in mind a rellatively low sttability of carrbonyls complexes, a recoil separator shhould provide the necessary y gentle condiitions for the in-situ formaation of carbonnyls. The heavvier homologuues of previouusly studied fiission productts, W, Re, Os, and Ir were produced in 24Mg ion beam inducedd fusion reacttions and guided through the TransAcctinide mistry Apparaatus (TASCA)) at GSI Darm mstadt [51, 522, 54]. The shoort-lived isotoopes of Separrator and Chem W andd Os have a significant Į-ddecay branch and a could be measured m with h the detection setup COM MPACT directtly coupled to TASCA (Figgure 8). Thus, the complete experimental setup for investigations off a new class of compounds of SHE, nam mely carbonyll complexes, was w tested wiith W and Os.. The ions sepparated nd volatile caarbonyl with TASCA weree thermalizedd in CO conttaining atmossphere inside the RTC an w transporteed by the gas flow to a chro omatography and detectionn setup. complexes were forrmed. They were I methods were w applied foor Į-decaying W and Os isootopes, and foor Re and Ir issotopes Gas-solid TC and IC r (F Figure 9). decayying by ȕ- andd Ȗ-emission, respectively Exxtended measu urements withh W(CO)6 were w performed at GARIS in RIKEN (Japan). Amonng the adsorpption measureements at diffferent gas flow rates and gas g compositiions, the efficciency of the in-situ carbonnyl formationn and the transsport time were determinedd [16, 54]. Atttempts to prodduce carbonylls with nucleaar fusion prod ducts W and Os in a reco oil chamber without w presep paration were also perform med. In these experiments a gas mixture containing CO C was introdduced directly in a gas cham mber behind a target p throuugh. The harssh conditions did not allow w the formattion of wheree a heavy ionn beam was passing carbonnyls with a reemarkable yieeld. In addition n, CO molecuules were desttroyed with thhe beam resulting in the foormation of caarbon clusters and metal oxiides [53, 55].

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Fig. 9. Distributions of Os(CO)5 and W(CO)6 in COMPACT (upper panel); break-through curves for Re(CO)5X and Ir(CO)4X in IC experiments (lower panels). The adsorption behavior was simulated by a Monte Carlo model, the distribution curves and the most probable adsorption enthalpy values are also given [51, 52].

The first investigation of a carbonyl complex with the transactinide element Sg was performed at GARIS (RIKEN, Japan) [16]. 265Sg was produced via the nuclear reaction 248Cm(22Ne,5n) and separated from the primary beam and from other reaction products with GARIS (Figure 10). At the exit of GARIS an RTC with an inner diameter of 100 mm was attached. A Mylar® window on a supporting grid separated the RTC and GARIS volumes. The RTC was continuously flushed with a He/CO gas mixture, and the volatile under ambient conditions species were transported by the gas flow to COMPACT, which serves as a TC column and a detection setup. The gas flow rates of the gas mixture He:CO = 1:1 were varied from 1 to 2.2 L/min. For the optimization of the chemical yield and of the transport time, W detector rates were measured firstly with a focal plane silicon strip detector in the RTC position, and then in COMPACT. The species entering COMPACT were deposited on a SiO2 detector surface according to their volatilities. The combination of a physical preseparation and chemical separation led to an extremely low background in Į-particle spectra (Figure 11)

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w of the experimenntal setup in the experiment e on syynthesis and charaacterization of Sg g(CO)6 [16]. Fig. 100. Schematic view

Too identify the deposition d paattern of adsorrbed Sg(CO)6 in a TC channnel, a search for Į-particlees with energies from 7.4 to t 10 MeV annd for SF fraggments correlaated in time and a position was w performedd. This o froom 265Sg. Thee probability that t one or more m of searchh revealed 15 correlated deecay chains originating these chains is of raandom origin from backgro ound was less than 10−5 [16, 54]. In addittion, three SF events were found in the same part off the TC chaannel and tenttatively assignned to the deecay of 265Sg or its mple of a uniqque decay chaain is shown in Figure 11. Here, 265Sg decayed d to thee longprogeenies. An exam 261 lived state of Rffa, which in tuurn decayed by b SF, which was observedd for the first time. Such a decay α evennt was chain can only be observed unnder negligiblle backgroundd conditions. No single α-particle f the one atttributed to 265Sg S that detectted above 7.4 MeV in all 32 detector paiirs during thiss run, except for 2 was tiime-correlatedd with the SF decay. A beaam integral off 4.35 × 1017 22 Ne was accuumulated in thhis run. All 188 events assiggned to 265Sg or its daughtters were takeen for the anaalysis of the adsorption a behhavior. They were found in i the secondd half of the TC T channel at a temperaturees below ௅30 °C. An enthaalpy of mol−1 was dedduced as the most m probablee value for Sg g(CO)6 on thee SiO2 adsorpption ௅ǻHads = 50 ± 4 kJ⋅m surfacce [16]. The lighter group 6 homologuess Mo and W were studied under the sam me conditionss using the saame setup.

Fig. 11. Sum spectrum of o α-particles andd SF-fragments off 32 detector pairrs collected for a beam b integral 4.3 35 × 1017 [54].

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Within the statistical error limits, the ௅ǻHads-values for Mo(CO)6, W(CO)6 and Sg(CO)6 complexes are almost identical: (50 ± 2 kJ⋅mol−1) for Mo(CO)6, (49 ± 2 kJ⋅mol−1) for W(CO)6, and (50±4 kJ⋅mol−1) for Sg(CO)6, respectively (Figure 12). The value for W(CO)6 agrees well with the value obtained in a TASCA experiment [52], however, the value for Mo(CO)6 is 7.5 kJ⋅mol−1 higher than the values from IC experiments with fission products, which is 42.5±2.5 kJ⋅mol−1 [51, 52]. Mo(CO)6 is the only compound that was studied by both IC and TC methods, and the reason for the disagreement in the adsorption enthalpy values obtained by different techniques is not clear so far.

Fig. 12. Deposition patterns of volatile hexacarbonyl complexes in COMPACT. Bars – experimental distributions; lines result of Monte Carlo simulations for the given adsorption enthalpy values [16].

Studying the thermal stability of carbonyl complexes can provide an answer how strong the chemical bond between a transition metal and the CO ligand is. Thus, the bond strength in Sg(CO)6 can be experimentally compared with the predicted one and with those of its lighter homologues W(CO)6 and Mo(CO)6. First decomposition experiments of in situ-formed carbonyls were performed for Re and Ir complexes in experiments at TASCA [51]. Re and Ir carbonyl complexes were guided through a quartz column with a quartz wool plug. The column temperature varied from room temperature to 600 °C. The transport yield through the quartz tube was determined for different temperatures. An activated charcoal trap was placed behind the quartz tube and monitored by a γ-ray detector. 50% of the Re or Ir carbonyl molecules were destroyed inside the column and were not collected in the trap at 390 °C and 300 °C, respectively. The decomposition studies were continued for Mo(CO)6 and W(CO)6. The thermal stability of Mo(CO)6 was investigated in interactions with different column materials and by varying different factors, such as CO concentration and the influence of impurities of O2 and CO2 [56]. Figure 13 shows the experimental setup for the decomposition studies performed with fission products at the University of Bern and the obtained results for 104Mo(CO)6 [56]. The thermal stability of a carbonyl

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complex is dependent on the column material where the gas-solid interaction takes place (Fig. 14). The less chemically active materials quartz and Ag towards CO and carbonyl complexes were selected for the decomposition studies. Finally, the Ag column was selected for the online studies with W(CO)6 and Sg(CO)6 precluding the use of quartz because of unclear side reactions on the hydroxylated surface. Investigations of the thermal stability of carbonyl complexes with short-lived isotopes 87,87Mo and 163,164 W were carried out during the first studies on the synthesis of Sg(CO)6 at GARIS in RIKEN. The temperature inside the deposition column made of Ag varied between 100 and 600 °C.

Fig. 13. Principal scheme of the setup used for decomposition studies. Regulated by mass flow controllers (1), gases are mixed before entering the recoil chamber of the ‘Ms. Piggy’ spontaneous fission source (2). Thermalized fission fragments forming volatile compounds are flushed out of the chamber into the 1 m long decomposition column (3) and finally, depending on the decomposition rates, reach the charcoal trap (4), where they are quantitatively retained. The radioactivity in the trap is monitored by a HPGe ȖǦray detector (5). The decomposition column could be byǦpassed with a PFA Teflon® column (6) held at room temperature. The exact same geometry compared to the decomposition column allows for measuring undisturbed production rates at given experimental conditions. Insert: sketch of the open decomposition column (3): a stainless steel tube (7) is inserted into a regulated resistance furnace (8). Inside the stainless steel tube there is a quartz column (9), used as a casing for metal foils inserts (10), which form thereby a metal column. All the connectors are based on Swagelok® parts for HeǦtight hermetic sealing [56].

Fig. 14. 104Mo(CO)6 decomposition curves on different metal surfaces as a function of column temperature [56].

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Figgure 15 showss decomposition curves measured for Moo and W carbo onyls. Decreasing the CO content c from 50 to 16 vol-% did not have a signifficant impact on the decoomposition beehavior of W(CO) W 6, suppoorting the assumption of a preferred heterogeneoous decomposition at thee chosen higgh CO conceentrations. Reggardless of thhe experimentaal parameters, the decompoosition reactioon of W(CO)6 starts consisstently at 350°C, which is 100°C higherr, compared too Mo(CO)6 [556]. Surprisinggly, at 0.5 l/m min gas flow rrate (Figure 15, 1 right paneel), the decom mposition curvve for 163,164W(CO) W ot recover to 100% 6 did no yield even at low temperaturess. A similar behavior wass observed when w the breaak-through cuurve of 87-88 M Mo(CO)6 was measured m at 0.5 0 l/min gas flow f rate and 50 vol-% CO O concentratioon in the carriier gas (Figurre 15, left pannel). This issuee is currently under u further investigation [56].

Fig. 15. 87,87Mo(CO)6 annd 163,164W(CO)6 decomposition d cuurves on a silver column c (left paneel) and the influennce of the experim mental mposition temperaature for 163,164W((CO)6 (right panell) [56]. conditions on the decom

5. Boh hrium (Bh, element 107) Thhe fourth transactinide element, bohrium, is expected too be a homoloog of Mn, Tc, and Re and, thhus, to belong to group 7 of o the Periodic Table. With h the identificaation of the nu uclides 266Bh (T1/2≈1 s) andd 267Bh 249 22 +14 ments of Bkk with Ne ions and the reccognition that the rapid form mation (T1/2= 17 −6 s) [57]] in bombardm of voolatile oxide hydroxides h iss apparently hindered [588], Eichler et al. paved thhe way to thhe first successful chemicaal identificatioon of Bh as oxychloride o coompound [59]. However, due d to the verry low 249 f 267Bh (produced in the reaction r Bk k(22Ne,4n)) [577], any formaation cross secctions of only about 70 pb for experiment aiming at a chemicaal identificatioon of Bh wass predestined to t be a "tour de force". Inn a one monthh long experiiment conduccted at the Paul Scherrer Institute (PSI), Switzerlannd, an internaational collabboration of raddiochemists measured m closse to 180’000 samples and observed a tootal of 6 geneetically linkedd α-decay chaains originatinng most likely from 267Bh affter chemical isolation. i Fouur decay chainns were registered at an iso othermal tempperature of 180 °C, two at a 150 °C andd none at 75 °C. Due to a small mination withh Po and Bi nuuclides, and a statistical treeatment of thiis backgroundd, 1.3 of the 4 decay contam chainss observed at 180 °C had tto be attributeed to accidentaal correlationss unrelated to the decay off 267Bh. At 1550 °C this co orrection was only 0.1 outt of 2 observved decay chaains. The dedduced enthalppies of adsorpption on the column surfface were −Δ ΔHads(TcO3Cl)) = 51±3 kJ··mol−1, −ΔHads(ReO3Cl) = 61±3 −1 +6 kJ·mool , and −ΔH Hads(BhO3Cl) = 75 −9 kJ·mool−1 (68% connfidence intervval). The estaablished sequeence in volatiility was thus TcO3Cl > ReO O3Cl > BhO3Cl C [59].

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T isothermal (°C) Fig. 16. Relative yields of the compounds 108TcO3Cl (z), 169ReO3Cl (z), and (most likely) 267BhO3Cl („) as a function of isothermal temperature. The error bars indicate a 68% confidence interval. The solid and dashed lines indicate the results of simulations with the microscopic model of Zvara [15] with the adsorption enthalpies given in the text. The shaded area represents the calculated relative yield concerning the 68% confidence interval of the standard adsorption enthalpy of BhO3Cl from −66 to −81 kJ·mol−1. Figure reproduced from [59].

Despite the fact that no binary, stable, mononuclear carbonyls are known for Tc and Re, these group 7 elements were observed to form volatile carbonyl complexes (see Figure 9, section 4.3). It is thus very likely, that also Bh can be isolated as a volatile carbonyl complex in a similar fashion as this was done for Sg. Corresponding experiments to accomplish this are under preparation. 6. Hassium (Hs, element 108) 6.1. The first Hs chemistry experiments Element 108, hassium, was synthesized for the first time in 1984 using the reaction 58Fe + 208Pb. The identification was based on observation of three decay chains, which were assigned to 265Hs [60]. Only one Į-decay chain was measured in the irradiation of 207Pb with 58Fe. The measured event was assigned to the even-even isotope 264Hs [61]. These Hs isotopes, 264Hs and 265Hs, are far from the valley of β-stable nuclei and, therefore, their half-lives are very short, of the order of milliseconds, thus they could not be objects for chemical studies with element 108. A much longer-lived Hs isotope, 269Hs with a lifetime of about 10 s, was observed as a grand-daughter in the experiment that led to the discovery of element 112, Cn [62]. This observation made chemical studies with Hs feasible, especially in the gas phase. The most promising nuclear fusion reaction for Hs chemistry studies was the most asymmetric, experimentally accessible one, 248Cm(26Mg,xn)274íxHs. However, calculations with the HIVAP code [63] predicted production cross sections of a few picobarns only, a big challenge for chemistry studies. The expected production rate of Hs nuclei in the fusion reaction 26Mg + 248Cm is a few atoms per day, requiring a very high efficiency of the chemical investigation of Hs nuclei with acceptable statistics. Intensive studies with Os and Ru (see [64] and references cited therein), the lighter homologs of Hs, were conducted at Flerov Laboratory of Nuclear Reactions (FLNR), Dubna, PSI/University of Bern, and at Lawrence Berkeley National Laboratory (LBNL), Berkeley. The volatility of their tetroxides was used

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in gass phase experriments for ann effective sepparation and transport t to a detection sysstem. The enhhanced volatiility was a bigg advantage foor the fast and d effective sepparation, but a new detection technique had to be devveloped. The CTS detectorr for OsO4 waas developed by b Kirbach ett al. [13]. Thee progress in on-line o chemiical separationn and detectioon resulted in a first successsful experimeent on the cheemical identifi fication of Hss that was coonducted in 2001 2 at GSI, Darmstadt, by b a large intternational co ollaboration [665]. A schem matic drawing of the experim mental setup is i shown in Fiigure 17.

Fig. 17. Experimental seetup used for the first chemical ideentification of eleement 108, Hs.

Nuuclear reactionn products reccoiling from the target weere thermalizeed in a gas voolume of the In situ VOlattilization (IVO O) device [655] and flushedd with a dry He/O H 2 mixturee. The reactionn products paassed a quartzz column con ntaining a quaartz wool pluug, which wass heated to 600 6 ºC and seerved as a filter for aerosool particles. At A the same time t it providded a surfacee to complete the oxidationn reaction off Hs to hassiuum tetroxide. The HsO4 molecules m weere further traansported by the carrier gas g through a PFA capillary to a deteection system.. For the adsorption of HsO4 from thee gas phase, TC T with a neegative f the tempeerature gradieent was used. The chromaatographic collumn also serrved as detection system for identiification of decaying d atom ms of Hs. The T COLD-arrray consisted of 36 pairs of siliconn PINphotoodiodes, suitabble for detecttion of Į-partticles and fisssion fragmentts. Always 3 neighboring diodes were connected toggether and moounted with th he opposite 3 ddiodes as a saandwich with a gap of 1.5 mm. m A tempeerature gradiennt from −20 ºC to −170 ºC was establishhed along the detector arrayy. A total of 7 decay chainss detected in this t experimeent were assignned to the deccay of 269Hs or o tentatively to that of 270H Hs [65, 66]. H However, the attribution a of decay chains to 270Hs was later l shown too be incorrect. From the obbserved depossition temperaature of Hs, itts adsorption enthalpy e was derived –ǻH Hads(HsO4) = 46 4 ± 2 kJ⋅mol-1 . The relativvely low adso orption tempeerature reflectted the physisorption proccess of very symmetric s tettroxide moleccules on the Si S 3N4 detectorr surface. The observed cheemical properrties of Hs aree similar to thhose of Ru annd Os and confirmed Hs as a member in the t group 8 off the Periodic Table of elem ments. Thhe next experim ment was aim med at the inveestigation of the t chemisorpption process of o HsO4 on a NaOH surfacce. It was con nducted by Zw weidorf et al. at a the GSI in 2004 [67]. In n this experim ment the same fusion reaction 26Mg + 248Cm and the saame beam eneergy as in the previous experiment [65] was w used. Thee setup

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for chemical separation of Hs in the form of HsO4 was also similar. The only differences were the use of a moisturized carrier gas and a detection system consisting of 16 silicon PIN-photodiodes facing stainless steel plates with a layer of NaOH. The volatile Os and Hs tetroxides were transported to the detection setup, where they were deposited on the NaOH surface, and their decays were detected by the PIN diodes. In analogy to OsO4, which forms an osmate Na2[OsO4(OH)2] with aqueous NaOH, presumably hassate Na2[HsO4(OH)2] was formed on the surface [67]. Altogether six decay chains were assigned to the decay of Hs. However, due to the 2ʌ detection geometry and the resulting low detection efficiency, mostly incomplete decay chains were detected. In addition, the energy of Į-particles was partially degraded in the alkali layer, resulting in a resolution of 80-130 keV (FWHM). For these reasons an unambiguous assignment of the observed decay chains was difficult. These two Hs chemistry experiments were an excellent example of the very effective combination of on-line gas phase chemical separation and detection allowing chemistry studies with SHE which can be produced on a level of one atom per day. They opened a new perspective for nuclear reaction studies and a search for new isotopes in the region around 270Hs after chemical separation. 6.2. Search for new Hs isotopes in the chemistry experiments A significantly longer half-live of the deformed doubly magic nucleus 270Hs compared to its even-even neighbours was predicted due to the stabilization at the N = 162 nuclear shell [68]. The experimental measurement of such differences would give strong evidence for the location of the shell closure. The used nuclear reaction 26Mg + 248Cm for the first Hs chemistry experiments is one of the possible reactions allowing synthesis of 270Hs. This and three other fusion reactions leading to the compound nucleus [274Hs] were theoretically studied by Liu and Bao [69] predicting the highest cross section for the reaction 48Ca + 226 Ra. The 4n-evaporation cross section for reaction 36S+ 238U was predicted to be significantly higher than that for the reaction 26Mg + 248Cm. These two reactions have been investigated at GSI Darmstadt by a collaboration led by the TU Munich group. The main goal of the research was investigating the decay properties of the nuclides 269,270Hs and their daughters, nuclear reaction studies and the search for new isotopes in the region of interest using an improved chemical separation and detection system. The 248 Cm(26Mg, xn)274íxHs has been investigated, while Hs isotopes were chemically isolated in form of the volatile tetroxide HsO4. To accomplish this goal, the new effective chemical apparatus for the separation and detection COMPACT has been built (Figure 2) [70]. The high overall efficiency of COMPACT allowed systematic investigations of the decay properties of Hs nuclei with reasonable statistics on a level of one event per day at the maximum production cross section of a few picobarns only. Altogether 26 genetically linked nuclear decay chains originating from Hs nuclei have been observed. The known decay properties of 269Hs and its daughters served to localize the 5n excitation function, thus allowing the assignment of newly observed decay signatures to 270Hs formed in the 4n channel and to 271 Hs formed in the 3n channel [70, 71]. Based on 12 decay chains originating from 269Hs, the decay properties of this Hs isotope and its daughters were refined. The existence of two isomeric states of 261Rf and 265Sg has been con¿rmed, which necessitated reconsidering the attribution of decay chains in previous experiments. The decay properties of 266Sg, the daughter of 270Hs, were measured for the first time. In previous experiments [65-67], several decay chains were assigned to 270Hs based on the decay properties for 266Sg and 262Rf as they were known at the time. These were reexamined in 2008 and reassigned to isomeric states in 265Sg and 261Rf, respectively [46]. The QĮ of the even-even isotope 270Hs, experimentally measured for the first time in the Hs chemistry experiments provided a proof for the predicted deformed shell around Z = 108 and N = 162. The increased stability leads to local minimum at the N = 162 neutron shell and a large difference in QĮ values between Ds and Hs isotones, see Figure 18a [70]. The decay of 271Hs was also observed for the first time [71]. The Į-particle energy from the decay of 271Hs is higher than that from 270Hs, demonstrating the stabilizing effect of the closed shell at N = 162. All decay properties for three Hs

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isotoppes and their daughters d are summarized in Figure 18bb. However, thhe measurement of the hallf-lives of Hss isotopes in TC experimeents was not possible duee to the conttinuous separaation and dettection techniique. 269Hs 9.7 +−9.7 3.2 s

265Sg 8.5 s 14.4 s

8.85 261Rf R 68 s 2 2.6 s

8.28

(a)

8.51; SF

8.69 9

271Hs H ~ 4* 4 s

8.95; 9.12 9.23

270Hs 7.6 +−4.9 2.2 s

8 8.93±0.10

9.13; 9.30

266Sg 0.30 +−0.11 0.06 s

267Sg 84 +−58 24 s

SF

8 8.20; SF

263Rf 8 +−40 4 s

SF

(b)

Fig. 18. (a) Comparison n of QĮ values from m theoretical calcculations [72] andd experimental daata [73-75] for ev ven-even nuclei. The T QĮ value oof 270Hs is depicteed with an open sqquare. (b) Decay properties of 269-2271Hs and their daaughters [76]. 2 Thhe measuremennt of the prodduction cross sections of thhe reaction 248Cm( C 26Mg,xn)274íx Hs at ¿vee beam energies allowed the evaluatioon of excitattion functionns for the 3--5n evaporatiion channels. This f experimeental data on fu fusion-neutron n-evaporation excitation funnctions measuurement is uniique, as only few are avvailable so farr for reactionss producing SHE S in hot fussion reactionss with cross seections at a leevel of one piicobarn, especcially for SHE E separated by y chemical meeans. Thhe decay prop perties of 270Hs H and its dauughter 266Sg have h been con nfirmed at thee Dubna Gas-Filled Recoiil Separator where w the reacction 226Ra(48Ca,xn)274íxHss was studied. The productt of 4n evapooration 6+−137 pb, channnel, 270Hs, wass synthesized at an excitation energy of E* = 41 MeV V with a crosss section of 16 +4.9 and thhe half-life haas been deterrmined, T1/2= 7.6−2.2 s [77], confirming th he higher preddicted cross section s [69, 78]. 7 Howeverr, for the prodducts of 3n and 5n channeels, only prodduction cross section limitss were obtainned. The fusio on reaction 2338U(36S,xn)2744−xHs was invvestigated in a chemistry experiment e wiith the COM MPACT detecttor. Only onee decay chainn from 270Hs was observeed at 51-MeV V excitation energy e +2.6 resulting in a produuction cross seection of 0.8−0.7 pb [79], muuch lower thann predicted in [69]. he efficiency in chemical experiments e iss strongly limiited by Unnlike the separration by physsical means, th the hhalf-life of thhe nuclei undder study. Th he search for the unknown even-even nucleus 268Hs H was t nuclear fuusion reaction 248Cm(25Mg,55n)268Hs. Acccording underrtaken in a cheemistry experriment using the to theeoretical prediictions 268Hs should s decay by Į-particle emission withh a half-life of o less than 1 s [80]. This ssearch experim ment was senssitive to volatiile species thaat can be transsported to the detector withhin 1 s. Howeever, the expeeriment was not n sensitive enough e to deteect decays froom 268Hs [81]. A one eventt cross sectioon limit of 1 pb was reachhed for Hs isootopes with half-lives h of about a one seccond in a twoo week experiment. This isotope was discovered lateer in an experriment perforrmed at the seeparator SHIP P (GSI +1.8 mstadt), one decay from 268H Hs was observeed with a halff-life of 0.38−0.17 s [75]. Darm

6.3. Evaluation E of thermodynami t ic properties of o HsO4 from recent experim ments Thhe measuremen nt of the adsoorption behavior of HsO4 was not a prioriity of these stu udies. Neverthheless, the diistribution off 26 detected Hs decays along a with itts lighter hom molog Os in the TC channnel of COM MPACT alloweed evaluating −ΔHads-valuees. Two differrent types of surfaces in thhe chromatoggraphic channnel have been n used in thesse studies: i) PIN diodes coated c with 30 0-50 nm of gold; g ii) PIN diodes coatedd with a layeer of aluminiuum of the sam me thickness having Al2O3 on the surfaace. The adsoorption

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30

Experiments:

20

rel. yield 172,173OsO4

25

0

number of events 269HsO4

Temperature

-20 -40

20

15

Monte-Carlo Simulation: OsO4 - ΔH = 48 kJ/mol HsO4

-60 -80

- ΔH = 56 kJ/mol

o Temperature, C

Number of events / Rel. yield, %

interaction of HsO4 and OsO4 was observed to be not dependent on the applied inert surface material at the given experimental sensitivities. The distribution of the detected Hs events along the detector is shown in Figure 19 in comparison with the deposition peak of OsO4 and the simulated distribution of HsO4. The distribution of the Hs events along the detector is rather wide compared to the Hs deposition peak reported in [65]. This can be explained by a much higher gas flow velocity in the detector channel of COMPACT and a not so steep temperature gradient in the first half of the detector. Using a Monte Carlo model of mobile adsorption [15] in a square channel, −ΔHads(HsO4) = 56 ± 2 kJ⋅mol-1 and −ΔHads(OsO4) = 48 ± 2 kJ⋅mol−1 have been evaluated. These values are 8 kJ/mol higher compared with the ones obtained in the first chromatography experiment where HsO4 and OsO4 were adsorbed on Si3N4 surface [65]. The possible reason for his difference is the used Monte Carlo simulation code for the determination of −ΔHads; in the first experiment the gas flow in the detector channel was described as a flow through a number of parallel cylindrical columns, whereas the correct parametrization for the gas flow in a rectangular channel was used in later simulations.

-100

10

-120 -140

5

-160 0

-180 2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

32

Detector number / Column length, cm Fig. 19. Thermochromatogram of HsO4 and OsO4 measured with the COMPACT detector set-up.

Taking into account the high symmetry of the MO4 molecule (M = Ru, Os, Hs), a pure van der Waals mechanism was proposed for describing the interaction between the tetroxide molecule and the surface of the adsorbent [82]. In this case the dielectric constant and the ionization potential of an adsorbent influence the strength of dispersion forces. The interaction between two noble gas atoms or non-polar molecules due to dispersion forces can be described by the London equation [83] for induced dipoles: E ( x) = −

C1 x6

(1)

where C1 is a specific constant for given interacting atoms or molecules with polarizability Įi and ionization potential IPi lying at a distance x, C1 =

3 α 1α 2 ( IP1 ⋅ IP2 ) 2 ( IP1 + IP2 )

(2)

Author name / Systems Engineering Procedia 00 (2015) 000–000

23

The total interaction between a molecule and a slab of infinite extent and depth can be obtained by summation over all molecule-molecular units of the slab interactions. This summation can be replaced by a triple integration using the simple dispersion formula (1). The triple integration gives: §π · − ¨ ¸ NC1 6 E ( X ) = © ¹3 x

(3)

where x is the distance between centers of the interacting atom or molecule and the surface plane and N is the number of interacting atoms or molecules per 1 cm3. A very important parameter for calculating dispersion forces is the distance between interacting atoms or molecules and the surface. It is especially difficult to precisely define a distance between an adsorbed atom or molecule and the surface plane of an eff adsorbent. In the approximation one can use effective van der Waals radii RVdW of atoms or molecules as the distance between the adsorbed species and the solid for adsorption enthalpy calculations [84]. For noble gas dimers, a van der Waals radius of a noble gas atom is defined as a half of the interatomic distance (d), RVdW = d/2, and thus, the sublimation enthalpy ǻHsub is equal to ½Ediss, because the energy needed for separation is divided between the two atoms. However, the determination of van der Waals radii is not always trivial, especially for molecules, but they can be estimated from crystallographic data, as a half of the mean interatomic distance between central atoms of molecules. In crystals, atoms (or eff molecules) are packed as spheres with an effective RVdW in a lattice. Each atom (molecule) has a different number of neighbours (n) depending on the lattice type. This multiple surrounding results in a significant energy gain increasing the sublimation enthalpy of crystalloids by a factor n compared to the corresponding dimer. For example, in the cubic face centered lattice of noble gases each atom has 12 neighbours in the crystal. Thus, for a solid to gas transition of noble gas crystals, the molar heat of sublimation is equal to six times the dissociation energy (Ediss) of the corresponding dimers: ǻHsub = 12 × (½(Ediss NA)), where NA is the Avogadro constant. By applying this approach, one can calculate the dissociation energy values according to equations (1) and (2), taking the know determined values for the eff polarizability and for the first ionization potential, and the estimated values of RVdW from the crystallographic data. In Figure 20, sublimation enthalpy values for noble gases and selected non-polar symmetric molecules calculated by this approach are shown versus the known experimental values. The values of van der Waals radii for noble gases are taken from [85, 86], and effective van der Waals eff radii RVdW of selected non-polar molecules and tetroxides are deduced from crystallographic data [87-91]. While noble gases have a cubic face-centered crystal structure, the tetroxide molecules MO4 (M = Ru, Os) are arranged in a monoclinic lattice. In both cases one atom or molecule has 12 neighbours at interatomic (-molecular) distance. The values of the sublimation enthalpy of molecular crystals, SF6, CH4, CF4 and OsO4 (blue symbols), can be perfectly fitted by a line obtained as linear approximation for noble gases (black symbols). The determination of the distance between an adsorbed molecule and the surface plane is even more problematic, because the molecules have no ideal spherical shape. However, effective van der Waals eff radii, RVdW , estimated from crystallographic data can be taken again as the distance between a physically adsorbed molecule and the solid plane. The adsorption enthalpy values for noble gases and selected nonpolar molecules adsorbed on quartz and ice were calculated according to equation (3) and compared with experimental values. The calculated values for the adsorption on quartz and ice are very similar and in very good agreement with values obtained in experiments in all cases. In many works the empirical correlation between −ΔHads and ΔHsub has been described by a linear approximation [58, 92]. Considering that ΔHsub is proportional to Ediss, −ΔHads should be proportional to (Eads)2. The correlation between eff −ΔHads-values calculated for noble gases and selected non-polar molecules on the basis the effective RVdW 1/2 and the experimental values of ΔHsub are shown in Figure 21. The data were fitted with a function y ~ x .

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Fig. 20. Correlation between calculated and experimental values of the sublimation enthalpy.

Fig. 21. Correlation between calculated adsorption enthalpies for noble gases and non-polar molecules on quartz and ice and their experimental sublimation enthalpies.

Author name / Systems Engineering Procedia 00 (2015) 000–000

7. Copernicium (Cn, element 112) 7.1. Production and discovery of copernicium Copernicium (Cn, element 112) was discovered at the Gesellschaft für Schwerionenforschung (GSI), Darmstadt, Germany in 1996 [93]. It was produced artificially in the nuclear fusion reaction of 70Zn with 208 Pb leading to the isotope with mass number 277. The measured half-life of about 200 μs was too short for its chemical investigation. Pioneering experiments performed at FLNR, Dubna, Russia using the nuclear fusion of 48Ca with actinide targets lead in recent years to the discovery of four new, more neutron-rich isotopes of Cn, which are partly long-lived enough for their chemical investigation [94]. Thus, element Cn is most efficiently produced in the 48Ca + 242,244Pu fusion reactions, where the α-decay of the primarily produced isotopes of flerovium (Fl, element 114), 287Fl and 289Fl, leads to Cn isotopes with the mass numbers 283 and 285, having half-lives of 3.8 s and 29 s, respectively. Cross sections of 58 pb have been reported for the production of these isotopes [94, 95] allowing production rates of about 1 atom per day at recently available heavy ion accelerators. 7.2. Predictions of chemical properties of copernicium The ordering of elements according to the Periodic Table suggests Cn to be an element of group 12 with the lighter homologues Zn, Cd, and Hg. This group of volatile metals culminating in the liquid metal Hg shows remarkable trends in chemical properties. Whereas Zn, the least volatile member of group 12, and Cd are quite reactive metals dissolving in diluted acids and Zn even in weak bases, Hg shows quite a high chemical resistivity to both, acids and bases. Despite this inertness as an element also an exciting chemistry with other elements is observed. Most exciting are here the intermetallic compounds, known as amalgams, the highly toxic organo-mercury compounds, and compounds stabilizing the Hg-Hg2+ ion, such as calomel Hg2Cl2. Here, Hg reveals a metallic bond between two mercury ions, which is a property unseen in the lighter homologues of group 12. There are chemical effects showing up nicely in this group, which are disrupting or enhancing trends established by the periodic laws if moving down the groups of the Periodic Table towards the heavier elements with higher atomic numbers and higher nuclear charges (Z). These so-called relativistic effects induced by the nuclear charge (Z) of the atoms are increasingly influencing the electronic structure of atoms (for an early review see [96]). Thus, for Cn having a predicted ground state configuration of [Rn]5f146d107s2 the electrons in the outer spherical 7s orbitals, which have a high probability to occur in the close vicinity of the highly charged nucleus are moving with velocities at considerable fractions of the speed of light. This is the so-called primary relativistic effect leading to a relativistic mass increase and thus to a shrinkage of the orbitals and their stronger binding to the nucleus. Hence, 7s-orbitals are less available to the world of chemical reactions. This primary effect has a secondary impact. The electron with larger angular momentum, i.e. Cn’s d and f electrons, perceive a smaller effective nuclear charge, since the primary effect leads to an increased shielding of the nucleus. This leads in turn to a weaker binding of the electrons filling the d and f orbitals which have larger angular momenta. A strong spin-orbit coupling revealed by electrons exposed to high nuclear charges represents a third effect complicating the prediction of chemical properties. Thus, the difficult absolute quantification of relativistic effects and electron correlation represent still the challenge to modern abinitio quantum chemical models. Therefore, the prediction of chemical properties for Cn shows a large spread. As a first guess for this prediction chemists use extrapolative methods to predict the unknown properties on the basis of the trends observed among the lighter homologues of the same group in the Periodic Table. These predictions include to some extent relativistic effects since they are also expressed in the lighter elements. Thus, Cn is predicted to reveal a noble metallic character with a chemical inertness clearly exceeding the one of Hg leading to an even more increased volatility [97]. Relativistic atomic calculations of Cn suggest an increased chemical stability of the elemental atomic state for Cn and

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some models predict even a noble-gas like inertness with exceptionally high promotion energies (Eprom) suggesting chemical properties similar to Rn [18] . Relativistic calculations of solid element 112 predicted a semiconductor-like band structure and hence, a non-noble-gas like behavior [98]. These strongly diverse predictions require experimental data, used as benchmark for the applied models. The experiment shall be able to distinguish between a noble volatile metal and a noble-gas like element. For this purpose the investigation of the gas adsorption properties of element 112 on noble metal surfaces was suggested in [99]. Theoretically, the adsorption interaction of Cn on metals was assessed extending the semi-empirical macroscopic Miedema approach of solid metal solution [100, 101] to the thermodynamic description of adsorption [102]. The physisorption interaction was estimated for the Cn-gold interaction [102] from the assumption of a noble-gas like behavior of Cn using an extension of an inert gas adhesion model suggested in [103]. Later on, ab-initio quantum chemical models were used to describe the interaction of Cn with noble metals. A compilation of predictions of chemical properties related to elemental Cn and its atomic interaction with metal surfaces is presented in Table 3. Despite showing the largest metallic interactions with Cu, Pd, and Pt, model experiments related to the adsorption of Rn [11] and Hg [104] on surfaces of these elements revealed, that these elements do not provide stable enough surfaces for long-term experiments envisaged with Cn due to surface oxidation. It was observed that gold surfaces are best suited for the demanding experiments. Table 3 Predicted thermodynamic data for elemental Cn and its interaction with metal surfaces using theoretical models based on extrapolations of trends observed in group 12 of the Periodic Table and based on ab-initio relativistic quantum chemical models. Interaction with

ΔHsubl

−ΔHads

De

Eprom

α

IP

(kJ⋅mol−1)

(kJ⋅mol−1)

(kJ⋅mol−1)

(eV)

(a.u.)

(eV)

Ref. [18]

8.57

Cn

[97]

22

[105]

11.97

[106]

6.86 23.57-29.19

[107]

27.66

[86] [98]

109 Quartz Au

43.5-46.7

[106]

41-43

[86]

68-83

0.36

[106] [108]

14-62 53-90

0.39

[109]

12

[102]

27

[11]

Pt

46

[102]

Pd

87

[102]

Rh

57

[102]

Ag

44

[102]

Cu

79

[102]

Ir

49

[102]

Ni

36

ΔHsubl: sublimation enthalpy; ΔHads: adsorption enthalpy, Eprom: promotion energy (s2ĺsp), α: dipole polarizability, IP: ionization potential

[102] De:

dissociation

energy

of

dimers,

Author name / Systems Engineering Procedia 00 (2015) 000–000

7.3. Experimental chemical identification of copernicium The very first experiments seeking for gas phase chemical properties of Cn have been performed at FLNR Dubna in 2001 [110, 111] and at GSI Darmstadt in 2002-2003 using COLD [112]. These experiments relied on the initial reports of the production of a long-lived isotope of Cn with the mass number 283 in the nuclear fusion reaction of 48Ca and 238U in the “3n”-evaporation channel having a halflife of 3 min and decaying solely by spontaneous fission (SF) [113]. Interestingly, both experiments observed an effect of a weak interaction of Cn with noble metal surfaces. However, the later reported different decay properties for 283Cn (T1/2=4 s, Eα=9.5 MeV, 9.3 MeV) from two independent experiments [114, 115] heavily questioned the initial chemical results, which have been obtained at transport times exceeding 30 s. Between 2001-2007 an increasing amount of data regarding the production of SHE in 48 Ca induced nuclear fusion reactions appeared (for a review see [116]). The reports revealed largest cross sections for the formation of 283Cn and 285Cn in the nuclear fusion of 48Ca with 242Pu and 244Pu after the αdecay of the initially produced 287Fl and 289Fl, respectively. Therefore, all later chemical studies were focusing on these production paths. In 2006 further gas adsorption experiments with 283Cn were performed using COLD. A technology was developed at the Paul Scherrer Institute and University of Bern to cover silicon based PIN diode detectors with thin layers of gold [104]. Pairs of PIN diode detectors of 1 cm x 1 cm size were used to produce sandwich-type modules (one side covered with gold layers). Put into an array, 32 such modules formed a square TC channel of about 32 cm length with an open channel cross section of 0.16 cm x 1.0 cm. Thus, the COLD TC channel could be modified to provide a stationary chromatography surface of metallic gold ready for in-situ α-particle and SF-fragment spectroscopy in a temperature interval from 35°C down to −180°C (see Figure 22).

Fig. 22. COLD TC channel (right) made out of 32 modules built from exchangeable sandwiched pairs of silicon PIN detectors (insert). The gas enters through a conical Teflon® coated channel mounted on the left side of COLD, intended to evenly distribute the gas velocity in the channel to nearly laminar flow conditions from the very first diode sandwich on.

The first successful experiments with Cn performed in 2006 and 2007 [2, 3] used the nuclear fusion 283 α Cn. Targets of 1.8 mg/cm2 242Pu, which consisted of 242PuO2 painted reaction 242Pu(48Ca,3n)287Fl ⎯⎯→ on 2 μm Ti thin foils, were irradiated at center-of-target energies (ECOT) of 240 MeV. The principle of the applied experimental setup is depicted in Figure 23. The products were recoiling out of the target with the

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momentum of the beam and were thermalized in a quartz covered recoil chamber flushed by a He/Ar mixture. In this inert carrier gas stream the volatile products were transported through an aerosol particle filter consisting of a quartz wool plug heated to temperature of 950°C. Subsequently, in a second separation step the volatility of the products was again challenged by passing the product stream through a 4-m long PFA®-Teflon capillary held either at room temperature or at 70°C. Only the products able to pass this separation step within their life time were injected into the COLD TC where they were separated according to their interaction with the gold surface in a temperature gradient reaching from 35°C down to −180°C. The remaining carrier gas was looped back to the recoil chamber after passing it through a thorough cleaning setup consisting of a Sicapent® cartridge and a Ta metal getter heated to 1000°C, to remove all reactive contaminations such as water and oxygen to sub ppm levels. The on-line measured dew point of <−101°C revealed water contamination levels of less than 0.1 ppm. The Cn isotope with the mass number 283 has a unique decay scheme. It decays with a half-life of about 3.8 s by α-particle emission (Eα=9.54, 9.35 MeV) to 279Ds, that undergoes spontaneous-fission (SF) decay with a half-life of 200 ms. In the TC experiments in 2006 and 2007 five events representing this decay scheme have been identified (see Figure 24). This observation represented the first independent confirmation for the formation of SHE in 48Ca induced nuclear fusion reactions with actinides as pioneered at FLNR, Dubna, Russia. The estimated random rates increased from <1×10-5 for the first two events measured in experimental campaign 2006, where otherwise no other SF decay was detected, to 5×10−2 estimated for the three events observed in the second experimental campaign in 2007. There, at higher gas flow rates, the aerosol particle filtering was not as efficient leading to overall 17 SF decays detected throughout this campaign. Figure 25 shows the deposition pattern that is explained by an Au +4 adsorption interaction enthalpy of Cn with gold surfaces of − ΔH ads (Cn) = 52−3 kJ⋅mol−1 (68% c.i.) [2, 3].

Heavy Ion beam Actinide Target Aerosol filter + getter

Pump

Getter Buffer

Recoilchamber

Sicapent® Dryingunit

Gas flow liq. N2

T=35°C

COLD

Fig. 23. The principal scheme of gas chromatography experiments with Cn adopted from [2].

Author name / Systems Engineering Procedia 00 (2015) 000–000

on ice

on gold 283 Cn

283 Cn

29

283 Cn

283 Cn

283 Cn

-28° 28°C

-5°C

-21° 21°C

-39° 39°C

-124° 124°C

9.37 MeV

9.47 MeV

9.52 MeV

9.52 MeV

9.35 MeV

279 Ds

279 Ds

279 Ds

279 Ds

279 Ds

τ: 0.592 s SF

τ: 0.536 s SF

τ: 0.072 s SF

τ: 0.088 s SF

τ: 0.773 s SF

108+123 MeV

127+105 MeV

112+n.d. MeV

94+51 MeV

85+12 MeV

Fig. 24. Decay chains of 283Cn observed in the chemistry experiments in 2006/2007, adopted from [3]. The observed Cn atoms have been shown to sensitively respond onto changed experimental conditions [3] (see Figure 25). The experiments were started at a carrier gas flow of 890 mL⋅min-1. A temperature gradient starting at −24°C down to −184°C was established in COLD. At these conditions the first event was observed in detector number #2 held at −28°C. The upper temperature of the gradient was increased to 35°C. The second event was detected on detector number #7 held at −5°C. In the second experimental campaign the gas flow was increased to 1500 mL⋅min-1 aiming at higher transport efficiency. At these conditions, events have been observed at detectors number #11 (−21°C) and #14 (−39°C). The third atom was transported and deposited on the ice-covered detector 26 (−124°C). 30

50

ice

gold 20

0 -50

283

Cn

10

-100

0

-200 2

4

6

8 10 12 14 16 18 20 22 24 26 28 30 32

30

gold

ice

50 0

20 283

-50

Cn

-100

10

-150 0

Temperature, °C

Rel. yield / detector, %

-150

-200 2

4

6

8 10 12 14 16 18 20 22 24 26 28 30 32

30

gold

ice

50 0

20

-50

283

Cn

10

-100 -150

0

-200 2

4

6

8 10 12 14 16 18 20 22 24 26 28 30 32

Detector #

Fig. 25. Events of 283Cn (black arrows) observed at three different experimental conditions compared to Monte-Carlo simulations (red lines) using a model suggested in [15] and considering the given experimental conditions using as −ΔHadsAu(Cn) = 52 kJ⋅mol-1. Figure adopted from [3].

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7.4. Experimental confirmations of the observed chemical properties of copernicium Between 2007 and 2013 several experimental campaigns have been performed at U400 accelerator at FLNR Dubna, Russia and at the Gesellschaft für Schwerionenforschung (GSI) Darmstadt, Germany aiming at the chemical identification of Fl, that is covered by a chapter later herein. We present here the data measured in these campaigns, which we assume undoubtedly being related to the chemical behavior of Cn. Thus, in 2007 and 2008 experiments using 244Pu targets were performed. In 2007 the initial 285 α Cn experimental setup was used. The isotope of Cn produced in the reaction 244Pu(48Ca,3n)289Fl ⎯ ⎯→ 281 decays by an α-particle emission of 9.19 MeV followed by SF decay of Ds having a half-live of 12.7 s. 281 Ds has an additional α-decay branch with an decay energy of 8.73 MeV followed very shortly by the SF decay of 277Hs (T1/2=3 ms). However, the background induced by the chemical transport of Po isotopes presumably as PoH2 [117] and as progenies of transported isotopes of Rn produced in multi-nucleon transfer reactions together with the non-zero SF background from actinide isotopes presumably transported by aerosol particles able to pass the high temperature filter prevented an unambiguous identification of 285Cn. Therefore, in 2008 experiments were prepared to use physical pre-separation with the Dubna Gasfilled Recoil Separator (DGFRS) prior to chemistry to deplete considerably the background [118]. A sketch of the applied setup is depicted in Figure 26. The 48Ca beam (1) passes through a rotating target assembly consisting of a Ti vacuum window (2) and a target backing (3) before entering the target material, 250-300 μg/cm2 244PuO2, which was electroplated on to 2 μm thin Ti backing foil. The arc shaped segments of these targets were mounted on a rotating target wheel setup. The beam (1) and nuclear reaction products (4) entering the cavity of the DGFRS magnet array (5: D, Q1,Q2) (41 Pa H2) are spatially separated in the gas-filled dipole magnet (D). The beam (low rigidity ions) is redirected to the Ta beam stop (6). Only nuclear reaction products having the required magnetic rigidity are focused by a quadrupole lense (Q1,Q2) onto the focal plane (7) of the separator. Here, the products pass through a 3 ȝm thick Mylar® window (8) mounted on a honey comb steel grid (80% transmissivity) and are stopped in the Ar filled recoil chamber (9). The carrier gas Ar (10) transports the thermalized products through a 4-m-long perfluoroalkoxy PFA®-Teflon capillary (1.56 mm inner diameter) (11) to a quartz column containing tantalum metal and a quartz wool plug (both heated up to 850°C) (12), serving as a getter/trap for water, oxygen, and other reactive species. Volatile and inert nuclear reaction products pass this getter and are transferred further through a 40 cm PFA capillary (13) (2 mm inner diameter) to the TC detection system COLD (14). The gas flow within the closed loop system was established by metal bellow pumps (16). The gas loop consists of a drying unit (15) for additional purification of the gas and a buffer volume (17) to prevent pressure waves and variations of the flow rate. Between the separator and COLD a concrete wall (18) of 2 m thickness shielded the sensitive spectroscopy electronics of the COLD from fast particles produced by beam-induced nuclear processes. One decay chain was observed that can be attributed unambiguously to the decay of 285Cn and its daughter 281Ds (see Figure 26, panel a ). The number of expected random correlations was estimated as 1.8·10−3. This decay chain was detected in the COLD on detector number #19 held at a temperature of −93°C. Two non-correlated SF events have been observed on detector number 4 and 6, held at temperatures of −8°C and −16°C, respectively. With respect to the included physical preseparation these non-specific decays likely belong to isotopes of Cn. However, these two SF-events have been excluded from the following discussion of chemical properties. In September 2009 at GSI Darmstadt a similar experiment was conducted using the TASCA separator [119] using a newly developed cryo-on-line detector, the Cryo-Online-Multidetector for Physics and Chemistry of Transactinides (COMPACT). The interaction of Fl and Cn on gold surfaces has been investigated [5]. The open cross section of this chromatography detector channel is 0.6 mm x 1 cm. This experiment is detailed in the following section about the investigations of Fl. However, it can be noted here that one decay chain attributed to 285Cn has been observed in the temperature gradient applied along

Author name / Systems Engineering Procedia 00 (2015) 000–000

31

COMPACT at −32°C (see Figure 27, panel a) using as carrier gas a mixture of 30% Ar and 70% He flowing at a rate of about 1500 mL⋅min-1. For a fourth experimental campaign at FLNR Dubna in 2009 a 1.5 mg/cm2 242Pu target was prepared by the painting technique in the form of PuO2 with a natural neodymium admixture of about 50 μg onto a 1.7 μm Rh backing foil. Pure Ar was used as carrier gas instead of mixtures of He and Ar, at a flow rate of 1960 mL⋅min-1. Otherwise the experimental conditions were the same as applied in 2007. This decision was taken to safely thermalize all transactinide products in the small volume of the recoil chamber. The overall beam dose on the target was 6.1×1018 48Ca. In the first part a beam dose of 3.4×1018 was applied with a ECOT of 240±2 MeV. During this part a gold filter was installed between the heated aerosol filter and the COLD. The temperature of 10°C ensured that no Cn, Hg, and At was able to pass this filter. No decay chains related to 283Cn were observed. In the second part this filter was heated to 100°C. Still the 182-184 Hg produced in the nat.Nd with 48Ca reaction and most of the At isotopes (211At) produced in nuclear transfer reactions of the projectile with the actinide target was still retained. After having applied 2.7×1018 48 Ca particles to the same target one decay chain related to 283Cn (see Figure 27, panel b) was detected on detector number #6 held at a temperature of −12°C.

1 2

4 D 5

3 6 Q1 Q2

11

7 8 10 18 17

10

12

16 13

-160°C

10°C

15

9

4 10

14

Fig. 26. Schematic of the experimental setup using physical preseparation with the DGFRS adopted from [118]. The numbering is described in the text.

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a)

19 (COLD)

Det#

285Cn

-93°C

-32°C

9.2 MeV

9.11 MeV

τ: 3.38 s SF

τ: 25.3 s SF

106 + 44 MeV

37 + 78 MeV

3.1E-6

1.3E-6

-12°C 9.3 MeV

61 + n.d. MeV

NR

14

285Cn

285Cn

(1)

30

30

(2)

285Cn

6E-3

(3)

285Cn

10

31

(4)

285Cn

(5)

285Cn

(6)

5°C

-62°C

-165°C

-165°C

-168°C

-39°C

9.05MeV

8.94 MeV

9.31 MeV

9.07 MeV

8.99MeV

8.96 MeV

281Ds

281Ds

281Ds

281Ds

281Ds

281Ds

τ: 45 s

τ: 2.78 s

τ: 94 s

τ: 44.2 s

τ: 7 s

τ: 7.63 s

SF 41+n.d.

SF 53+n.d.

SF 74+15

SF 80+n.d.

SF 81+88

NR

SF 81+50

1.5

0.3 Det#

11 285Cn

(7)

0.4

16

10

285Cn

283Cn

(8)

9

(9)

(10)

-72°C

-33°C

-39°C

9.2 MeV

9.38 MeV

9.54 MeV

9.57 MeV

281Ds

281Ds

279Ds

279Ds

τ: 1.61 s

τ: 0.31 s

τ: 0.005 s

SF 86+86

0.51

283Cn

-45°C

τ: 23.01 s SF 53+ n.d.

6 283Cn

τ: 0.073 s SF

1

Det#

Det#

279Ds

279Ds

c)

NR

285Cn

279Ds

NR

b)

52 (COMPACT)

SF 103+20

SF 71+n.d.

8E-3

Fig. 27. Additional decay chains related to 283,285Cn observed: a) in the experimental campaigns at FLNR (left) in 2008 and at GSI in 2009 (right) in experiments combining gas phase chemistry and physical pre-separation; b) in the experimental campaign at FLNR in 2009 using a 242Pu target having a 1.7 μm Rh backing; c) in the experimental campaign at FLNR in 2013 using the mixed 242,244Pu targets. The detector numbers (Det#) are given together with the random rates NR, which differ dependent on the used target and on the deposition position in the COLD due to varied background induced by transported and deposited isotopes of Po and Rn; the deposition temperatures are indicated in blue.

In 2013 a fifth experiment was conducted at FLNR Dubna. Two mixed Pu targets were prepared on 2 μm Ti foil backings by a standard molecular plating procedure out of is n-butanol solution of freshly produced nitrate [120]. The first target had a ratio of 244Pu:242Pu = 2:1. The second target consisted mainly of 244PuO2 with a small 5% admixture of 242PuO2. Both targets had a Pu thickness of 1.2 mg/cm2. The 48 Ca beam energy ECOT was kept at 242±3 MeV. The recoil chamber was increased to a length of 7 cm at a diameter of 4 cm leading to transport times of about 2 s. The quartz aerosol particle filter was prolonged

Author name / Systems Engineering Procedia 00 (2015) 000–000

33

and additional artificial chicanes were included in the used quartz tubes to efficiently deplete the contamination of the detector by actinides braking through the chemistry at the high gas flow rates of about 2 l/min. Precautions were taken to monitor sensitively the contamination level of water and oxygen by using on-line atmospheric pressure mass spectrometry (MKS, CIRRUS® 2) and new dew point meters (Easidew, Michell Instruments®). This allowed to deplete the transport of disturbing Po isotopes, which was observed connected to the presence of water traces in the previous experiments [117]. Thus the irradiation of the target was performed only at conditions where the dew point in the carrier gas was below −100°C and the oxygen content was below the observation limit for the mass spectrometry of 100 ppb. The PIN diode detectors on both sides of the sandwich assembly were covered with thin layers of gold. Otherwise, the same experimental setup with a gas mixture of Ar:He = 50:50 vol-% was used. In this experiment isotopes of 283Cn and 285Cn have been observed as depicted in Figure 27, panel c. Thermochromatographic distribution of the simultaneously observed events of 283Cn and 285Cn depicted in Figure 28 together with Monte-Carlo simulations considering the given experimental Au conditions and using − ΔH ads (Cn) = 52 kJ⋅mol−1 reveal a perfect confirmation of the initially obtained 285 result. Note that Cn has longer half-life by about a factor of seven compared to 283Cn. Thus, its deposition is slightly shifted towards lower temperatures as expected. The measured dew point limit in the carrier gas of −100°C lead to the assumption of an ice coverage in the last third of COLD beginning on detector number #21. From the observed deposition of Cn on this ice layer on detectors number #30 ice (Cn) = 30 kJ⋅mol−1. and #31 we estimated the interaction of Cn with ice as − ΔH ads 24

Temperature

283

Cn simulation Cn simulation 283 obs. Cn 285 obs. Cn

22

18

-50

16 14 12 10

Temperature, °C

20

Yield, %/Det

0

285

-100

8 6 4 2

-150

0 2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

32

Det # Fig. 28. Thermochromatographic deposition of 283,285Cn (bars) as observed in the experimental campaign in 2013 together with the patterns for both isotopes described by a Monte-Carlo Simulation of gas chromatography as suggested in [15] (lines) using −ΔHadsAu(Cn) = 52 kJ⋅mol-1. The vertical line indicates the measured limit of dew point in the used carrier gas of −100°C. Therefore, an ice layer is assumed starting downstream from detector number #21. The simulation assumes −ΔHadsice(Cn) = 30 kJ⋅mol-1. Figure adopted from [121].

7.5. Conclusions on chemical properties of Cn The adsorption interaction potential expressed as the standard adsorption enthalpy of Cn on gold at Au +4 zero surface coverage was deduced as − ΔH ads (Cn) = 52−3 kJ⋅mol−1 and confirmed by additional

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experimental data. This compares well with predictions based on modern relativistic density functional theory suggesting 68-83 kJ⋅mol−1 [106] and 53-90 kJ⋅mol−1 [109]. However, the Miedema model based approach suggested in [102] seems to underestimate the interaction potential with 15 kJ⋅mol−1. Modern relativistic calculations based on density functional theory calculate atomic properties for Cn such as ionization potentials and dipole polarizabilities ([105, 107], see Table 3). Using these properties empirical physisorption models can be applied to estimate the expected interaction of a hypothetical inert-gas like Au (Cninert) = 30±5 kJ⋅mol−1 [11]. The direct comparison of this value to the Cn with gold as − ΔH ads experimentally determined adsorption enthalpy indicates qualitatively that metal bonding must be partially involved in the interaction of Cn with gold, despite of Cn being a very volatile element. The estimated value for the interaction of Cn with ice is in good agreement with physisorption interactions predicted for this element [11, 86]. Properties of macroscopic phases of transactinides can be assessed through empirical correlations Au and macroscopic established between the experimentally assessable microscopic quantity − ΔH ads standard thermodynamic values, e.g. the standard sublimation enthalpy ΔHsubl for a variety of elements [122, 123]. This correlation was updated with the most recently measured data for the adsorption of single atomic species on gold [124-126] and is shown in Figure 29. Thus, the macroscopic volatility expressed Au +4 as its standard sublimation enthalpy can be deduced from this correlation using the − ΔH ads (Cn) = 52−3 −1 −1 kJ⋅mol as ΔHsubl(Cn) = 36±10 kJ⋅mol (68% c.i.). Recent solid state calculations of cohesive energies seem to underestimate the volatility of Cn and overestimate the solid state binding of Cn with Ecoh = 1.13 eV (109 kJ⋅mol−1) [98]. The trends established in the Periodic Table for group 12 are preserved (see Figure 30, [3] ) as already suggested in 1976 [97]. Other chemical properties can be deduced qualitatively from the behavior of Cn as observed in the experiments: 1) at room temperature: No strong reaction with quartz (walls of recoil chamber) and with copper (beam stop) and no considerable retention on PFA®-Teflon (transport tube, entrance channels of cryo-detectors); 2) at 900°C: No strong reaction with quartz and with Ta (getter; aerosol particle filter); 350

300

Tl Po

250

Bi

200

At

Pb

Au

-ΔHads, kJ/mol

In

experimental data weighted least square fit 95% c.i. 2 R =0.987

150

Hg

100 Xe

50

Rn

Kr 0 0

Au

-ΔHads(M) = (1.32±0.05) * ΔHsubl(M) + (4.18±3.68), kJ/mol 50

100

150

200

250

ΔHsubl, kJ/mol Fig. 29. Empirical correlation between the microscopic property adsorption enthalpy on gold and the macroscopic elemental volatility expressed as the standard sublimation enthalpy updated from [121, 123].

Author name / Systems Engineering Procedia 00 (2015) 000–000

35

120 100

Zn

ΔHsubl, kJ/mol

80

Cd 60

Hg

40

Cn

20 0 20

40

60

80

100

120

Z Fig. 30. Trends of volatility of elements in group 12 of the Periodic Table estimated from the observed chemical behavior of copernicium and plotted over the atomic number Z updated from [3].

8. Flerovium (Fl, element 114) 8.1. First observation of Fl in chemistry experiments Already in 1975 the chemical properties of SHE with Z = 112, 114, and 118 became a hot topic in SHE research, despite the fact that the heaviest known element at that time was seaborgium (Z = 106). According to early atomic calculations by Pitzer [18], the promotion energy to the valence state electron configuration s1/22 ĺ sp in Cn and p1/22 ĺ p2 in Fl will not be compensated by the energy gain of the chemical bond formation. He concluded that both Cn and Fl will be very inert, like noble gases, or volatile liquids bound by dispersion forces only. Many theoretical efforts were done in the last decade since isotopes of Cn and Fl with half-lives accessible for chemical investigations were synthesized [94]. +0.8 The up to now longest-lived Fl isotope synthesized is 289Fl with a half-life of 2.1−0.4 s [127]. A comprehensive review of theoretical works devoted to chemical properties of Fl can be found e.g. in [19]. The main conclusion is that Fl is much more inert than all homologs in group 14 and reveals properties of a volatile metal, which can be adsorbed on metals (e.g. gold) upon formation of weak metal bonds. Thus, according to theoretical calculations, the adsorption behavior of Cn and Fl on gold is rather similar. In 2007 two experiments aimed at studying the adsorption of Fl were conducted at FLNR Dubna by a collaboration led by the PSI group. One of these experiments is described in detail in section 7.3 (see Figure 24) of this publication, since it led to the observation of 3 events of 283Cn [3]. Fl isotopes were produced by the irradiation of 242Pu and 244Pu targets with a 48Ca ion beam. Beam doses of 3.8×1018 and 4.5×1018 were accumulated during irradiations of 242Pu and 244Pu targets, respectively. The volatile species were transported from a recoil chamber made of quartz to the detector setup by a gas flow (He:Ar = 1:1, 1.5 l/min) through an 8-m-long PFA®-Teflon capillary (1.56 mm inner diameter) to the TC detector setup COLD [4]. The top side of the detector channel was covered with a thin gold layer, whereas the bottom side had SiO2 coverage. The cold end of the detector channel was held a temperature of ௅170 °C.

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The overall transport time was measured to be 2.2 s. Short-lived 185Hg was simultaneously produced due to a natNd admixture to the target material for monitoring the retention capability of the detector gold surface. Multi-nucleon transfer reaction products, Rn and At isotopes, were also transported to the detector by the gas flow causing a background in the Į-particle spectra. The low water content of 0.1 ppm in the carrier gas resulted in a possible ice formation in the detector channel only below −95 °C. Despite several gas purification stages, an uncontrolled small gas impurity caused a contamination of the Au surface, which led to a lower retention capability for 185Hg after typically one week of operation [4]. Three observed decay chains were assigned to decays originating from Fl isotopes, one starting from +0.16 +0.17 287 Fl (T1/2= 0.48−0.09 s [94]) and two starting from 288Fl (T1/2= 0.69−0.11 s [127]). The transport efficiency from the recoil chamber to the detector setup was estimated as 5% and 15% for 287Fl and 288Fl, respectively. The more long-lived 289Fl could be transported even more efficiently (56%) but background from Į-particles in the region where decays of 289Fl and its daughter 285Cn were expected did not allow the unambiguous identification of decay chains starting from 289Fl [4]. Two decay chains assigned to 287Fl and 288Fl were registered on the detector pair 19 at very low temperatures of ௅88 °C and ௅90 °C, respectively. One decay chain from 288Fl was distributed over two detector pairs 3 and 6, pointing to a possible decay of 288Fl in flight (Figure 31). From the observation of three Fl atoms at the mentioned above positions, presumably adsorbed on the gold surface, the adsorption Au +20 Au +54 enthalpy value of Fl on gold was determined − ΔH ads (Fl) = 34−3 kJ⋅mol-1 or − ΔH ads (Fl) = 34−11 -1 kJ⋅mol for 68% or 95% confidence interval, respectively [4]. The large upper limit at 95% c.i. value Au demonstrates only the fact that volatile species with the adsorption enthalpy on gold up to − ΔH ads = 88 -1 kJ⋅mol could enter the detector channel. However, the authors showed that volatile species with an Au adsorption enthalpy value of − ΔH ads • 50 kJ⋅mol-1 are deposited in a diffusion-controlled adsorption process quantitatively within the first half of the detector channel. The determined experimental results suggested the formation of a weak physisorption bond between atomic Fl and the gold detector surface. The next experiment devoted to Fl chemistry was conducted by the same group at FLNR Dubna using the same nuclear reaction 244Pu(48Ca;3-4n)288,289Fl. The beam dose accumulated on the target during 35 days was 9.7 × 1018 particles. The chemical separation and detection setup was coupled to the Dubna GasFiller Recoil Separator (DGFRS) for the background reduction from volatile multi-nucleon transfer reaction products, like Rn and At (see also section 7.4 of this contribution and Figure 26). This combination allowed a significant reduction of the background in the Į-particle spectra (Figure 32) [118]. One decay chain starting from 285Cn was registered on the detector pair #19 at a temperature of ௅93°C (Figure 27, panel a). The probability that 285Cn could reach this low temperature was estimated to be about 21%, whereas a probability to observe an incomplete chain with the missed Į-particle from 289Fl was estimated to be 28%. Two SF events with coincident fragment detection were observed in detectors 4 and 6 at ௅8 °C and ௅16 °C, respectively, which were tentatively assigned to Fl isotopes or their progenies. In addition, 17 single high energy events over 50 MeV were detected in course of experiment as well and assigned to background from fast particles and/or electronic noise [118]. The additional pre-separation with DGFRS prior to chemical separation, on the one hand, suppressed transfer reaction products and resulted in much cleaner Į-particle spectra, but on the other hand, this combination reduced the overall efficiency of the experiment. The nature of the remaining background in the sum Į-particle spectrum between 6 and 8 MeV, and especially above 8 MeV, was not discussed. However, the number of expected random correlations during the entire experiment was estimated to be 1.8×10௅3. This experiment demonstrated that the sensitivity for the unambiguous identification of rare Fl decay chains can be increased by background reduction, however, other possibilities for a higher overall efficiency have to be found.

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30 20 10

3:

4:

Rel. yield / detector, %

2:

0 15 12 9 6 3 0 15 12 9 6 3 0 15 12 9 6 3 0

185

219

Hg

Rn

28 83

Cn

287

Fl

288

Fl

50 0 -50 -100 -150 -200 50 0 -50 -100 -150 -200 50 0 -50 -100 -150 -200

Temperature, °C C

1:

50 0 -50 -100 -150 -200

2 4 6 8 10 12 14 16 18 20 0 22 24 26 28 30 32

Detector # a Fl in the COL LD detector as ob bserved in experim ments by R. Eichhler et al. Fig. 31. Deposition patteerns of the elemeents Hg, Rn, Cn, and [4]. Forr a detailed discussion see text.

Fig. 32. Comparison of the α-particle sum m spectra from deetector 1 to 27 off the COLD arrayy [3, 128] normaliized to the applied beam p in 20007 [3, 4] without preseparation p (whhite spectrum) an nd with preseparattion (black spectrrum) dose froom experiments performed [118]. B Both experimentss used the nuclearr reaction 48Ca + 244Pu and were peerformed at the same gas flow connditions. The dataa are normallized to the target thickness of 0.444 mg/cm2 244Pu annd 1018 48Ca partiicles.

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Tw wo further Fl atoms a were deetected in a chhemistry expeeriment behind d the gas-filled separator TA ASCA at GS SI Darmstadt [5]. A target wheel with thhree segmentts containing 244Pu targets on Ti backing foils -1 were bbombarded with w a 48Ca10+ beam b of typiccally 2× 1012 particles⋅s p . A beam dose of o 4 × 1018 paarticles was aaccumulated during d 29 dayys. Two TC detectors d COM MPACT withh a similar design to the deetector COLD D, were couplled directly to the exit of TA ASCA, both covered c with a thin Au layeer. The first onne was attachhed to a Recoiil Transfer Chhamber (RTC) via a 2-cm-llong PTFE tuube and kept at a room tempeerature (+21 °C). The second one was connected too the exit of the first COM MPACT via a 30-cm-long PTFE t g gradient from +22 °C to ௅1162 °C capillary (2 mm innner diameter)) (Figure 33). A negative temperature CT detector duuring the lastt three weeks of the experriment, was eestablished aloong the seconnd COMPAC while for three daays only ௅86 °C was reacched due to a weak therm mal contact between b the second s MPACT array and a the cryosttat. COM

Fig. 33: Schematic draw wing of the TASC CA-COMPACT2 arrangement usedd for the gas chroomatographic invvestigation of the he 48Ca beam (1) passed p through th he rotating 244Pu target t (2) assemblly. The volatiliity of Fl and its reeactivity towards a Au surface. Th separattor TASCA consists of one dipole (D), where unwaanted nuclear reacction products (3)) were deflected, and two quadruppole (Q) magnetts. At the exit of TASCA, T a vacuum window (4) separated the low pressure p required in TASCA from the high pressuree appliedd in the recoil trannsfer chamber (RT TC) (5). After paassing the vacuum m window, Fl wass thermalized in thhe gas inside the RTC and waas transported in its i elemental statee with the carrier gas through a 2 cm c long PTFE tubbe (6) into a seriees of two COMPA ACT detectoor arrays (7) connected by a 30 cm m long polytetrafluuoroethylene (PT TFE) capillary (8) (2 mm i.d.). A negative temperatuure gradiennt was applied alo ong COMPACT II I using a liquid nitrogen n cryostat (9) at the exit. Figgure adopted from m [5]. © 2014 Am merican Chemiccal Society.

Too minimize lossses of Fl isottopes due to decay d before they t enter the first detectorr, a RTC madee from PTFE E with a volum me of only 299 cm3 was attaached to the exit e of TASCA A. For this puurpose TASC CA was operatted in the Sm mall Image Mode M having a smaller im mage size at the focal pllane and a reeduced transm mission through the separaator [119]. Th he recoils passsed a 30 × 40 4 mm2 Mylar® window with a thicknness of 3.3 μm m on a stainlless steel supp porting grid with w 80% tran nsmission effiiciency. A traansport time oof 0.81 s was determined with w the shortt-lived isotopees 182Hg and 186Pb produceed in a pulsedd beam regim me by irradiatiions of 142Nd and 144Sm tarrgets with 48Ca C ions, respeectively [5]. The T Pb and Hg H ions were thermalized in i the RTC gaas volume annd flushed outt and transporrted by a gas flow (He:Ar = 7:3, T, where they adsorbed onn the gold surrface in a difffusionflow rrate 1.3 L/miin) to the firsst COMPACT controolled depositioon process. Prrior to the cheemistry experriment, producction rates of Hg and Pb issotopes were measured in the RTC poosition behindd the RTC window. The Pb P and Hg ioons guided thhrough CA were impllanted in a (558 × 58) mm m2 double-sideed silicon strip p detector whhere their Į-pparticle TASC decayys were registeered. In COM MPACT, 27% of Hg atoms and 20% of Pb P atoms enteering the RTC C were observved with an error level of o ± 5%. Thhe distribution of Rn, thee third volatiile element for f the

A Author name / Syystems Engineerinng Procedia 00 (2015) ( 000–000

comparative study was measuredd in COMPA ACT during thee entire experriment – a sm mall amount off 219Rn 227 c t the carrierr gas. The unndesired produucts of to emanaating from a Ac source was added continuously multi--nucleon transsfer reactions were strongly y suppressed by b TASCA. The T small amoounts of non-vvolatile targett-like productss that still reacched the focal plane were not transported d to COMPAC CT by the gas flow. Thhe search proccedure for deccay chains revvealed two coorrelated chaiins, Į௅SF (#1) and Į௅Į௅SF F (#2). Both members of chain c #1 weree observed inn detector pairr #9 in the firrst COMPAC CT. The membbers of t mother nucleus n chain #2 were fouund distributeed over both COMPACT arrays: the Į particle of the f COMPAC CT. The last two t members of the initiatting the chain was detectedd in top detecttor #9 in the first chain were detecteed in detector pair #52 in the t second COMPACT at í32 °C. Theese two chainss were s for SF events revealeed two assignned to the deccay of 288Fl annd 289Fl, respecctively (see Fiigure 34). A search additiional events inn detector pairrs #1 and #64 with coincideent fragment detection. d In total, only thesse four SF evvents were obsserved, all witth two coinciddent fission fraagments.

Fig. 344: Observed correelated decay chainns assigned to Fl. (I)/(II) denote COMPACT I and COMPACT II, reespectively. Righht hand m [5]. side of the boxes: tempeerature of the deteector that registerred the event. Figgure adopted from

Thhe analysis of the sum Į-paarticle spectra revealed thatt the main peaaks originated d from 219Rn and a its 215 211 progeenies, Po an nd Bi. Due tto the extremeely low backgground in the energy e range above 7.5 MeeV, the observved decay chaains are highlyy significant. The T probabilitties for a rand dom origin, unnrelated to thee decay of Fl, are only 6.3·110-6 (chain #1) and 1.3·10-66 (chain #2) [55]. w the two o Fl decay chaains were observed (panel e) together with w the In Figure 35 thee position at which a Hg measuured distributiions (solid barrs) of Pb (pannel b), Hg (paanel c) and Rnn (panel d) aree shown. Pb and interaact strongly wiith gold but haave significanntly different adsorption a entthalpies on go old. The similaarity in the obbserved distribbution patternns points at thhe diffusion-coontrolled natu ure of the adsoorption processs. The diffussion to the waall controls thee process for both b Pb and Hg, H and they adsorb a upon first f contact with w the Au surfacce at the beginnning of COM MPACT I. A lower limit of − ΔH ads > 64 6 kJ⋅mol-1 waas deduced foor both Pb annd Hg [5]. ginating from 288Fl, the SF decay d from 2884Cn was regisstered, after a lifetime of 6550 ms, In chain #1 orig in thee same detectoor pair as the Į decay of thhe mother nucclide 288Fl. Taaking into account the half--life of 284 Cn and the adsoorption enthalppy value for Cn C atoms on gold, 284Cn would w travel at a room tempeerature i COMPACT T I within about 135 ms. Thhe fact, that thhe decay from 284Cn was obbserved 1 cm downstream in F in the adsorrbed state. In chain c #2 origiinating in thee same detectoor pair is indiccative for the decay of 288Fl

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from 2289Fl, the last two memberss, starting from m the daughteer nucleus 285Cn, C were foundd in detector pair p 52 at a teemperature off −32 °C. Appparently, uponn α decay of thhe mother nuccleus, the dauughter 285Cn reecoiled into thhe gas stream. No unambigguous conclusiion about wheere the Fl decaay occurred, in i the adsorbeed state or in flight, can be drawn. Durinng its lifetime of 11.6 s, 285Cn was transpported along the t detector chhannel COMPACT III. As shown in Figure 35 (panel ( e), the observed adssorption positiion for 285Cn agrees into C well with the calcculated depossition pattern for this Cn isotope usin ng the experiimentally meeasured adsorpption enthalpyy [2, 3]. This corroborates our assignmeent of the lastt two memberrs to 285Cn → 281Ds and heence that of th he mother beinng 289Fl [5]. Booth observed Fl F decays weree registered inn the isotherm mal section, in COMPACT I, I while zero decays d was observed o in COMPACT C II. Considerin ng the low experimental e statistics, a lower l limit for f the Au adsorpption enthalpyy of Fl on goold was foundd as − ΔH ads > 48 kJ·mol-1 at 95% c.l. Similar S calculations were pperformed forr 90% and 68% % confidencee intervals, ressulting in limiits of 49 kJ⋅mool-1 and 50 kJJ⋅mol-1, respecctively. The authors a concluuded that Fl demonstrates d metallic charracter upon addsorption on a gold surfacce due to a weeak metal-metal bond formaation and is a volatile v metall [5].

Fig. 35: The observed gas-chromatograp g phy behavior of Fl and Cn in COM MPACT compared d to that of Pb, Hg g and Rn. Measuured H (panel c), andd 219Rn (panel d) together t with the temperature proffile in the main paart of distribuutions (bars) of 185Pb (panel b), 182Hg the expperiment (panel a)) are shown. Figuure adopted from [5].

Author name / Systems Engineering Procedia 00 (2015) 000–000

8.2. Conclusion on chemical properties of Fl To conclude, the limited number of observed Fl events in chemistry experiments and contradicting observations did not yet allow an unambiguous determination of chemical properties for Fl. More advanced experiments with higher statistics at low background conditions are required. 9. Ununtrium (Uut, element 113) 9.1. Production Isotopes of Uut can be produced in nuclear fusion reactions of 48Ca with 237Np [129] and 70Zn with Bi [130-132] directly. Or indirectly in the decay of heavier transactinides Uup and Uus produced in the fusion of 48Ca with 243Am [133-136], and 249Bk [137, 138]. The production path via the 243 Am(48Ca,xn)291−xUut reaction was recently confirmed at GSI Darmstadt [139]. Among the reported isotopes of Uut there are candidates suitable for fast chemical investigations: 284Uut (T1/2=0.48 s), 285Uut (T1/2=5.5 s), and 286Uut(T1/2~20 s). The IUPAC is in the process of reviewing the claims for discovery of Uut. Therefore, this element has so far no assigned name. 209

9.2. Predictions of chemical properties of Uut From the position of Uut in group 13 of the Periodic Table along with the elements boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (Tl) the trends of increasing metallic character and increasing elemental volatility are expected to be preserved [97]. Hence, Uut is expected to be a volatile metal [140]. An electronic ground state configuration of [Rn]5f146d107s27p1/21 is expected [141]. With respect to the relativistic stabilization of the spherical s- and p1/2 orbitals the trivial prediction would be that Uut is usually a monovalent element similar to its homologue Tl or similar to alkali metals with a quite high first ionization potential [140]. The predictions for the volatility of Uut are quite close to each other if deduced from trends established in group 13 of the Periodic Table or from relativistic density functional theory (see Table 4). Further predictions for compound formation [142-146] reveal stable compounds for Uut in oxidation state +1, rarely +3. Atomic adsorption of elemental Uut on metal surfaces has been suggested as a possible investigation method to assess the chemical character of SHE including Uut [102]. The successful experiments investigating the adsorption of Cn and Fl on gold surfaces triggered also the predictions for calculating the interaction between Uut and gold using relativistic density functional theory on various levels of theory [146-150]. Results are summarized in Table 4.

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Table 4 Predicted thermodynamic data for elemental Uut and its interaction with various surfaces. Interaction with Uut

ΔHsubl

−ΔHads

De

IP

(kJ⋅mol-1)

(kJ⋅mol-1)

(kJ⋅mol-1)

(eV)

142

7.4

111±8 Quartz Au

Ref. [140] [97]

89

[102]

176

[106]

158.6

1.825

[146] [102]

164 1.38−1.57

[149]

1.31−1.4

[148]

Pt

128

[102]

Pd

195

[102]

Rh

267

[102]

Ag

175

[102]

Cu

233

[102]

Ir

266

[102]

Ni

237

[102]

ΔHsubl: sublimation enthalpy; ΔHads: adsorption enthalpy, De: dissociation energy of dimers, IP: ionization potential

9.3. Experimental chemical identification of Uut Between 2010 and 2012, a series of experiments was conducted using the nuclear fusion reaction of Ca with 243Am and 249Bk targets. The FLNR gas-loop system was installed at the U400 beam line. This system comprises similarly to the IVO-COLD system a quartz covered cylindrical recoil chamber of 2 cm diameter and 6 cm length. The recoiling reaction products were thermalized in a 70/30-vol% He/Ar mixture flowing at a flow rate of 1.8 l/min and flushed to a quartz wool filter mounted in a 750°C hot tubular oven to separate particulate matter accidentally produced in beam induced sputtering processes. Volatile products of the nuclear reaction were transported through Teflon® transport capillaries of 2 mm inner diameter held at 70°C connecting the aerosol particle filter and the detector units. As detectors served arrays of gold covered PIN detector pairs (1 cm x 2 cm in size) held at different temperatures. One of the experiments was performed using the COLD setup as shown in Figure 23. Altogether, beam doses of 3.2×1018 (Isothermal detector at 0°C), 4.7×1018 (two isothermal detectors in a row held at 20°C and at 0°C, respectively) and 5.6×1018 (COLD, temperature gradient from 35°C down to -110°C) have been applied to ~1.5 mg⋅cm−2 243AmO2 targets prepared on 2 μm Ti-foils by the painting and molecular plating techniques. In the fourth experiment ~1.5 mg⋅cm−2 249Bk2O3 targets have been prepared by molecular plating and additional painting onto 2 μm Ti-foils. Here, the isothermal detector was held at 0°C. Considering the measured reaction cross-sections [135, 139] and all known efficiencies, e.g., targetgrid transmission, transport velocity, detection efficiency, these experiments missed to observe 10 to 20 284 Uut events, which was indicative for either a very high volatility of Uut similar to noble gases or more likely to a lower volatility of the elemental state of Uut, not allowing for an efficient transport of Uut through Teflon® capillaries at 70°C. In the experiments with the 249Bk target at an applied beam dose of 9×1018 48Ca one decay chain was observed (see Figure 36, adopted from [117]), which is quite similar to the decay chain 286Uut observed in the decay pattern from 294117 [151], representing the 3-n evaporation channel. Unfortunately, the U-400 cyclotron was limited in 48Ca beam energy leading to excitation 48

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energies between 27 and 35 MeV in the Bk targets only. Therefore, no 4-n evaporation channel could be observed, which would have an estimated factor of 4 higher cross section at 39 MeV excitation energy [151]. 3n

249Bk + 48Ca

294

Į1

117

290

Į2 Bot. Bot. 8 Į3 Bot. Bot. 8 Į4 Bot. Bot. 8 Į5 Bot. Bot. 8 Į6 270

274

278

Mt

115

286

113 9.62 MeV

282

Rg

8.89 MeV 64.90 s

9.52 MeV 6.49 s

Bh 8.77 MeV 13.59 min

Db SF 101.0 + 89.1 MeV 79.25 h Fig. 36. Decay chain tentatively attributed to 286Uut as progeny of 294117 observed in the isothermal detector held at 0°C after a transport through a 4-m-long Teflon® capillary held at 70°C and through a quartz wool filter held at 750°C.NR=3.8E-3

In 2013 another experiment was conducted following the same principal scheme as described above [152]. An isothermal module containing 16 gold covered detector pairs was held at room temperature. Along a 32 detector pair module connected further downstream from the isothermal one, a temperature gradient was applied between 20°C and −50°C. 243AmO2 of 1.5 mg/cm2 thickness was electroplated on 2 μm Ti foils and was used as target for a 48Ca beam with ECOT = 248 MeV with intensities of 1.3×1012 particles⋅s-1. In the data of the two- month irradiation several decay chains have been identified revealing similar decay properties as reported in [135, 136] (see Figure 37). The results published by Dmitriev et al. [152] presently are under critical discussion in the community. Briefly, decay chains b), e), and f) heavily hinge on the decay properties of 276Mt. The state with a half+8 life of (T1/2= 6 − 2 s) was not observed in [139] and was associated in [135, 136] with a 9.81 MeV or 9.68 MeV α-particle (a third event was detected in a side detector only). In chains b), e), and f) 276Mt is rather long-lived but associated with α-particle energies, which are too low, even considering a possible energy loss across the detector channel. In chain d) the α-particle energy of 284Uut (from the position at which the daughter events were registered, bottom 5 and 7, the mother event in top 6 should most likely be a full energy event) and the fission fragment energies are rather low. This currently leaves only chain c) in agreement with currently available decay data. Data from an experiment that has been conducted recently at the Berkeley Gas-filled Separator (BGS) may shed additional light on the decay properties and assignment of Uup decay chains.

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( a) 291

115

288

115

10.33 - 10.58 MeV 171 ms (+42,-28)

( b)

( c)

1

284

9.97 (5) 9.81 (7) MeV 0.97 sec (+0.25, -0.17)

113

284

2

9. 973 MeV TOP11.7

284

2

280

280

Rg

missing α

280

3

276

9.17 - 9.95 MeV 0.54 sec (+0.14-0.09) 6 sec (+8-2)

Mt

4

272

276

Mt

8.747 MeV 20 sec TOP11.7

Bh

5

Db

268

27 h (+5-4)

113

9.474 MeV TOP11.6

284

Rg

9.139 MeV 268 34.8 sec BOT11.7

272 5

268

Mt

missing α

113

9.640 MeV TOP11.10

284

272

Bh

5

Db

8.759 MeV 9.4 sec BOT11.5 15 h 20 m TOP11.6 46.2 MeV BOT11.6 14.1 MeV

Rg

Mt

Bh

4

272

Bh

5

268

113

9.754 MeV BOT11.13

9.404 MeV 1 sec TOP11.12

280

Rg

9.193 MeV 14.2 sec TOP11.13

3

276

Bh

4

13 h 35 m TOP11.3 88.9 MeV BOT11.3 70.2 MeV

Db

( f)

3

276

8.853 MeV 17.2 sec TOP11.3

Bh

2

280

3

missing α

Mt

Bh

2

280

9.688 MeV 10.7 sec TOP11.4

Rg

( e)

2

268

276 4

85 h 13 m 54 s TOP11.7 36.3 MeV BOT11.7 9.6 MeV

Db

(d)

284

9.231 MeV 81 sec TOP11.7

Bh

272

5

9. 851 MeV TOP11.2

3

4

8.73 -9.15 MeV 12.0 sec (+3.1-2.1)

Bh

113

2

9.09 - 9.87 MeV 3.6 sec (+0.9-0.6)

Rg

3

268

113

Db

9.075 MeV 2.3 sec BOT11.11

8.720 MeV 2 sec TOP11.12 1 h 55 m TOP11.12 80.15 MeV BOT11.11 91.6 MeV

276

Mt

Bh

4

272

Bh

5

268

Db

9.026 MeV 24.2 sec TOP11.13

8.781 MeV 13.2 sec BOT11.12 23 h 32 m TOP11.13 72.9 MeV BOT11.13 90 MeV

Fig. 37. Decay chains (b-f) attributed to 284Uut as progeny of 288115 observed in the isothermal gold covered detector held at room temperature after a transport through a 4-m-long Teflon® capillary held at 70°C and through a quartz wool filter held at 600°C. For comparison the reported decay properties [135, 136] are shown in (a).The abbreviation TOPXY.Z means top detector, number of ADC (XY), detector number (Z), i.e. TOP11.6 translates into an event detected in the top detector number 6 recorded by the first 16 channel ADC(11). (Figure adopted from [152].)

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9.4. Conclusions on chemical properties of Uut The so far observed properties for Uut need certainly further confirmations. A tentative explanation for the observed behavior of Uut in these first experiments is that likely the formation of UutOH is required for an effective transport of Uut to the detection devices through the Teflon® capillary at 70°C. It cannot be excluded that this formation is kinetically hindered and/or suppressed in the clean inert carrier gases leading to large losses not accounted for. A new experiment series is scheduled for spring 2015 with the intention to perform similar chemistry experiments with the additional use of preseparation applying DGFRS as described in [118]. The two envisaged experiments will, at otherwise identical experimental conditions, either include a heated quartz tube between the recoil chamber and the detection module or exclude it. In experiments with the closest homologue of Uut, thallium, the formation of TlOH was facilitated by the presence of fused silica [126]. Thus, in the first experiment the formation of volatile UutOH may be expected, whereas the second experiment will investigate the elemental state of Uut. Otherwise, the water vapor contents in the carrier gas can also be increased to facilitate the volatilization of Uut in the chemical form of its hydroxide. Furthermore, preparations are under way to investigate the elemental state of Uut in the gas phase by vacuum chromatography, where the hydroxide formation can be prevented. 10. Outlook and Conclusions While the body of experiments and data on the gas-phase chemistry of transactinide elements is already impressive, pushing the limits to investigations of even heavier and shorter-lived elements will require the improvement of existing technologies and the development of new techniques. For the not yet studied elements, like Mt, Ds, and Rg, isotopes with T1/2 suitable for chemical studies have already been identified. Currently, also element 115 may come within reach of present day technologies, provided a constant supply of 249Bk target material can be maintained. The 249Bk target material is bred through multiple neutron capture reactions in special high-flux reactors. The development of vacuum chromatography and the successful coupling of this technique to kinematic preseparators will allow investigations of elements with isotopes that have half-lives shorter than one second. The knowledge about the gas-phase chemistry of lighter transactinides will grow and new classes of compounds, such as volatile carbonyls, open new perspectives [52] especially to first chemical studies of elements such as Mt. For elements Lv through 118, new isotopes suitable for chemical studies must first be discovered. On the long run, new accelerators delivering higher beam intensities and even more exotic target materials such as 250Cm, 251,252Cf, and 254Es will allow to produce nuclides closer to the line of betastability [153] in super-heavy element factories. Such a super-heavy element factory is currently already under construction at Flerov Laboratory of Nuclear Reactions, in Dubna, Russia and a superconductiong cw-LINAC is planned to replace the GSI UNILAC in Darmstadt, Germany, mainly for SHE studies. Possible electron-capture branches in the members of the α-particle decay chains may lead to the formation of longer-lived, yet unknown nuclides of Mt, Ds, Rg, and Cn, that could be stored and studied in traps. A key factor will be not only the availability of highly intense heavy ion beams, but also the fabrication of targets and target setups that will be able to safely accept these beams. Furthermore, highly efficient kinematic preseparators dedicated to chemistry experiments will be a necessity. The spectacular developments in theoretical chemistry which are able to provide more accurate predictions of experimentally measurable properties, will allow a much deeper understanding of the influence of relativistic effects on chemical properties of SHE.

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