Chemical studies of the heaviest elements

Nuclear

Physics

A734

(2004)

124-135 www.elsevier.comilocate/npe

Chemical

studies

of the heaviest

elements

Y. Nagamea, H. H&baa*, K. Tsukadaa, M. Asai”, A. Toyoshimaat, S. Gotoat, K. Akiyamaa, T. Kanekoa, M. Sakama ‘@, M. Hirata”, T. Yaita”, I. Nishinaka”, S. Ichikawa” and H. Nakahara” “Advanced Science Research Center, Tokai, Ibaraki 319-1195, Japan

Japan

Atomic

Energy

Research

Institute,

Chemical studies of the transactinide elements (2 2 104) at JAERI (Japan Atomic Energy Research Institute) are reviewed and prospects for extending the studies are briefly considered. Aqueous chemistry of element 104, rutherfordium (Rf), and element 105, dubnium (Db), produced in the 248Cm(‘80,5n)261Rf and 248Cm(1gF,5n)262Db reactions, respectively, has been studied by ion-exchange chromatography on an atom-at-a-time basis. Characteristic ion-exchange behavior of Rf and Db in acidic solution is discussed.

1. INTRODUCTION Presently, we know of 24 man-made transuranium elements. According to the actinide concept [l], the 5f e 1ec t ron series ends with element 103, lawrencium (Lr), and a new 6d transition series is predicted to begin with element 104, rutherfordium (Rf), and is called transactinide elements or superheavy elements [a]. The currently discovered 13 transactinide elements, elements 104 through 116, are placed in the periodic table under their lighter homologues in the 5d electron series, Hf to Hg, and in the successive groups 13 to 16, Tl - PO. (see Fig.1). Studies of chemical properties of the transactinide elements offer unique opportunities to obtain information about trends in the periodic table of the elements at the limits of nuclear stability, and to assess the magnitude of the influence of relativistic effects on chemical properties. According to the calculations of electron configurations of heavier elements, it is predicted that sudden changes in the structure of electron shells may appear due to relativistic effects which originate from the increasingly strong Coulomb field of the highly charged atomic nucleus [5]. Therefore, it is expected that heavier elements show a drastic rearrangement of electrons in their atomic ground states, and as electron configurations are responsible for chemical behavior of elements, such relativistic effects can lead to surprising chemical properties. Increasing deviations from the periodicity of chemical properties based on extrapolation from lighter homologues in the periodic table *Present address: Cyclotron Center, RIKEN, Wako, Saitama 351-0198, Japan. TDepartment of Chemistry, Osaka University, Toyonaka, Osaka 560-0043, Japan *Permanent address: Department of Chemistry, Niigata University, Niigata 950-2181, Japan fpermanent address: Department of Radiologic Science and Engineering, The University of Tokushima, Tokushima 770-8509, Japan 0375-9474/s - see front matter doi:10.1016/j.nuclphysa.2004.01.021

0 2004 Elsevier

B.V.

All rights reserved.

I! Nagame et al./Nuclear

57

58

58

60

hn’hanides La

Ce

Pr

Nd

Figure 1. Periodic (Ds) [3]. Recently,

61

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Physics A734 (2004) 124-135

82

63

64

65

66

67

58

69

70

71

Pm Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

table of the elements. Element 110 has been just named the synthesis of elements 115 and 113 has been reported

darmstadtium by the Dubna

group141.

are consequently predicted [6-S]. It would be no longer possible to deuce detailed chemical properties of transactinide elements simply from the position of the periodic table. The transactinide elements must be produced by bombarding heavy radioactive actinide targets with high-intensity heavy-ion beams and must be identified by measurement of their decay or that of their known daughter nuclei with unambiguous detection techniques. Both half-lives and production rates of isotopes of still heavier elements are rapidly decreasing, and so each atom produced decays before a new atom is synthesized. It means any chemistry to be performed must be done on an “atom-at-a-time” basis. Thus, rapid, very efficient and selective chemical procedures are indispensable to isolate desired transactinide isotopes. The pioneering works so far performed have concentrated on the question how well the periodic table accommodates the transactinide elements as transition metals in the seventh period: first generation chemistry of the transactinide elements. The results with a few atoms have justified placing the elements Rf through element 108, hassium (Hs), experiment of into groups 4 to 8 of the periodic table [a]. I n contrast, the first challenging element 112 indicates that at certain conditions this element behaves morelike radon (Rn) rather than mercury (Hg). In Table 1, the experiments on the recent chemical studies of element 106, seaborgium (Sg), e1ement 107, bohrium (Bh), Hs, and element 112 are summarized together with some relevant quantities. The data obtained in the first generation experiments, however, are not enough for the

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Table 1 Recent chemical studies z Nuclide Half-life 106

265Sg

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of Sg, Bh, Hs, and element 112. (s) Reaction cr (pb) Production 24sCm(22Ne,5n)

~240

5 h-’

rate a

Reference [9-131

106

T3g

21

248Cm(22Ne,4n)

~25

0.5 h-l

P4

107

267Bh

17

24gBk(22Ne,4n)

x70

1.5 h-’

P51

108

26gH~

11

24sCm(26Mg,5n)

~6

3 d-’

VI

108 270H~ 3.6 248Cm(26Mg,4n) x4 112 283112 %3 min 238U(48Ca,3n) %2 “Assuming typical values of 0.8 mg/cm2 for the target 3~10’~ s-l [18].

2 d-l 1 d-l thickness

1161

P71 and beam intensities

of

discussion of such as relativistic effects and thermodynamical properties of the transactinide elements even on Rf and Db; those are fragmentary with less statistics. In fact, the experiments show still conflicting results and are criticized due to unsatisfactory experimental conditions [19]. Therefore, it is of great interest to study detailed chemical properties of the transactinide elements with high statistics and to compare these with properties deduced from extrapolations and from modern relativistic molecular orbital calculations: second generution chemistry of the transactinide elements. Recently, we have successfully produced the transactinide nuclides, “‘Rf and 262Db, by using the 248Cm(‘80,5n) and 248Cm(‘gF,5n) reactions, respectively at JAERI [20], and conducted detailed ion-exchange experiments of Rf together with the group-4 elements Zr and Hf in acidic solution [21-231, and Db with the group-5 elements Nb and Ta based on the atom-at-a-time chemistry. In this report, the second generation chemistry of Rf and Db at JAERI is outlined.

2. CHEMICAL

STUDIES

2.1. Production

of “‘Rf

OF Rf AND

Db AT JAERI

and “‘Db

Longer-lived transactinide isotopes needed in chemical studies are existing in the neutronrich region and they are produced by the so-called hot fusion reactions of such as 180, “F 22Ne, and 26Mg projectiles with actinides targets of 244Pu, 248Cm, 24gBk, and so on. Figure 2 shows the schematic of the experimental set-up for the production of “‘Rf and onto 2s2Db. A 248Cm target of 590 pg/ cm2 in thickness prepared by electrodeposition a 2.2-mg/cm2-thick beryllium backing foil was bombarded by 180 and “F beams delivered from the JAERI tandem accelerator. The reaction products recoiling out of the target were stopped in He gas (~1 bar), attached to KC1 aerosols, and were transported through a gas-jet transport system to the apparatus MANON (Measurement system for Alpha-particle and spontaneous fissioN events ON-line), where the reaction products were deposited on polyethylene terephthalate foils of 120 pg/cm2 in thickness and 20 mm in diameter at the periphery of an 80-position stainless steel wheel of 80-cm diameter. The wheel was periodically rotated to position the foils between six pairs of Si PIN photodiodes for o-particle detection.

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Gas **Cm

Target

on Be Backing Gas-jet

Outlet

-

Si PIN Photodiodes

Cattier 120 mg/cm*,

Foil 20 mm i.d.

Figure 2. Schematic representation of a target and recoil chamber He/KC1 gas-jet system and the apparatus MANON (Measurement particle and spontaneous fissioN events ON-line).

arrangement with a system for Alpha-

The sum of o-particle spectra measured in the six top detectors in the bombardment of 248Cm with 94-MeV ‘*O ions is shown in Fig. 3(a). The total beam dose was 2.4~10~~. In the a-energy range of 8.10-8.40 MeV, cy lines from 78-s 261Rf (8.28 MeV) and its daughter 25-s 257N~ (8.22, 8.27, and 8.32 MeV) are clearly seen. A total of 166 events were registered both in the top and bottom detectors in the singles measurement and 57 ,(261Rf)-(r(257N o ) correlation events were detected. Assuming a 100% o-decay branch (la) for both 261Rf and 257N~, the production cross sections of “‘Rf were evaluated to be 8f2, 13f3, and 8f2 nb at the beam energies of 91, 94, and 99 MeV, respectively. In Fig. 3(b), the measured cross sections are plotted as a function of the “0 bombarding energy together with the literature data [24,25], where the relative cross section values in [25] are normalized to the present results. The data are smoothly connected with the maximum cross section of about 13 nb at around 94 MeV [20]. In the production of 262Db, 19 mother-daughter correlations of o-energies between 262Db (8.45, 8.53, and 8.67 MeV) and “*Lr (8.565, 8.595, 8.621, and 8.654 MeV) were detected at the “F energy of 106 MeV. With assumption of I,=64% in 262Db and I,=lOO% in “*Lr [26], the production cross section of 2s2Db was evaluated to be 1.4f0.4 nb [20]. The measured cross section value is larger by a factor of about 6 than that by Dressler et al. [27], while that is nearly equal to the value of about 2.2 nb reported by Trubert et al. [28]. The production rate in the present set-up was approximately 2 atoms per min for “‘Rf and 0.25 atom per min for “‘Db . As the gas-jet transport yield was evaluated to be ~35% [20], 0.7 atoms per min of 26rRf and 5 atoms per hour of “‘Db were anticipated for the chemistry experiments.

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t 03

Energy

(MeV)

Figure 3. (a) Sum of o-particle spectra measured in the bombardment of the 248Cm target with 94-MeV “0 ions, and (b) cross sections of the 248Cm(‘80,5n)26’Rf reaction as a function of the bombarding energy of 180. The data taken from the literature [24,25] are also shown.

2.2. Atom-at-a-time Chemistry of Rf and Db As mentioned above, the chemical study needs to be carried out on the phenomena that gives the same results for only a few atoms and for macro amounts of atoms; the question arises as to whether a meaningful chemistry with single atoms is possible. For singe atoms, the classical law of mass action is no longer valid, because the atom cannot exist in the different chemical forms taking part in the chemical equilibrium at the same time. Guillaumont et al. suggested for single-atom chemistry to introduce a specific thermodynamic function, the single-particle free enthalpy [29]. An expression equivalent to the law of mass action is derived, in which activities are replaced by probabilities of finding the species in the appropriate phases. According to this law, an equilibrium constant (distribution coefficient) of the atom between two phases is correctly defined in terms of the probabilities of finding the atom in one phase or the other. If a static partition method is used, this coefficient must be measured in repetitive experiments. Since dynamical partition methods can be considered as repetitive static partitions, the displacement of the atom along the chromatographic column gives a statistical result [29]. For short-lived atoms, the partition equilibrium must be reached during the life-time of the isotopes, which requires high reaction rates. Based on the kinetics of a single-step exchange reaction, Borg and Dienes suggested that at certain conditions a measurement of the partition of the atom between the phases with very few atoms will already yield an equilibrium constant close to the “true” value provided that both states are rapidly sampled [30]. Th e a b ove interpretation indicates that chromatographic systems with fast kinetics are ideally suited for single-atom separation as there is rapid, multiple sampling of the adsorbed or mobile chemical species [19]. To perform fast and repetitive ion-exchange chromatographic separation of Rf and Db, we have developed the apparatus AIDA (A u t omated Ion-exchange separation apparatus

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coupled with the Detection system for Alpha spectroscopy) that consists of a modified ARCA (Automated Rapid Chemistry Apparatus) which is the miniaturized computeron-line a-particle decontrolled liquid chromatography system [31] and an automated tection system. In the modified ARCA as schematically shown in Fig. 4, there are two different paths to supply solutions; the first eluent goes through the collection port to the micro column, while the other one is directed to the column without going through the collection port to avoid cross-contamination.

Front

view

Side

view

HelKCl Jet in 11.5 M HCI (charge)

20 micro-columns 1.6 mm0 x 7.0 mm

Figure 4. Schematic of the modified ARCA in AIDA (Automated Ion-exchange apparatus coupled with the Detection system for Alpha spectroscopy).

separation

Due to the influence of relativistic effects, deviations from the periodicity of the chemical properties based on extrapolations from the lighter homologues are expected. Thus, the experimental approach should involve a detailed comparison of the chemical properties of the transactinide elements with those of their lighter homologues under identical conditions. We have investigated the anion-exchange behavior of Rf and Db together with their lighter homologues in the same on-line experiments. In the present study, two kinds of targets were used; for example in the case of Rf experiment, one is the 248Cm “‘Gd of 36 pg/cm2 in thickness) mixed target to simultaneously and Gd (39.3%-enriched and Gd(“O,xn) reactions, produce 78-s 261Rf and 3.24-min 16’Hf via the 248Cm(180,5n) thickness) and natGe (660+g/cm2 respectively, and the other is the natGd (370+g/cm2 thickness) mixed target to produce 16’Hf and 7.86-min 85Zr through the natGd(laO,xn) and natGe(lSO, xn ) reactions, respectively [22]. In the following, we outline the experimental procedures and a part of the result on the anion-exchange behavior of Rf in hydrochloric acid (HCl) solution. 26*Rf was produced in the 94-MeV “O-induced reaction of 248Cm. The reaction products recoiling out of the target were transported by the He/KC1 gas-jet system to the collection port of AIDA.

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130

3

4

5

6

7

8

HCI concentration

9

IO

2,ii 0

(M)

Figure 5. (a) Adsorption behavior of Rf, Zr and Hf as and (b) that in 8 M HNOs. The data for 261Rf and ‘“‘Hf are shown by closed symbols, while those for 85Zr and by open ones. The adsorption of Th (closed triangle) in off-line experiment [22].

2

4

HNO,

6

6

concentration

10

12

14

(M)

a function of HCl concentration, obtained from the Cm/Gd target 16’Hf from the Ge/Gd target are (b) was obtained by the separate

After collection for 125 s, the port was mechanically moved on the top of one of the micro columns, where the products were dissolved with 170 ~1 of hot (- 80 “C) 11.5 M HCl and were fed onto the 41.6~7 mm chromatographic column filled with the anion-exchange resin MC1 GEL CA08Y (particle size of about 20 pm) at a flow rate of 1.0 ml/min. Then, the products were eluted with 290 ~1 of 4.0 - 9.5 M HCl and those remaining in the column were eluted with 250 ~1 of 4.0 M HCl through the second path (see Fig. 4). The effluents collected on Ta disks were evaporated to dryness, and the pair of disks were automatically subjected to cu-spectrometry with eight 600-mm2 passivated ion-implanted planar silicon (PIPS) detectors. The anion-exchange experiments with “Zr and ‘“‘Hf were conducted under the same conditions as those with “rRf . The effluents were collected in polyethylene tubes and were assayed by y-ray spectroscopy [22]. Each separation was accomplished within 20 s and the a-ray measurement was started within 1 min after the collection of the products at the AIDA collection port. From ion-exchange experiments carried out 245, 366, 400, 395, 328, and 159 times at 11.5, 9.5, 9.0, 8.5, 7.0, and 4.0 M HCl, respectively, a total of 186 (Y events from 261Rf and 35 cy-o correlation events [22]. Figure 5(a) its daughter 257N~ were regis tered, including shows the adsorption behavior of Rf, Zr and Hf as a function of HCl concentration. The result clearly indicates that the adsorption of Rf is quite similar to those of the group-4 elements Zr and Hf. It means the anionic chloride complexes of the tetravalent Rf, Zr, and Hf are formed in concentrated HCl: [M(OH)Cl#and [MC16]‘(M=Rf, Zr, and Hf). On the other hand, the adsorption behavior of the pseudohomologue tetravalent thorium (Th) in HCl concentration of > 8 M was quite different from that of Rf, Zr and Hf; Th does not form the anionic complexes in this region. Another interesting feature is observed in the adsorption trend on the anion-exchange resin among Rf, Zr and Hf as

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131

Figure 6. Fourier transformed K-edge EXAFS spectra of the Zr complexes in 9.0, 10.0, and 11.5 M HCI. Evaluated structures of the metal (M=Zr and Rf) complexes are also shown. A denotes the phase shift parameter.

shown in Fig. 5(a). The adsorption order that reflects the stability of the chloride complex formation is Rf 2 Zr > Hf [22]. Non Th(IV)-like behavior of Rf was also probed with anion-exchange experiments in 8 M nitric acid (HNOs) as shown in Fig. 5(b). Although Th(IV) forms the anionic complexes and is strongly adsorbed on the anion-exchange resin, Rf remains in solution confirm that as expected for a typical group-4 element [22]. Th e above results definitely Rf is the member of the group-4 element, but not like the pseudo-homologue Th [32]. In order to understand the chemical species and structure of Rf complexes in solution, we measured the extended x-ray absorption fine structure (EXAFS) spectra of Zr complexes at the KEK (High Energy Accelerator Research Organization) Photon Factory. EXAFS spectra give information on the local environment around the central atom such as the atomic number and the number of neighboring atoms and their distances from the central atom. Figure 6 shows the Fourier transformed K-edge EXAFS spectra of the Zr complexes in 9.0, 10.0, and 11.5 M HCl solutions, which represent the radial distributions of the atoms surrounding the central Zr atom. The evaluated distances corresponding to the chemical bonds of Zr-0 and Zr-Cl are indicated by vertical lines. The intense peak of Zr-0 is observed at 9.0 M HCl, while at 11.5 M HCl that of Zr-Cl is clearly seen. It is found that the Zr complex structure in HCl solution is changing from such as Zr(OH)4.2HsO to the anionic chloride complex [ZrCls]‘-, which is consistent with the results of the ion-exchange experiments. Therefore, we can assume the structure of the Rf complexes on the analogy of the Zr ones as depicted in Fig. 6. Although the present approach is the indirect method to characterize the Rf complex structure, this would be the first attempt to examine the chemical species of the transactinide elements.

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Figure 7 shows the variation of the adsorption behavior of Rf, Zr and Hf as a function of hydrofluoric acid (HF) concentration. The adsorption of Zr and Hf is absolutely equal in the whole rage of HF concentration, while that of Rf decreases over 4 M HF [23]. The lower adsorption of Rf indicates that the fluoride complex formation of Rf would be weaker than that of Zr and Hf. The relativistic density-functional calculations on the electronic structures of hexafluoride complexes ([MFs12-, M=Rf, Zr, and Hf) have been performed to evaluate the stability of the complexes [33]. The results indicate that the trend in the overlap population between the valence d orbitals of M4+ and the valence orbitals of F- is found to be ZrzHf>Rf, suggesting that the Rf complex is less stable than those of Zr and Hf in the [MFs]‘s t ructure. It should be noted here, however, the calculations without relativistic effects show no differences in the stability among the structures. The similar result has been derived by Pershina et al. considering the free energy change of fluorination reactions of Rf, Zr, and Hf [34]. The different behavior between Rf and its homologues Zr and Hf suggests that the relativistic effect strongly influences the fluoride complex formation of Rf [23]. F ur th er consideration is needed to quantitatively understand this interesting result.

+ + IO'

HF concentration

(M)

Figure 7. Adsorption behavior of Rf, Zr and Hf as a function of HF concentration. The data for 261Rf and 16’Hf obtained from the Cm/Gd target are shown by closed symbols, while those for “Zr and “jgHf from the Ge/Gd target are by open ones.

Recently, we conducted the anion-exchange experiment of 34-s 262Db produced in the 24sCm(‘gF,5n) reaction with the cross section of 1.5 nb together with the homologue 6.76min “‘Ta in Gd(lgF,xn). From more than 17 hundred separations, the adsorption of Db on the anion-exchange resin at 14 M HF was obtained based on 10 (Y singles attributed to the decay of 2s2Db and its daughter 3.9-s “‘Lr as shown in Table 2. In the AIDA experiment, 14 M HF solution was used as the first eluent and the second one was 6 M HNOs/0.015 M HF to elute the products remaining in the column. The adopted column

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Y Nagame et al. /Nuclear Physics A 734 (2004) 124-135 size was much smaller one, 41.0x3.5 values of the lighter homologue Nb determined. Although the adsorption those in Rf, that is close to the value NbzTa>Db-Pa.

mm. In the separate experiments, the adsorption and the pseudo-homologue Pa at 14 M HF were of Db is quite different from those of Nb and Ta as of pseudo-homologue Pa. The adsorption order is

Table 2 Adsorption of Db, Nb, Ta, and the pseudo-homologue CA08Y at 14 M HF concentration. 262Db “Nb”

177Ta

233Pa

Adsorption

90f2

=30b

(%)

‘Produced in the “Zr(p,n) tracer 233Pa.

45:;;

x90 reaction.

bObtained

Pa on the anion-exchange

in the off-line

experiment

using

resin

radio-

Presently, we are conducting the relativistic molecular orbital calculations to understand the characteristic behavior of Rf and Db in acidic solution, and to see if the anionexchange behavior of Rf and Db is affected by relativistic effects. 2.3. Future plans In the study of Rf aqueous chemistry, experiments of reverse-phase extraction chromatography with AIDA are planed to study complex formation ability of Rf with various kinds of organic extractants. We will continue the ion-exchange experiments of Db. Improvements to the overall production rates and a possible multiple 248Cm-target system are being considered to study the chemical properties of the short-lived “‘Db and 265Sg. Development of a new ion-exchange separation apparatus, an improved AIDA based on a continuous ion-exchange system, is also required. Gas-phase chemistry of the transactinide elements with a continuously operating isothermal gas chromatographic apparatus is being studied [35]. Th e coupling of a mass analyzer to the isothermal gas chromatograph system is planned to characterize the chemically separated species. 3. PERSPECTIVES From the view point of chemistry, we still need the detailed chemical studies even in the lighter transactinide elements to obtain thermodynamical quantities and to clarify the influence of relativistic effects. For the element at the upper end of the periodic table, e.g. 2=114, the challenging experiment will be conducted using the isotope 28g114 (T,,, = 2-23 s) produced in the 244Pu(48Ca,3n) reaction with a cross section of about 1 pb elements, we need much higher [36]. To study c h emical properties of heavier transactinide intense heavy-ion beams and have to develop new target designs to withstand the intense beams. The new approach to the transactinide chemistry is the coupling of chemical The recoil separators serve as a pre-separator apparatuses to physical recoil separators. for the chemical experiments. The first and successful experiment was made by coupling the SISAK (Short-lived Isotopes Studied by the AKUFVE Technique) system [37] to the

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BGS (Berkeley Gas-filled Separator) [38] for Rf chemistry [39]. The excellent gas-filled separator GARIS [40] at RIKEN is the powerful apparatus for this program. In the nuclear aspects, chemical techniques have significantly contributed to the nuclear decay study of neutron-rich isotopes and to the characterization of new nuclides, such as 263Rf [41], 263Db [42], 2s5@sSg [9,43], 266,267Bh [l&44], and 26g1270Hs [16,45]. The advantages of chemical techniques compared with physical kinematic separators arise from the possibility of using thicker targets, high beam intensities spread over larger target areas, and in providing access to nuclides emitted under large angles and low velocities that are produced in hot fusion and in multi-nucleon transfer reactions. Chemical techniques are quite feasible for the decay studies of relatively long-lived nuclei not only around a doubly magic deformed nucleus with Z=lOS and N=162 [46] but also around a spherical superheavy elements with Z N 114. It can also give a clear identification of the atomic number of relatively long-lived spontaneous fission (SF) nuclides at the end of o-decay chains of superheavy nuclei (2 2 110). The present study at JAERI has been carried out in collaboration with Niigata University, Osaka University, Tokyo Metropolitan University, University of Tokushima, Kanazawa University, Tsukuba University, Gesellschaft fiir Schwerionenforschung (GSI), and Mainz University. This work was partly supported by the JAERI tandem accelerator collaboration program and the program on the scientific cooperation between JAERI and GSI in research and development in the field of ion beam application. We would like to acknowledge the crew of the JAERI tandem accelerator for providing the stable and intense “0 and “F beams.

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