Journal of Nuclear and Radiochemical Sciences, Vol. 5, No. 2, pp. R19-R25, 2004

Reviews
ReviewGas Phase Chemistry of Superheavy Elements
A. Türler*
Institut für Radiochemie, Technische Universität München, D-85748 Garching, Germany
Received: October 20, 2004
The knowledge and understanding of the chemistry of the superheavy elements, especially of the early transactinides Rf, Db, and Sg and their compounds, both experimentally and theoretically, is nowadays quite extensive.
While rapid chemical separations in aqueous solution were (and are) the method of choice for detailed investigations of Rf and Db, the development of gas-phase chemical separation methods have allowed first chemical
studies of increasingly heavier transactinide elements such as Bh and Hs. In all cases, the separated transactinide
element was unequivocally identified by observing the characteristic nuclear decay properties of one of its
isotopes. The current paper will review the progress made in gas phase chemical separations of transactinide
elements and highlight future perspectives.

1. Introduction
The discovery of new elements and their correct placementin the periodic table have originally been the domain ofchemists. Indeed, the discovery of new elements up tomendelevium (Md, Z = 101) was accomplished by chemicalmeans.1 The decreasing production cross sections, that nowadays are of the order of one picobarn or even less for the heaviest known nuclei, and the rapidly diminishing half-lives mademanual chemical investigations of elements beyond Z = 105
virtually impossible. Thus, the synthesis and identification of
new elements has completely shifted to nuclear physics and led
to the discovery of more than ten new elements. However, theplacement of a new element in the periodic table is not only
based on its proton number but also on its chemical properties,
which are determined by its electronic structure. Relativity
which is important in determining the nuclear shell structure
and thus the decay properties of superheavy elements (SHE), is
extremely important in defining the chemical properties.
Therefore, not only the physics but also the chemistry of SHE
is extremely interesting and rewarding to be studied. With Z =104, Rf, starts a new series of d-elements, the so-called transactinides (see Figure 1). Since the liquid drop fission barrieralso disappears around Z = 104, the term SHE is often used asa synonym for transactinide elements.
Despite the numerous difficulties involved in chemicallyisolating one single, relatively short-lived atom from a plethoraof unwanted by-products of the nuclear production reaction,
remarkable progress was made in recent years. This progress
can be attributed to the discovery of enhanced nuclear stability
close to the vicinity of the previously predicted deformed
shells at Z = 108 and N = 162,2 which provided chemists with
sufficiently long-lived isotopes of seaborgium (Sg, Z = 106),3
bohrium (Bh, Z = 107)4 and even hassium (Hs, Z = 108).5 Bydeveloping fast, automated, and sensitive techniques chemists
have discovered or significantly contributed to the characterization of the new nuclides 265,266Sg,6 266,267Bh,7 and 269,270Hs.8
As in experiments of our nuclear physics colleagues, theproduction of a heavy nucleus was detected by registering its
nuclear decay, which for many super-heavy nuclei consists ofunique alpha-particle decay chains quite often terminated byspontaneous fission (SF). The knowledge and understandingof the chemistry of SHE, especially of the early transactinides
Rf, Db, and Sg and their compounds, both experimentally andtheoretically, are nowadays quite extensive as documented by
the first textbook entitled “The Chemistry of Superheavy
Elements” edited by Schädel.9 While rapid chemical separations in aqueous solution were (and are) the method of choicefor detailed investigations of Rf and Db,10, 11 the developmentof gas-phase chemical separation methods have allowed firstchemical studies of all transactinide elements up to Hs. 12−14
The discovery of longer-lived (half-lives of several seconds)
isotopes of elements 110 through 114 in the 48Ca-induced reactions on actinide targets at the Flerov Laboratory of Nuclear
Reactions in Dubna has opened fascinating new perspectives
to experimentally study the chemistry of even the heaviest
elements in the periodic table.15 Currently, experiments are
conducted to elucidate the chemical properties of element 112(E112) 16−18 and also element 114 (E114) in their elemental
state. There are already indications that E112 behaves chemically not at all similarly to its lighter homologue Hg. Since allof the decay chains of the newly discovered neutron-rich
nuclides end in an uncharted area of the chart of nuclides achemical identification of one of the members of these decay
chains could clearly pinpoint the atomic number of the newly
synthesized elements. Therefore, a chemical separator system
can be regarded as a somewhat slow, but efficient Z-separator.
Since chemical separator systems are nearly independent on
the production reaction, thick targets and very asymmetric
reactions can be employed to access neutron-rich heavy nuclei.

Periodic Table of the Elements
1

2

H

2

13 14 15 16 17 He

3

4

5

Li

Be

11

12

B

Na Mg 3

4

19

20

21

5

6

7

9

N

O

F

Ne

13

14

15

16

17

18

12 Al

Si

P

S

Cl

Ar

30

32

33

34

35

36

23

24

V

Cr Mn Fe Co Ni

Cu Zn Ga Ge As Se Br Kr

40

41

42

47

46

29

10

22

45

28

48

31

C

9

Ca Sc Ti

44

27

11

8

38

43

26

10

7

K

39

25

8

6

37

49

50

51

52

53

54

Rb Sr

Y

Zr

Nb Mo Tc Ru Rh Pd Ag Cd In

Sn Sb Te I

Xe

55

57-71

72

73

82

86

56

Cs Ba
87

88

Fr

Ra

89-103

74

75

76

77

Hf Ta W

Re Os Ir

104

107

105

106

78

79

80

81

Pt

Au Hg Tl

110

111

83

84

85

Pb Bi

Po At

Rn

114

115

116

118

69

70

108

Rf Db Sg Bh Hs

109

112

113

Mt Ds Rg
57

58

59

60

61

62

63

64

65

66

67

68

71

Lanthanides La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
89

Actinides*Corresponding author. E-mail: Andreas.Tuerler@radiochemie.de.
FAX: +49-89-289-12204.

18

1

90

91

92

Ac Th Pa U

93

94

95

96

97

98

99

100

101

102

103

Np Pu Am Cm Bk Cf Es Fm Md No Lr

Figure 1. Periodic table of the elements. See text for detailed explanations.

© 2004 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/30/2004

Türler

R20 J. Nucl. Radiochem. Sci., Vol. 5, No. 2, 2004
Figure 1 displays a modern periodic table of the elements.
Elements whose chemical behavior has not yet been experimentally investigated are shown below their expected positions. The element names are those accepted, or in the case ofelement 111 (roentgenium, Rg), recommended by IUPAC.
Provisional names such as ununbium (Uub, Z = 112) are notindicated.
In the following the observed chemical properties of Hs andthe expected behavior of heavier elements, the available chemical separation techniques, and the latest results of recent
chemistry experiments on E112 are presented.
2. Chemical Properties of SHE and their Compounds
Studies of transactinide elements by chemical means rely
strongly on predictions of their chemical properties. In orderto plan a separation scheme, the stability of compounds andtheir physicochemical properties have to be judged on the basisof predictions. Traditionally, the chemical properties of an
unknown element and its compounds were predicted by
exploiting the fundamental relationships of the physicochemical data of the elements and their compounds within thegroups and the periods of the periodic table. Since relativistic
effects are increasing proportionally with Z 2, extrapolations ofchemical properties from lighter homologue elements must failat some point. Here, the only alternative are fully relativistic
calculations. Generally, three relativistic effects are observed.
First, due to the high probability density near the highly
charged nucleus the s1/2 and p1/2 orbital radii are contracting
and the binding energies are increasing. These are called“direct” relativistic effects.19 Second, the spin-orbit splitting
produces two levels for each 1 quantum number. Third, due tothe increased screening of the nuclear charge by the s1/2 and p1/2
orbitals the d and f orbitals are expanding spatially and areenergetically destabilized. This was named an “indirect” relativistic effect,19 but this effect directly influences the outermost
electrons that are largely responsible for the chemical behaviorof an atom. At present it is not possible to directly computephysicochemical properties of a molecule. All-electron calculations of molecules containing transactinide elements are
exceedingly difficult due to the large number of electrons
involved. Nonetheless, also due to the enormously increasing
computing power, theoretical calculations have progressed
now to the point that a number of relevant atomic and molecular properties can be computed. This paper will focus onsome aspects of the expected chemical properties of elementswith Z ≥ 108. Comprehensive reviews on the calculated chemical properties of the early transactinides can be found in theliteratures.20, 21
Sometimes Mother Nature is rather kind to chemists andprovides them with favorable and very clear cut cases. Such acase constitutes the chemistry of Hs. If Hs has any resemblance to its lighter homolog element Os then it should also
form molecules of the type MO 4 (M = Os, Hs). Due to theperfect tetrahedral symmetry of OsO4, this compound is very
volatile, and only interacts very weakly with non-reducing
surfaces, similar to a noble gas. Relativistic molecular orbital
calculations by Pershina et al.22 confirmed that indeed HsO4
must be a very stable molecule and that its volatility should bevery similar to its lighter homologue compound OsO4. This
exceptional chemical property distinguishes Os and Hs fromother d-elements and allows a clear assignment to group 8 of
the periodic table. Thus, experimentalists successfully took on
the daunting task to build a separator system that allowed theisolation of Hs as volatile HsO4.
An illustrative example of the increasing influence of relativistic effects is shown in the case of the so-called coinage
metals Cu, Ag, Au, and Rg (Z = 111). In Figure 2, the groundstate electronic configurations and the first exited states of Cu,

Figure 2. Energy levels of group 11 elements Cu, Ag, Au, and E111
(Rg). In the case of Au also the calculated non-relativistic energy
level scheme is shown. The calculated relativistic energy levels for
E111 are taken from Ref. 23.

Ag, Au, and Rg23 are shown. For Au also the hypothetical
non-relativistic case has been calculated. Cu, Ag, and Au all
have the same d10s1 ground state configuration. As can beclearly seen, the spin-orbit splitting of the d9s2 first exited stateis increasing with the proton number. A possible reason for
the red color of Cu and the colorless appearance of Ag lies inthe substantially higher first excited state of Ag which leads toan absorption which no longer lies in the visible region. If thistrend would continue, Au should be a colorless metal which isless noble than Cu and Ag due to its lower ionization energy.
However, the relativistic stabilization of the s-orbitals and theincreasing spin-orbit splitting make Au a yellow metal with amore noble character than Cu or Ag. The strong relativistic
stabilization of the filled s2-orbital and the further increased
spin-orbit splitting lead to an inversion of states in Rg. Thus
Rg is expected to have a 6d97s2 ground state; the higher ionization energy compared to Au (≈10.6 eV vs. 9.21 eV) and thelower electron affinity (≈1.55 eV vs. 2.31 eV) will probably
make Rg an even nobler metal. The trends observed in group11 are also persisting in groups 12 through 14.
Another chemically very favorable case constitutes E112 as
an expected homolog of Hg. E112 is expected to have an
ionization potential which is more than 1.5 eV higher than thatof Hg.24 Similar to Hg, E112 should not have a bound anion.24
Therefore, E112 is expected to be rather inert in its elemental
state. It is well known that Hg is a very volatile element, but itinteracts strongly with other metals such as Zn or Au to form
amalgams. Yakushev et al. demonstrated that single atoms ofHg can quantitatively be adsorbed on a Au surface from astream of He.16 First relativistic calculations of dimers Hg–Au
and E112–Au indicate a lower, but still sizeable binding energyof the E112–Au dimer.25 Therefore, current experiments focus
on the property whether E112 also forms strong metal bonds
with Au or whether E112 is so inert, that it behaves more like aheavy noble gas such as Rn.
Similar considerations apply to E114, which also has ahigher ionization potential and no electron affinity, rendering it
chemically less metallic and more inert compared to its lighter
homolog Pb. Qualitatively these trends were realized early on,
already in 1975 Pitzer26 published a short paper with the title“Are elements 112, 114, and 118 relatively inert gases?”.
Molecules of the type MX4 (X = F, Cl) were calculated to be
thermodynamically stable for M = Pb, whereas with M = E114
the +2 oxidation state will be favored.27
3. Experimental Techniques
A chemistry experiment with a transactinide element can bedivided into four basic steps: synthesis of the transactinide
element, rapid transport of the synthesized nuclide to thechemical apparatus, fast chemical isolation of the desired
nuclide and preparation of a sample suitable for nuclear spec-

J. Nucl. Radiochem. Sci., Vol. 5, No. 2, 2004 R21

Gas Phase Chemistry of Superheavy Elements
Beam

TargetBeam
Trajectory

EVR
Trajectory

Q1

Recoil Transfer
Chamber (RTC)

M1

M2

Figure 3. The GSI rotating target wheel.

troscopy, and detection of the nuclide via its characteristic
nuclear decay properties.
In order to gain access to the longer-lived isotopes of transactinide elements, exotic, highly radioactive target nuclides suchas 244Pu, 243Am, 248Cm, 249Bk, 249Cf, or 254Es are bombarded
with intense heavy ion beams such as 18O, 22Ne, 26Mg, or 48Ca.
On the one hand as intense beams as possible are to be used;on the other hand the destruction of the very valuable andhighly radioactive targets has to be avoided. A rotating 248Cm
target with a rotating vacuum window that can stand pressure
differences of up to 1.5 atm has recently been used in heavy
element chemistry experiments at the Gesellschaft für
Schwerionenforschung (GSI). This rotating target carrying up
to 6 mg of target material has allowed the use of high-intensity
beams for heavy element chemistry experiments.9 A picture ofa rotating 248Cm target after irradiation with up to 1 pµA of a26
Mg beam (a factor of four larger than possible with a fixed
target) is shown in Figure 3. The target wheel is synchronizedwith the time structure of the accelerator (UNILAC) in order tospread one beam pulse over the area of one target segment.
Reaction products recoiling out of the target are stopped in arecoil chamber filled with helium gas. Non-volatile products
such as Sg or Bh are adsorbed on aerosol particles of about
100 nm diameter and transported through a capillary over relatively long distances to the chemistry apparatus. As aerosol
materials potassium chloride (KCl) or carbon are often used.
Aerosol particles of suitable size can be generated by sublimation of KCl that is heated in flowing He to about 640˚C.
Carbon aerosols are generated by spark discharge in flowing
He. Unfortunately, high transportation yields cannot be reliably maintained over long time periods, also the yield isstrongly dependent on the beam current and usually decreasing
with increasing beam intensity.
Basically, four different approaches which involve the direct
detection of the nuclear decay of the isolated nuclides have beensuccessful in studying the chemical properties of transactinide
elements. Two of the systems work in the liquid phase, whereas
the other two are designed to investigate volatile transactinide
compounds in the gas phase. The predominant share of today’s
knowledge about the chemical behavior of Rf, Db, and Sg inaqueous solution was obtained with the Automated Rapid
Chemistry Apparatus (ARCA).9 This micro-computer controlled
set-up allows fast repetitive separations on miniaturized chromatography columns. Depending on the chemistry, the columns
are filled with ion exchange resin or organic extractants on an
inert support material. Often several thousand separations are
performed. After chemical separation the fraction containing thetransactinide element is evaporated and the sample placed in an
α-/SF spectroscopy system. This last step, which was performed
manually, has now also been automated in a set-up called

BGS-RTC
Window(Mylar)Aerosol Gas-Jet
to Experiment

Figure 4. Schematic of the recoil transfer chamber (RTC) coupled to
the Berkeley Gas-filled Separator (BGS). The separator delivers aclean beam of evaporation residues (EVR) to the RTC. The RTC is
separated from the separator which is operated at a pressure of about 1
mbar by means of a thin mylar window of typically > 1 µm thickness.
This thin window is supported by a thin wire grid. For chemical
studies with the SISAK system, the RTC is flushed with an aerosol
gas-jet and stopped EVR’s are rapidly transported to the continuously
operating liquid-liquid extraction system.

AIDA developed in Japan.10
A continuously operating liquid-liquid extraction systemusing miniaturized centrifuges and liquid scintillation flow
through detection cells (SISAK) was recently successful toseparate and identify 4-s 257Rf.28 The latest version of SISAK
is very well adapted to study nuclides with half-lives as short
as 1 s. In order to suppress the large background created by βparticles, a recoil chamber was coupled to the Berkeley Gasfilled Separator (BGS) (Figure 4). Coupling a chemistry set-up
with a recoil separator may open new frontiers in direct chemical reactions with a large variety of compounds, since one nolonger has to deal with the harsh ionizing conditions created by
the passage of the primary beam. Furthermore contaminants
that cannot be effectively removed due to their chemical similarity to SHE, such as Po or Rn isotopes, are largely removedby the recoil separator.
In order to study elements beyond Sg, faster and much more
efficient chemistry set-ups had to be used. Such systems are
gas-phase chromatographic separations. Since transactinide
nuclei are usually stopped in gas, a fast and efficient link to agas chromatography system can be established; either by directtransport of volatile species or volatile compounds that wereformed in-situ in the recoil chamber, or by a transport withaerosol particles. Despite the fact that the transition metals(groups 4 to 11) are very refractory, there exist few stable inorganic compounds that are appreciably volatile at experimentally manageable temperatures below 1100˚C. These are thehalides and oxyhalides of groups 4 to 7, the oxide hydroxides
of groups 6 and 7, and the oxides of group 8. Moreover,elements 112 to 118 are expected to be rather volatile in theelemental state and thus gas-phase separations will play a
crucial role in investigating the chemical properties. Early on,
gas-phase chemistry played an important role in the investigations of SHE. The technique was pioneered by Zvara and coworkers in Dubna to investigate Rf- and Db-halides whichwere formed in-situ behind the target and detected by latentfission tacks left in a thermochromatography column.9
Later the On-Line Gas-chemistry Apparatus (OLGA) was
developed and successfully used to study halides and oxyhalides
of elements Rf,9 Db,9 Sg,12 and also Bh.13 A schematic of the
OLGA set-up coupled to the Ro-tating wheel Multidetector
Analyzer (ROMA) is shown in Figure 5. Aerosols carrying
reaction products form the target to the OLGA set-up arecollected on a quartz wool plug inside the reaction oven kept at
1000˚C. Reactive gases are introduced in order to form volatilecompounds. The chromatographic separation takes place
downstream in the adjoining isothermal section of the column.

Türler

R22 J. Nucl. Radiochem. Sci., Vol. 5, No. 2, 2004

Figure 5. Schematic of the OLGA set-up coupled to the ROMA
detection system.

At isothermal temperatures ≥ 300˚C most of the halides andoxyhalides of Rf, Db, Sg, and Bh are volatile and travel troughthe column essentially without delay, while less volatilecompounds are retained much longer and are removed by decayof the interfering nuclide. At the exit of the column the isolated
molecules are attached to new aerosol particles and transported
to a detection system. Thus all investigated nuclides wereunambiguously identified by registering α-particle decay chains.
The OLGA set-up was sensitive to cross sections of theorder of 100 pb. In order to assess nuclides that are producedwith few picobarn cross sections the overall efficiency had tobe improved by at least one order of magnitude. This was
accomplished by introducing the in-situ formation of volatilecompounds and their condensation and detection in a thermochromatography detector, as in the early experiments byZvara [see Ref. 9 and references cited therein]. Sufficiently
volatile reaction products such as Hg or E112 can be transported directly with the flowing gas. In experiments with Hs,
recoiling atoms were converted in-situ with an admixture of O2
gas to the volatile HsO4. Instead of an isothermal temperature
profile as in OLGA, a negative longitudinal temperature
gradient is established along the column. Thus compounds aredeposited in the chromatography column according to theirvolatility, forming distinct deposition peaks. If the column
consists of silicon detectors then the nuclear decay of the separated nuclide can be registered. The detector number indicates
the temperature at which the volatile compound was deposited.
1

2

Thus, every detected nuclide reveals also chemical information. A schematic of the In-situ Volatilization and On-line
detection apparatus (IVO)28 is shown in Figure 6. The IVO
set-up was successfully used to study the chemical propertiesof HsO4.14 The overall efficiency (including detection of acomplete 3 member α-particle decay chain) is of the order of
30 – 50%.
Similar set-ups were used in first attempts to chemically
identify E112. If E112 is chemically similar to Hg it should be
adsorbed on Au or Pd surfaces. In test experiments short-lived
Hg isotopes could be isolated in the elemental form from otherreaction products and transported in He quantitatively througha 30 m long Teflon capillary at room temperature.
Adsorption of Hg nuclides on silicon detectors, proved experimentally not feasible, since Hg was adsorbed on quartz
surfaces only at temperatures of –150˚C and below. However,Hg adsorbed quantitatively on Au, Pt, and Pd surfaces at room
temperature. As little as 1 cm2 of Au or Pd surface was sufficient to adsorb Hg atoms nearly quantitatively from a streamof 1 L/min He.16 Therefore, detector chambers containing apair of Au or Pd coated PIPS detectors were constructed.
Eight detector chambers (6 Au and 2 Pd) were connected inseries by Teflon tubing. The detector chambers were positioned inside an assembly of 84 3He filled neutron detectors (ina polyethylen moderator) in order to simultaneously detect
neutrons accompanying spontaneous fission events (see Figure7). 16, 17 In the latest experiment aiming at measuring thevolatility of E112, one row of the silicon detectors of the Cryo
On-Line Detector (COLD) was removed and replaced by a Au
surface. A negative temperature gradient was establishedreaching from room temperature down to –187˚C.18
3.1. Recent Results on the Chemistry of Hassium. In thecourse of an experiment to produce the nuclides 269,270Hs in thereaction 248Cm(26Mg, 4n) and 248Cm(26Mg, 5n) conducted in
May of 2001 at GSI, valid data was collected during 64.2 h.8, 14During this time 1.0 × 1018 26Mg beam particles passed throughthe 248 Cm target. The IVO set-up was used to synthesize
volatile HsO4, which were adsorbed in the COLD detector.
Only α-lines originating from 211At, 219,220Rn, and their decay
products were identified. While 211At and its decay daughter
211
Po were deposited mainly in the first two detectors, 219,220Rn
and their decay products accumulated in the last three detectors, where the temperature was sufficiently low to partly
adsorb Rn. During the experiment, seven correlated decay
chains were detected (see Figure 8). All decay chains wereobserved in detectors 2 through 4 and were assigned to the
decay of either 269Hs or the yet unknown 270Hs. The characteristics of the three decay chains agreed very well with the decay
properties of 269Hs and its daughter nuclides observed in two

5
4

4

3

6

7

11

8
10

9

Figure 6. Schematic of the experimental set-up used for Hs chemistry.
The beam (1) passes through the rotating vacuum window (3.68
mg/cm 2 Be) and the target (2) assembly ( 248 Cm 239 µg/cm 2 , 692µg/cm2, and 730 µg/cm2 on 2.82 mg/cm2 Be). Hs nuclei formed infusion reactions recoil out of the target into a gas volume (3) and areflushed with a He/O2 mixture (4) to a quartz column (5) containing aquartz wool plug (6) heated to 600˚C by an oven (7). There, Hs isconverted to HsO4 which is volatile at room temperature and transported with the gas flow through a perfluoroalkoxy (PFA) capillary (8)to the Cryo On-Line Detector (COLD) registering the nuclear decay(α and spontaneous fission) of the Hs nuclides. The array consisted of
36 detectors arranged in 12 pairs (9), each detector pair consisted of 3
PIN diode sandwiches. Always 3 individual PIN diodes (top andbottom) were electrically coupled. A thermostat (10) kept the entranceof the array at 20˚C; the exit was cooled to −170˚C by means of liquid
nitrogen (11). Depending on the volatility of HsO4, the molecules
adsorbed at a characteristic temperature. Figure from Ref. 14.

Figure 7. Detector arrangement for the detection of 283112 consistingof pairs of Au and Pd coated PIPS detectors inside an assembly of 84
3
He filled neutron detectors.16

J. Nucl. Radiochem. Sci., Vol. 5, No. 2, 2004 R23

Gas Phase Chemistry of Superheavy Elements
269

9.18 MeV (3B)

α1

α2

50

94 MeV (3T) 85 MeV (3B)
7.920 s8.90 MeV (3B)
17.078 s257

No

Date: 12-May-2001
Time: 22:00:28

Date: 13-May-2001
Time: 10:02:07

8.21 MeV (3T)
55.567 s-40

Sg

8.50 MeV (3B)
0.846 sα3-208.88 MeV (3T)

Rf

Rf

SF

No

60
Hs

261

261

257

α4

8.68 MeV (3B)
9.325 sα28.69 MeV (3T)
4.409 s

Rf

253

265

Sg

261

8.50 MeV (3T)
2.365 s

α1

265

Sg

α3

9.10 MeV (3B)

α1

265

α2

269

Hs

-60

40

-80
30

-100
-120

20

-140

Fm

10

Date: 12-May-2001
Time: 09:55:03

-160
269

270

270

Hs

Hs

α1

9.16 MeV (2T)

8.66 MeV (2T)
25.665 sRf

α2

8.64 MeV (4B)
11.911 s

α2
261

Rf

Date: 12-May-2001
Time: 02:33:08

SF
94 MeV (4T) 92 MeV (4B)
1.216 sDate: 12-May-2001
Time: 17:09:09

or

270

Hs

Hs
α1

Sg

or

266

266

Sg

Sg

8.72 MeV (4T)

α2
261

Rf

Rf

or

262

Rf

Rf

SF

SF
53 MeV (4B)
3.124 sDate: 12-May-2001
Time: 14:59:39

0

1

2

3

4

5

6

7

8

9

10

11

12

-180

9.14 MeV (3B)

265

Sg

or

or

262

SF
94 MeV (2T) 93 MeV (2B)
0.199 s

Hs

α1
265

262

269

Hs

or

270

8.97 MeV (4B)

Sg

Sg

262

α1
266

266

α2

Temperature /˚C

Hs

Relative Yield /%269

108 MeV (3T)
42.615 s
Date: 14-May-2001
Time: 07:08:21

Figure 8. Observed decay chains attributed to the decay of 269Hs and270
Hs produced in the reaction 26Mg + 248Cm Ref. 8.

decay chains of 277112.5 The unusual α-decay energy of 8.52
MeV, which fits well into the systematic of ground-state αdecay energies, as well as a rather large spontaneous fission
branch could be confirmed for the nuclide 261Rf.5 The previously very well established decay properties of 261Rf (T1/2 = 78s, Eα = 8.28 MeV, SF < 10%) must therefore be assigned to along-lived isomeric state. Two additional decay chains weretentatively attributed to the decay of 270Hs. The last two decay
chains were incomplete and a definite assignment to 269Hs or
270
Hs could not be made. The tentative assignment of two
decay chains to 270 Hs is based on the following reasoning:
according to the measurements of Lazarev et al.3 the daughter
nuclide of 270Hs, 266Sg decays at least partially by emitting αparticles of 8.63 ± 0.05 MeV to 262 Rf which decays with ashort half-life (T1/2 = 2.1 ± 0.2 s) by spontaneous fission.3, 30
The nuclide 262Rf is known to fission symmetrically.30 In both
decay chains the α-decay energy attributed to 266Sg is in agreement with literature data.3 The short correlation times of both
SF events and the symmetric energy division among the fragments point to 262Rf as terminating nuclide. Nevertheless, only
measurements of the complete excitation function of the 26Mg
+ 248 Cm reaction will provide more definite answers. An
assignment of all observed decay chains to only 269Hs cannot
be ruled out. No additional three-member decay chains with a
total length of 300 s were registered in detectors 2 to 10. The
background count-rate of α-particles with energies between
8.0 and 9.5 MeV was about 0.6 h−1 per detector, leading tovery low probabilities of 7 × 10−5 and 2 × 10−3 for any of thefirst five chains and any of the last two chains, respectively,
being of random origin. In addition, four fission fragmentswith energies > 50 MeV that were not correlated with apreceding α-particle were registered in detectors 2 through 4.
Assuming an overall efficiency of ≈ 40%, production crosssections of 4 pb for α-decaying 270Hs and of 6 pb for α-decaying
269
Hs were calculated at a beam energy of 143.7–146.8 MeV(with an estimated accuracy of a factor of ≈ 3).
The longitudinal distribution of the 7 decay chains originatingfrom Hs is depicted in Figure 9. The maximum of the Hs distribution was found at a temperature of −44 ± 6 ˚C. The distribution of 172OsO4 (T1/2 = 19.2 s) measured before and after theexperiment showed a maximum in detector 6 at a deposition
temperature of −82 ± 7 ˚C. The higher deposition temperatureof about 40˚C and the thus about 7 kJ/mol higher adsorption
enthalpy seem to indicate a slightly lower volatility of HsO4

Detector number

Figure 9. Relative yields of HsO 4 and OsO 4 for each of the 12
detector pairs. Measured values are represented by bars: 269 HsO 4 :black; 270HsO4: dark grey; 269 or 270HsO4: white; 172OsO4: light grey. The
dashed line indicates the temperature profile (right-hand scale). The
maxima of the deposition distributions were evaluated as –44 ± 6˚C
for HsO4 and –82 ± 7 ˚C for OsO4. Solid lines represent results of aMonte Carlo simulation of the adsorption process.

compared to its lighter homologue OsO4. This experimental
result was somewhat unexpected, since according to both classical extrapolations and relativistic molecular calculations, HsO4
was predicted to be about as volatile as OsO4. Nevertheless, thehigh volatility of the Hs oxide species clearly suggests that it is
HsO4 since, by analogy with the known properties of the Os
oxides, all other Hs oxides are expected to be much less volatileand unable to reach the detector system. The observed formation of a volatile Hs oxide (very likely HsO4) provides strong
experimental evidence that Hs behaves chemically as an ordinary member of group 8 of the periodic table.
3.2. Recent Results on the Chemistry of E112. A firstattempt to chemically identify one of the long-lived isotopes of
E112, namely 283112 (SF, T1/2 ≈ 3 m),31 in the elemental state
was made by Yakushev et al.16 using the 48Ca + 238U production reaction. It should be noted that in very recent experiments at the Dubna gas-filled separator, this long-lived SF
activity could no longer be identified. Instead 283112 decayedby α-particle emission (Eα = 9.54 MeV, T1/2 ≈ 6 s) to shortlived 279110 (SF, T1/2 ≈ 0.3 s).15
In an experiment conducted in January of 2000 a total beamdose of 6.85 × 1017 48Ca ions was accumulated. The chemicalyield for the simultaneously produced 185Hg (T1/2 = 49 s) was
80%. If E112 behaved chemically like Hg and all efficiencies
measured for Hg were also valid for E112, detection of 2 – 4
SF events could be expected assuming the cross section value
for the production of 283112 measured in.16 However, no SF
events were observed. Therefore, no unambiguous answer asto the chemical and physical properties of E112 was obtained.
In the next experiment, the question whether E112 remainedin the gas phase and passed over the Au surfaces was addressed.17
For this purpose, a special ionization chamber to measure SF
fragments of nuclei remaining in the gas was added at the exitof the Au coated PIPS detector array. A total beam dose of 2.8
× 1018 48Ca ions were accumulated. Again zero SF events wereregistered on the Au and Pd coated PIPS detectors, confirming
the result of the first experiment. However, 8 SF events accompanied by neutrons were registered in the ionization chamber,while only one background count was expected. Therefore, themajority of the SF events were attributed to the decay of a
nuclide of E112, since there are no other known volatile nuclides
decaying by SF. From these experiments it appears that theinteraction of E112 with a Au or Pd surface is much weaker thanfor Hg. In the light of the latest results from Dubna the question
arises whether the observed SF events could be attributed to the
decay of 279Ds, the daughter of 6 s 283112. Calculations of theR24 J. Nucl. Radiochem. Sci., Vol. 5, No. 2, 2004
transport time of a volatile nuclide from the target to the ionization chamber show that indeed this scenario cannot be excluded
with certainty, but the experiment was designed to be mostsensitive for a 3-min nuclide.
Obviously, in a next step, the adsorption temperature of
E112 on Au surfaces has to be measured experimentally. Thiswas done with a modified version of the cryo thermochromatography detector in an experiment conducted recently at
GSI.18 A 1.5 mg/cm2 thick 238U target was bombarded with 2.8
× 1018 48Ca particles. A total of 11 events with energies > 40
MeV were registered with an estimated background of about 3
events. Seven events were registered at the cold end of thedetector where also Rn was deposited. However, none of these
events was correlated with a preceding α-particle. Therefore,
the detected events cannot yet be conclusively attributed to the
decay of E112 or its daughter.
4. Summary and Outlook
The spectacular results obtained at Flerov Laboratory in
Dubna32 using 48Ca ion beams and actinide targets to synthesize isotopes of elements 112, 114, 115, and 116 give chemists
the historic opportunity to experimentally investigate thechemistry of even the heaviest known elements. Especiallysince some of the nuclides and their decay products seem tohave half-lives of the order of seconds to even hours. In thelast couple years, chemists have made a quantum leap and are
now working with nuclides that can be produced with picobarn(10−36 cm2) cross sections only, as demonstrated by the firstchemical investigations of hassium. There are indications thatthe chemical investigation of E112 and also E114 seem toreveal strong relativistic effects that significantly alter thechemical behavior of these elements compared to their lighter
homologues. The unambiguous identification of the separated
nuclides by measuring their decay properties yields additional
valuable information about the nuclear structure of SHE. In
order to reduce interferences from the plasma created by thebeam and from undesired by-products of the nuclear reaction,
chemists are investigating possibilities to couple their equipment with recoil separators. First experiments with the recoiltransfer chamber coupled to the BGS were very successful. At
GSI components of the dismantled HECK separator33 are being
used to set-up a dedicated separator that suits the needs ofchemistry.
Due to the decreasing production cross sections, alsochemists have to rely on future accelerators that will deliver
even higher beam intensities. A substantial amount of beamtime is required for each heavy element experiment. Target
designs that allow the irradiation of exotic radioactive materials with highest possible beam intensities must be developed.
Intense, neutron-rich radioactive heavy ion beams will be theonly way to approach the predicted N = 184 neutron shell andproduce long-lived SHE. Prospects to experimentally study
the chemical properties of all currently known elements in the
periodic table, even though only single atoms can be produced,
are fascinating and challenging our creativity.
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