Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 2, pp. A21-A28, 2005

Radiochemical Studies of the Transactinide Element, Rutherfordium (Rf) at JAERIY. Nagame*
Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan
Received: June 30, 2005; In Final Form: August 27, 2005
Radiochemical studies of element 104, rutherfordium (Rf), at JAERI (Japan Atomic Energy Research Institute)are reviewed. The transactinide nuclide 261Rf has been successfully produced in the reaction 248Cm(18O, 5n) at the
JAERI tandem accelerator. The excitation function of the reaction has been measured with a rotating wheel
catcher apparatus and the maximum production cross section has been determined to be about 13 nb. On-line ionexchange experiments on Rf together with the group-4 elements Zr and Hf in acidic solutions have been
conducted with a rapid ion-exchange separation apparatus. From the systematic study of the anion-exchange
behavior of Rf, it has been found that the properties of Rf in HCl and HNO3 solutions are quite similar to those ofZr and Hf, definitely confirming that Rf is a member of the group-4 elements. However, we have observed an
unexpected chemical behavior of Rf in HF solutions; the fluoride complex formation of Rf is significantly
different from those of Zr and Hf. The current progress in the Rf aqueous chemistry is briefly summarized andprospects of extending chemical studies of transactinide elements at JAERI are considered.

1. Introduction

1

18

1

Presently, we know more than 20 man-made transuranium
elements. According to the actinide concept,1 the 5f electron
series ends with element 103, lawrencium (Lr), and a new 6delectron transition series is predicted to begin with element
104, rutherfordium (Rf). The elements with atomic numbers Z≥ 104 are called transactinide elements or recently calledsuperheavy elements. 2 The currently known transactinide
elements, elements 104 through 112, are placed in the periodic
table under their lighter homologues in the 5d electron series,
Hf to Hg. Elements from 113 to 118 except for 117 that havebeen recently synthesized3−5 would be in the successive 7p
electron series (see Figure 1), although the discoveries of
elements with Z ≥ 112 are waiting to be confirmed.6
Studies on the chemical properties of the elements at theuppermost end of the periodic table are extremely interesting
and challenging subjects in the fields of nuclear and radiochemistry. It is also of special interest to assess the magnitudeof the influence of relativistic effects on chemical properties.
From the calculations of electron configurations of heavier
elements, it is predicted that sudden changes in the structure ofelectron shells may appear due to relativistic effects which
originate from the increasingly strong Coulomb field of ahighly charged atomic nucleus.7 Therefore, it is expected thatheavier elements show a drastic rearrangement of electrons intheir 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 areconsequently predicted.2, 8−10 It would be no longer possible todeduce detailed chemical properties of the transactinide
elements simply from the position in the periodic table.
The transactinide elements must be produced at accelerators
using reactions of heavy-ion beams with heavy target materials
and must be identified by measurement of their decay or thatof their known daughter nuclei with unambiguous detection
techniques. Chemical experiments with the transactinide*Corresponding author. E-mail: nagame.yuichiro@jaea.go.jp.
FAX: +81-29-282-5927.

2

H

2

13

14

15

16

17

He

3

4

5

6

7

8

9

10

C

N

O

F

Ne

Li

Be

B

11

12

13

14

15

16

17

18

Na

Mg

3

4

5

6

7

8

9

10

11

12

Al

Si

P

S

Cl

Ar

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

K

Ca

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

Ga

Ge

As

Se

Br

Kr

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

Zr

Nb

Mo

Tc

Pd

Ag

Cd

In

Sn

Sb

Rb

Ru

Rh

55

56

57

72

73

74

75

76

77

78

79

80

81

82

83

84

85

86

Cs

Ba

Sr

La

Hf

Ta

W

Re

Os

Ir

Pt

Au

Hg

Tl

Pb

Bi

Po

At

Rn

87

88

89

104

105

106

107

108

109

110

111

112

113

114

115

116

Fr

Ra

Ac

Rf

Db

Ds

Rg

112 113 114 115 116

Lanthanides

57

58

59

60

61

62

63

64

65

66

67

68

69

70

71

La

Ce

Pr

Nd

Pm

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Actinides

Y

Sg

Bh

Hs

Mt

Te

I

Xe
118

118

89

90

91

92

93

94

95

96

97

98

99

100

101

102

103

Ac

Th

Pa

U

Np

Pu

Am

Cm

Bk

Cf

Es

Fm

Md

No

Lr

Figure 1. Periodic table of the elements.

elements are generally divided into the following 4 basic steps:i) synthesis of transactinide nuclides, ii) rapid transport of thesynthesized nuclides to chemical separation apparatuses, iii)
fast chemical isolation of a desired nuclide and preparation ofa sample suitable for nuclear spectroscopy, and iv) detection of
nuclides through their characteristic decay properties. Becauseof the short half-lives and the low production rates of the transactinide nuclides, each atom produced decays before a new
atom is synthesized. This means that any chemistry to be
performed must be done on an "atom-at-a-time" basis. Thus,
rapid and very efficient radiochemical procedures must be
devised. Recent comprehensive reviews on the chemistry of
the transactinide elements are seen in References 2, 11, and 12.
In order to carry out chemical experiments of the transactinide elements with single atoms, we developed some experimental apparatuses: a beam-line safety system for the usage ofa gas-jet coupled with a radioactive target and recoil chamber,a rotating wheel catcher apparatus for the measurement of α
and spontaneous fission (SF) decay of transactinide nuclei, andan automated rapid ion-exchange separation apparatus basedon high performance liquid chromatography coupled with anon-line α-particle detection system.13
The transactinide nuclide, 78-s 261Rf, that is commonly used
for recent chemical studies of element 104, was successfully
produced in the reaction 248Cm(18O, 5n) at the JAERI tandem
accelerator for the first time in Japan. The excitation functionof the reaction was measured with the rotating wheel catcher
apparatus MANON (Measurement system for Alpha particle
and spontaneous fissioN events ON-line), and the maximum© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 11/07/2005

NagameA22 J. Nucl. Radiochem. Sci., Vol. 6, No. 2, 2005
production cross section was determined to be 13 nb at around
94-MeV 18O energy.14 On-line anion-exchange experiments of
Rf together with the group-4 elements Zr and Hf in acidic solutions, HCl, HNO3, and HF, were conducted with the rapid ionexchange separation apparatus, AIDA (Automated Ion-exchange
separation apparatus coupled with the Detection system for
Alpha-spectroscopy).15, 16 AIDA enables us to perform cyclic
discontinuous column chromatographic separations of shortlived nuclides in aqueous solutions and automated detection of
α-particles within a typical cycle of 1 – 2 min. The adsorption
behavior of Rf as a function of the acid concentration has beensystematically studied with AIDA.15, 16
In this paper, the progress in the radiochemical studies of Rf
and future research plans for extending chemical studies of thetransactinide elements at JAERI are summarized.
2. Production of the Transactinide Nuclide 261Rf
A schematic representation of the experimental set-up
including a target and recoil chamber arrangement and therotating wheel catcher apparatus MANON is shown in Figure
2. A 248Cm target of 590-µg/cm2 thickness prepared by electrodeposition of Cm(NO3)3 from isopropyl alcohol onto a 2.2mg/cm2-thick beryllium backing foil was bombarded by an 18O
beam delivered from the JAERI tandem accelerator. The 18O6+
beam passed through a 2.0-mg/cm2 HAVAR entrance window
foil, 0.09-mg/cm 2 He cooling gas and the beryllium target
backing, and finally entering the 248Cm target material. The
beam intensity was 250 – 300 particle nA. The reaction products recoiling out of the target were stopped in a volume of Hegas (≈ 1 bar) that had been loaded with potassium chloride(KCl) aerosols generated by sublimation from the surface of
KCl powder at 640 ˚C. The products adsorbed onto the surfaceof the aerosols were swept out of the recoil chamber with the
He gas flow (2.0 L/min) and were transported through a Tefloncapillary (2.0 mm i.d., 20 m long) by a He/KCl gas-jet system
to MANON. The transported nuclei were deposited on polyethylene terephthalate foils of 120 µg/cm2 thickness and 20 mm
diameter mounted on the circumference of a stepwise rotating
stainless steel wheel that has 80-position collection sites with an
80-cm diameter. The wheel was rotated every 110 s to position
the foils between six pairs of Si PIN photodiodes for α-particle
detection. Each detector had an active area of 18 × 18 mm2. A
detection efficiency of approximately 40% for α-particles wasachieved. The energy resolution (FWHM) was about 30 keV
for the top detectors and was 100 keV for the bottom ones.
To determine the gas-jet transport yield, we first collected
the recoil of 252Md (T1/2 = 2.3 min) produced in the reaction

238

19

U( F, 5n) behind the target in an aluminum catcher foil to
get a reference value (100%), while 252Md transported by thegas-jet was collected on a glass filter. 252Fm (T1/2 = 25 h), theEC decay daughter of 252Md, in both samples was chemicallyseparated and subjected to α-spectroscopy. By comparing the
production rate measured after transport through the gas-jetwith the absolute production rate from the catcher foil, thetransport efficiency was determined to be approximately 35%.
The production cross sections were evaluated from themeasurement of the mother-daughter correlations of α-particle
energies between 261Rf (Eα = 8.28 MeV) and 25-s 257No (Eα =8.22, 8.27, and 8.32 MeV).17 A photograph of the MANON
apparatus is shown in Figure 3.
The sum of α-particle spectra measured in the six top detectors for the production of 261Rf at 94-MeV 18O beam energy isshown in Figure 4. The total beam dose was 2.35 × 1016. In
the α-energy range of 8.10 – 8.40 MeV, α lines from 261Rf andits daughter 257 No are clearly seen. No contributions fromother nuclides in this energy window are observed, although
there exist several α lines originating from the transfer reaction
products from the Pb impurity in the 248Cm target. A total of
166 events were registered both in the top and bottom detectors in the singles measurement and 57 α – α correlation eventswere detected at this beam energy. Assuming a 100% α-decaybranch for both 261Rf and 257No,17 the production cross sectionsof 261Rf in this reaction were evaluated to be 8 ± 2, 13 ± 3, and
8 ± 2 nb at the 18 O beam energies of 91, 94, and 99 MeV,
respectively. The contribution of the direct production of257
No via the 248Cm(18O, α5n) reaction is assumed to be negligible in the studied energy region. In Figure 5, the crosssections are plotted as a function of the 18O bombarding energy
together with the literature data,18, 19 where the relative crosssection values in Reference 19 are normalized to the present
results. The maximum cross section of about 13 nb at the 18O
energy of 94 MeV was obtained.14 This results in a production
rate of about 2 atoms per min under the given conditions.

Figure 3. View of the MANON (left) and the part of the detectors
inside the apparatus (right).

30

MANON: Measurement system for Alpha-particle
and spontaneous fissioN events ON-line

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

257

Rf
78 s

261

257

At
214m

5

253

Fm
3.0 d

Po

Po

10

At +

211m

Po

No
26 s

212m

Catcher foil
120 µg/cm2, 20 mm i.d.

261

214

Si PIN Photodiodes

16

211m

Wheel rotation

At +Recoils

15
Fr

HAVA R
window foil
2.0 mg/cm2

20

218m

Gas-jet inlet(He/KCl)Counts per 20 keV

Water cooledbeam stop

18

94-MeV O (2.35 × 10 p / 4.0 h)

Rf +

25

18O beam215

Gas-jet outlet

211m

248Cm target (590 µg/cm2) on Be backing

No

Po

He cooling gas

0
7

8

9

10

α-Energy / MeV

11

12

Figure 4. Sum of α−particle spectra measured in the bombardmentof the 248Cm target with 94-MeV 18O ions.

J. Nucl. Radiochem. Sci., Vol. 6, No. 2, 2005 A23

Radiochemical Studies of the Transactinide Element
10

2Classical chemistry
Phase 1

Phase 2Single atom chemistrytimePhase 1

Phase 2

1

Cross section / nb

2

10

3

1

4
5
6

10

0

Activity a1 > Activity a2
Equivalent

Present result
Ghiorso et al.
Silva et al.
10

Figure 6. Concept for single atom chemistry. Courtesy of M. Schädel.

18

19

-1

85

90

95

100

Probability P1 > Probability P2

105

110

E lab / MeV
Figure 5. Excitation function for the 248Cm(18O, 5n) reaction. The
data taken from References 18 and 19 are also shown.

3. Aqueous Chemistry of Element 104 Rf
The pioneering works, the first generation experiments, so
far performed have concentrated on the question how well the
periodic table accommodates the transactinide elements astransition metals in the seventh period. The results with a few
atoms have justified placing the elements Rf through element
108, hassium (Hs), into groups 4 to 8 of the periodic table.2
However, the first generation experiments still show conflicting
results and some of them are criticized due to unsatisfactory
experimental conditions.20 There are few convincing data for
examining relativistic effects and for considering thermodynamic chemical properties of the transactinide elements evenin those of Rf and element 105, dubnium (Db). Therefore, it isof great importance to study detailed chemical properties of thetransactinide elements with high statistics and to compare
these with properties deduced from extrapolations of periodicity and from modern relativistic molecular orbital calculations: the second generation experiments. In the following, thesecond generation studies on Rf at JAERI are outlined.
3.1. On-line Production of 261Rf, 169Hf, and 85Zr. As mentioned above, one of our experimental approaches is a detailed
comparison of the chemical properties of Rf with those of the
expected lighter homologues Hf and Zr. Thus, the chemical
experiments on Rf should be conducted together with thehomologues under strictly identical conditions. For the on-line
production of Rf and the homologues, we used two different
targets; one was the mixed target of 248 Cm (610 µg/cm 2 inthickness) and Gd (39.3%-enriched 152 Gd of 36 µg/cm 2 inthickness) prepared by electrodeposition onto a 2.4-mg/cm 2
beryllium foil for the simultaneous production of 261Rf and 3.24min 169Hf via the 248Cm(18O, 5n) and Gd(18O, xn) reactions,
respectively. The other was the natGd (370-µg/cm2 thickness)
and natGe (660-µg/cm2 thickness) mixed target to produce 169Hf
and 7.86-min 85Zr through the natGd(18O, xn) and natGe(18O, xn)
reactions, respectively. The natGd was electrodeposited on a
2.7-mg/cm2 beryllium backing and then, on the resulting natGd
target, natGe was deposited by vacuum evaporation.15, 16 The
reaction products recoiling out of the target were transported
through a Teflon-capillary to the chemistry laboratory by the
He/KCl gas-jet system as described in Section 2.
3.2. Atom-at-a-time Chemistry of Rf. The chemical study
needs to be carried out on phenomena that give the same
results for only a few atoms and for macro amounts of atoms;
the question arises as to whether a meaningful chemistry withsingle atoms is possible. For single atoms, the classical law ofmass 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 forsingle-atom chemistry the introduction of a specific thermodynamic function, the single-particle free enthalpy. 21, 22 An
expression equivalent to the law of mass action is derived, inwhich activities are replaced by probabilities of finding thespecies 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 canbe considered as repetitive static partitions, the displacement of
the atom along the chromatographic column gives a statisticalresult.2, 11 The concept for single atom chemistry is depicted in
Figure 6.
For short-lived atoms, the partition equilibrium must be
reached during the lifetime of the nuclides, which requires fast
reaction kinetics. Based on the kinetics of a single-stepexchange 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 bothstates are rapidly sampled.23 The above interpretation indicatesthat chromatographic systems with fast kinetics are ideallysuited for single atom separation as there is rapid, multiple
sampling of the adsorbed or mobile chemical species.2, 11
To perform rapid and repetitive chemical experiments withRf, we employed the apparatus AIDA15, 24 that consists of amodified ARCA (Automated Rapid Chemistry Apparatus)25
which is a miniaturized computer controlled liquid chromatography system, and an automated on-line α-particle detection
system. The schematic drawing of AIDA is given in Figure 7.
There are two column magazines, each containing 20 microcolumns. In the modified ARCA as shown in Figure 8, twodifferent paths to supply solutions are available; the first eluent
goes through the collection site to the microcolumn, while thesecond is directed to the column after one-step forward movement of the column magazine to avoid cross-contamination at
the collection site.
3.3. Anion-exchange Behavior of Rf in HCl and HNO 3.
The experimental procedures and the result on the anionexchange behavior of Rf in HCl solution are summarized here.
The reaction products recoiling from the target were transported by the He/KCl gas-jet system to the collection site of
AIDA (see Figure 8). After collection for 125 s, the site wasmechanically moved to the top of one of the microcolumns,
where the products were dissolved with 170 µL of hot (≈ 80
˚C) 11.5 M HCl and were fed onto the 1.6 mm i.d. × 7.0 mm
chromatographic column filled with the anion-exchange resin

NagameA24 J. Nucl. Radiochem. Sci., Vol. 6, No. 2, 2005
He/KCl gas-jet
Signal out

Pulse motors

600 mm2 PIPS detectors

Preamp.

Air cylinder
Eluent bottles
Micro-columns

Halogen lamp

He gas heater
Ta disk reservoir

Figure 7. Schematic drawing of AIDA (Automated Ion-exchange
separation apparatus coupled with the Detection system for Alpha
spectroscopy).

5

261Rf 8.280 MeV

5 cm

Resin, MCI GEL CA08Y (particle size: 20 µm)
20 micro columns
1.6 mm i.d. × 7.0 mm

254Fm 7.150, 7.192 MeV

210Fr 6.543 MeV, 211Fr 6.534 MeV

257No 8.220, 8.270, 8.320 MeV

Discard(a)
214Po 7.687 MeV (Daughter of 222Rn)

Ta diskα/γ-spectroscopy

252Fm 6.998, 7.039, 255Fm 7.016 MeV

10246Cf 6.675 MeV

Fraction 2

209Fr 6.646 MeV

Fraction 1

15

246Cm 5.387, 5.344 MeV

Magazine

248Cm 5.035, 5.078 MeV

20
Gas exhaust

SliderEffluent

151Dy 4.067 MeV, 150Dy 4.233 MeV

Eluent11.5 M HCl (≈ 80˚C) 170 µL, 1.0 mL/min
1st: 4.0 - 9.5 M HCl 290 µL
2nd: 4 M HCl 250 µL
1.1 mL/min

Counts / 10 keV

Collection site

25

Side view

He/KCl gas-jet in

244Cm 5.763, 5.805 MeV

Front view

Figure 9. Photograph of the sample collection part of AIDA.

218Po 6.003 MeV (Daughter of 222Rn)

Sampling table

0

Figure 8. Schematic of the modified ARCA in AIDA.

4.0

5.0

6.0

7.0

8.0

9.0

α-Energy / MeV
25(b)257No 8.220, 8.270, 8.320 MeV

261Rf 8.280 MeV

5214Po 7.687 MeV (Daughter of 222Rn)

10

254Fm 7.150, 7.192 MeV

244Cm 5.763, 5.805 MeV

15218Po 6.003 MeV (Daughter of 222Rn)

20Counts / 10 keV

MCl GEL CA08Y (particle size of about 20 µm) at a flow rateof 1.0 mL/min. Then, the products were eluted with 290 µL of
HCl through the second path with concentrations between 4.0
M and 9.5 M at a flow rate of 1.1 mL/min. The effluent wascollected on a Ta disk as fraction 1 and evaporated to dryness
using hot He gas and a halogen heat lamp for α-particle
measurement (see Figure 7). The remaining products in thecolumn were eluted with 250 µL of 4.0 M HCl at a flow rate of
1.1 mL/min. The effluent was collected on another Ta disk
and evaporated to dryness as fraction 2. The pair of disks wereautomatically transported to the α-spectroscopy stationequipped with eight 600-mm2 PIPS (passivated ion-implanted
planar silicon) detectors. Counting efficiencies of eachdetector ranged from 30% to 40% depending on geometricdifferences of the dried α sources. Alpha-particle energy resolution was 80 – 200 keV FWHM. 15, 16 After the α-particle
measurement, the 493 keV γ-radiation of 169Hf was monitoredwith Ge detectors to determine the elution behavior of Hf andits chemical yield. The anion-exchange experiments with 85Zr
and 1 6 9 Hf produced from the Ge/Gd mixed target wereconducted under the same conditions as those with 261Rf and169
Hf. The effluents were collected in polyethylene tubes andwere assayed by γ-ray spectroscopy.15 Each separation was
accomplished within 20 s and the α-particle measurement wasstarted within 80 s after the collection of the products at the
AIDA collection site. The chemical yield of 169Hf including
deposition and dissolution efficiencies of the aerosols wasapproximately 60%.15 A photograph of the effluent collection
part of AIDA is shown in Figure 9.
Figures 10(a) and 10(b) show the typical α spectra observed in
the elution with 11.5 M HCl and with 4.0 M HCl, respectively.26
There are only a few α events from the α decay of 261Rf – 257No

0
4.0

5.0

6.0

7.0

8.0

9.0

α-Energy / MeV

Figure 10. α-particle spectra of samples prepared from the elution
fractions: (a) 11.5 M HCl and (b) 4.0 M HCl. Data taken from Reference26.

at 11.5 M HCl, which means Rf is strongly adsorbed on theanion-exchange resin, while at 4.0 M HCl, Rf is not retained in
the column.
In order to evaluate the contribution to the adsorption distribution of 261Rf on the anion-exchange resin of the α events from257
No formed from the α decay of 261Rf during the collection at
AIDA before the chemical separation, the adsorption behaviorof No on the same anion-exchange resin was examined. On-line
experiments with 255No produced in the reaction 248Cm(12C, 5n)were separately conducted according to those with 261Rf in HCl

J. Nucl. Radiochem. Sci., Vol. 6, No. 2, 2005 A25

Radiochemical Studies of the Transactinide Element
27

and HNO3 solutions, and it was found that this contribution
was negligible.27 As shown in Figure 10(a), the Cm isotopes,
recoil products of the target, the isotopic composition of the248
Cm target is as follows: 244Cm (1.12 at.%) and 246Cm (1.31
at.%), and the Fm isotopes, transfer reaction products from theCm target, are mostly eluted with 11.5 M HCl. The present
result is consistent with the fact that the chloride complex
formation of element 104 in 12 M HCl was stronger than thatof the trivalent actinides as reported by Hulet et al.28 The trivalent rare earth element isotopes in the figure, 150,151 Dy, thetransfer reaction products from the 152Gd target, are also eluted
with 11.5 M HCl.
From the 1893 cycles of the anion-exchange experiments, a
total of 186 α events from 261Rf and its daughter 257No wereregistered including 35 α – α correlation events.15 Figure 11(a)shows the adsorption behavior of Rf, Zr, and Hf as a functionof HCl concentration. The ordinate shows the adsorption
probabilities of these elements, %ads = 100A2 /(A1 + A2), whereA1 and A2 are the eluted radioactivities observed in fractions 1
and 2, respectively. It should be noted here that the data of Hffrom both targets are in very good agreement. The adsorption
behavior of Rf is quite similar to those of the group-4 elements
Zr and Hf. The adsorption of these elements rapidly increases
with increasing HCl concentration above 7 M: typical anionexchange behavior of the group-4 elements. This means thatanionic chloride complexes of the tetravalent Rf, Zr, and Hf
are formed above 7 M HCl. On the other hand, the adsorptionof the tetravalent pseudo-homologue thorium (Th) in HCl
concentrations above 8 M is quite different from that of Rf, Zr,
and Hf as shown in Figure 11(b); Th does not form anioniccomplexes in this region of concentration. The distribution
coefficients (K d ) for Zr, Hf, and Th in Figure 11(b) wereobtained by the batch method using radiotracers of 88Zr, 175Hf,

Adsorption probability

120

(a)

100
80

261Rf (Cm/Gd)
169Hf (Cm/Gd)

60

85Zr (Ge/Gd)234

and Th, respectively.15 Another interesting feature is theadsorption sequence on the anion-exchange resin among Rf,
Zr, and Hf. The adsorption order that reflects the strength ofthe chloride complex formation is Rf ≥ Zr > Hf. The present
outcome, however, contradicts the prediction with the relativistic molecular density-functional calculations by Pershina
et al. where the sequence of the chloride complex formation is
expected to be Zr > Hf > Rf.29
In order to obtain information on the chemical species andstructure of Rf complexes in HCl, we performed measurementsof the extended X-ray absorption fine structure (EXAFS) spectraof Zr and Hf chloride complexes at the KEK (High Energy
Accelerator Research Organization) Photon Factory.30, 31 It has
been found that the Zr and Hf complex structure in HCl solution changes from presumably [M(H2O)8]4+ (M = Zr and Hf) to
the anionic chloride complex [MCl6]2− with the increase in the
HCl concentration from 9 to 12 M, which is consistent with theresults of the anion-exchange experiments. Therefore, we
assume the structure of the Rf complex, in analogy to those ofZr and Hf, is the octahedral [RfCl6]2− in conc. HCl. The difference in the strength of the chloride complex formation of Zr
and Hf was also demonstrated; the affinity of the Cl− ion for Zr
is higher than that for Hf: [ZrCl6]2− > [HfCl6]2−,31 which agrees
with the adsorption sequence in Figures 11(a) and 11(b) andthe result by Huffman et al.32 This confirms that the sequencein the chloride complex strength among these elements is Rf ≥Zr > Hf.
Non Th4+-like behavior of Rf was also probed with anionexchange experiments in 8 M HNO3. From 217 experiments, a
total of 20 α events from 261 Rf and 257 No were observed
including 5 time-correlated α pairs. Although Th4+ formed
anionic complexes and was strongly adsorbed on the anionexchange resin, Rf was eluted from the column with 8 M HNO3
as expected for a typical group-4 element as shown in Figure
12.15 The adsorption value for Th was obtained by batch experiments using radiotracer 234Th.15 The above results definitely
confirm that Rf is a member of the group-4 elements, but does
not resemble the pseudo-homologue Th.
3.4. Fluoride Complex Formation of Rf. The experimental
procedures with AIDA are basically the same as thosedescribed in Section 3.3 and the details are given in Reference

169Hf (Ge/Gd)

40

120

20

100
0
5

10

4

10

3

10

2

10

1

10

0

Adsorption probability

K d / mL g -1

10

(b)
88Zr
175Hf
234Th

80

261

Rf (Cm/Gd)

169

Hf (Cm/Gd)85

60

Zr (Ge/Gd)169

Hf (Ge/Gd)234

Th (off-line)

40

20

10

-1

0
3

4

5

6

7

8

9

10

11

12

HCl concentration / M
Figure 11. (a) Adsorption probability (%ads) of Rf, Zr, and Hf on theanion-exchange resin CA08Y as a function of HCl concentration. The
data for 261Rf and 169Hf obtained from the Cm/Gd target are shown by
closed symbols, while those for 85Zr and 169Hf from the Ge/Gd target
are by open symbols. (b) Distribution coefficients (Kd) for Zr, Hf, andTh on the anion-exchange resin CA08Y as a function of HCl concentration. Data taken from Reference 15.

0

2

4

6

8

10

12

14

HNO3 concentaration / M
Figure 12. Adsorption probability (%ads) of Rf, Zr, and Hf on theanion-exchange resin CA08Y at 8 M HNO3 concentration. The datafor 261 Rf and 169 Hf obtained from the Cm/Gd target are shown by
closed symbols, while those for 85Zr and 169Hf from the Ge/Gd target
are by open symbols. The adsorption of Th (closed triangle) was
obtained by the separate off-line experiment. Data taken from
Reference 15.

NagameA26 J. Nucl. Radiochem. Sci., Vol. 6, No. 2, 2005
120

(b)

(a)

261Rf (Gd/Cm)
169Hf (Gd/Cm)

Adsorption probability

100

85Zr (Ge/Gd)
169Hf (Ge/Gd)

80

89mZr (Y)167Hf (Eu)60
261Rf (Cm/Gd)

40

169Hf (Cm/Gd)
85Zr (Ge/Gd)20

169Hf (Ge/Gd)

0
10

0

10

1

10

0

10

[HF] ini / M

1

10

2

[HF] ini / M

Figure 13. Adsorption behavior of Rf, Zr, and Hf on the anion-exchange resin CA08Y as a function of the initial concentration of HF, [HF] ini
obtained with (a) the 1.6 mm i.d. × 7.0 mm column and with (b) the 1.0 mm i.d. × 3.5 mm one. The data for 261Rf and 169Hf obtained from theCm/Gd target are shown by closed symbols, while those for 85Zr and 169Hf from the Ge/Gd target are by open ones. The adsorption values of 89mZr
and 167Hf produced from the 89Y and natEu targets, respectively, are also included in (b). Data taken from Reference 16.

H+ + F − →
← HF

(1)

−
HF + F − →← HF2 .

(2)

According to the dissociation constants K1 = 935 M−1 and K2 =3.12 M−1 for eqs (1) and (2), respectively,38 above 1 M [HF]ini,
the concentration of the anionic HF2− species is more than one
order of magnitude higher than that of the free F −. Thus, the Kdvalues for Rf, Zr, and Hf in Figure 14 are plotted as a functionof the concentration of HF2−, [HF2−].24 The Kd values for Rf
were deduced from the adsorption probability in Figure 13 byassuming that the kinetics in the fluoride complex formationand the anion-exchange processes of Rf are as fast as those of
the homologues Zr and Hf as described in Reference 16, while
those for Zr and Hf were from the on-line column chromatographic experiments with AIDA. A decrease of the Kd values
for these elements with [HF2−] is explained as the displacement

10 3

K d / mL g -1

16. In this study, we used two different microcolumns, 1.6
mm i.d. × 7.0 mm and 1.0 mm i.d. × 3.5 mm, to measure awide range of Kd values. From the 4226 cycles of the anionexchange experiments, a total of 266 α events from 261Rf andits daughter 257No were registered including 25 time-correlated
α pairs of 261Rf and 257No. Figures 13(a) and 13(b) show thevariation of the adsorption probability of Rf, Zr, and Hf as afunction of the initial concentration of HF, [HF]ini for the twocolumns. 16 The adsorption values of Zr and Hf were alsomeasured in separate experiments using 89m Zr and 167 Hf
produced in the 89Y(p, n) and natEu(19F, xn) reactions, respectively.16 The contribution of the α events from the daughter
nucleus 257 No formed from the α decay of 261 Rf before thechemical separation procedures was also evaluated to be lessthan 5% in HF solution.33 As shown in Figure 13, the adsorption values of Zr and Hf obtained in the different experiments
agree well, and the adsorption of Zr and Hf is absolutely equalover a wide range of HF concentrations and steeply decreases
with [HF]ini, while that of Rf decreases at much lower [HF]ini.
The lower adsorption of Rf indicates that the fluoride complex
formation of Rf is weaker than those of Zr and Hf. The
present result is consistent with those reported in References
34 – 37 where a different behavior of Rf as compared to that of
the homologues in the fluoride complex formation wasobserved.
It is well known that the dissociation of HF is as follows,38

10 2

10 1

10 0

Rf 1.6 i.d. × 7.0 mm
Rf 1.0 i.d. × 3.5 mm
Zr 1.6 i.d. × 7.0 mm
Zr 1.0 i.d. × 3.5 mm
Hf 1.6 i.d. × 7.0 mm
Hf 1.0 i.d. × 3.5 mm

10 -1[HF2- ] / M

10 0

Figure 14. Distribution coefficients (Kd) for Rf, Zr, and Hf on theanion-exchange resin CA08Y as a function of the concentration of
HF2−. The data from the 1.6 mm i.d. × 7.0 mm and 1.0 mm i.d. × 3.5
mm columns are depicted by open and closed symbols, respectively.
Data taken from References 16 and 24.

of the metal complex from the binding sites of the resin by thecounter anion HF2− as,n−
Rn – MF4+n + n⋅HF2− →← n⋅R – HF2 + MF4+n(3)(M = Rf, Zr, and Hf),
where R denotes the resin. As shown in Figure 14, the Kdvalues of these elements decrease linearly with [HF2−] in thelog Kd vs. log [HF2−] plot. It should be noted that the slopes
for Zr and Hf are clearly –3 (dashed line), while that for Rf issignificantly different, i.e. -2 (solid line). Equation 3 implies
then that Rf is likely to be present as the hexafluoro complex,[RfF 6 ] 2− similar to the well known [ZrF 6 ] 2− and [HfF 6 ] 2− atlower [HF]ini, while Zr and Hf are likely to be present in theform of the heptafluoro complexes, [ZrF7]3− and [HfF7]3− assuggested in References 39 and 40. The activity coefficientsof the involved chemical species, however, are not available inthe case of the higher concentration of HF, thus the definite
identification of the anionic fluoride complexes is presently an
open question.
Recently, we have studied the anion-exchange behavior of Rf
in the mixed HF/HNO3 solutions to further the understanding of

J. Nucl. Radiochem. Sci., Vol. 6, No. 2, 2005 A27

Radiochemical Studies of the Transactinide Element

the fluoride complex formation of Rf. The Kd values for Rf andthe homologues Zr and Hf have been measured as a function of
the fluoride ion concentration [F −].41 The results indicate thatat constant concentration of the nitrate ion [NO3−] = 0.01 M,
the formation of the hexafluoro anionic complexes of Zr andHf occurs at a fluoride ion concentration of about 10−5 M, whilethat of Rf started at around 10−3 M. There was about two-orders ofmagnitude difference in the fluoride ion concentration of Rf andthe homologues for the formation of the hexafluoro complexes.
This also clearly demonstrates that the formation of the fluoride
complexes of Rf is much weaker than those of the homologues
Zr and Hf.41
Relativistic density-functional calculations of the electronic
structures of hexafluoro complexes ([MF6]2−, M=Rf, Zr, and Hf)have been performed to evaluate the stability of the complexes.
The results indicate that the sequence in the overlap populations
between the valence d orbitals of M4+ and the valence orbitals of
F − is found to be Zr ≈ Hf > Rf, suggesting that the Rf complex
is less stable than those of Zr and Hf in the [MF6]2− structure.42
Although the calculations qualitatively agree with the experimental results, further theoretical approaches are needed toquantitatively understand the present interesting results.
4. Summary and Perspectives
Anion-exchange behavior of rutherfordium 261Rf producedin the 248Cm(18O, 5n) reaction was systematically studied basedon an atom-at-a-time scale. The adsorption probabilities of Rf
in anion-exchange chromatography were measured with highstatistics, which has made it possible to perform a detailed
comparison of the properties of Rf with those of the homologues
Zr and Hf. It was found that the anion-exchange behavior of Rf
is quite similar to those of Zr and Hf in HNO3 and HCl solutions, clearly confirming that Rf is a member of the group-4
elements, and Rf does not behave like the tetravalent pseudohomologue Th. On the other hand, a large difference in theadsorption behavior of Rf and the homologues on the anionexchange resin was observed in HF solution; the fluoride
complex formation of Rf is much weaker than those of thehomologues. According to the HSAB (Hard and Soft Acids
and Bases) concept,43 the fluoride anion is a hard anion andinteracts stronger with (hard) small cations. Thus, a weakerfluoride complex formation of Rf as compared to Zr and Hf
would be reasonable if the size of the Rf4+ ion is larger thanthose of Zr4+ and Hf4+ (0.08 nm vs. 0.07 nm) as predicted by
Johnson and Fricke. 44 The fact that the chloride complex
formation of Rf is slightly stronger than those of the homologues would also be explained by this concept, because thechloride anion is a relatively soft anion. (The different adsorption behavior of Zr and Hf in HCl is not interpreted with thisconcept, since the radii of Zr4+ and Hf4+ are known to be very
similar.) To enhance the understanding of the chemical properties of Rf, the complex formation study with the softer halide
anions (Br − and I − ) will give some valuable information.
Quantitative theoretical calculations are also required to gain
more knowledge on the complex chemistry of Rf.
In further studies of Rf aqueous chemistry, experiments onreversed-phase extraction chromatography with AIDA are
under way to study complex formation ability of Rf withvarious kinds of organic extractants. Also experiments arebeing performed to obtain some thermodynamic equilibrium
constants of Rf in fluoride complex formation. On the basis ofthese quantitative data, the influence of relativistic effects on
chemical properties of Rf will be discussed.
Anion-exchange experiments with 34-s 262Db produced in
the 248Cm(19F, 5n) reaction with the cross section of 1.3 nb are
also being conducted with AIDA.30, 45 To obtain more accuratedata, however, we need to develop a new device to shorten thetime for the preparation of the α sources. An improved AIDA

based on a continuous ion-exchange system is under construction.
Acknowledgment. The research work reported was carriedout by many colleagues that make up the Heavy Element
Nuclear Chemistry Group at JAERI. The list includes K.
Akiyama, M. Asai, S. Goto, H. Haba, M. Hirata, S. Ichikawa,
Y. Ishii, I. Nishinaka, M. Sakama, T. K. Sato, A. Toyoshima,
and K. Tsukada. The present study is supported in part by the
JAERI-University Collaboration Research Program and by 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 thank all colleagues participatedin the experiment and contributed to this study from Wako,
Niigata, Osaka, Kanazawa, Tokyo, Tsukuba, Shizuoka,
Tokushima, Darmstadt, Mainz, Bern/Villigen, and Munich.
We are grateful to M. Schädel for his continuous help and valuable suggestions from the beginning of this work. We also
acknowledge W. Brüchle, H. W. Gäggeler, H. Kudo, J. V. Kratz,
Y. Oura, A. Shinohara, K. Sueki, A. Türler, and A. Yokoyama
for their kind and constant support. Our thanks go to the staffof the JAERI tandem accelerator, Advanced Science ResearchCenter, and Nuclear Physics Group at JAERI, and other allcolleagues for their continuous help. The author appreciates
kindness of P. J. Karol for reading and correcting the manuscript. Last, he is sincerely indebted to H. Nakahara for histhoughtful and constant advice and encouragement throughout
the course of this work.
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