Journal of Nuclear and Radiochemical Sciences,Vol. 3, No. 1, pp. 129–132, 2002

129

Status and Prospects of Heavy Element Nuclear Chemistry Research at JAERIY. Nagame,∗ M. Asai, H. Haba, K. Tsukada, S. Goto,† M. Sakama,‡ I. Nishinaka, A. Toyoshima,
K. Akiyama, and S. Ichikawa
Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195,
Japan
Received: March 3, 2002; In Final Form: May 9, 2002
The present status of heavy element nuclear chemistry research at JAERI (Japan Atomic Energy Research Institute)is reviewed. Successful production of the transactinide nuclei, 261 Rf and 262 Db via the reactions of 248 Cm(18 O, 5n)
and 248 Cm(19 F, 5n), respectively at the JAERI tandem accelerator is presented. Aqueous chemistry of Rf is being
carried out with a newly developed rapid ion-exchange separation apparatus. Anion-exchange behavior of Rf in HCl
solution is shortly discussed. Recent experimental results on decay studies of neutron-deficient actinide nuclei using
the gas-jet coupled JAERI-ISOL are also given. Prospects for the near future studies are briefly considered.

1. Introduction
Nuclear chemistry study of the transactinide elements inJapan is currently being performed using the JAERI tandem accelerator. We have developed some experimental apparatuses:
a beam-line safety system for the usage of the gas-jet coupledradioactive 248 Cm target and recoil chamber, a rotating wheel
catcher apparatus for the measurement of α and spontaneous
fission decay of transactinides, and an automated rapid ionexchange separation apparatus based on the high performanceliquid chromatography coupled with an on-line α-particle detection system.1
The schematic of the experimental setup for the study of thetransactinide elements is shown in Figure 1. The transactinide
nuclei, 78-s 261 Rf and 34-s 262 Db which are commonly used forrecent chemical studies of element 104 and 105, have been successfully produced for the first time in Japan through the reactions of 248 Cm(18 O, 5n) and 248 Cm(19 F, 5n), respectively. The
production cross sections for each reaction have been accurately
measured with the newly developed rotating wheel catcher apparatus MANON (Measurement system for Alpha particle andspontaneous fissioN events ON line).2
On-line anion-exchange experiments of Rf together with thegroup 4 elements Zr and Hf in HNO3 and HCl solutions havebeen conducted with AIDA (Automated Ion exchange separation apparatus coupled with the Detection system for Alpha
spectroscopy). The results have clearly shown that ion-exchange
behavior of Rf is quite similar to that of Zr and Hf, and is different from that of the tetravalent pseudo-homologue Th, indicating
Rf is the member of the group 4 elements.3
Studies of α/EC (electron capture)-decay properties ofneutron-deficient actinide nuclei have been carried out using
the gas-jet coupled JAERI-ISOL system and recently the newisotopes 233,236 Am and 237 Cm have been unambiguously identified.4–6
In this report, the present status of nuclear chemistry studies of heavy elements and research plans for the near future at
JAERI are summarized.

livered from the JAERI tandem accelerator. The reaction products recoiling out of the target were stopped in He gas (∼1 bar),
attached to KCl aerosols, and were transported through a Teflon
capillary to the rotating wheel apparatus MANON. The transported nuclei were deposited on polyethylene terephthalate foilsof 120 µg/cm2 thickness. The wheel was periodically rotated toposition the foils between six pairs of Si PIN photodiodes for
α-particle detection. The production cross sections were evaluated from the mother-daughter correlations of α energies between 261 Rf and 257 No, and 262 Db and 258 Lr. The experimental
details are described in Reference 2.
The typical α-particle spectrum measured in the six top detectors during irradiation of 1.6 ×1017 19 F ions for the productionof 262 Db in the 106 MeV 19 F-induced reaction of 248 Cm is shownin Figure 2(a). In the α-energy range of 8.40–8.70 MeV, α lines
from 34-s 262 Db (8.45, 8.53, and 8.67 MeV) and its daughter
3.9-s 258 Lr (8.565, 8.595, 8.621, and 8.654 MeV) are observed.
Figure 2(b) shows the mother-daughter (α-α) correlation plot
for the 106 MeV 19 F + 248 Cm reaction. A total of 30 α-α correlation events were registered. The literature values for the αenergies of 262 Db and 258 Lr are indicated by the solid lines.
From the mother-daughter correlation events, the production
cross section of 261 Rf was evaluated to be 13 ± 3 nb at the 18 O energy of 94 MeV, while that of 262 Db was 1.3 ± 0.4 nb at 106 MeV
19
F (Ref. 2).
3. Aqueous Chemistry of Rf
Study of the chemical properties of the transactinide elements
is challenging both experimentally and theoretically.7 Due to inHe Cooling Gas
248Cm Target on Be Backing

Gas-jet Outlet
18O and 19F Beams

Water Cooled
Beam StopHAVAR Window
2.0 mg/cm2

Gas-jet Inlet(He/KCl)

Recoils

2. Production of 261 Rf and 262 Db

AIDAAs shown in Figure 1, the 248 Cm target of 590 µg/cm2 thickness prepared by electrodeposition onto a 2.2 mg/cm2 thickberyllium backing foil was bombarded by 18 O and 19 F beams de∗Corresponding author. E-mail: nagame@popsvr.tokai.jaeri.go.jp.

FAX: +81-29-282-5927.
†Permanent address: Department of Chemistry, Niigata University,
Niigata 950-2181, Japan.
‡Permanent address: Department of Radiologic Science and Engineering, University of Tokushima, Tokushima 770-8509, Japan.

MANON

Solution Inlet

Gas Out

Wheel Rotation
Effluent

Si PIN Photodiodes

Column Magazine

Catcher Foil
120 µg/cm2, 20 mm i.d.

Effluent
Ta Disk

Figure 1. Schematic drawing of the experimental setup.

c 2002 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 6/30/2002

130 J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

Nagame262

Po

α-Energy (daughter) / MeV

Lr

At
214m

At +
Po

Db +

Lr
3.9 s

211m

214

258

Po

Po
213

262

At +

211m

258

α

215

20

Db
34 s

212m

Fr

α

218

217

Fr

30

10

0
7.0

(b)

262

Rn

211m

(a)

218m

Counts per 20 keV

40

Db

9.0

Po

50

8.0

9.0

10.0

11.0

α-Energy / MeV

12.0

8.8
258

8.6

Lr

8.4

8.2

8.0
8.0

8.2

8.4

8.6

8.8

9.0

α-Energy (mother) / MeV

Figure 2. (a) Sum of α-particle spectra measured in the bombardment of the 248 Cm target with 106 MeV 19 F ions. (b) Mother-daughter correlations
for the above reaction. The α energies of the mother and daughter for each correlated event are plotted. The literature values of the mother anddaughter α-particle energies are shown by the solid lines.

creasingly strong relativistic effects, increasing deviations fromthe periodicity of the chemical properties based on extrapolations from the lighter homologues are predicted. Thus, the experimental approach should involve a detailed comparison of
the chemical properties of the transactinides with those of theirlighter homologues under identical conditions. We have investigated the anion-exchange behavior of Rf together with thelighter homologues Zr and Hf in the same on-line experiments.3
To perform fast and repetitive ion-exchange separation of Rf,
we have developed the apparatus AIDA that consists of a modified ARCA (Automated Rapid Chemistry Apparatus) whichis the miniaturized computer-controlled liquid chromatographysystem8 and an automated on-line α-particle detection system.
In the following, we briefly introduce the experiment and apart of the result on the anion-exchange behavior of Rf in HCl
solution. 78-s 261 Rf was produced in the 94 MeV 18 O-induced
reaction of 248 Cm. As shown in Figure 1, the reaction products
recoiling out of the target were transported by the He/KCl gasjet system to the collection site of AIDA, where the products
were dissolved with 170 µL of hot (∼80 ◦ C) 11.5 M HCl andwere fed onto the 1.6 mmφ × 7 mm chromatographic column
filled with the anion exchange resin MCl GEL CA08Y (particle

257No 8.220, 8.270, 8.320 MeV

254Fm 7.150, 7.192 MeV

214Po 7.687 MeV (Daughter of 222Rn)

252Fm 6.998, 7.039, 255Fm 7.016 MeV

209Fr 6.646 MeV
246Cm 6.675 MeV

210Fr 6.543 MeV, 211Fr 6.534 MeV

244Cm 5.763, 5.805 MeV(a)

261Rf 8.280 MeV

5218Po 6.003 MeV (Daughter of 222Rn)

10

246Cm 5.387, 5.344 MeV

15

248Cm 5.035, 5.078 MeVCounts / 10 keV

20

151Dy 4.067 MeV, 150Dy 4.233 MeV

25

size: about 20 µm) at a flow rate of 1.0 mL/min. The effluent collected on a Ta disk as fraction 1 was evaporated to dryness, and the remaining products in the column were eluted with
280 µL of 4.0 M HCl as fraction 2. The pair of Ta disks wereautomatically subjected to α spectrometry with eight 600-mm2
passivated ion-implanted planar silicon (PIPS) detectors.
Figures 3(a) and 3(b) show the α spectra observed in the first
fraction with 11.5 M HCl and that in the second fraction with
4.0 M HCl, respectively. There are only a few α events from
261
Rf–257 No at 11.5 M HCl, which means Rf is strongly adsorbed by the anion-exchange resin, while at 4.0 M HCl, Rf
is not retained in the column. The contribution of the α eventsfrom 257 No formed from the α decay of 261 Rf before the chemical processing was negligible.9 Further we confirmed that No
was entirely eluted with 11.5 M HCl.9 As shown in Figure 3(a),the Cm isotopes, recoil products of the target (the isotopic composition of the 248 Cm target is follows: 244 Cm (1.12 at.%) and246
Cm (1.31 at.%)) and the Fm isotopes, transfer reaction products from the Cm target are mostly eluted with 11.5 M HCl.
The present result is consistent with the fact that the chloride
complexation of element 104 in 12 M HCl was stronger thanthat of the trivalent actinides.10 The rare earth element isotopes
150,151
Dy, the transfer reaction products from 152 Gd in the 248 Cmtarget are also eluted with 11.5 M HCl. The on-line experimentswith Zf and Hf isotopes were carried out under identical conditions as those with 261 Rf (Ref. 3).
As reported in Reference 3, the adsorption behavior of Rf
as a function of HCl concentration was similar to that of thegroup 4 elements Zr and Hf, and was quite different from thatof the tetravalent pseudo-homologue Th in HCl concentrationof >8 M, indicating that the anionic chloride complexes of thetetravalent Rf, Zr, and Hf are formed in concentrated HCl. The
Multiple target chamber
HAVAR Window0Gas-jet Inlet (He/PbI2)

25

6Li Beams(b)

Targets

257No 8.220, 8.270, 8.320 MeV

261Rf 8.280 MeV

214Po 7.687 MeV (Daughter of 222Rn)

5

254Fm 7.150, 7.192 MeV

10218Po 6.003 MeV (Daughter of 222Rn)

15
244Cm 5.763, 5.805 MeVCounts / 10 keV

20
Gas-jet Outlet

Thermal ion source
Mass-separated Ions

Skimmer

0
4.0

5.0

6.0

7.0

8.0

Detection Systems

He Cooling Gas

M+
30 keV

Magnet9.0

α-Energy / MeV

Figure 3. α-particle spectra of samples prepared from the two elution
fractions: (a) 11.5 M HCl and (b) 4.0 M HCl.

Differential Pumping

Figure 4. Schematic view of the gas-jet coupled JAERI-ISOL.

Status and Prospects of Heavy Element Nuclear Chemistry Research

J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

131

TABLE 1: EC/α decay properties of the neutron-deficient Am and Cm isotopes measured with JAERI-ISOL and those compared with theliterature data.

Nuclideα-energy / MeVJAERI-ISOL
Reference
6.780 ± 0.0176(7.100)b6.46018
6.457 ± 0.01416(6.700)b6.15016
6.4119

Half-life / min
JAERI-ISOL
Reference
3.2 ± 0.86
(1.72)a2.32 ± 0.0818
10.3 ± 0.616
15 ± 522173.6 ± 0.2
4.4 ± 0.85172.9 ± 0.2
(15)a

233

Am
Am
235
Am
236g
Am
236m
Am
237
Cm
238
Cm
a
Predicted.20b
Evaluated from the Q values in Reference 21.
234

6.660 ± 0.0104
6.560 ± 0.0104

other interesting feature was observed in the adsorption trend
on the anionic-exchange resin among Rf, Zr, and Hf. The adsorption order that reflects the stability of chloride complexation
was Rf > Zr > Hf. Presently, we are conducting the relativistic
molecular orbital calculations to understand the present results
and to see if the anion-exchange behavior of Rf is affected byrelativistic effects.
4. EC/α-decay Studies of Neutron-deficient Actinides
There still remain many unknown isotopes to be discoveredin the region of neutron-deficient actinides which predominantly
decay through the electron capture (EC). Decay properties ofthese nuclides lead to considerable advances in the understanding of proton excess heavy nuclei: verification of the proton drip
line, nuclear structure of large deformed nuclei such as hexadecapole deformation, and fission barrier heights of neutrondeficient nuclei far from stability.
To search for new isotopes and study EC/α-decay propertiesof neutron-deficient actinides, we have developed a composite
system consisting of a gas-jet transport apparatus and a thermal
ion source in the on-line isotope separator (JAERI-ISOL).5, 11This gas-jet coupled JAERI-ISOL system enables us to determine simultaneously mass number via the isotope separator andatomic number by the measurement of x rays associated with
the EC/β± decay of a nucleus. Some new neutron-rich rareearth isotopes produced in the proton-induced fission of 238 U
were identified with this system.12–15 In the following, the EC/αdecay studies of the neutron-deficient actinides are summarized.
The experimental setup is schematically drawn in Figure 4.
The 6 Li beams delivered from the JAERI tandem accelerator
bombarded the 233,235 U and 237 Np targets set in a multiple target
chamber. The used reaction systems are 233 U(6 Li, xn)233−235 Am,
233

>3%

Am

3.2 min EC <97%6.780 MeV

229

70%
6.890 MeV

Np EC 30%

4.0 minα-branching ratio / %JAERI-ISOL
Reference>36<0.04160.039 ± 0.01218
0.40 ± 0.0516
0.004160.042 ± 0.00619(6.800)b6.520 ± 0.05023

235

U(6 Li, 5n)236 Am, and 237 Np(6 Li, xn)237,238 Cm. The reaction
products recoiling out of the targets were thermalized in He gas
loaded with PbI2 aerosol clusters. The products attached to theaerosols were swept out of the target chamber and transported
to the thermal ion source of ISOL through a Teflon capillary(1.5 mm i.d. and 8 m length). The transported nuclides were ionized in the thermal ion source, and mass-separated atoms werecollected on an aluminum coated Mylar tape in a tape transportsystem or a rotating catcher foil apparatus. In the tape transportsystem, we used Ge detectors for the x/γ-ray measurement and
Si PIN photodiode detectors set in the rotating catcher foil apparatus were employed for the measurement of α particles. Detailed experimental procedures are described in References 6,11.
As a typical example, the identification of the new isotope
233
Am produced in the 233 U(6 Li, 6n) reaction is given in the follows. The isotope 233 Am was identified through an α-α correlation analysis. The α decay of 233 Am is followed by the five successive α decays starting from 229 Np as shown in Figure 5: 229 Np(T1/2 = 4.0 min) → 225 Pa (T1/2 = 1.7 s) → 221 Ac (T1/2 = 52 ms)→ 217 Fr (T1/2 = 22 µs) → 213 At (T1/2 = 125 ns) → 209 Bi (stable).
Since the last four nuclides decay via α-particle emission withthe short half-lives, the α-α correlation events among these nuclides can be unambiguously identified. Figure 6 shows an αparticle spectrum constructed from the observed α-α correlation
events in the mass-separated A = 233 fraction. The α-particle energies of 233 Am and 229 Np are clearly observed and those corresponding to the decays of the other nuclides are also seen. The
α energy of 233 Am was determined to be 6.780 ± 0.017 MeV,
and from the decay curve of the intensity of this α line, the halflife was 3.2 ± 0.8 min. Since no Pu Kx rays following the EC
decay of 233 Am was observed, the α-decay branching ratio wasestimated as Iα > 3% based on the detection efficiency of the Pu
Kx rays.6
In Table 1, the half-lives, α-decay energies, and α-decaybranching ratios measured with JAERI-ISOL and those fromthe literature data are summarized. In the case of 235 Am, theα-decay process was observed for the first time and the α20

225

100%
7.645 MeV
7.440 MeV
7.375 MeV
217

100%

221

Ac

52 ms

Fr

22

s

8.315 MeV
213

100%

225

Am

15

Pa
221

229

Ac

Np

10

5

213
217

At

Fr

At

125 ns

9.080 MeV

209

A=233
233

1.7 s

Counts / 20 keV

100%
7.245 MeV
7.195 MeVPa

Bi

stableFigure 5. α-decay chain originating from the α decay of 233 Am.

0
6.0

6.5

7.0

7.5

8.0

8.5

9.0

9.5

α-Energy / MeV
Figure 6. α-particle spectrum constructed from the measured α-αcorrelation events associated with the α decay of 233 Am.

132 J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002
branching intensity was derived from the ratio between the observed α and Pu Kx-ray intensities.16 The half-life value of235
Am was determined to be 10.3 ± 0.6 min based on the decay curve of the α line of 6.457 MeV and that of Pu Kα x rays
following the EC decay of 235 Am. With the γ-γ coincidence
technique, it was found that there are two EC-decaying statesin 236 Am (Ref. 17). The α-decay process in 234 Am was not observed, although that was reported by Hall et al.18 as shown in
Table 1. In the α decay of 236 Am, a very weak α group wasobserved at 6.150 MeV, while the 6.41 MeV α group was measured in Reference 19.
5. Future Plans
In the study of Rf aqueous chemistry, anion-exchange experiments in HF solution will be soon conducted. Plans for additional experiments are being made to understand complex structure of Rf together with Zr and Hf in HNO3 , HCl, and HF solutions. Measurements of XAFS (X-ray Absorption Fine Structure) spectra of Zr and Hf compounds in these solutions will be
performed at KEK (High Energy Accelerator Research Organization) Photon Factory. Relativistic molecular orbital calculations are also being carried out to gain understanding of complex
chemistry of Rf, and to see if there are unexpected differences
within the group 4 elements.
In the next stage of aqueous chemistry, we plan to conduct
Db ion-exchange experiments. Improvements to the overall production rates and a possible multiple target system are being
considered to study the short-lived 34-s 262 Db produced in the248
Cm(19 F, 5n) reaction with the small cross section of about
1.5 nb. Development of a new ion-exchange separation apparatus, an improved AIDA system is required. We are now cooperating with the Niigata group on the gas phase chemistry of thetransactinide elements. An apparatus based on the isothermal
gas chromatography is now under construction.
Experiments of α-γ spectroscopy, and spontaneous fission
and α-decay properties of heavy and transactinide nuclei will be
conducted with the gas-jet coupled JAERI-ISOL, an α-γ spectrometry apparatus, and MANON coupled with a gas chemistry
separator system.
Acknowledgement. The present study has been carriedout in collaboration with Niigata University, Osaka University, Tokyo Metropolitan University, University of Tokushima,
Kanazawa University, Nagoya University, Hiroshima University, Gesellschaft für Schwerionenforschung (GSI), Mainz University, Paul Scherrer Institut (PSI), Bern University, and Institute of High Energy Physics, Beijing. We would like to acknowledge the crew of the JAERI tandem accelerator for providing the stable and intense 6 Li, 18 O, and 19 F beams. 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.
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