Journal of Nuclear and Radiochemical Sciences, Vol. 14, No. 2, pp. 7-11, 2014

ArticlesCoprecipitation Behaviors of Zr, Hf, and Th with Sm Hydroxide for Chemical Study ofElement 104, Rf
Y. Kasamatsu,*,a K. Toyomura,a N. Shiohara,a T. Yokokita,a Y. Komori,a A. Kino,a T. Yoshimura,bN. Takahashi,a H. Haba,c Y. Kudou,c H. Kikunaga,d T. Mitsugashira,e T. Ohtsuki,f K. Takamiya,fand A. ShinoharaaaGraduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan
Radioisotope Research Center, Osaka University, 2-4 Yamadaoka, Suita, Osaka 565-0871, Japanc
Nishina Center for Accelerator-Based Science, RIKEN, Wako, Saitama 351-0198, Japand
Research Center for Electron Photon Science, Tohoku University, 1-2-1 Mikamine, Taihaku-ku, Sendai, Miyagi
982-0826, Japane
Institute for Material Research, Tohoku University, Oarai, Ibaraki 311-1313, Japanf
Research Reactor Institute, Kyoto University, Kumatori-cho, Sennan-gun, Osaka 590-0494, Japanb

Received: February 25, 2014; In Final Form: July 7, 2014
In superheavy element chemistry, rapid single-atom chemistry combined with rapid identification via alpha-particle measurement is required to be performed. Herein, we propose a new protocol for rapid preparation of coprecipitates of superheavy elements with Sm hydroxide. The coprecipitation behaviors of Zr, Hf, and Th (homologuesof element 104, Rf) with Sm hydroxide in aqueous ammonia (NH3) and sodium hydroxide (NaOH) solutions were
investigated by the conventional suction filtration method using 88Zr, 175Hf, and 228Th radiotracers. The precipitation yields obtained by employing an aging time of only 10 s were consistent with those obtained after aging for 10
min, suggesting the rapid chemical reactions in the present coprecipitation. Approximately 100% coprecipitation
yields of Zr and Hf were achieved in the solutions with dilute hydroxide concentrations, and the yields decreased
with higher hydroxide concentrations. In comparison, the Th yield remained constant at almost 100% under all of
the conditions. In addition, a semiautomatic suction filtration apparatus for rapid preparation of precipitated samples was developed and evaluated using the short-lived isotopes 89m,gZr and 173Hf, which were transported onlinefrom the nuclear reaction chamber by a He/KCl gas-jet system. Using this apparatus, sample preparation wascompleted within 80–120 s, and precipitation yields consistent with those determined on the basis of the manual
method were obtained, suggesting the applicability of the present method to Rf experiment.

1. Introduction
The influence of relativistic effects on electron orbital shells
becomes significant for heavy elements because of the higher
positive charges on their nuclei. Therefore, the chemical properties of the transactinide elements with atomic numbers Z ≥104, i.e., superheavy elements, are expected to deviate from thecharacteristic periodicity of the lighter homologues in the periodic table, and their chemical studies have attracted muchattention.1,2 However, experimental evaluation of these species
is fraught with difficulties. Because of the extremely low production rates and short half-lives (within a few min) of theseheavy nuclides, chemical evaluations of the superheavy elements must be conducted on a one-atom-at-a-time basis,3 andmust be rapidly completed. Thus, simple chemistry employing
partition methods such as solvent extraction and ion-exchange
chromatography have been utilized and the chemical separations were repeated hundreds or thousands of times under identical conditions. This approach requires two types ofapparatuses: an online system having the capability for rapidtransport of nuclear reaction products from the reaction chamber to the chemistry laboratory, and a rapid chemistry apparatus equipped with a radiation detection system.1,2 Additionally,for unambiguous identification of superheavy elements on a
one-atom-at-a-time basis, it is necessary to measure the energies and lifetimes of α or sometimes spontaneous fission
decays. For these measurements using a Si detector, the sample*Corresponding author. E-mail: kasa@chem.sci.osaka-u.ac.jp
TEL: +81-6-6850-5418; FAX: +81-6-6850-6999

must be dried and flat without salt contaminants. Liquid scintillation counting is an alternative analytical approach, but theenergy resolution of this technique is very poor compared withthat using the Si detector. Moreover, α-particle measurement
using liquid scintillation is easily disturbed by β- and γ-rays.
Above-mentioned experimental difficulties limit the variety ofchemical investigation of superheavy elements.
Thus far, primarily on the basis of gas-phase and aqueousphase chromatographic experiments, transactinide elementswith Z up to 108 have been assigned as d-block metal elements
belonging to the 7th period of the periodic table.1,2 Experiments
employing solution chemistry have focused mainly on the lightest transactinide element, rutherfordium (Rf, Z = 104).4–10 On
the basis of the similarity of the behavior of Rf to the homologues Zr and Hf, these experiments revealed that Rf is a group
4 element. More recently, detailed ion-exchange experimentsof Rf have been succeeded, yielding interesting results; theanion- and cation-exchange behaviors of Rf in HF and HF/
HNO3 solutions are clearly different from those of the homologues Zr and Hf.11–13 In particular, the distribution coefficientsof Rf in the cation exchange lay between those of the homologues and the pseudo-homologue Th.13 Detailed studies on Rf
in various chemical systems are still required to clarify thechemical properties of Rf, which leads to more in-depth understanding of the characteristic chemical properties of superheavy elements.
The purpose of the present study is to establish a new experimental methodology for investigating the chemical properties
of superheavy elements, specifically, the coprecipitation properties of 261Rf (T1/2 = 68 s)14 by the formation of hydroxide and© 2014 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 08/01/2014

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Kasamatsu

J. Nucl. Radiochem. Sci., Vol. 14, No. 2, 2014

ammine complexes. Coprecipitation technique has been widelyused for chemical separation and so on in radiochemical
researches, and the coprecipitation is one of the importantchemical properties.15,16 In general, however, the energy resolution of a precipitate sample in the α spectrum is poor, andindeed, the precipitation method has not yet been applied tosuperheavy element chemistry. For this problem, we successfully established a simple coprecipitation method with Smhydroxide, which facilitates α spectrometry with high energy
resolution.17 Herein, this method was applied to the investigation of the coprecipitation behaviors of Zr, Hf, and Th with Smhydroxide. In addition, a semiautomatic apparatus for repetitive preparation of precipitate samples was developed.
Experiments utilizing the apparatus connected to the gas-jet
transport system from the nuclear reaction chamber were performed on 89m,gZr and 173Hf samples produced by the accelerator as a model experiment for 261Rf.
2. Experimental
2.1. Coprecipitation experiment for Zr, Hf, and Th with
Sm hydroxide. Carrier-free radiotracers of 88Zr (T1/2 = 83.4 d)and 175Hf (70.0 d) were produced in the nuclear reactions of89
Y(p,2n) 88Zr and 175Lu(p,n)175Hf, respectively, using the AVF
cyclotron at the Research Center of Nuclear Physics (RCNP),
Osaka University. The proton beam energy was 22 MeV andthe irradiation time was ca. 1 h for the production of 88Zr. For
175
Hf, the corresponding parameters were 14 MeV and 7 h. The
beam current was ~1 µA for both irradiations. The nuclides
were then purified by chemical separation protocols similar to
those in Reference 18. The radiotracer 228Th (1.9 y) was chemically separated from its grandmother nuclide 228Ra (5.75 y) as
follows. First, 8 M nitric acid (HNO3) containing 228Th, 228Ra,
and other daughter nuclides was fed onto the strongly basic
anion-exchange column (Muromac 1 × 8, 200–400 mesh, 5mm i.d. × 15-mm height, NO3− form). 228Ra and other nuclides,
except for 228 Th and some relatively short-lived daughter
nuclides, were then eluted with 8 M HNO3. After decay of
those short-lived nuclides, the column was washed with 8 M
HNO3, and 228Th was subsequently eluted with 2 M hydrochloric acid (HCl). This separation was repeated to purify 228Th.
After completion of the chemical purification, the 88Zr, 175Hf,
and 228Th radiotracers were mixed together in 0.01 M HCl andused in the following coprecipitation experiment.
Coprecipitated samples with Sm hydroxide were preparedusing various basic solutions such as dilute and concentrated
aqueous NH3 and 0.1, 1.0, 6.0, and 12.0 M NaOH solutions. A
20 µL aliquot of the Sm standard solution (Sm 1000 ppm, 1 M
HNO3) was added into 220 µL of an aqueous solution (~0.4 MHCl) containing the radiotracers in a polypropylene (PP) beaker. The solution was stirred and 2 mL of the basic solution
was then added to produce the precipitate. When the dilute
aqueous NH3 was used, we checked the basicity of the solution
using a pH-test paper. After aging the precipitate for 10 min at
25 °C, the precipitate sample was subjected to suction filtration
with a PP membrane filter (eichrom, Resolve® Filters 0.1 µm).
The obtained sample was dried on a heater at 100 °C, and was
subjected to γ-ray and α-particle measurements. The experiments with the aging time of 10 s were also performed at 25 °C
to determine suitable experimental conditions for Rf. The
radioactivities of the filtrate and PP beakers used were alsomeasured. The standard radioactivities of 88Zr, 175Hf, and 228Th
were determined at the same detection efficiency by preparing
standard samples with the same diameter as the precipitate
sample. In the present study, we define a precipitation yield, Y,
and the ratio of adsorption on the PP beaker, Rad, according to
eqs 1 and 2, respectively.

Y=

Appt
× 100
Astd

(1)

Rad =

Acup
× 100
Appt + Afil + Acup

(2)

In the above equations, Appt, Afil, and Acup represent the radioactivities of the precipitate, filtrate, and PP beaker used, respectively, and Astd represents the standard radioactivity. The errors
in the yields were determined by taking into account the standard deviation in each γ or α count (0.4–6.3%) and in the variation of the precipitation yield (5%). It is difficult in superheavy
element chemistry to determine the Afil value by rapidly measuring α-activity of the filtrate containing salt. Thus, we
decided to use the Y value which is a relative value to the standard radioactivity, whereas the Astd value is required to be separately determined. The Y value can be determined even for
short-lived nuclides. However, the Y value would show a lower
value if physical adsorption of the complex is strong. The Rad
value was, therefore, determined to evaluate the effect of thephysical adsorption of the Zr, Hf, and Th complexes on the PP
beaker on the analytical result. For Th, Rad was determined byevaluating the ratio of Appt to Astd because Afil of Th could not
be determined by α-particle measurement because of interference from NaCl and/or NH4Cl salts in the α spectrometry.
The energy resolution of the 5423-keV α peak of 228Th wasmeasured from the full width at half maximum (FWHM) for
each precipitate sample. The distance between the Si detector
and the sample was ~20 mm.
2.2. Online precipitation experiment using the apparatus. A computer-controlled suction filtration apparatus for the
preparation of precipitated samples of heavy elements (termed
C H I N ba s e d on t he Japa ne s e word for p r e c ipit a t e,
“CHINDEN”) was developed using cylinders operated by compressed air, valves operated by on/off operation of 24 V DC
power and so on. A compact data acquisition system(CompactDAQ) and LabVIEW software from National
Instruments Corporation were used for the operation of theapparatus. The suction filtration part of the apparatus and simple movement are schematically illustrated in Figure 1. Thisapparatus performs rapid and repetitive filtration. Filter sticks
were fabricated using polytetrafluoroethylene (PTFE) to prevent the adsorption of the radiotracers on the wall of the stick,and the stick was washed after each usage.
The online experiment using CHIN was performed with relatively short-lived nuclides 89m,g Zr (4.2 min and 78.4 h) and173
Hf (23.6 h) produced using the AVF cyclotron at RCNP.
These nuclides produced in the reactions of natSr(α,xn) 89m,gZr
and natYb(α,xn)173Hf were rapidly and continuously transported
from the nuclear reaction chamber to the chemistry laboratory
using the He/KCl gas-jet system in the same manner employedin superheavy element chemistry.1,2 The products were collected on the collection site of the dissolution apparatus. Afterdissolution by 200 µL of dilute HCl for 12 s, the precipitate
samples were prepared at room temperature in a manner similar to that used in the tracer experiment described above, withstirring for 10 s. The samples were filtered with the filtration
apparatus, and were subsequently dried on a heater under hot
air. The coprecipitation yields were determined from the γ-ray
radioactivity of the precipitated sample relative to the total
radioactivity of the precipitated and filtrate samples.
3. Results and discussion
3.1. Coprecipitation experiment for Zr, Hf, and Th with
Sm hydroxide. The hydroxide ion concentrations of six solutions prepared by the addition of dilute and concentrated aqueous NH3 and 0.1, 1.0, 6.0, and 12.0 M NaOH solution were

Coprecipitation Behaviors of Zr, Hf, and Th with Sm Hydroxide forJ. Nucl. Radiochem. Sci., Vol. 14, No. 2, 2014

9

SolutionsampleFilterFilter stick

Valve

Suction

Cylinders operated
by compressed air
Figure 1. Schematic illustration of the filtration section of rapid suction filtration apparatus.

calculated as 2.3 × 10 −3, 0.016, 0.079, 0.84, 5.3, and 11 M,
respectively, using the dissociation constant of aqueous NH3.19
The concentration of NH3 in the samples prepared by adding
dilute and concentrated aqueous NH 3 were 0.28 and 13 M,
respectively. The Rad values are shown in Table 1. The ratio ofphysical adsorption of Zr, Hf, and Th on the surface of the PP
beaker for samples aged for 10 s was generally less than 3%,
and was slightly lower than those of samples aged for 10 min.
These low adsorption ratios especially for the samples aged for10 s suggests that physical adsorption of these group 4 elements during coprecipitation with Sm hydroxide was smallenough to be negligible in the determination of the coprecipitation yields. The coprecipitation yields of Zr, Hf, and Th when
10-min aging was employed are shown in Figures 2(a), andthose obtained with 10-s aging are displayed in Figures 2(b).
Figures 2(a) and (b) demonstrate that the yields obtained with
10-min and 10-s aging are in good agreement. This suggeststhat the chemical reactions during coprecipitation of Zr and Hf
are fast, and the present conditions are suitable for evaluating
the coprecipitation behavior of 261Rf with T1/2 =68 s . Based on
this result and the small adsorption especially for 10-s aging, it
was found that accurate precipitation yields of the group 4 elements could be obtained under the present conditions by the
rapid method.
Almost complete coprecipitation of Zr and Hf with Smhydroxide was achieved when aqueous NH3 solutions and 0.1

M NaOH were used. These results are consistent with the wellknown properties of these elements, i.e., hydroxide precipitation in basic solutions. 20,21 With the use of more highly
concentrated NaOH solutions, the yields of Zr and Hf decreased
as the OH− concentration increased, whereas the Th yields wereconstant at ca. 100%. This is attributed to the fact that Zr andHf form hydroxide complex ions such as [M(OH) 5] − (M4+ =Zr4+ and Hf 4+) 22,23 in solutions with high hydroxide concentration. Indeed, we also observed the decrease of the precipitateof Zr and Hf on the macro scale in concentrated hydroxide
solutions. The yields of Hf were consistently lower than thoseof Zr in the same solutions. This is qualitatively consistent
with the larger solubility product of [Hf(OH)5] − (logKs5 = −3.2)
than that of [Zr(OH)5] − (logKs5 = −3.6),22 whereas that of Th isquite small (logKs5 = −5.8).22 The ~100% coprecipitation yieldsof Zr, Hf, and Th with the use of aqueous NH3 indicate that theammine-coordinated complex ions, which dissolve in aqueous
solution (e.g., Zn, Cu, and Ag hydroxide dissolve in concentrated NH3 solutions), are not dominantly formed even in concentrated NH 3 solution. On the basis of coprecipitation
experiments with Sm hydroxide using various radioactive lanthanide nuclides, it was confirmed beforehand that the yieldsof Sm are almost constant around 100% under all of the conditions studied. The obtained results indicate that the hydroxide
and ammine complexation properties of group 4 elements canbe qualitatively investigated through their coprecipitation

TABLE 1: Adsorption ratios of Zr, Hf, and Th on polypropylene beakers
Added basic
solution
dilute NH3
conc. NH3
0.1 M NaOH
1 M NaOH
6 M NaOH
12 M NaOH

Adsorption ratio, Rad / %Zr
6.4 ± 1.7
4.6 ± 1.3
5.0 ± 1.4
3.5 ± 0.6
3.8 ± 0.4
4.5 ± 1.0

10-min aging
Hf
1.6 ± 1.1
1.4 ± 1.0
2.2 ± 1.4
0.4 ± 1.7
0.5 ± 1.6
0.2 ± 2.2Th
4.1 ± 0.3
2.6 ± 0.3
1.7 ± 0.2
1.8 ± 0.3
0.7 ± 0.3
0.2 ± 0.1Zr
4.3 ± 1.1
1.3 ± 0.6
2.1 ± 1.0
2.5 ± 0.4
0.5 ± 0.1
1.5 ± 0.3

10-s aging
Hf
2.6 ± 1.3
1.4 ± 0.9
1.4 ± 1.1
0.5 ± 1.5
0.3 ± 1.1
0.3 ± 1.6Th
2.7 ± 0.3
2.3 ± 0.3
1.2 ± 0.2
0.7 ± 0.2
N. D.
0.1 ± 0.1

Kasamatsu

J. Nucl. Radiochem. Sci., Vol. 14, No. 2, 2014

behaviors with Sm hydroxide under the present experimental
conditions.
The FWHM value of the 5423-keV α peak of 228Th for eachsample is shown in Table 2. The energy resolution was adequate for all samples, because the resolution with the present Si
detector is at least 18 keV and our upper acceptable limit is 80
keV for the identification of 261Rf. The resolution for the 10min aged samples was relatively poor compared to that for the
10-s aged ones because of the growth of the precipitates during
aging.
Overall, it was found that without compromising the yield,
well-resolved alpha spectra can be obtained for coprecipitatedsamples by aging the hydroxide precipitate for only 10 s at
25 °C. These conditions are all suitable for Rf experiment.
3.2. Online precipitation experiment using the apparatus. The coprecipitation yields obtained in the online experiments are shown in Figure 3. The yields in Figure 3 and theirdependence on the composition of the added basic solution areapproximately consistent with those in Figure 2(b). This suggests that the present experimental method using CHIN under
the present conditions should be applicable to the coprecipitation experiment of Rf. Because the dissolution yield of a product transported by the gas-jet system has some variation,
repetitive determination of the coprecipitation yields and averaging are required to obtain more accurate values in the online
experiment. In the present online experiment, the timerequired for the sample preparation from the start of the dissolution was 80–120 s, except in the case employing 12 M NaOH.
With the use of 12 M NaOH, more than 4 min was required forthe suction filtration because of the high viscosity of the solution; consequently, the use of 12 M NaOH is unsuitable for Rf
experiment. Based on the present experiment, we were able todetermine the conditions for suction filtration, sample drying,etc., for rapid preparation of the precipitate samples.
Considering the half-life of 261Rf (68 s), more rapid preparationof the coprecipitate is desirable. Shortening the preparation
time by methods such as automating the pump and hot gas
operations is now in progress.

120

(a)

100
80
60
40
Yield, Y / %

10

20

Zr-88

0
120

Hf-175(b)

100

Th-228

80
60
40
20
0

dilute
NH3

conc.
NH 3

0.1 M
NaOH

1.0 M
NaOH

6.0 M
NaOH

12.0 M
NaOH

Figure 2. Coprecipitation yields of Zr, Hf, and Th based on the addition of various basic solutions. Yields obtained with 10-min aging areshown in (a) and with 10-s aging in (b).

TABLE 2: Energy resolution of alpha peak for 228Th precipitate samples
Added basic
solution

Energy resolution / keV
at 5423-keV α peak
10-min aging

10-s aging

29
46
28
32
30
30

21
17
19
19
18
25

dilute NH3
conc. NH3
0.1 M NaOH
1 M NaOH
6 M NaOH
12 M NaOH

4. Conclusion120

Zr-89m
Hf-173

100

Yield / %In the present study, coprecipitation with Sm hydroxide wasestablished as a new experimental methodology for superheavy
element chemistry. The conditions for the evaluation of thehydroxide coprecipitation properties of element 104, Rf, were
determined. The coprecipitation behaviors of the group 4 elements Zr and Hf and the pseudo-homologue Th with Smhydroxide were investigated using radiotracers of 88Zr, 175Hf,
and 228Th. It was found that 10-s aging is sufficient to obtain
the equilibrium precipitation yields for these elements.
Approximately 100% yields were achieved for Zr, Hf, and Th
in dilute NH3, NaOH, and concentrated NH3 solutions, and theyields of Zr and Hf decreased at higher OH− concentrations,whereas that of Th was constant at 100%. From these results,
the following conclusions are drawn: 1) The Zr and Hf ionsform neutral hydroxide complexes in the basic solutions andform anionic hydroxide complexes in concentrated NaOH solutions. 2) The pseudo-homologue of Rf, Th shows clearly different behavior from those of Zr and Hf; almost complete
coprecipitation with Sm hydroxide was achieved under all of
the conditions.
In the online model experiment of Rf employing the nuclear
reaction products Zr and Hf, which were transported using thegas-jet system, and semiautomatic suction filtration apparatus
CHIN, coprecipitation yields consistent with those obtained in
the offline experiments were obtained. It was found that thepresent conditions, except for the case using 12 M NaOH, aresuitable for the Rf experiment. We believe that the present
method can be applied to the evaluation of the formation of

80
60
40
20
0
dilute
NH3

0.1 M
NaOH

1.0 M
NaOH

6.0 M
NaOH

12.0 M
NaOH

Figure 3. Coprecipitation yields obtained in the online experiment.

ammine and hydroxide complexes of Rf.
Acknowledgement
We express our gratitude to the staffs of the Accelerator
Facility in RCNP for the productions of 88,89m,gZr and 173,175Hf.
This work was supported by JSPS KAKENHI Grant Number
24655050.

Coprecipitation Behaviors of Zr, Hf, and Th with Sm Hydroxide for

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