Ⅲ-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 53

Anion exchange of element 104, Rf, at 0.11 M H2 SO4 by usingbatch-type solid-liquid extraction apparatus, AMBER
T. Yokokita,∗1 Y. Kasamatsu,∗2 E. Watanabe,∗2 Y. Komori,∗1 Y. Shigekawa,∗1 Y. Wang,∗1 D. Mori,∗1
H. Ninomiya,∗2 S. Hayami,∗2 K. Tonai,∗2 K. Ghosh,∗1 A. Shinohara,∗2 and H. Haba∗1
The relativistic eﬀect of orbital electrons is relatively more pronounced for heavy elements. In particular, the chemical properties of superheavy elements(SHEs) with atomic number Z ≥ 104 are expected deviate from the periodicity of their lighter homologues
in the periodic table. Thus, it is important and interesting to investigate the chemical properties of these elements. Thus far, ion-exchange experiments on SHEs
have been conducted to determine the distribution coeﬃcients (Kd ), which are deﬁned as the ratio of the elemental concentrations of the two phases.1) However,those values at equilibrium have not been obtained inmost studies. To obtain the Kd values for Rf at equilibrium, a batch-type solid-liquid extraction apparatus
called AMBER was developed,2) and the equilibrium
Kd values on the chloride complexation of Rf were successfully obtained in an Aliquat 336/HCl system.3)
To study the sulfate complexation of Rf, we planto perform anion-exchange experiments with Rf andits homologous elements. By using AMBER, we havealready performed online anion-exchange experimentswith Zr and Hf in H2 SO4 as the model experiments
for Rf and determined the anion-exchange experimental condition of Rf as 0.1–0.7 M H2 SO4 .4) In this work,
we performed online anion-exchange experiments withRf and Hf at 0.11 M H2 SO4 by using AMBER to examine the equilibrium time for the distribution of theelements and determine their Kd values.
In the online anion exchange of Rf and Hf, we simultaneously produced 261 Rf and 169 Hf (T1/2 = 3.24 min)by the bombardment of a mixture of 248 Cm and nat Gd
mixture with 18 O beams delivered by the K70 AVF
cyclotron at RIKEN. The products were transported
to a chemistry room by a He/KCl gas-jet system. The
transported products were deposited on the collection
site of AMBER’s dissolution equipment for ∼3 minand were dissolved with 0.24 mL of 0.11 M H2 SO4 .
The solution sample was injected into a chemical reaction container containing the anion-exchange resin(MCI GEL CA08Y). After shaking the container witha shaker for 10, 30, and 90 s, only the solution phase
was discharged from the container, after being passed
through a PTFE ﬁlter by compressed air for 10 s. The
discharged solution was collected in a Ta disk on theround table of an automated rapid α/SF detection system5) and evaporated quickly to dryness using hot Hegas and a halogen heat lamp. Subsequently, using thedetection system’s robot arm, the Ta disk was sub∗1∗2RIKEN Nishina Center
Graduate School of Science, Osaka University- 165 -jected to rapid γ-ray measurement with a Ge detector
to monitor 169 Hf. We also performed control experiments with 10-s shaking without the resin to determine
the standard radioactivity of the solution sample. The
Kd values were determined from the radioactivities in
the resin and solution phases, the volume of the solution phase, and the mass of the dry resin.
We conducted 327 anion-exchange and 135 control
cycles, and observed a total of 96 α events from thedecay of 261 Rf and its daughter nuclide 257 No (T1/2 =24.5 s), including 14 time-correlated α-α correlations.
The event ratios of 261 Rf between the resin and solution phases were estimated from the α events. The Kdvalues of Hf were constant in all the time ranges studied (10–90 s), indicating that equilibrium in the anion
exchange of Hf was accomplished within 10 s. Thoseof Rf were also constant in all the time ranges studied,yielding values of approximately 10 mL g−1 . This indicates that Rf is not adsorbed on the resin. In turn, thisis considered to suggest that Rf does not form an anionic sulfate complex at 0.11 M H2 SO4 , though there is2−
a possibility that counter ions of HSO−
4 and SO4 are
preferentially adsorbed on the resin. It should be notedthat that we cannot exclude the possibility that the kinetics of Rf is signiﬁcantly slower than that of Zr andHf. The Kd values at 0.1 M H2 SO4 follow the order ofZr > Hf ≫ Rf, and this sequence is consistent with thetrend predicted by theoretical calculation.6) Furthermore, in a cation-exchange study in H2 SO4 /HNO3 , thesequence of the Kd values was reported to be Rf > Hf≥ Zr.7) These results indicate that the sulfate complexation of Rf is weaker than those of lighter homologues.
From the present work, we found that Rf is not adsorbed on the anion-exchange resin at 0.11 M H2 SO4within 10–90 s. However, it is not clear whether Rf
avoids forming the anionic sulfate complex in this studied condition. We need additional anion-exchange experiments of Rf at a lower counter-ion concentration.
References
1) Y. Nagame et al., Nucl. Phys. A 944, 614 (2015).
2) Y. Kasamatsu et al., Radiochim. Acta 103, 513 (2015).
3) T. Yokokita et al., Dalton Trans. 45, 18827 (2016).
4) T. Yokokita et al., RIKEN Accel. Prog. Rep. 52, 187(2019).
5) H. Haba et al., RIKEN Accel. Prog. Rep. 45, 204 (2012).
6) V. Pershina et al., Radiochim. Acta 94, 407 (2006).
7) Z. J. Li et al., Radiochim. Acta 100, 157 (2012).