Ⅲ-3. Radiochemistry & Nuclear Chemistry
RIKEN
Accel. Prog. Rep. 56 (2023)

RIKEN Accel. Prog. Rep. 56 (2023)

Improved chemical separation scheme of Pa isotopes from a 232 Th
target toward observing the radiative decay of 229m Th using 229 Pa
Y. Shigekawa,∗1 X. Yin,∗1 A. Nambu,∗1 Y. Wang,∗1 and H. Haba∗1
The ﬁrst excited state of the 229 Th nucleus (229m Th)
has an excitation energy of ∼8.3 eV (150 nm),1) potentially leading to an ultraprecise nuclear clock. We aim
to observe the radiative decay (γ rays) of 229m Th anddetermine its radiative half-life; this is an importantparameter to develop the nuclear clock by doping aCaF2 crystal with 229 Pa, which decays to 229m Th withnegligibly small recoil energy.2–4) In a previous study,4)
we ionized 233 Pa and implanted it into a CaF2 crystal
with a total eﬃciency of 0.53(1)% using the surface
ionization technique. However, when we tried the ionization and implantation of 229 Pa that was producedin the 232 Th(p, 4n)229 Pa reaction and chemically puriﬁed,3) the eﬃciency was limited to ∼0.1%. A probable reason for the low eﬃciency is that some amountof 232 Th and impurities remain in the puriﬁed 229 Pa
solution and inhibit the ionization of Pa. Therefore,
we improved the chemical separation scheme in thisstudy by adding a separation step using the CL resin(Triskem).5) Moreover, for faster separation, we increased the ﬂow rate of eluents during column chromatography by a factor of ∼10, compared with ourprevious separation experiments.2,3)
The chemical separation scheme was developedusing 230 Pa (T1/2 = 17.4 d) and 233 Pa (T1/2= 27.0 d) produced in the 232 Th(d, 4n)230 Pa and232Th(d, n)233 Pa reactions at the RIKEN AVF cyclotron. Two 232 Th foils (70 mg/cm2 × 2, ∼140 mg)
were irradiated with 9.1 µA of a 24-MeV deuteronbeam for 70 min. The foils were dissolved with concentrated HCl and 0.05 M (NH4 )2 SiF6 6) 62 days after
the irradiation. The chemical separation of the Pa isotopes was performed following the scheme shown inFig. 1, which consists of three column separations using anion-exchange resin (Column A and C) and theCL resin (Column B). The ﬂow rate of every eluent
was set to 1–2 mL/min by pushing the air in the column using a peristaltic pump. The radioactivity of Pa
isotopes and ﬁssion products (FPs) for each fraction
was measured via γ-ray spectroscopy using a Ge detector to check chemical yields and radioactive impurities.
The ﬁnal Pa fraction was subjected to inductively coupled plasma mass spectrometry (ICP-MS) to check thechemical purity.
The separation by Column A removed the majority of 232 Th and FPs, and the Pa fraction that eluted
from Column A included radioactive impurities of 95 Zr(T1/2 = 64.0 d), 95 Nb (T1/2 = 35.0 d), and 103 Ru (T1/2= 39.2 d). The separation with Column B reduced∗1RIKEN Nishina Center- 145 -Fig. 1. Chemical separation scheme of Pa.

95

Zr and 103 Ru to levels less than the detection limits of the γ-ray measurement. 95 Nb, which behaves
similarly to Pa, was separated with Column C by adecontamination factor of 220, which is high enoughfor the experiment of 229 Pa. It was reported that Nb
can be separated with the Eichrom DGA resin;5) thus,
we checked the performance of this method. However,the decontamination factor was measured to be only
12, and thus, we selected the method in Fig. 1. The
chemical yields of Pa isotopes for Columns A, B, and C
were 95(4)%, 96(3)%, and 95(3)%, respectively (totalyield 86(5)%), meaning that the separation at the highﬂow rate does not lead to the loss of Pa. We expectthat the whole separation will be ﬁnished in ∼4 h,which is ∼3 times shorter than that of the previous
scheme, minimizing the decay loss of 229 Pa. For theICP-MS measurement of the puriﬁed Pa solution, theimpurities of >100 ng were Mg (627 ng), Al (284 ng),Zn (141 ng), and Th (167 ng). Therefore, the amountⅢ-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 56 (2023)

of 232 Th was reduced to the same level as other impurities.
We are now developing a new Pa implantation apparatus that enables in varying the implantation energyof 229 Pa in the range of 0.5–30 keV, not limited to15 keV.4) In a preliminary experiment using the puriﬁed 230, 233 Pa solution and the new apparatus, thetotal ion implantation eﬃciency was measured to be∼0.2%, which is higher than the previous value using
229Pa (∼0.1%), probably due to the improved chemical
puriﬁcation scheme. Higher eﬃciency may be further
obtained by improving and optimizing the new apparatus.
References
1) B. Seiferle et al., Nature 573, 243 (2019).
2) Y. Shigekawa et al., RIKEN Accel. Prog. Rep. 54, 143(2021).
3) Y. Shigekawa et al., RIKEN Accel. Prog. Rep. 55, 124(2022).
4) Y. Shigekawa et al., RIKEN Accel. Prog. Rep. 55, 126(2022).
5) T. Mastren et al., Anal. Chem. 90, 7012 (2018).
6) V. Radchenko et al., Radiochim. Acta 104, 291 (2016).

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