III-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Production and photon measurement of 229 Pa toward the observationof radiative decay of 229m Th
Y. Shigekawa,∗1 T. Yokokita,∗1 Y. Komori,∗1 and H. Haba∗1
The first excited state of 229 Th (229m Th) has an extremely low excitation energy of ∼8.3 eV (150 nm),1)
which may enable a nuclear clock with unprecedentedly low uncertainty. So far, the radiative half-life of229mTh, which is an important parameter to develop
the nuclear clock, has not yet been determined. To
directly observe the radiative decay (γ-ray emission)of 229m Th, the internal conversion (IC) process witha half-life of ∼7 µs2) must be prohibited by placing
229m
Th in the chemical environments where the electron binding energy is higher than the excitation energy of 229m Th. 229m Th doped into fluoride crystals isa candidate for such chemical environments. We areaiming to dope a CaF2 crystal with 229m Th by doping with 229 Pa, which decays to 229m Th by electroncapture with a negligibly small recoil energy. Suitable doping can be realized by implanting high-energy229
Pa ions into a crystal and then annealing it. In
this study, we developed a method for producing 229 Pa(T1/2 = 1.5 d) in the 232 Th(p, 4n)229 Pa reaction andseparating it from the target. We also measured lowenergy photons from Pa isotopes on a CaF2 crystal toevaluate the background toward the future γ-ray measurement of 229m Th.
Two 232 Th metallic foils (thickness: 69.07 mg/cm2 ,purity: 99.5%) were irradiated with a 30-MeV proton beam having an intensity of 1 µA for 10 h at theRIKEN AVF cyclotron. After the irradiation, we measured γ-ray spectra for the 232 Th foils and fractions
resulting from the subsequent chemical separation process using a Ge detector.
The chemical separation process for one of the foils
was performed as follows. First, we dissolved the foilin 2 mL of 11.3 M HCl plus 300 µL of 1 M HF andheated the solution to dryness. The sample was dissolved in 2 mL of 11.3 M HCl, following which 1.1 gof Al(NO3 )3 · 9H2 O was added as a masking agent forFig. 1. Setup of the photon measurement of 229 Pa.
∗1RIKEN Nishina CenterFig. 2. Count rates of photons as a function of the elapsed
time for the 151-nm (red circle) and 171-nm (bluesquare) filters. Red and blue lines show the decay curvesof 230 Pa fitted to the data.

the remaining F− ions. After the solution was driedup and dissolved in 2 mL of 11.3 M HCl, the solution was fed into an anion-exchange column (Muromac1X8, 100–200 mesh, ∼1.0 mL). We poured 10 mL of11.3 M HCl into the column to elute Th isotopes, Acisotopes, and some fission products. Next, we added
10 mL of 6 M HCl to elute Zr, following which 5 mLof 8 M HNO3 was added to elute Zr, Mo, Ru, Sb, andTe isotopes. After we added 1 mL of 11.3 M HCl, Paisotopes were eluted with 8 mL of 9 M HCl/0.1 M HF.
The chemical yield of Pa isotopes in the whole process
was 94(2)% (residual Pa isotopes were observed in the8 M HNO3 eluate). The radioactivity of chemicallyseparated 229 Pa at the end of bombardment was evaluated to be 30(1) MBq, while those of 232 Pa, 230 Pa(T1/2 = 17.4 d), 95 Zr (T1/2 = 64.03 d), and 97 Zr(T1/2 = 16.75 h), included as impurities, were 2.18(3),0.89(4), 0.0054(7), and 0.40(2) MBq, respectively.
The 229 Pa sample dissolved with 27 M HF wasdropped on a CaF2 crystal, which was then annealed
at 900◦ C for 1 h in a He gas flow (3 L/min). Photons from the crystal were measured with a photomultiplier (PMT, Hamamatsu R10454) in vacuum (Fig. 1).
Band-pass filters for the photons of 151 ± 20 and
171 ± 20 nm (eSource Optics) placed between the crystal and the PMT were switched every 5 min. The radioactivities of 229 Pa, 230 Pa, and 232 Pa were 10.7(8),35(2), and 0.381(8) kBq at the start of the measurement (11 days after the proton irradiation).
As shown in Fig. 2, the count rates of photons forboth filters are ∼10 counts per second (cps), which- 143 -1

III-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 54 (2021)

are much higher than the dark count rate of the PMT(0.96 cps). The detected photons would originate fromthe Cherenkov radiation caused by the passage of beta
particles though the CaF2 crystal. The long decay
time of photons (half-life >7 d) in Fig. 2 indicates that
the Cherenkov photons dominantly originate from thebeta decay of 230 Pa (0.004 photons per beta particle).
If we implant 100 kBq of 229 Pa into a CaF2 crystal,anneal it, and start the measurement one day after theproton irradiation, the background photons from 232 Paand 230 Pa are estimated to be ∼30 cps, which is muchhigher than the estimated count rate of γ rays from229m
Th (2 cps). Thus, we plan to perform the mass
separation of 229 Pa to reduce the amount of 232 Pa and230
Pa by a factor of >10 when we implant 229 Pa into a
CaF2 crystal. Implanting 229 Pa into a crystal with aneﬃciency of 0.1–1% allows us to observe a growth anddecay curve of photons of several cps only for the 151nm ﬁlter, resulting in the unambiguous identiﬁcationof the γ rays from 229m Th.
References

1) B. Seiferle et al., Nature 573, 243 (2019); A. Yamaguchi
et al., Phys. Rev. Lett. 123, 222501 (2020); T. Sikorskyet al., Phys. Rev. Lett. 125, 142503 (2020).
2) B. Seiferle et al., Phys. Rev. Lett. 118, 042501 (2017).

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