III-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Development of a photon measurement apparatus for observing theradiative decay of 229m Th produced from 229 Pa
Y. Shigekawa,∗1 Y. Wang,∗1 X. Yin,∗1 A. Nambu,∗1 T. Yokokita,∗1 and H. Haba∗1
The ﬁrst excited state in the 229 Th nucleus (229m Th)
has an excitation energy of ∼8.3 eV (150 nm),1) which
potentially enables an ultraprecise nuclear clock. We
have been aiming to observe the radiative decay (γrays) of 229m Th and to determine its radiative half-life,
which is an essential parameter to develop the nuclear
clock, by doping a CaF2 crystal with 229 Pa, which decays to 229m Th with negligibly small recoil energy.2)
In this study, we developed a photon measurement apparatus for observing the γ rays of 229m Th. We also
investigated the background photons originating fromthe decay of 229 Pa (T1/2 = 1.5 d) and other impurities such as 232 Pa (T1/2 = 1.31 d) and 230 Pa (T1/2 =17.4 d).
Figure 1 shows a schematic view of the developed
apparatus. Two photomultipliers are placed inside
a vacuum chamber: one is for measuring the γ raysof 229m Th in the vacuum ultraviolet (VUV) range(PMT1, Hamamatsu R10454), and the other is formeasuring scintillation photons produced from 229 Pa
and other isotopes in a CaF2 crystal (PMT2, Hamamatsu R7154). The events of PMT1 that coincidedwith those of PMT2 correspond to high-energy radiation that can produce scintillation photons, and thus,
such events can be excluded from the analysis of theγ-ray events of 229m Th. Band-pass (BP) ﬁlters for photons of 151 ± 20 and 171 ± 20 nm (eSource Optics) areplaced between PMT1 and the CaF2 sample. The ﬁlters can be switched using a linear drive driven by astepper motor; the γ rays of 229m Th are expected tobe detected only for the 151-nm BP ﬁlter. PMT1 andPMT2 can be cooled to −25◦ C using a Peltier cooler,
reducing the dark count rate to 0.09 s−1 for PMT1 and0.25 s−1 for PMT2. Considering the half-life of 229m Th(103 –104 s), the 229 Pa-doped CaF2 can be rapidly introduced to the measurement position (∼10 min) as
follows without the leakage of the large vacuum chamber (Fig. 1). First, the sample ﬁxed on a linear drive isFig. 1. Schematic view of the developed apparatus (left)
and a photograph of the sample loading system (right).
∗1

RIKEN Nishina Center- 124 -Fig. 2. Count rate of photons measured using PMT1 forthe 151-nm (circle) and 171-nm (square) BP ﬁlters with(closed) and without (open) anticoincidence using theevents measured by PMT2. The sum of the exponential
decay functions of 229 Pa and 230 Pa is ﬁtted to the countrate with anticoincidence for each ﬁlter (solid lines).

placed inside a small vacuum chamber, which is thenrapidly evacuated. Next, a gate valve is opened, andthe sample is moved to the large chamber for photonmeasurement.
We investigated the background photons originating from 229 Pa and other isotopes, which may interfere with the observation of the γ rays of 229m Th. The
production and chemical separation of 229 Pa were performed similarly to those in a previous study.2) First,
two 232 Th metallic foils (total 138 mg/cm2 ) were irradiated with 1 μA of a 30-MeV proton beam for
10 h at the RIKEN AVF cyclotron. Next, the foils
were dissolved with concentrated HCl and fed onto ananion-exchange column (Muromac 1X8, 100–200 mesh,∼1.0 mL). After pouring concentrated HCl, 6 M HCl,and 8 M HNO3 to the column, Pa isotopes were eluted
with 9 M HCl/0.1 M HF. In this study, we performed
an additional anion-exchange process to reduce radioactive impurities such as 97 Zr as follows. First, Pa
isotopes were dissolved in 0.1 M HCl/0.1 M HF and fed
onto an anion-exchange column (Muromac 1X8, 100–200 mesh, ∼0.5 mL). After pouring 0.1 M HCl/0.1 M
HF, Pa isotopes were eluted with 0.4 M HCl/0.1 M HF.
The ratio of radioactivity of 229 Pa isotopes to that ofother radioactive elements was ∼105 after the chemical separation. Thereafter, we dropped the 229 Pa solution on a CaF2 crystal, annealed it, and started aphoton measurement four days after the proton irradiation. Compared with the previous measurement,2)

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III-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 55 (2022)

the amount of 229 Pa was dominant over other isotopes (229 Pa 48(3) kBq, 232 Pa 2.51(6) kBq, and 230 Pa
6.6(4) kBq); thus, we could evaluate the backgroundphotons produced by high-energy radiation from 229 Pa
more precisely.
As shown in Fig. 2, the count rate of photons detected by PMT1 for the 151-nm BP ﬁlter was ∼9 s−1
at the start of the measurement. Anticoincidence using PMT2 reduced the count rate to ∼1/3. For eachBP ﬁlter, the sum of the exponential decay functions of229
Pa and 230 Pa was well ﬁtted to the data, indicatingthat the photons originate from 229 Pa and 230 Pa. The
ratio of photons from 229 Pa to those from 230 Pa was∼1. In the presence of these background photons, if weuse a CaF2 sample doped with 100 kBq of 229 Pa, oursimulation indicated that we can observe the γ raysof 229m Th without the mass separation of 229 Pa anddetermine its half-life with a relative error of ∼25%;
the development of the apparatus for measuring the γrays of 229m Th is nearly complete.
References
1) B. Seiferle et al., Nature 573, 243 (2019).
2) Y. Shigekawa et al., RIKEN Accel. Prog. Rep. 54, 143(2021).

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