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

RIKEN Accel. Prog. Rep. 56 (2023)

Preparation of a small high-density 229 Th target for the X-ray
pumping of the 229 Th nuclear clock isomerY. Shigekawa,∗1 K. Okai,∗2 Y. Fukunaga,∗2 K. Yoshimura,∗2 and H. Haba∗1
The ﬁrst excited state of the 229 Th nucleus (229m Th)
has an excitation energy of ∼8.3 eV,1) which allows
laser excitation and spectroscopy of the nucleus. One
application is ultraprecise nuclear clocks with unprecedented uncertainty.2) Our group aims to observe the γrays emitted from 229m Th, which is an important stepfor realizing nuclear clocks. In 2019, we succeeded in
actively producing 229m Th by populating the 29.2-keVsecond excited state of 229 Th using X-rays at SPring8.3) With this technique, we are now trying to detect γrays of 229m Th produced in a 229 Th-doped CaF2 crystal, in which the internal conversion process of 229m Th
would be inhibited. For every beamtime at Spring-8,
we have to precisely adjust the energy of X-rays for theresonant excitation of 229 Th to the 29.2-keV state. For
the energy adjustment, we have used a source containing 0.24 µg of 229 Th placed in a hole with a diameterof 0.4 mm on a graphite plate. Owing to the small
amount of 229 Th, searching for the resonant X-ray energy has required more than 14 h. The above 229 Th
source was prepared by performing the ∼7000 sets ofdropping a small amount of 229 Th solution (5 nL perdrop) into the hole and evaporating it by heating. The
amount of 229 Th that can be poured into the small
hole was limited because the nanoscale drop was notstable and the density of the 229 Th sample was lower
than expected, probably owing to the repeated rapid
evaporation processes. In this study, we developed a
new simple method and successfully prepared a small
high-density 229 Th source, which is used to search forthe resonant X-ray energy for the 29.2-keV state.
The new source-preparation method is drying one
drop (∼1 µL) of nitric acid containing 229 Th on a
Teﬂon plate. Owing to the high water repellency of
Teﬂon, the shape of the drop remains spherical during the evaporation process, and the drop size becomesmuch less than 1 mm just before it is completely dried.
We prepared two types of Teﬂon plates shown in Fig. 1.
The type A plate has a small taper hole made bydrilling, where the 229 Th solution is dropped. The holein the type B plate was manually prepared by heating
the Teﬂon plate and deforming it with a conical die.
We found that a drop on the type B plate was evaporated and retained a spherical shape. In contrast, adrop on the type A plate slightly expanded, and thesize of the residues became larger than 1 mm. The images obtained by scanning electron microscopy (SEM)
for the type A plate (Fig. 2) showed scratches made by∗1∗2

RIKEN Nishina Center
Research Institute for Interdisciplinary Science, Okayama
University- 147 -Fig. 1. Schematics of the Teﬂon plates of types A (left) andB (right).

Fig. 2. SEM images of the Teﬂon plates of types A (left)
and B (right).

drilling, which would be the reason for the lower water
repellency. The type B plate showed a smooth surface(Fig. 2); thus, we adopted the type B plate to prepare
the 229 Th source.
229
Th was ﬁrst puriﬁed via anion exchange chromatography using high-purity acids (metal impurities<100 ppt). The puriﬁed 229 Th was dissolved with10 µL of 1-M HNO3 . The solution was dropped ona Teﬂon sheet and evaporated. Then, the residue wasdissolved with 1 µL of 1-M HNO3 and dropped on thetype B plate placed on a hot plate. We washed theTeﬂon sheet with 1 µL of 1-M HNO3 , which was thenadded to the drop on the type B plate. The temperature of the hot plate was ﬁrst maintained at ∼45◦ C for
20 min to slowly evaporate the majority of the solution. It was then increased to ∼120◦ C over 11 min andmaintained at that temperature for 9 min. The temperature was then raised to 200◦ C over 13 min andmaintained at that temperature for 19 min for complete dryness.
The 229 Th source shown in Fig. 3 had a residue witha diameter of ∼0.7 mm. The amount of 229 Th in thesource was measured to be 1.61(2) µg by γ-ray spectroscopy, which is 6.7 times larger than the amountof the previous 229 Th source that we have used.3) Xray ﬂuorescent measurements performed at SPring-8
showed that a large amount of 229 Th was condensed
within a diameter of 0.65 mm. By using this new229
Th source, we were able to ﬁnish the X-ray en-Ⅲ-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 56 (2023)

ergy search for the excitation to the 29.2-keV state
four times faster than that for the previous source.

Fig. 3. Photograph of the prepared 229 Th target.

References
1) B. Seiferle et al., Nature 573, 243 (2019).
2) E. Peik et al., Europhys. Lett. 61, 181 (2003).
3) T. Masuda et al., Nature 573, 238 (2019).

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