Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 53

Half-life determination of nuclear excited states of 229 Th by thecoincidence measurement between α particles and γ rays from 233 U
Y. Shigekawa,∗1,∗2 A. Yamaguchi,∗3 K. Suzuki,∗4 H. Haba,∗1 T. Hiraki,∗4 H. Kikunaga,∗5 T. Masuda,∗4
S. Nishimura,∗1 N. Sasao,∗4 A. Yoshimi,∗4 and K. Yoshimura∗4
Studying the nuclear structure of 229 Th is importantin that reflection-symmetric and reflection-asymmetricshapes are expected to coexist1) and the first nuclear excited isomeric state of 229 Th (229m Th) has an extremely
low energy of 8.3 eV,2) which would enable the development of nuclear laser spectroscopy and an ultraprecise
nuclear clock. In this study, we measured the half-livesof nuclear excited states of 229 Th, populated through thealpha decay of 233 U, for further understanding of the nuclear properties of 229 Th. This work was also aimed atdetermining the half-life of the 29.2-keV state (Fig. 1).
This half-life was one of the key parameters for exciting 229 Th from the ground state to the 29.2-keV statewith synchrotron radiation to actively produce 229m Th,
which was recently realized by our group.3)
For the half-life determination, we performed a coincidence measurement between α particles and γ rays froma 233 U source prepared by electrodeposition. The 233 U
source was placed inside a vacuum chamber enclosed by
5-cm Pb blocks for background γ-ray reduction. Emitted α particles from the source were measured with apassivated implanted planar silicon detector mounted inside the chamber, and γ rays that were emitted from thesource and passed through a polyimide window out ofthe vacuum were measured with a LaBr3 (Ce) scintillator and photomultiplier. After amplification, α-particle

Fig. 1. Excited states of 229 Th the half-lives of which weredetermined, and α and γ transitions used for determining
the half-lives (unit: keV).
∗1
∗2
∗3
∗4
∗5RIKEN Nishina Center
Graduate School of Science, Osaka University
Quantum Metrology Laboratory, RIKEN
Research Institute for Interdisciplinary Science, Okayama University
Research Center for Electron Photon Science, Tohoku University- 51 -Fig. 2. Time spectrum of γ-ray signals following α-particle
signals for 4783.5-keV α particles and 42.4-keV γ rays
(histogram). A single exponential decay function convoluted with a Gaussian function is fitted to the data(curve).

and γ-ray signals were recorded with Computer-Aided
Measurement And Control (CAMAC) modules to obtaintime spectra of γ-ray signals following α-particle signals
for each combination of α-particle and γ-ray energies.
Figure 2 shows a time spectrum for 4783.5-keV α particles and 42.4-keV γ rays. The half-life obtained fromthe fitting of a single exponential decay function convoluted with a Gaussian function was almost the sameas the reference value (172(6) ps).4) From a time spectrum for 4783.5-keV α particles and 29.2-keV γ rays,
we obtained the half-life of the 29.2-keV state, which isconsistent with that obtained from our synchrotron excitation of 229 Th (82(4) ps).3) These coincidences withthe previous results indicate that our experiments anddata analysis were correctly performed. The half-life of
the 97.1-keV state was obtained from γ rays of 54.7 and97.1 keV detected in coincidence with 4729-keV alphaparticles. The obtained half-life was seemingly shorter
than the reference value (147(12) ps).4) This may be because our measurement has better and time resolutions
than the previous measurement.4)
We could clearly observe the peak of 66.1-keV γ rays
by gating the γ-ray spectrum with α particles of approximately 4664 keV, which allowed us to obtain the half-lifeof the 163.2-keV state for the first time. We will continue detailed analysis to precisely determine each halflife and discuss the nuclear properties of 229 Th based on
the half-lives determined in this study.
References

1) G. A. Leander et al., Nucl. Phys. A 413, 375 (1984).
2) B. Seiferle et al., Nature 573, 243 (2019).
3) T. Masuda et al., Nature 573, 238 (2019).
4) H. Ton et al., Nucl. Phys. A 155, 245 (1970).