The metastable first excited state of thorium-229, 229mTh, is just a few electronvolts above the nuclear ground state1,2,3,4 and is accessible by vacuum ultraviolet lasers. The ability to manipulate the 229Th nuclear states with the precision of atomic laser spectroscopy5 opens up several prospects6, from studies of fundamental interactions in physics7,8 to applications such as a compact and robust nuclear clock5,9,10. However, direct optical excitation of the isomer and its radiative decay to the ground state have not yet been observed, and several key nuclear structure parameters—such as the exact energies and half-lives of the low-lying nuclear levels of 229Th—remain unknown11. Here we present active optical pumping into 229mTh, achieved using narrow-band 29-kiloelectronvolt synchrotron radiation to resonantly excite the second excited state of 229Th, which then decays predominantly into the isomer. We determine the resonance energy with an accuracy of 0.07 electronvolts, measure a half-life of 82.2 picoseconds and an excitation linewidth of 1.70 nanoelectronvolts, and extract the branching ratio of the second excited state into the ground and isomeric state. These measurements allow us to constrain the 229mTh isomer energy by combining them with γ-spectroscopy data collected over the past 40 years.
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The synchrotron radiation experiments were performed at the BL09XU and BL19LXU lines of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (proposals 2016B1232, 2017B1335, 2018A1326 and 2018B1436) and RIKEN (proposal number 20180045). We thank all members of the SPring-8 operation and supporting teams. The experiment received support from the KEK Photon Factory (proposal number 2017G085) and the Institute for Materials Research, Tohoku University (18F0014), where indispensable detector tests and target preparation were performed. We especially thank S. Kishimoto for support at KEK, T. Kobayashi for technical assistance at SPring-8 and K. Beeks for discussion during the preparation of the manuscript. This work was supported by JSPS KAKENHI grants JP15H03661, JP17K14291, JP18H01230 and JP18H04353. T.S. and S.S. gratefully acknowledge funding by the EU FET-Open project, grant number 664732 (nuClock). A. Yoshimi and A. Yamaguchi acknowledge the MATSUO foundation and Technology Pioneering Projects in RIKEN, respectively.
The authors declare no competing interests.
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Peer review information Nature thanks Jason Burke, Feodor Karpeshin and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
a, Timing diagram. Line (A) shows an analogue pulse from a Si-APD chip and line (B) represents the accelerator reference clock. The example shows two pulses with different photon energies within a cycle. b, Block diagram. For each pulse, three parameters are stored for the post-analysis: the timing of the pulse (CFD), the pulse height (ATC) and the TE timing. Preamp, preamplifier; Amp., amplifier; TDC, time-to-digital converter; USB; universal serial bus.
The X-ray beam is diffracted by a single Si crystal. Two p–i–n (PIN) photodiodes monitor the diffracted beams. The rotary table (black disk) adjusts the angle between the Si crystal and the X-ray beam so that the diffraction condition is satisfied. The two swivel stages adjust the tilt angles between the X-ray beam, the reciprocal lattice vector of the crystal and the rotation axis of the rotary table.
Temporal profiles measured at incident X-ray energy on resonance (blue histogram) and off resonance (black histogram). Inset, resonance curve (black dots) with a Gaussian fit (blue curve).
a, Energy spectrum of the prompt signal (black line). The coloured lines show various X-ray emission lines convoluted with the Si-APD energy response function: the photoelectric absorption lines listed in Extended Data Table 2 (blue), Compton scattering (magenta) and the Kα and Kβ lines of Cu, Zn and Fe (green). The strengths of these lines are adjusted to give the best fit to the data. The sum of all lines (red) reproduces the data above 7 keV well . b, NRS energy spectrum, obtained by subtracting the off-resonance data from the on-resonance data. The coloured lines are fits of the X-ray emission lines. We note that there is no contribution from Compton scattering or the Cu, Zn and Fe lines. Both experimental datasets are normalized to a 3,600-s run. The error bars represent statistical uncertainty of one standard deviation.
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Masuda, T., Yoshimi, A., Fujieda, A. et al. X-ray pumping of the 229Th nuclear clock isomer. Nature 573, 238–242 (2019) doi:10.1038/s41586-019-1542-3
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