Laser Excitation of the Th-229 Nucleus

The 8.4 eV nuclear isomer state in Th-229 is resonantly excited in Th-doped ${\mathrm{CaF}}_2$ crystals using a tabletop tunable laser system. A resonance fluorescence signal is observed in two crystals with different Th-229 dopant concentrations, while it is absent in a control experiment using Th-232. The nuclear resonance for the ${\mathrm{Th}}^{4+}$ ions in Th:${\mathrm{CaF}}_2$ is measured at the wavelength 148.3821(5) nm, frequency 2020.409(7) THz, and the fluorescence lifetime in the crystal is 630(15) s, corresponding to an isomer half-life of 1740(50) s for a nucleus isolated in vacuum. These results pave the way toward Th-229 nuclear laser spectroscopy and realizing optical nuclear clocks.

Figure 1

Excitation scheme (a) and experimental apparatus (b) for VUV laser spectroscopy of the isomeric state in Th-doped crystals. The VUV source consists of two cw lasers with the frequencies ${\nu }_1$, ${\nu }_2$, two pulsed dye amplifiers that introduce frequency shifts $({\delta }_{B1},{\delta }_{B2})$ due to Brillouin mirrors, a THG stage, and a xenon gas cell. The scanning is provided by tuning of the difference frequency ${\mathrm{\Omega }}_2$ via ${\nu }_2$. The spectroscopy vacuum chamber contains the Th-doped crystal mounted on a cold finger, signal collection optics, and a PMT. (c) False color CCD camera image of the crystal during VUV laser excitation and a schematic representation of the crystal structure of ${\mathrm{Th}}^{4+}$ ions doped into the ${\mathrm{CaF}}_2$ lattice with $2{\mathrm{F}}^{-}$ for interstitial charge compensation.

Figure 2

(a) VUV fluorescence signals from the Th-229-doped X2 crystal, recorded in frequency scans from higher to lower frequency (squares) and lower to higher frequency (dots). The measurement time between frequency steps is shorter than the isomer decay time (see Fig. 3), which leads to an asymmetry in the resonance curves. (b) The resonance asymmetry is removed, together with the radioluminescence background, from the plots (a) in postprocessing. (c) Result of a control experiment with the Th-232-doped V12 crystal.

Figure 3

Th-229 fluorescence decay curve after resonant excitation (blue trace) for 500 s measured with crystal X2 at a temperature of 150 K. Gray trace: result of a control experiment with 200 GHz off-resonant excitation, showing no long-lived photoluminescence. Inset: fluorescence decay times for crystal temperatures between 108 and 168 K. No changes in the decay time are observed within the measurement uncertainty. An overall decay time constant $\tau =630(15)\mathrm{s}$ is observed.