Lifetime Measurement of the ${}^{229}\mathrm{Th}$ Nuclear Isomer

The first excited isomeric state of ${}^{229}\mathrm{Th}$ possesses the lowest energy among all known excited nuclear states. The expected energy is accessible with today’s laser technology and in principle allows for a direct optical laser excitation of the nucleus. The isomer decays via three channels to its ground state (internal conversion, $\gamma$ decay, and bound internal conversion), whose strengths depend on the charge state of ${}^{229m}\mathrm{Th}$. We report on the measurement of the internal-conversion decay half-life of neutral ${}^{229m}\mathrm{Th}$. A half-life of $7\pm 1\mu \mathrm{s}$ has been measured, which is in the range of theoretical predictions and, based on the theoretically expected lifetime of $\approx 10^4\mathrm{s}$ of the photonic decay channel, gives further support for an internal conversion coefficient of $\approx 10^9$, thus constraining the strength of a radiative branch in the presence of internal conversion.

Figure 1

(a) Visualization of the detection scheme reported in Ref. [34]. An ion beam is formed from ${}^{229(m)}\mathrm{Th}$ ions emerging from a ${}^{233}\mathrm{U}$ $\alpha$-recoil source. As long as ${}^{229m}\mathrm{Th}$ remains in a charged state, the lifetime is longer than 1 min. In the moment when the ions neutralize (e.g., by collecting the ions on a metal surface), internal conversion is triggered and the lifetime will be in the range of $\mu \mathrm{s}$. The emitted electron can be detected. (b) Scheme of the experimental setup that was used. A detailed description is given in the text. (c) Simulation of the isomer decay time characteristics of ${}^{229}\mathrm{Th}$ bunches. The simulation is based on a measured bunch shape, the 2% of the ${}^{229}\mathrm{Th}$ ions that, according to the nuclear level scheme, are in the isomeric state and an assumed half-life of $7\mu \mathrm{s}$ after neutralization. The electron detection efficiency is assumed to be 25 times larger than the one for ion detection. (d) Corresponding measurement of the isomeric decay with a bunched ${}^{229(m)}{\mathrm{Th}}^{3+}$ ion beam (blue curve) and a comparative measurement with ${{}^{233}\mathrm{U}}^{3+}$ (red curve).

Figure 2

Measurement of the isomeric decay with a bunched (a) ${}^{229(m)}{\mathrm{Th}}^{2+}$ and (b) ${}^{229(m)}{\mathrm{Th}}^{3+}$ ion beam (red curves). Corresponding comparative measurements performed with ${{}^{230}\mathrm{Th}}^{2+}$ and ${{}^{230}\mathrm{Th}}^{3+}$ are also shown (blue lines). Note that for all the logarithmic plots above, bins containing 0 counts were subsequently set to 0.1 counts in order to visualize the baseline.

Figure 3

Signal obtained when collecting ${}^{229(m)}{\mathrm{Th}}^{2+}$ ion bunches with different kinetic energies. The isomeric decay signal strength remains constant in all measurements, while the ionic impact signal decreases with the kinetic energy of the ion bunch.

Figure 4

Logarithmic plot of the temporal decay characteristics for ${}^{229(m)}{\mathrm{Th}}^{2+}$ (left) and ${}^{229(m)}{\mathrm{Th}}^{3+}$ ions (right) together with the fit curves (red) applied to determine the isomeric half-life of ${}^{229m}\mathrm{Th}$ after charge recombination on the MCP detector surface. The fit range is indicated by the vertical black dashed lines.