A Laser Excitation Scheme for ${}^{229m}\mathrm{Th}$

Direct laser excitation of the lowest known nuclear excited state in ${}^{229}\mathrm{Th}$ has been a long-standing objective. It is generally assumed that reaching this goal would require a considerably reduced uncertainty of the isomer’s excitation energy compared to the presently adopted value of $(7.8\pm 0.5)\mathrm{eV}$. Here we present a direct laser excitation scheme for ${}^{229m}\mathrm{Th}$, which circumvents this requirement. The proposed excitation scheme makes use of already existing laser technology and therefore paves the way for nuclear laser spectroscopy. In this concept, the recently experimentally observed internal-conversion decay channel of the isomeric state is used for probing the isomeric population. A signal-to-background ratio of better than $10^4$ and a total measurement time of less than three days for laser scanning appear to be achievable.

Figure 1

Schematic drawing of the experimental concept, proposed to probe the direct laser excitation of the isomeric state in ${}^{229}\mathrm{Th}$. See text for explanation.