Thorium on steroids: A new proposal by E.V. Tkalya

Many research groups around the world try to get an “optical” handle on the Th-229 isomeric state, but it turns out that the detection of optical photons emitted upon de-excitation of the isomer is no mean feat. As an alternative, one might shift gears and aim to detect the electron that is emitted during internal conversion (IC) of the isomer. Such an approach has been suggested by the LMU group at last week’s DPG conference in Darmstadt, Germany.

The IC electron would have a kinetic energy of a few eV only: an energy scale that nuclear physicists might feel uncomfortable with. “Is there a way to boost this energy?”, E.V. Tkalya (a well-known expert in the field of Th-229 research, based in Moscow) asked himself. Obviously, the few-eV isomer energy cannot be changed, but he found a different way: in a recent publication (see the preprint on the arXiv), he suggests magnification the ground and isomer states’ hyperfine structures into the 100 eV range. The required magnetic field is generated by substituting the innermost 1S electron of the Th-229 atom by a muon. The muon’s orbit is effectively within the nucleus, generating a magnetic field of a few 10 GT. The lifetime of the muon would be on the order of 100 ns.

The enormous magnetic field splits the ground state into a hyperfine doublet with an energy gap of some 350 eV. The hyperfine splitting of the isomer would be only a few eV. As a consequence, the upper ground-state hyperfine state appears above the isomer doublet, allowing for a bizarre scenario: the ground state may populate the isomer by simple relaxation! As the muon disappears again after 100 ns, the nucleus might remain in the isomeric state. It is estimated that on today’s existing muon factories (e.g. PSI in Switzerland), on the order of 10 nuclei in the isomeric state could be produced per second.

Apart from shifting the energy of the IC electrons into the 100 eV range, there is another appealing asset to this approach: during the (admittedly short) lifetime of the muon, the hyperfine structure of the isomer is about 5 eV, and could be driven by laser light. Hyperfine transitions are thus pushed from the microwave into the optical domain.

Time will tell if the Th-299 research, technologically quite involved already, can benefit from muonic atoms.