Physics: From the atomic to the nuclear clock

Atomic clocks are currently our most precise timekeepers. The present record is held by a clock that is accurate to within a single second in 20 billion years. Researchers led by Ludwig-Maximilians-Universitaet (LMU) in Munich physicists Peter Thirolf, Lars von der Wense and Benedict Seiferle have now experimentally identified a long-sought excitation state, a nuclear isomer in an isotope of the element thorium (Th), which could enhance this level of accuracy by a factor of about ten. Their findings are reported in the scientific journal Nature. The team also includes scientists based at Johannes Gutenberg University Mainz, the Helmholtz Institute Mainz and the GSI Helmholtz Centre for Heavy-Ion Research in Darmstadt, Germany.

The heart of timekeeping

The second is our basic unit for the measurement of time, and is tied to the oscillation period of electrons in the atomic shell of the element cesium (Cs). The best atomic clock currently in use boasts a relative precision of 2×10-18. "Even greater levels of accuracy could be achieved with the help of a so-called nuclear clock, based on oscillations in the atomic nucleus itself rather than oscillations in the electron shells surrounding the nucleus," says Thirolf. "Furthermore, as atomic nuclei are 100,000 times smaller than whole atoms, such a clock would be much less susceptible to perturbation by external influences."

However, of the more than 3300 known types of atomic nuclei, only one potentially offers a suitable basis for a nuclear clock -- the nucleus of the thorium isotope with atomic mass 229 (Th-229), which, however, does not occur naturally. For over 40 years physicists have suspected this nucleus to exhibit an excited state whose energy lies only very slightly above that of its ground state. The resulting nuclear isomer, Th-229m, possesses the lowest excitation state in any known atomic nucleus. "Th-229m is further expected to show a rather long half-life, between minutes and several hours. It should thus be possible to measure with extremely high precision the frequency of the radiation emitted when the excited nuclear state falls back to the ground state," Thirolf explains.

First direct detection of the transition

However, direct detection of the thorium isomer Th-229m has never been achieved. "Up until now, the evidence for its existence has been purely indirect," says Thirolf. Together with his colleagues, he has now succeeded in detecting the elusive nuclear transition in a complex experiment. They made use of uranium-233 as a source of Th-229m, which is produced in the radioactive alpha decay of uranium-233. In an experimental tour-de-force, the scientists isolated the isomer as an ion beam. "Using a microchannel plate detector, we were then able to measure the decay of the excited isomer back to the ground state of Th-229 as a clear and unambiguous signal. This constitutes direct proof that the excited state really exists," says Thirolf. "This breakthrough is a decisive step toward the realization of a working nuclear clock," he adds. "Our efforts to reach this goal in the framework of the European Research Network nuClock will now be redoubled. The next step is to characterize the properties of the nuclear transition more precisely -- its half-life and, in particular, the energy difference between the two states. These data will allow laser physicists to setting to work on a laser that can be tuned to the transition frequency, which is a prerequisite for an optical control of the transition."