One step closer to the development of an ultra-precise nuclear clock

Measuring time using oscillations of atomic nuclei might significantly improve precision beyond that of current atomic clocks


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 physicist PD Dr. Peter Thirolf and his team at LMU Munich and including scientists and engineers from Johannes Gutenberg University Mainz, the Helmholtz Institute Mainz, and the GSI Helmholtz Center for Heavy Ion Research in Darmstadt 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.

Oscillations as the heart of timekeeping

The second is our basic unit for the measurement of time. In today’s conventional atomic clocks, the time of a second 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 almost 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," said 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 3,300 known types of atomic nuclei only one potentially offers a suitable basis for a nuclear clock, and that is the nucleus of the thorium isotope with atomic mass 229 (Thorium-229), which, however, does not occur naturally. For over 40 years physicists have suspected this nucleus to exhibit an excited state with energy 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. Furthermore, Th-229m is expected to show a rather long half-life from between minutes to 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.

First direct detection of the nuclear transition

Direct detection of the thorium isomer Th-229m has never before been achieved. "Up until now, the evidence for its existence has been purely indirect," said Thirolf. In a complex experiment, the researchers involved have now succeeded in detecting the elusive nuclear transition. They made use of uranium-233 as a source of Th-229m, which is produced in the radioactive alpha decay of uranium-233. "The uranium-233 was chemically purified by our team including Mainz- and Darmstadt-based experts and was deposited as an ultrapure thin layer on a titanium-covered silicon wafer as used in the semiconductor industry. This uranium-233 source was then transferred to Munich, where it was mounted in the experimental apparatus, providing the desired Th-229m", explained Professor Christoph Düllmann, the head of the groups in Mainz and Darmstadt.

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," said Thirolf. "This breakthrough is a decisive step toward the realization of a working nuclear clock," emphasized the LMU physicist. "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, i.e., its half-life and, in particular, the energy difference between the two states. These data will allow laser physicists to set to work on a laser that can be tuned to the transition frequency, which is an important prerequisite for an optical control of the transition." Professor Thomas Stöhlker, research director at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, added: "These new findings are very valuable for our experiments with TH-229m planned at the GSI/FAIR storage ring, particularly those concerning the determination of the energy of the nuclear transition."