Researchers have accomplished the high-precision measurement of a thorium (Th) isotope's nuclear transition - opening the possibilities for more accurate nuclear clocks.

Average clocks tick every second through the use of a balance wheel or a quartz oscillator. However, for ultra-precision requirements, such as maintaining television broadcasts, navigation systems, and advanced scientific studies, atomic clocks are used. Following the change in electron energy levels in the shell, these clocks can reach accuracies of almost one in 1018. In perspective, these atomic clocks have not yet lost a second throughout the lifetime of the Universe.

In 2003, scientists proposed the idea of following the ticking of thorium-229 nuclei. The concept will improve existing clocks by order of magnitude, to 1019, but has not been accomplished until now. The main restriction was precisely determining the transition frequency, allowing the nucleus' direct excitation with narrow-band lasers.

Measuring Thorium-229 Isomer Energy

For atomic clocks, atoms of a single element - usually ytterbium or strontium - are irradiated using lasers, exciting electrons in the shells.  Specific wavelengths of electromagnetic radiation, introduced by the lasers, create energy level transitions in the atom's shells. These energized electrons then oscillate between two energy states.

3D Strontium Atomic Clock
(Photo: National Institute of Standards and Technology via Wikimedia Commons)
JILA's three-dimensional (3-D) quantum gas atomic clock consists of a grid of light formed by three pairs of laser beams. A stack of two tables is used to configure optical components around a vacuum chamber. Shown here is the upper table, where lenses and other optics are mounted. A blue laser beam excites a cube-shaped cloud of strontium atoms located behind the round window in the middle of the table. Strontium atoms fluoresce strongly when excited with blue light.

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Nuclear clocks, in theory, should function similar to atomic clocks, but following the oscillation in the nucleus itself instead of the electrons in the atomic shell. The problem, however, is that most nuclei require high transition energies, from kilo electron volt to megaelectronvolt range. The energy needed to make them oscillate, achievable with gamma-ray radiation instead of lasers, makes them impractical for timekeeping purposes. The only exception is the thorium-229 nuclei, being the only known nucleus to have a transition energy requirement in the electronvolt range.

A team of researchers from German and Austrian universities have proposed measuring the nuclear transition of thorium-229 using a cryogenic magnetic calorimeter of their design. To calculate the change, they first begin with measuring the emitted gamma radiation from the isotope uranium-333 decaying into other isomers - same molecular formula but having different arrangements - of thorium-229. Their particular target in this process is the metastable isomer thorium-229m. This method has been tested before, yielding 7.6 electronvolts in a 2007 attempt and 7.8 electronvolts in 2009.


Narrowing Down the Th229m Values, In-Line With Previous Works

With the novel spectrometer, gamma rays contact an absorbing plate to generate heat, which produces a magnetization change in the sensors, translated as the transition energy. Their measuring technique yielded a transition energy value of 8.1 electronvolts from an excitation wavelength of 153.1 nanometers. It follows a previous work, using a different method, to have a measurement of 8.28 electronvolts from a wavelength of 149.7 nanometers.

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Researchers explained that their work "complements the conversion electron experiment in that the isomer energy is extracted directly from the experimental data, without resorting to calculations." They also noted that so far, the remaining significant uncertainty lies in the statistical error. This can be reduced significantly as the researchers continue and repeat the measurements by using these methods.