Development of Nuclear Clocks Using Thorium-229
Atomic clocks traditionally keep time by counting the ‘ticks’ of electrons moving between energy levels. However, there is an interest in using nuclear transitions for such measurements due to their potential for greater stability.
Thorium-229 as a Candidate
- Candidate: Thorium-229 (229Th) is a promising candidate for nuclear clocks.
- Unique Feature: The nucleus of 229Th has an excited state only about 8.4 electron-volts (eV) above the ground state, allowing excitation by vacuum-ultraviolet (VUV) laser light.
Challenges in Detection
- Detection Issues: The direct detection of thorium excitation in solids is challenging due to internal conversion.
- Internal Conversion: Instead of emitting a photon, the thorium nucleus often transfers its energy to an electron, which may be ejected from the material.
Research and Methodology
- Innovative Approach: Researchers from Germany, the UK, and the US treated internal conversion as a signal.
- Experiment Setup:
- Utilized thorium dioxide with an energy gap of about 6 eV.
- Employed VUV laser pulses to excite thorium nuclei and counted escaped electrons from decayed nuclei.
- Used timed electric fields to suppress bursts of photoelectrons and guide electrons to a detector.
- Findings: A resonance at 2,020,407.5 GHz was observed, consistent with previous studies.
- Conversion Lifetime: The internal conversion lifetime was found to be 12.3 µs, indicating high precision with potential deviation of one second every 15.8 billion years.
Implications and Future Prospects
- Commentary: Independent researchers emphasized the expansion of materials available for stable, high-precision nuclear clocks.
- Miniaturization Potential: The design allows monitoring by measuring emitted electron currents, facilitating miniaturization compared to current techniques.
