Currently, time standards (i.e. the way a unit of time is defined) are set by ‘atomic clocks’. These rely on using electromagnetic waves to excite electronic transitions in an atom. The atom chosen is caesium, Cs. But electronic transitions define the chemistry of the elements, and can therefore be subject to external influences. This leads to atomic clocks being constructed in very complicated ways so as to minimise these effects. Nuclear transitions, i.e. transitions between the energy levels within an atomic nucleus, have the potential of offering greater stability, and considerably improved accuracy (potentially up to 6 orders of magnitude).
Electronic transitions in atoms correspond to a range of energies (and therefore frequencies). For example, for the simplest atom, hydrogen, there are transitions in the infra-red, visible and ultra-violet regions of the spectrum, but all can be probed by laboratory spectroscopic equipment. The issue with nuclear transitions is that, at least potentially, the energies involved can be much higher, making the transitions less straightforward to probe using available sources of electromagnetic radiation. At the very least, a laser will be required, and even then, the energies involved may not all be accessible.
The discovery that made the development of nuclear clocks a real possibility was that the isotope 229Th has a low lying state with a transition energy of 7.6 eV. Although this is in the vacuum ultraviolet region, it is easily accessible by laser spectroscopic methods. This isotope of thorium is rare; it results from the alpha decay of 233U, which is in itself not universally abundant. Not only is it rare, but it is expensive ($50M per gram!). It’s not available for mail order from a chemicals catalogue, for example, and relatively few laboratories in the world have supplies available.
So, how might the clock be constructed, and why is ii of interest to a materials modeller like myself? Well, the thorium nucleus must be embedded in suitable crystal lattice, so once a suitable material is identified, the question arises as to where the nucleus will substitute. One of the groups interested in developing nuclear clocks identified me as someone who could help with this question. The background to their research is given here. They had already identified LiCAF (LiCaAlF6) as a suitable host (it is transparent with a high band gap), so where would a thorium nucleus substitute in the lattice, and if it didn’t have the same charge as the ion it was replacing, how would the charge be compensated? These details are outside the scope of this posting, but can be summarised by saying that the thorium is expected to substitute at a calcium site, and the charge compensated is achieved by two additional fluorine atoms occupying interstitial sites in the lattice. You can read all about it here (contact me if you would like a PDF of the published paper).
So, where are we with nuclear clock development? With collaborators from the USA and Brazil, we will grow LiCAF crystals with 229Th nuclei embedded in them. Once this has been done, testing and ultimately device construction can commence. I will publish further posts as we progress with this exciting project!