What's special about Thorium-229?

As a UK academic, a combination of government and university policies push me towards publishing in particular journals, at least for a fraction of my research. One of the top journals that they (and indeed I) would like me to publish in is Physical Review Letters, as it is a highly respected and highly read journal. It publishes articles across all areas of physics, and with nuclear physics being only a part of all physics activity, and a somewhat small one at that, there are often no nuclear physics articles in an edition of the journal. This makes it somewhere that I don't always look for the latest nuclear physics research, but partly for the reasons stated above, I do look every now and then.

As I type this, there are indeed no articles on nuclear physics in the latest complete edition. If I look back to the last issue, then there are a couple of articles in the Nuclear Physics section. What interests me more from a nuclear physics point of view, though, is the article listed next - in the atomic physics section. The article is entitled "Wigner Crystals of ²²⁹Th for Optical Excitation of the Nuclear Isomer".

Thorium-229 (²²⁹Th) is a special isotope. Of all know nuclides, it has the lowest-lying excited state above the ground state, at only around 7eV. That's around 10,000 times less energy than it usually takes to make a nucleus excite into an excited state. It's comparable to the sort of energy an atom needs to excite an electron. The strange thing is that nuclei, being so much smaller than atoms usually require much shorter wavelengths - and hence higher energies - of light to cause excitations. What this means is that sooner or later we will be able to directly excite and control nuclei with light pulses in the same way that we can do with atoms. The scope for applications is immense, from UV lasers, to more accurate atomic clocks, to stable quantum computers. Nuclei are so much better isolated from their environment than atoms that devices relying on quantum effects are easier to make.

If the history of the development of fields that were at the cutting edge of smallness (e.g. as atomic physics once was) into practical applications is anything to go by, Thorium-229 will be the start of a technological leap in the forthcoming years. Watch that isotope!

229Th

Thorium-229 is one of my favourite isotopes. With the lowest-energy first excited state of any known nucleus, it's the isotope of choice for interacting directly with things such as lasers, atoms and molecules, whose characteristic energies are much smaller than normal nuclear transition energies. One of the most promising practical uses of ²²⁹Th is in making a new time standard which is more accurate than existing atomic clocks. My friend and fellow blogger Rob Jackson has just published a paper on the chemical side of implanting ²²⁹Th in material for possible use in such a clock standard. He's blogged about it here.