While there is little doubt about the existence of the Th-229 isomeric state, a direct measurement of its energy and lifetime are still pending. The current best value of the isomer wavelength is 159(10) nm, where the error stated here is purely statistical. The uncertainty needs to improved by at least one or two orders of magnitude before is makes sense to commence direct laser spectroscopy. So far, experiments employing synchrotron radiation for excitation of the isomer have not succeeded to detect the photonic de-excitation of the isomer. While each experiment might suffer from individual challenges, is seems that non-radiative decay of the isomer might dominate over the radiative branch whenever the thorium atom is adsorbed onto (or doped into) a medium.
The Vienna group now put forward a new approach to measure the isomer emission with a spectrometer. The method of isomer population itself has been used dozens of times before: U-233 undergoes alpha decay into Th-229, and, with a 2%-probability, into the isomeric state. The technological advancement is to go from surface-implated Th ions to Th ions doped into crystals. Leaping from 2-D to 3-D increases the flux of isomer gammas by two orders of magnitude! The new approach is experimentally much easier to implement and requires no pre-knowledge or “guessing” of the isomer lifetime. In a practical experiment, the signal is expected to be increased by more like 3 orders of magnitude.
A number of careful experiments were required to show that this new approach is indeed feasible. At first, it was shown that uranium can be doped into CaF2 at concentrations of 1000 ppm. Second, the transparency of 233U:CaF2 crystals in the VUV was shown to be sufficiently good to allow gammas to leave the crystal. The alpha decay of U-233 leads to scintillation of the crystal, and it could be shown that this luminescence appears only in a spectral region that is well-separated from the expected isomer transition.
The wavelength region around 160 nm is, however, covered by Cherenkov radiation. This radiation does not originate directly from the alpha decay, but from beta decays in the chain of Th-229. Luckily, Th-229 has a half-life (8000 years) much larger than a typical experiment. Sadly, commonly available U-233 is contaminated with U-232, which decays into Th-228 and further down the entire chain on timescales comparable to the experiment. It is the beta decays in the unavoidable Th-228 contamination that form the Cherenkov background. With a bit of theory input, the absolute amplitude of Cherenkov emission can be calculated numerically. The only input parameter required is the amount of U-232 contamination and the date of the last removement of thorium ingrowth.
Modelling a future experiment with reasonable parameters, such as a U-232 contamination of 10 ppm, a 3-month period since the last purification, and a standard commercial VUV spectrometer, it is found that even if 99% of the isomers undergo non-radiative de-excitation, a clear signal can be detected within a few days of measurement. This value jumps to many months if the non-radiative decay comes in at 99.9%.
Demonstrating the feasibility of this approach required contributions from experiment, theory, and radiochemistry: a truly multi-disciplinary effort! The preprint, backed by a good deal of supplemental material, can be retrieved from the arXiv pre-print server.