Recently there was big news that the optical transition in Thorium $229$ nuclei had been identified by laser excitation. The excitation energy is $8.355733554021(8)\,\mathrm{eV}$. According to the nuclear clock wiki page, the next lowest lying nuclear gamma transition is in Uranium $235$, and the energy is about $76\,\mathrm{eV}$, well beyond the limit of optical lasers (possibly accessible to free electron lasers?)
The Thorium transition was first suggested in the paper "Features of the low-energy level scheme of 229Th as observed in the α-decay of 233U." It was discovered, as far as I understand, because on an x-ray spectrometer there were two decays to different nuclear states but the decays appeared to have the same energy - indicating the two energy levels were very close together.
I'm wondering if it's at all possible that there are other optical transitions in nuclei which have not been discovered. I see two reasons such a thing might remain undiscovered:
- There might be an isotope with a low-energy gamma decay which doesn't happen to lie along the decay chain of something extensively studied like Uranium $233$, so nobody mapped out the energy levels of such an isotope.
- There could be an optical nuclear transition in an isotope that otherwise decays fairly quickly (like by beta decay), so we wouldn't have been able to carefully study the isotope.
Are any of these things possible? I want someone to tell me if back of the envelope considerations rule out these ideas, making the Thorium transition rare among all isotopes. Or perhaps I want someone to tell me how well we have investigated all possible isotopes - maybe if such a thing existed we would already know it.
