[adrian_b]

The radiation emitted when nuclei transition between their internal energy levels is known as gamma rays.

The gamma rays normally have energies per photon many orders of magnitude greater than for visible light and also much greater than for X-rays (which are produced by electrons accelerated by very high voltages when hitting a target).

The thorium 229 nucleus is the only one that can emit gamma rays that are so low in energy that their energy is not only lower than for X-rays, but it is also lower than for many sources of ultraviolet light. For instance the ultraviolet light used in state-of-the-art lithography for semiconductor manufacturing has much higher frequency (shorter wavelength), by about ten times.

These gamma rays of the Th229 have a wavelength that is not much shorter than the 184-nm ultraviolet light that can be obtained with a mercury-vapor lamp.

What is important is that for such a frequency/wavelength it is possible to build laser sources, which enables the design of an atomic clock that will use thorium 229 nuclei instead of neutral atoms or ions of other elements (like ytterbium, lutetium, strontium, aluminum).

[bawana]

Arent gamma, x, and uv ALL em radiation? What makes a gamma with a wavelength near uv still allow it to be called gamma? Why dont we say the nucleon emits uv at 148 when it transitions to its ground state?

[snarkconjecture]

You could say that as well. Gamma is often (usually?) defined as any radiation that originates from nuclear state transitions rather than electrons; this tends to be very high energy but can overlap the range of EM radiation from electronic transitions. Th229 is the extreme outlier.