# What is the lowest energy gamma ray?

If we use the "origin-based" definition of gamma rays (i.e. photons emitted from nuclear decay, regardless of wavelength), which decay produces the lowest-energy, longest-wavelength emission? This comment Are the Properties of the EM Spectrum Fluid? alludes to low-energy gammas in what would usually be considered the ultraviolet band; are there or could there be any in the visible?