Radiative Decay of the ²²⁹ᵐTh Nuclear Clock Isomer in Different Host Materials

A comparative vacuum ultraviolet spectroscopy study conducted at ISOLDE-CERN of the radiative decay of the ²²⁹ᵐTh nuclear clock isomer embedded in different host materials is reported. The ratio of the number of radiative decay photons and the number of ²²⁹ᵐTh embedded are determined for single crystalline CaF₂, MgF₂, LiSrAlF₆, AlN, and amorphous SiO₂. For the latter two materials, no radiative decay signal was observed and an upper limit of the ratio is reported. The radiative decay wavelength was determined in LiSrAlF₆ and CaF₂, reducing its uncertainty by a factor of 2.5 relative to our previous measurement. This value is in agreement with the recently reported improved values from laser excitation.