The natural isotope 103Rh has two low-lying levels with energies of 357.4 keV and 295.0 keV, the lifetimes of which are 107 and 9.7 ps, respectively. Thus the lower level has a considerably shorter lifetime than the upper one, a situation favorable for generating population inversion. Nuclear Resonance Fluorescence (NRF) experiments have been performed in which 26 high-lying levels have been identified, from which either the upper or the lower level or both can be populated by radiative decay. An evaluation of the data shows that this "feeding" mechanism indeed resulted in inversion on the 62.4 keV transition.
Stedile, F., Fill, E. E., Belic, D., von Brentano, P., Fransen, C., Kneissl, U., ... Werner, V. (2001). Generating inversion on a nuclear transition - photopumping of Rh-103. Journal de Physique IV, 11(PR2), 271-274. https://doi.org/10.1051/jp4:2001251