Emission of gamma rays by electron-nuclear double transitions

In this paper we discuss the relevance of electron-nuclear double transitions for the problem of the amplification of gamma rays. We consider the electron-nuclear coupled system of 151Eu in a CaF2 lattice, in the presence of a strong static magnetic field. We analyze the possibility of pumping this electron-nuclear coupled system from the ground atomic electron Zeeman level to a higher electron level with the aid of a microwave magnetic field, while the nucleus is simultaneously undergoing a gamma-ray transition to the nuclear ground state. It is shown that an overall inversion condition can be satisfied even in the absence of inversion of the nuclear populations, provided that the ratio of the electron populations in the excited and ground electron Zeeman states has a sufficiently small value