Proposal for a Nuclear Gamma-Ray Laser of Optical Range

A possibility of amplification of the 7.6 eV $\gamma$-radiation by the stimulated $\gamma$-emission of the ensemble of the ${}^{229m}$Th isomeric nuclei in a host dielectric crystal is proved theoretically. This amplification is a result of: 1) the excitation of a large number of the ${}^{229m}$Th isomers by laser radiation; 2) the creation of the inverse population of nuclear levels in a cooled sample owing to the interaction of thorium nuclei with the crystal electric field or with an external magnetic field; 3) the emissions/absorption of the optical photons by thorium nuclei in the crystal without recoil; 4) the nuclear spin relaxation through the conduction electrons of the metallic covering.