Creation of the inverse population in the $^{229}$Th ground-state doublet

Abstract

A new method for obtaining of inverse population between quadrupole sublevels of the ground state $5/2^+(0.0)$ and the isomeric state $3/2^+(7.6$ eV) of the $^{229}$Th nucleus in a dielectric crystal with a large band gap by means of external source of the VUV radiation is proposed. The method is based on an efficient depopulation of the upper sublevels of the ground state of $^{229}$Th by resonant photons from narrowband laser or broader tunable free electron laser. Sublevels of the isomeric state play the role of intermediate states. In addition, we have considered a case of excitation of the isomeric state (without creation of inverse population) by broadband source of light with the anti-Stokes scattering. The proposed scheme of optical pumping results in (1) inverse population of nuclear sublevels without using extremely low temperatures, and (2) shows a new way for the creation of the gamma-ray laser of optical range at the nuclear transition in $^{229}$Th (a) in crystals with isovalent substitution of host ions (eg Si$^{4+}$ ions in the SiO$_2$ crystal replaced by the $^{229}$Th$^{4+}$ ions) and (b) in crystals such as Na$_2$ThF$_6$, where the substitution is not necessary.