We present analytical estimates and numerical calculations showing that the energy level density in open-shell atoms increases exponentially with the increase of excitation energy. As an example, we use the relativistic Hartree-Fock and configuration interaction methods to calculate the density of states of Th and Th II. The result is used to estimate the effect of electrons on the nuclear transition which is considered for the nuclear clock.

• Received 24 March 2010

DOI:https://doi.org/10.1103/PhysRevLett.104.213002