Energy separation of the doublet of intrinsic states at the ground state of ${}_{}{}^{229}{}_{}{}^{}\mathrm{Th}$

It has been known from some time that the intrinsic state labeled by the asymptotic quantum numbers ${(3/2\mathrm{}}^{+}$[631] lies quite close (<0.1 keV) to the ${(5/2\mathrm{}}^{+}$[633] ground state of ${}_{}{}^{229}{}_{}{}^{}\mathrm{Th}$. Using the energies of selected γ rays emitted following the α decay of ${}_{}{}^{233}{}_{}{}^{}\mathrm{U}$, we have obtained a value of 1±4 eV for the energy separation of these two intrinsic states.