The energy splitting of the ${}^{229}\mathrm{Th}$ ground-state doublet is measured to be $7.6\pm 0.5\mathrm{eV}$, significantly greater than earlier measurements. Gamma rays produced following the alpha decay of ${}^{233}\mathrm{U}$ ($105\mu \mathrm{Ci}$) were counted in the NASA/electron beam ion trap x-ray microcalorimeter spectrometer with an experimental energy resolution of 26 eV (FWHM). A difference technique was applied to the gamma-ray decay of the 71.82 keV level that populates both members of the doublet. A positive correction amounting to 0.6 eV was made for the unobserved interband decay of the 29.19 keV state ($29.19\to 0\mathrm{keV}$).

• Received 24 October 2006

DOI:https://doi.org/10.1103/PhysRevLett.98.142501