Energy splitting of the ground-state doublet in the nucleus 229Th

The energy splitting of the Th-229 ground-state doublet is measured to be 7.6 +/- 0.5 eV, significantly greater than earlier measurements. Gamma rays produced following the alpha decay of U-233 (105 mu 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 -> 0 keV).