Journal article Open Access
Beck, B. R.; Becker, J. A.; Beiersdorfer, P.; Brown, G. V.; Moody, K. J.; Wilhelmy, J. B.; Porter, F. S.; Kilbourne, C. A.; Kelley, R. L.
The energy splitting of the 229Th 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 233U (105 muCi) 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).