Measurement of the Th-229 Isomer Energy with a Magnetic Microcalorimeter

We present a measurement of the low-energy (0–60 keV) γ-ray spectrum produced in the α-decay of U-233 using a dedicated cryogenic magnetic micro-calorimeter. The energy resolution of ∼10 eV, together with exceptional gain linearity, allows us to determine the energy of the low-lying isomeric state in Th-229 using four complementary evaluation schemes. The most precise scheme determines the Th-229 isomer energy to be 8.10(17) eV, corresponding to 153.1(32) nm, superseding in precision previous values based on γ spectroscopy, and agreeing with a recent measurement based on internal conversion electrons. We also measure branching ratios of the relevant excited states to be b29 =9.3(6)% and b42 < 0.7%.