Measurement of the 229Th Isomer Energy with a Magnetic Microcalorimeter

We present a measurement of the low-energy (0-60 keV) gamma-ray spectrum produced in the a decay of U-233 using a dedicated cryogenic magnetic microcalorimeter. The energy resolution of similar to 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. spectroscopy, and agreeing with a recent measurement based on internal conversion electrons. We also measure branching ratios of the relevant excited states to be b(29) = 9.3(6)% and b(42) < 0.7%.