Measurement of the ${}^{229}$Th isomer energy with a magnetic micro-calorimeter

Abstract

We present a measurement of the low-energy (0--60keV) $\gamma$ ray spectrum produced in the $\alpha$-decay of ${}^{233}$U using a dedicated cryogenic magnetic micro-calorimeter. The energy resolution of $\sim$$10$eV, together with exceptional gain linearity, allow us to measure the energy of the low-lying isomeric state in ${}^{229}$Th using four complementary evaluation schemes. The most accurate scheme determines the ${}^{229}$Th isomer energy to be $8.10(17)$eV, corresponding to 153.1(37)nm, superseding in precision previous values based on $\gamma$ 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 $b_{42}=0.3(3)\%$.

5 pages, 5 figures + supplementary

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