A Search for the Decay of Metastable 229 m Th with Superconducting Tunnel Junctions

The desire to build nuclear clocks with an unprecedented accuracy of 1 part in 1019 is currently driving the interest in accurate measurements of the first excited nuclear state in 229Th. We have used superconducting tunnel junction (STJ) radiation detectors to search for the associated decay of 229mTh and measure its energy accurately. In this experiment, recoiling 229mTh ions from the alpha decay of 233U are embedded in the STJ detectors, and their subsequent decay into the ground state is expected to produce a signal at an energy of 7.8±0.5 eV. This approach is particularly promising, because all decay products are captured inside the STJ so that the measured energy does not depend on the decay mode or on chemical effects. Although this approach works well to characterize the decay of metastable 235mU, no signal from 229mTh has been observed. This negative result can be explained by recent measurements of the 229mTh half-life of only 7±1μs. We discuss our experiments and the modifications required to measure the energy of metastable 229mTh with an accuracy of order ± 10 meV.