Half-life estimation of the first excited state of ^{229}Th by using α-particle spectrometry

To search for a direct-decay signal from the isomer Th-229(m), alpha-particle spectra of Th-229(m,g) produced from 93 mg of U-233 have been measured by using a rapid and high-resolution alpha-particle spectrometry, which can distinguish alpha lines of Th-229(m) from those of its ground state. Although alpha events were not obtained in the expected energy region for Th-229(m) with the exception of those derived from Th-229(g), we can estimate that the half-life of Th-229(m) is shorter than 2 h at 3 sigma confidence level under the chemical condition of chloride or hydroxide.