Search for a ''3.5-eV isomer'' in 229Th in a hollow-cathode electric discharge

A hollow-cathode electric discharge, a well-established source in optical spectroscopy, was used to populate the ''3.5-eV isomer'' in 229Th with use of nuclear excitation by electron transition (NEET). The radiochemically purest 229Th sample was loaded into the hollow cathode in which the electric discharge excited the 229Th to atomic states some of which could be expected to lie close to the excitation energy of the sought isomer. Although there remain some uncertainties, our experiments indicate that the isomer was populated by NEET and its α decay observed after switching off the electric discharge with a corresponding isomer half-life 1 min < or approx. T1/2m < or approx. 3 min. From the present NEET condition, the isomer appears to lie between 3 eV and 7 eV. The probability of the isomer population by NEET is discussed