High-precision measurement of 234U/238U ratio by thermal ionization mass spectrometer equipped with a WARP filter

High-precision measurement of 234U/238U isotopic abundance ratio was carried out for three uranium samples, separated from a lake sediment and two zircon specimens. The thermal ionization mass spectrometer used has nine new bucket-type Faraday collectors and a Daly ion-counting system detector positioned behind axial Faraday and WARP filter (a kind of energy filter). The WARP filter is designed to suppress the tailing effect of adjacent strong ion. The aberration of the measured ratio from the theoretical ratio was as small as 0.02%, for a zircon from Finland. A 0.20% excess of 234U in the lake sediment is judged to be reasonably significant. The amount of uranium used is of the order of 200-300 ng, and the measuring time required is 80 minutes as a whole. Half-life of 234U is evaluated to be 2.44416x105 y. (author)