[U] ULTRATRACE ANALYSIS This term could be used generally to describe the whole area of trace analysis using microsized samples, but could be more precisely specified as trace analysis with sample size £ 104 g and constituent content £ ppm (0.01%). C. See subtrace analysis.
 UNSEALED SOURCE A radioactive source which is not encapsulated, in contrast to a sealed source.
 UNSTABLE Radioactive.
 URANIUM SERIES The natural decay chain commencing with 238U and ending with stable 206Pb. The mass number of each member of the series is representable as 4n+2 where n is an integer.
