- ULTRATRACE ANALYSIS This term could be used generally to describe the whole area of trace analysis using micro-sized samples, but could be more precisely specified as trace analysis with sample size £ 10-4 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.