Determination of beta sources activity: instability due to measurement geometry changes inside cuvette

Liquid scintillators are most frequently used for determining the activity of low- and high-energy beta emitters (3H, 14C, 32P). Time instability is shown of recorded count rates. The sorption of an active sample on the cuvette wall results in a disturbed radioisotope distribution in the scintillation mixture, thus altering the measurement geometry and reducing measurement efficiency. The effects differ in low- and high-energy beta emitters in that, that electrons with high initial kinetic energy induce Cherenkov radiation in the cuvette wall and increase count rates in a certain measurement channel. The geometry alteration due to sorption may be reduced by initial correction or by acidifying the scintillation mixture during measurement, however, it decreases measurement efficiency. (H.S.)