Liquid Scintillation Counting

Abstract

Carbon —14 and tritium are β ⁻ emitting radioisotopes with β ⁻ emissions of very low energy which are extremely difficult to detect with any form of window counter, due to self-absorption of the β ⁻ particles and their absorption within the counter window. The window problem has been overcome to some extent by counting solid samples within a windowless G-M or proportional counter, where efficiencies of 15% for ¹⁴C and 2.5% for ³H have been reported. To reduce the self-absorption losses it is desirable to mix the active sample homogeneously with the detecting material. This can be done by counting the sample in the gaseous phase. For example, ¹⁴C samples may be converted into ¹⁴CO₂ by combustion or by evolution from a labelled carbonate. The gaseous activity can then be intimately mixed with the filling gas of any type of gas ionisation detector, thus minimising the effect of β ⁻ absorption and resulting in high counting efficiencies. Tritium can also be counted in the gaseous state as hydrogen, water vapour or a hydrocarbon.