In order to assay the tritium fuel content in laser fusion targets and/or to measure the pressurization of laser fusion targets of the type which use deuterium and tritium (DT) gas mixtures, without destroying the targets, the flux of beta particles which emerges from the target is measured with the aid of a gas flow proportional counter. The count rates are related to the tritium content and the pressurization. The tritium content in terms of the mass of the tritium in the target can be derived from the counting rate.