Free parameter, figure of merit and ionization quench in liquid scintillation counting

A statistical study of the detection process demonstrates that the free parameter is essential to compute the counting efficiency in both CIEMAT/NIST and TDCR methods. An analysis of the computed counting efficiencies shows the uselessness of old definition of the figure of merit. A new definition is required and we adopt the idea of taking quantities related with the output of the photomultiplier. In addition, we justify the application of the chemical quenching simulation with the electronic variation of the photomultiplier gain. Finally, we describe a new procedure to determine the figure of merit and the optimum ionization-quenching factor from the pulse spectrum of different radionuclides. The robustness of the new procedure is tested with three different sets of stopping power for low-energy electrons.