RESUMO
A liquid scintillation counting method consisting of three photomultiplier tubes for beta counters and a NaI(Tl) gamma counter has been developed for the standardization of radionuclides with the beta-gamma coincidence technique. The beta detection efficiency functions are obtained by means of a geometry-variation method developed in the present work; an array of beta detectors is moved uniformly at the same time from a centrally located counting vial to 50 mm. The method has been applied in the standardization of 60Co and 134Cs. Unquenched liquid scintillation samples with nominal count rates from 1000 to 6000 s-1 were prepared. The observed beta detection efficiencies with this method are from 90 to 45% in the case of 60Co, and from 84 to 50% for 134Cs. The output of each beta channel is summed together and compared with gamma data by the coincidence analyzer. The dead time of each counting channel is adjusted to be 20 micros, sufficiently long to suppress the afterpulses in the beta counting channel. The activity of each sample is determined by using the Cox and Isham formula. The obtained results are in good agreement with KRISS certified values.