New type of random coincidences in 4πβ-γ coincidence facility

A 60Co solution of a specific activity of 582.5 Bq/mg was used in sample preparation. The coincidence facility was provided with a common beta and a dual gamma channel. In the measurements it is possible to use simultaneously two differently adjusted discrimination levels of the timing single channel analyzer of type Canberra 2037A or Canberra 1436. For the first measurement of a set of samples, the discrimination levels were set to 110 keV for channel 1 and 1240 keV for channel 2. In subsequent measurements, the values were set to 550 keV and 1024 keV, respectively. The results of absolute activity measurements were observed to systematically differ from the correct values if higher discrimination level had been adjusted for the gamma channel. The difference was attributed to the loss of true coincidences due to accumulation of pulses in the CO timing mode. The calculation is described of coincidence loss and an example is shown of calculating corrections for the results of measurements in the CO timing mode. (E.J.). 6 figs., 3 tabs., 5 refs