The difference in the dead time observed between the two detectors should be partially attributed to the different fraction of the electron beam absorbed in the detector wall due to the variation of the wall thickness (Table 1). For example, from electron range calculation (csda) in stainless steel it is deduced that electrons with energy lower than 1.1 and 1.5 MeV for 3He and 4He counters, respectively, are totally absorbed in the detector wall. Hence the fraction of 90Y beta decay spectrum (maximum energy 2.28 MeV) that would penetrate the detectors cover would be 38% and 17%, respectively.