Another important factor which can increase the response of 3He detector is the higher probability of photons produced by ionization–excitation processes by electrons, which have passed through the metal cover, to interact with the gas mixture in the 3He detector with its high Kr (Z ¼ 36) content. The absolute dead time in both detectors is smaller when the beta ray source is placed on the base of the detector (parallel irradiation). In this case a fraction of the electrons and/or electrons kinetic energy is absorbed in the dead layer of the gas as they pass through it, before entering the effective volume of the detector. For example, electrons with residual kinetic energy (after penetrating the base) less than 60–70 keV would be totally absorbed in this layer (csda). In addition, photons produced on the cylindrical side of the counter (perpendicular irradiation) have about 1.5 times larger probability of interaction with the gas or the metal cover than the ones produced on the base due to the different geometry factor. Finally, the dead time growth rate, or equivalently the count rate growth, in case of the irradiation performed perpendicularly to the 3He detector axis, is much more rapid, indicating that in this case there is relatively larger number of smaller pulses than in the case of the parallel irradiation, in accordance with the above justification.