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If photoabsorption of the incident X-ray occurs too close to the entrance window of the proportional counter, electrons of the secondary electron cloud (generated during the thermalization of the primary electron) can be lost by diffusion to the window. (The ratio of the diffusion coefficient to the drift velocity of electrons in the gas is constant for the entire energy range of the counter.) In addition, secondary electrons can also be lost to the front cathode wire grid (K1; Fig. 3.1). In both cases, the detected pulse height (and therefore associated energy) will be shifter to lower values producing a low pulse height tail. However, if the size of the secondary electron cloud is large compared to the spacing of the cathode wires, all events will be affected by the grid to roughly the same degree, and the situation can reasonably approximated by a simple increase in the mean energy required to create an electron-ion pair.
The effect of these two processes is most noticeable in the lab for monochromatic lines at low energy. However, for flight data, the effect is most noticeable for hard, absorbed sources (such as the Crab) where there is a nonzero flux (or ``tail'') in the 1/4 keV band where because of high foreground absorption there are no true source photons.