55Fe vs cosmics
From Sauli's Principles of operation of multiwire proportional and drift chambers,
energy loss at atmospheric pressure for minimum ionizing particles is 2.44 keV/cm in Ar and 3.01 keV/cm in CO2 so for a 50/50 mix, and the shortest path through the centre of one straw (1.6 cm), cosmics should deposit a total of 4.4keV at a rate of 2.7 keV/cm.
The 55Fe source produces 6.0keV xrays. (55Fe decays by electron capture to 55Mn which de-excites by x-ray emission; Kα at 5.9 keV or Kβ X-ray at 6.5 keV with relative intensity 150:42 (Kaye&Laby 1973(!)) so the intensity-weighted average is (5.90*100+5.89*50+6.49*42)/192= 6.0 keV.)
The x-rays undergo photoelectric absorption by Ar; Ar emits a photoelectron and then de-excites either by Auger emission (87%) or fluorescence (13%); fluorescence produces the Ar escape peak at 3.0keV when the initial photoelectron causes an avalanche but the 3keV photon does not.
3keV electrons have a range of ~ 100 um in Ar (Sauli) which at a uniform rate of deposition (bad approximation to give a rough idea) is 300 keV/cm and 100 x greater than that for cosmics. So although the total energy deposited is similar for 55Fe and cosmics, the rate of deposition is very different.
