Complex sentence: This relies on interaction of the photon radiation with the tube wall material to produce electrons which then ionise the fill gas, as the density of gas in the tube is usually low, allowing most high energy gamma photons to pass through undetected.
Split into simpler sentences: This relies on interaction of the photon radiation with the tube wall material to produce electrons which then ionise the fill gas. This is necessary as the density of gas in the tube is usually low, allowing most high energy gamma photons to pass through undetected.