Complex sentence: High energy beta particles can also be detected by a thin walled tube which has no dedicated end window, and although the tube walls have a greater stopping power than an end window, they allow these more energetic particles to penetrate to reach the fill gas.
Split into simpler sentences: High energy beta particles can also be detected by a thin walled "windowless" tube; which has no dedicated end window. Although the tube walls have a greater stopping power than an end window, they still allow these more energetic particles to reach the fill gas.