The Roos electron chamber1 is used as a reference electron chamber. It is recommended by the IAEA2 for high precision electron dosimetry in radiation therapy. The chamber has a 4 mm wide guard ring to exclude any perturbation effect even at low electron energies. The polarity effect is negligible (< 0.5 % at 10 MeV). The energy response is only influenced by the stopping power ratios water / air. The chamber is waterproof for absolute dose and depth dose measurements in a water phantom. The coated acrylic entrance window has a thickness of 1.1 mm. The nominal useful energy range is from 2 MeV to 45 MeV.
