as possible to reduce photon directionality though any beta window response is obviously highly directional. Vented chambers are susceptible to small changes in efficiency with air pressure and correction factors can be applied for very accurate measurement applications. Sealed low-pressure chamber These are similar in construction to the vented chamber, but are sealed and operate at or around atmospheric pressure. These chambers also have the advantage of not requiring a vent and desiccant. To improve detection efficiency, they are filled with a noble gas because the highly electronegative oxygen in air easily captures free electrons, forming negative ions. The strength of the beta window limits the differential pressure from atmospheric pressure that can be tolerated, and common materials are stainless steel or titanium with a typical thickness of 25 µm. High-pressure chamber The efficiency of the chamber can be further increased by the use of a high-pressure gas. Typically a pressure of 8-10 atmospheres can be used, and various noble gases are employed. The higher pressure results in a greater gas density and thereby a greater chance of collision with the fill gas and ion-pair creation by incident radiation. Because of the increased wall thickness required to withstand this high pressure, only gamma radiation can be detected. These detectors are used in survey meters and for environmental monitoring. Chamber geometry Most commonly used for radiation therapy measurements is a cylindrical or "thimble" chamber. The active volume is housed within a thimble shaped cavity with an inner conductive surface (cathode) and a central anode. A bias voltage applied across the cavity collects ions and produces a current which can be measured with an electrometer. Parallel-plate chambers (PPIC) are shaped like a small disc, with circular collecting electrodes separated by a small gap, typically 2mm or less. The upper disc is extremely thin, allowing for much more accurate near-surface dose measurements than are possible with a cylindrical chamber. Monitor chambers are typically PPICs which are used to continuously measure such as a radiation beam's intensity. For example, within the head of linear accelerators used for radiotherapy. Multi-cavity ionization chambers can measure the intensity of the radiation beam in several different regions, providing beam symmetry and flatness information. Research and calibration chambers Early versions of the ion chamber were used by Marie and Pierre Curie in their original work in isolating radioactive materials. Since then the ion chamber has been a widely used