integral of the electric current) between the electrodes, we can determine the particle's kinetic energy because the number of ion pairs created by the incident ionizing charged particle is proportional to its energy. The energy resolution of a proportional counter, however, is limited because both the initial ionization event and the subsequent 'multiplication' event are subject to statistical fluctuations characterized by a standard deviation equal to the square root of the average number formed. However, in practice these are not as great as would be predicted due to the effect of the empirical Fano factor which reduces these fluctuations. In the case of argon, this is experimentally about 0.2. Photon detection Proportional counters are also useful for detection of high energy photons, such as gamma-rays, provided these can penetrate the entrance window. They are also used for the detection of X-rays to below 1 Kev energy levels, using thin-walled tubes operating at or around atmospheric pressure. Radioactive contamination detection Proportional counters in the form of large area planar detectors are used extensively to check for radioactive contamination on personnel, flat surfaces, tools, and items of clothing. This is normally in the form of installed instrumentation because of the difficulties of providing portable gas supplies for hand-held devices. They are constructed with a large area detection window made from such as metalized mylar which forms one wall of the detection chamber and is part of the cathode. The anode wire is routed in a convoluted manner within the detector chamber to optimize the detection efficiency. They are normally used to detect alpha and beta particles, and can enable discrimination between them by providing a pulse output proportional to the energy deposited in the chamber by each particle. They have a high efficiency for beta, but lower for alpha. The efficiency reduction for alpha is due to the attenuation effect of the entry window, though distance from the surface being checked also has a significant effect, and ideally a source of alpha radiation should be less than 10mm from the detector due to attenuation in air. These chambers operate at very slight positive pressure above ambient atmospheric pressure. The gas can be sealed in the chamber, or can be changed continuously, in which case they are known as "gas-flow proportional counters". Gas flow types have the advantage that they will tolerate small holes in the mylar screen which can occur in use,