the number of original events and the total ion current. For this reason, the applied voltage, the geometry of the chamber and the diameter of the anode wire are critical to ensure proportional operation. If avalanches start to self-multiply due to UV photons as they do in a Geiger–Muller tube, then the counter enters a region of "limited proportionality" until at a higher applied voltage the Geiger discharge mechanism occurs with complete ionization of the gas enveloping the anode wire and consequent loss of particle energy information. Therefore, it can be said that the proportional counter has the key design feature of two distinct ionization regions: Ion drift region: in the outer volume of the chamber – the creation of number ion pairs proportional to incident radiation energy. Avalanche region: in the immediate vicinity of the anode – charge amplification of ion pair currents, while maintaining localized avalanches. The process of charge amplification greatly improves the signal-to-noise ratio of the detector and reduces the subsequent electronic amplification required. In summary, the proportional counter is an ingenious combination of two ionization mechanisms in one chamber which finds wide practical use. Gas mixtures Usually the detector is filled with a noble gas; they have the lowest ionization voltages and do not degrade chemically. Typically neon, argon, krypton or xenon are used. Low-energy x-rays are best detected with lighter nuclei (neon), which are less sensitive to higher-energy photons. Krypton or xenon are chosen when for higher-energy x-rays or for higher desired efficiency. Often the main gas is mixed with a quenching additive. A popular mixture is P10 (10% methane, 90% argon). Typical working pressure is 1 atmosphere (about 100 kPa). Signal amplification by multiplication In the case of a cylindrical proportional counter the multiplication, M, of the signal caused by an avalanche can be modeled as follows: Where a is the anode wire radius, b is the radius of the counter, p is the pressure of the gas, and V is the operating voltage. K is a property of the gas used and relates the energy needed to cause an avalanche to the pressure of the gas. The final term gives the change in voltage caused by an avalanche. Applications Spectroscopy The proportionality between the energy of the charged particle traveling through the chamber and the total charge created makes proportional counters useful for charged particle spectroscopy. By measuring the total charge (time