The relationship between a detector’s applied voltage and pulse height is very complex. Pulse height and the number of ion pairs collected are directly related. As was written, voltages can vary widely depending upon the detector geometry, gas type, and pressure. The figure schematically indicates the different voltage regions for alpha, beta, and gamma rays. There are six main practical operating regions, where three (ionization, proportional, and Geiger-Mueller region) are useful for detecting ionizing radiation. These regions are shown below. The alpha curve is higher than the beta and gamma curve from the recombination region to part of the limited proportionality region due to the larger number of ion pairs produced by the initial reaction of the incident radiation.
In the proportional region, the charge collected increases with a further increase in the detector voltage, while the number of primary ion pairs remains unchanged. Increasing the voltage provides the primary electrons with sufficient acceleration and energy to ionize additional atoms of the medium. These secondary ions formed are also accelerated, causing an effect known as Townsend avalanches, which creates a single large electrical pulse. Even though there is a large number of secondary ions (about 103 – 105) for each primary event, the chamber is always operated such that the number of secondary ions is proportional to the number of primary events. It is very important because the primary ionization is dependent on the type and energy of the particles or rays in the intercepted radiation field. The number of ion pairs collected divided by the number of ion pairs produced by the primary ionization provides the gas amplification factor (denoted by A). The gas amplification in this region can increase the total amount of ionization to a measurable value. The charge amplification process greatly improves the detector’s signal-to-noise ratio and reduces the subsequent electronic amplification required. The voltage must be kept constant when instruments are operated in the proportional region. If a voltage remains constant, the gas amplification factor also does not change. Proportional counter detection instruments are very sensitive to low levels of radiation. Moreover, proportional counters are capable of particle identification and energy measurement (spectroscopy). Different radiation energies and different radiation types can be distinguished by analyzing the pulse height since they significantly differ in the primary ionization.