Gas-Filled Detectors PDF

Gas-Filled Detectors Absorption of radiation in a gas-filled chamber is accomplished by establishing an electrostatic field between the wall of the chamber and a positive electrode located on the axis of the chamber and insulated from it as illustrated in Figure 12-1.The gas is usually at a pressure...

Gas-Filled Detectors Absorption of radiation in a gas-filled chamber is accomplished by establishing an electrostatic field between the wall of the chamber and a positive electrode located on the axis of the chamber and insulated from it as illustrated in Figure 12-1.The gas is usually at a pressure of one atmosphere or less, but with appropriate care can be pressurized to enhance interactions.When radiation is absorbed in the gas contained by the chamber, ion pairs (a positive ion and an electron) are produced, which are collected and amplified for recording the signal produced. The collection of ion pairs in a gas-filled detector is a function of the applied voltage as shown in Figure 12-2, which characterizes the operation of such detectors for beta particles (lower curve) and for alpha particles (upper curve).If there is no voltage across the chamber, the ion pairs will recombine, and no charge will flow in the external circuit for either type of radiation.As the voltage is increased, say to a few volts, some ion pairs will still recombine but others will flow to the electrodes and be collected.At a voltage of perhaps 10 V or more, recombination becomes negligible, and all of the electrons produced by ionization will reach the central electrode.As the voltage is increased to several tens of volts the number of ion pairs collected is independent of the applied voltage, and the curve will remain horizontal as long as the radiation source produces ionizing radiation at a steady rate.If 10 ion pairs are formed initially, the response will be steady as shown by the lower curve; if the source strength or ionizing rate (e.g., an alpha emitter) is a factor of 10 higher, 100 ion pairs will be formed, and the detector response will also be a factor of 10 higher as shown in the upper curve.These curves are parallel to each other, i.e., the current collected is directly related to the ionization produced by the incoming radiation.This region in which the current measured is directly related to the amount of ionization produced is called the ionization region; its magnitude will be quite different for equal source strengths of alpha particles, beta particles, x-rays, and gamma rays due to differences in the number of ion pairs produced by each as they interact in the medium. Increasing the voltage of a gas-filled detector above the ionization region causes the electrons released by the primary ionizations to acquire enough energy to produce additional ionizations as they collide with the gas molecules in the chamber. The number of electrons collected increases roughly exponentially with the applied voltage because each initial electron is then accelerated to produce a small “avalanche” of electrons, most of which are liberated close to the central electrode. Each electron liberated by the incoming radiation produces its own independent avalanche such that at a given voltage of the detector the ionization produced is amplified by a constant amount, i.e., the number of ion pairs collected is proportional to the initial ionization.G as-filled detectors operated at these voltages are characterized as being operated in the proportional region.When the voltage across the chamber reaches several hundred volts the gas multiplication effect increases very rapidly, and as more electrons produce avalanches, the latter begin to interact with one another creating a region of limited proportionality.As voltage is increased further the charge collected becomes independent of the ionization initiating it, and the two curves not only become identical but form a plateau as voltage is increased.This is the Geiger–Mueller region and it is characterized by the plateau.At voltages above the plateau the detector produces a region of continuous discharge. Fig.