A common type of radiation detectors are the gas-filled tubes. These are cylindrical gas-filled capacitors where one electrode is the cylinder itself and the other elec­trode is a wire that is electrically insulated from the cylinder. Under the effect of ionizing radiation, the atoms or molecules of the gas are ionized, producing posi­tive ions and electrons. When direct potential is applied across the tube, the positive ions will migrate toward the negative electrode (anode), while the elec­trons toward the positive electrode (cathode) induce an electric current. Thus, the migration of ions to the electrodes results in an electric impulse that can be mea­sured, for example, by a simple electric circuit (see Section 14.4). The time needed for the ionization and migration to the electrodes determines the dead time, which is about 10_4 s. The formation and shape of the electric impulses and the regenera­tion of the detector are illustrated in Figure 14.2.

The ionization detectors can measure alpha and beta radiation. Since gamma radiation does not form ions directly, only the secondary electrons emitted in the scattering processes and in the photoelectric effect lead to ionization (Section 5.4), and the efficiency of the measurement of gamma radiation is low.

In Figure 14.3, the quantity of the ions that reaches the electrodes of the gas — filled tube is shown as a function of direct voltage. The function has characteristic ranges, of which some can be used for radiation detection. Range I is the range of

recombination. In this range, the voltage is too low to collect all the ions and elec­trons initiated by the radiation. Thus, this range is not used for the detection and measurement of radiation.

Voltage then increases to a threshold value where all ions and electrons can reach the electrodes. By continuing to increase the voltage, a plateau is reached where no additional ions and electrons are formed (Range II). This is the voltage range, where ionization chambers work. As seen in Figure 14.3, there is a big dif­ference in the numbers of ions produced by alpha and beta radiations. Thus, the ionization chambers can differentiate between alpha and beta particles. In the volt­age range of the ionization chambers, the number of the primary ions is propor­tional to the energy of the radiation; therefore, the energy of the radiation can be determined. However, the number of the primary ions is usually low, so the appli­cation of the ionization chambers is limited. Therefore, ionization chambers are used to determine the total activity; for example, in dosimeters and signals where different amplitudes are not discriminated, all impulses are counted together. In other words, the measurements are made in an integrated way.

By further continuing to increase the voltage in these detectors, the primary ions ionize additional gas atoms or molecules when flying toward the electrodes, which results in the formation of secondary ions. As a result, the number of the collected ions increases and the current increases by several orders of magnitude. This range (Range III) is the so-called proportional range. The detectors working in this range are called “proportional counters.” Since the number of the secondary ions is pro­portional to the number of the primary ions, and consequently to the energy of radi­ation, the proportional counters give some information on the energy of the radiation. By increasing the voltage, the proportionality becomes less clear. Similarly, the number of ions produced by the alpha and beta particles gets closer. This range is the semi-proportional range (Range IV) and is not appropriate for radiation detection.

Beyond the semi-proportional range, a new plateau is formed (Range V). This is the so-called Geiger—Muller range, the range of Geiger—Muller counters or tubes. As seen in Figure 14.3, the electric impulses are independent of both the type and the energy of the radiation. Therefore, the Geiger—Muller counters can measure the total activity or intensity of the alpha and beta radiation. Since the range of the alpha particles is short (as discussed in Section 5.2), the measurement of alpha particles needs very thin windows (<1.5 mg/cm2). If the window of the detector is thicker, only the beta particles are detected.

By an additional increase of voltage, continuous discharge is formed (Range VI), which can damage the detector.