Operating regions of gaseous ionization detectors

Case Discussion

Recombination region - incident radiation knocks out free electrons from gaseous molecules. However, the low electrical field applied is not enough to accelerate the free electrons to collide with all gaseous molecules into their respective ions and additional free electrons (ion-electron pairs). Some ions will recombine with electrons to form gaseous molecules back again. Besides, only a small number of ion-electron pairs reach the electrodes to be registered as a reading. Since the number of ion-electron pairs formation cannot be accurately predicted, this region is not used in detecting radiation doses.

Ionization region - formation of primary ion-electron pairs is "saturated" at this level of applied voltage. Further increase of the voltage applied does not increase the number of ion-electron pairs. Instead, the number of ion-electron pairs formed is dependent on the type and energy of the radiation. The number of ion-electron pairs detected by the electrodes is equal to the number of ion-electron pairs produced. This voltage region is used by ionization chamber to detect radiation doses.

Proportional region - electrons produced by primary ion-electron pairs at this region have enough energy to knock out more electrons (secondary electrons) from other gaseous molecules, known as "Townsend avalanches". These secondary electrons contribute to the increase in detector reading when increased voltage is applied, although the number of primary ion-electron pairs is unchanged. The number of secondary ion-electron pairs produced is proportional to the number of primary ion-electron pairs (known as gas multiplication). This voltage region is used by proportional counter to detect different types of radiations and their doses. The different types of radiation can be distinguished by their pulse heights.

Limited proportional region - increase in voltage applied causes a non-linear rise in the charge collected because electrons are drawn faster to the postive electrode when compared to positive ions drawn to the negative electrode. The high concentration of positive ions near the negative electrodes distorts the proportionality of gas multiplication. Thus, this region is not used in radiation measurement.

Geiger-Muller region - electrons produced by primary Townsend avalanches at this region have enough energy to produce secondary avalanches by the emission of ultraviolet rays from the atoms of the primary avalanches. High concentration of positive ions near the avalanches help to terminate the avalanches process after some time, thus preventing continuous discharge (see below). Different types of radiations produces similar charges at the electrodes. Therefore, unlike proportional counter, Geiger counter cannot distinguish between different types of radiation.

Continuous discharge region - electric field at this region accelerates the electrons enough to produce continuous avalanches, causing continous discharges into the electrodes; irrespective whether there is presence of any radiation. Thus, this region is not useful for radiation detection.