DETECTOR VOLTAGERadiation DetectorsIonizationRegionAs voltage is increased in the ionization region (Region II), there is no appreciableincrease in the pulse height. The field strength is more than adequate to ensure collectionof all ions produced; however, it is insufficient to cause any increase in ion pairs due togas amplification. This region is called the ionization chamber region.ProportionalRegionAs voltage increases to the proportional region (Region III), the pulse height increasessmoothly. The voltage is sufficient to produce a large potential gradient near the anode,and it imparts a very high velocity to the electrons produced through ionization of the gasby charged radiation particles. The velocity of these electrons is sufficient to causeionization of other atoms or molecules in the gas. This multiplication of electrons iscalled gas amplification and is referred to as Townsend avalanche. The gas amplificationfactor (A) varies from 103 to 104. This region is called the proportional region since thegas amplification factor (A) is proportional to applied voltage.LimitedProportionalRegionIn the limited proportional region (Region IV), as voltage increases, additional processesoccur leading to increased ionization. The strong field causes increased electron velocity,which results in excited states of higher energies capable of releasing more electrons fromthe cathode. These events cause the Townsend avalanche to spread along the anode. Thepositive ions remain near where they were originated and reduce the electric field to apoint where further avalanches are impossible. For this reason, Region IV is called thelimited proportional region, and it is not used for detector operation.Geiger-MüllerRegionThe pulse height in the Geiger-Müller region (Region V) is independent of the type ofradiation causing the initial ionizations. The pulse height obtained is on the order ofseveral volts. The field strength is so great that the discharge, once ignited, continues tospread until amplification cannot occur, due to a dense positive ion sheath surroundingthe central wire (anode). V4 is termed the threshold voltage. This is where the numberof ion pairs level off and remain relatively independent of the applied voltage. Thisleveling off is called the Geiger plateau which extends over a region of 200 to 300 volts.The threshold is normally about 1000 volts. In the G-M region, the gas amplificationfactor (A) depends on the specific ionization of the radiation to be detected.IC-06 Page 16 Rev. 0
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