are two electrodes, between which there is a potential difference of several hundred volts. When ionising radiation enters the tube, some molecules of the gas are ionised, either directly by the incident radiation or indirectly by further collisions. This creates positively charged ions and electrons, which are known as ion pairs, in the gas. The strong electric field created by the tube’s electrodes accelerates the positive ions towards the cathode and the electrons towards the anode.
This causes current to flow between the anode and cathode. The amount of current is converted to pulses, or counts, by an external amplifier and counter.
At a certain point, an avalanche effect occurs in which travelling ions collide with other particles and cause further ions. These increase the count rate.
cathode atom anode mica window
ionising radiation
electron R
500 V ion
counter
FIG. 31.20 As radiation enters the mica window, ion pairs form and cause a current to flow
Solid state detector
A solid state detector is used to detect the amount and type of radiation by using a semiconductor p-n junction (Fig. 31.21). It works in the following way:
• Radiation coming into the detector hits the depletion layer. (This was explained in detail in Chapter 25 Semiconductors.)
• Electron-hole pairs are produced, which allow current to flow through the junction.
FIG. 31.21 A solid state detector uses a semiconductor p-n junction to detect the amount and type of radiation
• Just like a G-M tube, a solid state detector converts the amount of current to counts by an external amplifier and counter.
