- When nuclear radiation passes through a gas enclosed between two electrodes, it ionizes the gas molecule. If a potential difference is applied across the electrodes the positive ions move towards the negative electrode and electrons toward the positive electrode.
- Thus, pulses of electric current are produced which detect & measure the ionizing radiation. Such detectors are called gas-filled detectors and include ionization chamber, proportional counter & G.M. Counter. Each of these counters has different ionization condition which can be explained as :
- Let us consider the variation of ionization current with the potential difference applied across the electrode. The curve I correspond to ionization current produced by the charged particle of lower energy, while the curve II corresponds to the charged particle of higher energy.
- If there is no potential difference across the electrodes the ions will recombine & there will be no current. As the p.d is increased to few volts, some of the positive ions & electrodes reach the opposite electrodes so that a small ionization current is obtained.
- At a certain p.d. V1, all the ions formed in the gas reaches the electrode and a saturation current is obtained. As the p.d. is further increased the current remains constant till the p.d. reached V2. Thus between V1 and V2 ionization current is independent of applied p.d. & is proportional to the number of ions (primary) formed in the gas, and hence In the region between V1 and V2 ionization chamber works.
- When the p.d. is increased above V2 the primary ions formed by the charged particles gains so much energy so that they themselves begin to ionize the gas molecule by the collision. The secondary ions so formed starts further ionization and so on.
- In this way avalanche of ionization is produced and the current increases up to a p.d. V3, the number of secondary ions formed remains proportional to the number of primary ions. Thus in V2 and V3 region, proportional counter works.
- Above V3, the avalanche begins to spread rapidly. In this region secondary ion begins to lose proportionality feature. In the region V3 and V4 no counter works. Above V4, the number of secondary ions becomes independent of the number of primary ions. In this region, G.M counter works i.e. V4 and V5.Above V5 continuous discharge starts.
- Curve I corresponds to charged particles of lower energy.
- Curve II corresponds to charge particle of higher energy.
This is how we have explained Gas-Filled Detectors as a part of nuclear radiation detectors. Some of the similar posts are: