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When nuclear radiation passes through a gas enclosed between two electrodes, it ionise the gas molecule. If a potential difference is applied across the electrodes the positive ions move towards the negative electrode and electrone toward positive electrode. 
Thus, pulses of electric current are produced which detect & measure the ionising radiation. Such detectors are called gas-filled detectors, and include ionisation chamber, proportional counter & G.M. Counter. 
Each of these counters have different ionisation condition which can be explained   as :
Let us consider the variation of ionisation current with the potential difference applied across the electrode. 
The curve corresponds to ionisation current produced by the charge particle of lower energy, while the curve II corresponds to the charge 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 ionisation 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. V2 is reached. Thus between V1 and V2 ionisation current is independent of applied p.d. & is proportional to number of ions (primary) formed in the gas, and hence In the region between V1 and V2 ionisation 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 begins to ionise the gas molecule by collision. The secondary ions so formed starts further ionisation and so on. 
In this way avalanche of ionisation is produced and the current increases upto a p.d. V3 ,the number of secondary ions formed remains proportional to 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 begin to loose proportionality feature. 
In the region V3 and V4 no counter works.
Above V4 ,the number of secondary ions become 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.