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The proportional counter is used to detect & measure the energiesof particles (α and β ) and also of photons.

It operates under a potential difference across the electrodes which is such that the secondary ionisation takes place but the output pulse is always proportional to the number of primary ions produced initially by entering particle (or photon). 
The pulse height gives a measure of energy of particles or photons entering the counter. 
The proportional counter is thus able to distinguish directly by virtue of the height of the pulse between α and β particles &  γ rays. Since they differ in ionising power. 
Since α particles causes maximum ionisation pulse height is maximum. On other hand β and gama particle causes minimum ionisation, hence pulse height is wlso minimum for β particle and  γ rays.