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G.M Tube is a most useful of the instument used for decting and measuring the  energies α,β,Ƴ and X-rays .it is sufficently sensentive to detect individual  αand β particles . It is also used to compare the activity of radioactive substance.

 

Construction

It consist of a metallic cylindrical cathode tube enclosed in a glass envelop. A fine tungsten wire (anode wire) is stressed  along the axes of the tube. The rays to be detected enters the tube through the glass envelop.

The tube contain monoatomic gas such as argon or neon at a pressure of about 10cm of Hg, together with a small trace of halogen such as Cl or Br. The halogen X act as quenching agent.
A potential difference of several 100 volt is applied between the anode wire and cathode wire.

 

Fig. G. M. Counter
When an ionizing particle enter the G.M Tube, it ionizes some of the argon atoms, forming positive ions and free electrons .The free electrons further causes ionization and a large avalanche (sudden) of electron is produced.
Besides this some of the free electrons colliding the argon atom excite them. The excited atom return to their normal state emitting light photon. If a photon is absorbed  by another  excited atom is ionized releasing more electron whicvh further produce avalanche.

The total number of ion produced is now is independent of initial number of ions (primary ions) formed by the entering particle.

The electron being very light are collected at the anode leaving behind a space charge(positive charge). In a short time the space charge become enough dense to cancel the electric field around the anode.
The ionization then stop. Therefore the positive ions are drawn to the cathode and ionization again starts.
The time during which the ionization remain suspended is called dead time.

Thus the entry of a single particle in the tube cause a pulse of current , which is amplified by pulse amplifier and the amplified pulse is sended to rate meter (scale).

The rate meter records the arrival of each individual pulse separately so gives , the exact number of particle entering the G.M Tube.The combination of G,M tube and recorder is known as G.M counter.

Self Quenching Action of G.M Tube

The G.M Tube must produce a single pulse due to the entering of a single particle. It consist not give a number of pulse but should recover quickly to record the next entering particle.
Unfortunately , the positive Argon ion which eventually strikes the cathode gains electron from it and become neutral argon atom.