G.M Tube (Geiger Mueller Counter Tube) is a most useful of the instrument used for detecting and measuring the energies α, β, Ƴ, and X-rays. It is sufficiently sensitive to detect individual α and β particles. It is also used to compare the activity of the radioactive substance.
It consists of a metallic cylindrical cathode tube enclosed in a glass envelope. 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 envelope.
The tube contains a monoatomic gas such as argon or neon at a pressure of about 10cm of Hg, together with a small trace of halogens such as Cl or Br. The halogen X act as a quenching agent.
A potential difference of several 100 volts is applied between the anode wire and cathode wire.
|Fig. G. M. Counter|
When an ionizing particle enters the G.M Tube, it ionizes some of the argon atoms, forming positive ions and free electrons. The free electrons further cause ionization and a large avalanche (sudden) of the electron is produced.
Besides this, some of the free electrons colliding with 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 which further produces an avalanche.
The total number of ion produced is now is independent of an initial number of ions (primary ions) formed by the entering particle.
The electron is very light are collected at the anode leaving behind a space charge(positive charge). In a short time, the space charge becomes 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 sent to rate meter (scale).
The rate meter records the arrival of each individual pulse separately so gives, the exact number of the particle entering the G.M Tube. The combination of the 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 consists not give a number of pulses but should recover quickly to record the next entering particle.
Unfortunately, the positive Argon ion which eventually strikes the cathode gains an electron from it and become neutral argon atom.