The basics of proportional chamber

In this particular article The basics of proportional chamber, we will study the proportional chamber in detail and in the easiest way possible.

Principle

When the voltage across the two electrons of a gas-filled detector is increased beyond the saturation region i.e. operated at voltages between about 200-800 volt. (fig.)The electrons produced by ionization are accelerated to such high velocities that they start producing further electron-ion pairs on collisions with atoms of gas in the chamber. This process is called the gas amplification.

The basics of Proportional Chamber

With a given energy of ionizing particle the gas amplification results in a larger ionization current. As the potential difference between the electrodes is increased the ionization current also increases. The total number of electron-ion pairs generated is proportional to the initial number of electron-ion produced by the ionizing particle or radiation. Thus the hight of the output pulse is proportional to the primary ionization i.e. the energy lost by the ionization participle. Thus the gas-filled ionization chamber operating in this voltage region is called a proportional counter. 

Construction

The proportional counter consists of a cylindrical metallic (copper) gas-filled tube as shown in fig. Thin central wire insulated from the copper cylindrical acts as the anode. The wire is maintained at a  high positive voltage with respect to the cathode. the electric field in the vicinity of thin axial wire is very high. The cylindrical tube is provided with a thin mica window so that ionizing particle or radiations may enter in it.

Various gases are used filling the tube. But in general, a mixture of simple gas such as hydrogen or argon for high gas amplification and a polyatomic gas such as CO2,  CH4, pentane etc. for providing stability of operations is used. The excited states of argon can be de-excited rapidly through collisions with gas molecules of a polyatomic gas and thus are gone ions cannot start ionization avalanche.

Any electron liberated by ionization is the tube fill be accelerated towards the positive central wire and will liberate additional electrons by collisions with the atoms of the gas. The central wire is connected to a pulse amplifier and a coupling circuit an R-C circuit.

Theory

When a charged particle of kinetic energy E(eV) enters an ionization chamber, electron-ion pairs are generated in it due to ionization by the charged particle. Let nearly Ed energy is needed for the generation of one electron-ion pair in the ionization chamber.

Hence the number of primary electron-ion pairs generated in the chamber will be about E/Ed. Due to a very high electric field, an avalanche of electrons develops near the anode. As a result collision of ions with primary electrons. If a single primary electron generates M secondary electron-ion pair near the anode. Then the total numbers of electrons liberated in the chamber will be ME/Ed. The coefficient M is called multiplication factor.

∴   Charge collected at the anode,

q=eME/Ed

where e is the magnitude of electronic charge

If a voltage pulse is generated across the resistance R by the accumulation charge q at the anode for t second, then

Voltage pulse V=IR q/τR

∴                         V=eMER/Edt volt

If capacitance C is connected between the anode and the cathode, then

Voltage pulse V=q/C=eME/EdC volt

From this formula, the energy of entering charge particle can be determined by measuring the height of the voltage pulse. And the time of pulse generating.

Uses and advantages of proportional counter
  1. It is used as an energy spectrometer for the measurement of the energy of soft radiations like β-particles, X-rays, γ-rays, mesons and protons as it has excellent energy resolution.
  2. The proportional chamber is used to detect the low energy X-rays, γ-rays, and β-particles.
  3. It can be used for counting α-particles in presence of ϒ-rays and β-particles.
  4. It is also used for detection of neutrons, fission fragments.
The disadvantage of the proportional counter

The main defect of the proportional counter is the dependence of gas multiplication factor on the applied voltage. Therefore the applied voltage must be maintained constant.

 

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