Proportional Counter Construction
It is a cylindrical metallic tube filled with a mixture of argon and methane gas in the ratio of 9: l at one atmospheric pressure or less. A thin metallic wire is fixed along the axis of the tube and insulated from it as shown in Fig. i (a). The wire acts as an anode is maintained at a high positive potential with respect to the tube which acts as a cathode. The wire is connected to pulse amplifier and counting circuit through the resistance R. The cylindrical tube is provided with a thin mica window so that ionizing particle or radiations can enter inside it.
Proportional Counter Operation
The field produced around the central wire is given by the relation
E = V/r ln(b/a)
where V = applied a potential difference
b = radius of the cylinder
a = radius of an axial wire
r = distance from the center Wire
- When a charged particle or radiations such as on, B-particle or y-photon enters in an ionization chamber, ionization of gas takes place resulting in an ion ‘pair formation.
- The positive particle moves towards the chamber wall whereas the negatron moves towards the central wire.
- In the proportional region, the applied voltage is so high that the primary particle gains spare K.E. to produce secondary particle by collision with the atoms of the gas leading to gas multiplication.
- During this particle multiplication or gas amplification, the number of ions can increase exponentially.
- This method is additive and is termed avalanche.
- This avalanche occurs at a certain purpose of the anode and depends on the radius of the anode wire, radius of the tube, applied voltage and therefore the nature of the gas.
- AN avalanche takes place close to the anode, all the secondary electrons are accumulated at the anode inside about one ns time.
- This produces a very little voltage pulse.
- The main voltage pulse is generated by the flow of secondary positive ions towards the cathode, which takes about 500 sec.
- Since the avalanche takes place near the anode,’ the shape of the voltage pulse does not depend on the track of the primary ions.
- The proportional counter can be used in several applications since the voltage pulse produced across ‘R’ is proportional to the number of primary electrons.
- It is also proportional to the energy lost by the initially charged particles.
- Thus the proportional counter permits each the count and energy determination of particles even of very low energy.
- It can be used as a spectrometer particularly for B-rays and magnetic radiations such as X-ray.
- They are useful for counting a-particles in the presence of particles and y-rays (latter can be biased off by the use of discriminator).