PROPOTIONAL COUNTER PRINCIPLE When voltage between electrodes increases

PROPOTIONAL COUNTER

PRINCIPLE When voltage between electrodes increases beyond ionization chamber region, the primary electrons produced due to ionization are accelerated towards anode and cause further ionization along their path. So primary ionization is multiplied by a factor dependent on applied voltage and geometry of apparatus (i. e. proportional counter)thereby producing a large output pulse. The height of output pulse is proportional to number of primary electrons which is further proportional to energy of the ionizing particle.

CONSTRUCTION 1. 2. 3. 4. 5. 6. Cathode(C): It is a hollow conducting cylinder It about 20 cm long and 2 cm in diameter. Anode(W): It is a fine tungsten wire fixed along the axis of the cylinder C and well insulated from it. These counters are usually filled with gases like methane, methane-argon etc at a pressure of I atmosphere. The applied voltage is adjusted in proportional region. Amplifier: It amplifies the electric pulses generated in the counter. Discriminator: It cuts of unwanted low voltage pulses. Counter: It counts the desired output pulses.

WORKING: The electric field strength E at radial distance r from the central electrode is given by(fig. a) E=v /r loge ( b/a) 0 where a and b are radii of central wire and cylindrical electrode. The field strength close to anode increases rapidly as r decreases. So field strength near central wire become very high. At such a high electric field , primary electrons are accelerated towards central wire and gain enough energy to ionize other atoms. The ions so produced cause further ionization and so on. Thus each original ion pair leads to formation of a large number of secondary ion pairs.

secondary electrons by collision. These secondary electrons give rise to photons during their collisions with gas atoms. These photons in turn can produce photo electrons in the volume of gas of counter. It can be proved that total number of secondary electrons reaching the anode is proportional to the number of initial electrons and hence the initial primary ion pairs. Hence this counter is called proportional counter. This counter thus generates single pulses of height proportional to the energy of ionizing particles.

APPLICATIONS 1. 2. 3. The proportional counters are used in measuring the energy of low energy radiation. Heavy particles such as alpha rays, fission fragments are also detected by proportional counter. These detectors can also detect neutrons.