Transcript PROPOTIONAL COUNTER

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.