72
5 Ionization Counters
Background
-particles
P u ls e s d u e to
- p a r ti c le s
Energy Volt
E max
50
0
5
100
Fig. 5.11 A schematic representation of some hypothetical square pulses produced by α-particles,
β-particles, and cosmic radiation (i.e., background radiations). Curve drawn with broken lines
indicates the typical shape of a spectrum of α-particle and β-particles
100 V). From this typical graph, as shown in Fig. 5.11, the pulse height analyzer if
used in an integral mode can eliminate the entire group of pulses produced by βparticles as well as those produced by background cosmic radiations. For example,
setting the gate of the pulse height analyzer, around 50 V, would cut down all the
pulses of height smaller than this value and allow the scalar unit to receive pulses
of height greater than 50 V, which will be due to α-particles only. This process also
discriminates the background radiation because their pulses would also be very small.
It should not be assumed that every time an α-particle is counted by the proportional counter, the pulse height would be greater than 50 volts. The pulse height
depends upon the anodic potential and amplification factor used for counting purpose (Fig. 5.8). However, it is always true that under a given condition, pulse height
produced by α-particles of an isotope would always be the same and would be much
bigger than that of the β-particles (of comparable energy) or γ -radiation. This further
adds a feather to the proportional counter, i.e., for counting α-particle one does not
need any shielding of counter from the background radiations (i.e., cosmic radiation),
because it can be eliminated by the pulse height analyzer.
5.8.7 Advantages of Gas Flow Proportional Counter
Although we have discussed some of the advantages of this counter, we enumerate
them itemwise in greater depth for the benefit of the readers.
• Since gas is allowed to flow from one end of the counter to the other, the sample
to be counted can be brought very near the anode wire, to minimize the losses
of α-particles or β-particles due to self-absorption. This is because intensity of
5 Ionization Counters
Background
-particles
P u ls e s d u e to
- p a r ti c le s
Energy Volt
E max
50
0
5
100
Fig. 5.11 A schematic representation of some hypothetical square pulses produced by α-particles,
β-particles, and cosmic radiation (i.e., background radiations). Curve drawn with broken lines
indicates the typical shape of a spectrum of α-particle and β-particles
100 V). From this typical graph, as shown in Fig. 5.11, the pulse height analyzer if
used in an integral mode can eliminate the entire group of pulses produced by βparticles as well as those produced by background cosmic radiations. For example,
setting the gate of the pulse height analyzer, around 50 V, would cut down all the
pulses of height smaller than this value and allow the scalar unit to receive pulses
of height greater than 50 V, which will be due to α-particles only. This process also
discriminates the background radiation because their pulses would also be very small.
It should not be assumed that every time an α-particle is counted by the proportional counter, the pulse height would be greater than 50 volts. The pulse height
depends upon the anodic potential and amplification factor used for counting purpose (Fig. 5.8). However, it is always true that under a given condition, pulse height
produced by α-particles of an isotope would always be the same and would be much
bigger than that of the β-particles (of comparable energy) or γ -radiation. This further
adds a feather to the proportional counter, i.e., for counting α-particle one does not
need any shielding of counter from the background radiations (i.e., cosmic radiation),
because it can be eliminated by the pulse height analyzer.
5.8.7 Advantages of Gas Flow Proportional Counter
Although we have discussed some of the advantages of this counter, we enumerate
them itemwise in greater depth for the benefit of the readers.
• Since gas is allowed to flow from one end of the counter to the other, the sample
to be counted can be brought very near the anode wire, to minimize the losses
of α-particles or β-particles due to self-absorption. This is because intensity of