5.8 Proportional Counter
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of each height (known as pulse height analyzer or a discriminator) or it can give
summation of all pulses (i.e., integral values of the pulses) above some preset pulse
height values. In the first case, one can get an idea about the spectrum of energy,
while in second one can get total activity by subtracting pulses from the set pulse
height. The later form can be used to subtract background activity from total count
rate of the radioactive material. In other words, under the integration mode it can
separate all pulses above a certain height, whereas under discrimination setting it
would allow the pulses to be grouped according to their respective heights.
For convenience, the pulse height analyzer has a range of 0–100 V, indicating that
it has 100 channels and each channel represents a particular height of pulse, in terms
of potential starting from 0 to 100. If each channel is calibrated in terms of energy,
then one would get an idea about the energy of pulses coming out of each channel.
There are pulse height analyzers, with 512 or 4000 channels, etc. Such analyzers can
differentiate pulse heights of small energy difference. For example, pulses produced
by an α-emitters of 2.4 and 2.35 MeV energies can be distinguished by 4000 channel
analyzer more easily than with 100 channel analyzer. If pulse height analyzer is set
at 10 volts, then it will allow all the pulses of height equal to 10 volts into the scalar.
However, every instrument has its limit of tolerance, designated by a certain value
of the window’s width. If width of window is set to 1, then the limit of pulse height
being allowed to pass to the scalar would be 10 ± 1.0 volt. Thus, width of the window
helps in discriminating one value from another. Normally, the width of the window
can be adjusted to 0.1, 1.0, or 10.0 as the case may be.
Now, we can discuss the application of pulse height analyzer in counting a radioactive sample either containing or emitting both α-particles and β-particles. β-particles
have energy from zero to a maximum value called E max (Fig. 5.3C), whereas αparticles have a definite fixed energy (Fig. 5.3B). Pulses produced by the β-particles
will, therefore, be of different heights and their distribution would follow the same
pattern as that of β-spectrum (Fig. 5.3D). Probability of interaction of β-particles
with ionizing gas and hence, transferring its energy to the gas molecule in the counter
is less as compared to α-particles of comparable energy. Therefore, height of pulses
produced by β-particles would be lower than that produced by α-particles of comparable energy. Moreover, α-particles being monoenergetic, height of their pulses
would also follow the same trend. γ -rays, on the other hand, (or cosmic radiation)
though may have higher energy than any of these radiations, since they possess the
highest penetrating power (i.e., probability of transferring energy to the ionizing gas
will be less than even β-particles), their pulse height would be even smaller than that
produced by β-particles.
A qualitative distribution of pulses produced by α-particles, β-particles, and cosmic radiation (i.e., the background radiation) is shown in Fig. 5.11. This will help to
explain features of pulse height analyzer more clearly. Fig. 5.11 reveals the typical
pulse height distribution produced by α-particles, β-particles, and cosmic radiations.
The highest pulses of similar height are due to α-particles. The pulses produced by
β-particles have a distribution of their height but are smaller than the pulses of αparticles. The background pulses are also of various heights but much smaller than
pulses produced by α-particles. The energy of these pulses is expressed in volts (0–
71
of each height (known as pulse height analyzer or a discriminator) or it can give
summation of all pulses (i.e., integral values of the pulses) above some preset pulse
height values. In the first case, one can get an idea about the spectrum of energy,
while in second one can get total activity by subtracting pulses from the set pulse
height. The later form can be used to subtract background activity from total count
rate of the radioactive material. In other words, under the integration mode it can
separate all pulses above a certain height, whereas under discrimination setting it
would allow the pulses to be grouped according to their respective heights.
For convenience, the pulse height analyzer has a range of 0–100 V, indicating that
it has 100 channels and each channel represents a particular height of pulse, in terms
of potential starting from 0 to 100. If each channel is calibrated in terms of energy,
then one would get an idea about the energy of pulses coming out of each channel.
There are pulse height analyzers, with 512 or 4000 channels, etc. Such analyzers can
differentiate pulse heights of small energy difference. For example, pulses produced
by an α-emitters of 2.4 and 2.35 MeV energies can be distinguished by 4000 channel
analyzer more easily than with 100 channel analyzer. If pulse height analyzer is set
at 10 volts, then it will allow all the pulses of height equal to 10 volts into the scalar.
However, every instrument has its limit of tolerance, designated by a certain value
of the window’s width. If width of window is set to 1, then the limit of pulse height
being allowed to pass to the scalar would be 10 ± 1.0 volt. Thus, width of the window
helps in discriminating one value from another. Normally, the width of the window
can be adjusted to 0.1, 1.0, or 10.0 as the case may be.
Now, we can discuss the application of pulse height analyzer in counting a radioactive sample either containing or emitting both α-particles and β-particles. β-particles
have energy from zero to a maximum value called E max (Fig. 5.3C), whereas αparticles have a definite fixed energy (Fig. 5.3B). Pulses produced by the β-particles
will, therefore, be of different heights and their distribution would follow the same
pattern as that of β-spectrum (Fig. 5.3D). Probability of interaction of β-particles
with ionizing gas and hence, transferring its energy to the gas molecule in the counter
is less as compared to α-particles of comparable energy. Therefore, height of pulses
produced by β-particles would be lower than that produced by α-particles of comparable energy. Moreover, α-particles being monoenergetic, height of their pulses
would also follow the same trend. γ -rays, on the other hand, (or cosmic radiation)
though may have higher energy than any of these radiations, since they possess the
highest penetrating power (i.e., probability of transferring energy to the ionizing gas
will be less than even β-particles), their pulse height would be even smaller than that
produced by β-particles.
A qualitative distribution of pulses produced by α-particles, β-particles, and cosmic radiation (i.e., the background radiation) is shown in Fig. 5.11. This will help to
explain features of pulse height analyzer more clearly. Fig. 5.11 reveals the typical
pulse height distribution produced by α-particles, β-particles, and cosmic radiations.
The highest pulses of similar height are due to α-particles. The pulses produced by
β-particles have a distribution of their height but are smaller than the pulses of αparticles. The background pulses are also of various heights but much smaller than
pulses produced by α-particles. The energy of these pulses is expressed in volts (0–