6.11 Optimum Conditions for Counting
109
window width such that it covers the area of the photopeak ( A or B), because
window width can set the lower and upper value from the photopeak value [ i.e.,
A
± (widow width)]. This method reduces background activity to about 20–40
cpm and can tolerate, to some extent, small fluctuations in the voltage change.
Nevertheless, it is advisable to use voltage stabilized units for this method also.
• Pulse height analyzer could be used as a discriminator bias where voltage in the discriminator is set at a value corresponding to the starting of the photopeak (Fig. 6.4).
Under this condition, all pulses greater than the set voltage would be recorded.
For example, if the discriminator bias voltage is fixed at potential corresponding
to the beginning of the photopeak, the counter will record all pulse greater than
this voltage to its upper limit (i.e., 100 V). The advantage of this method is that
this type of counting is not affected by any fluctuations in power supply as it did
while counting at the photopeak. Moreover, if the activity present in the sample
is very low to be detected effectively at the photopeak, then also this method is
preferred. But the background count under this condition would be much higher
than what one would get by counting the sample at the photopeak. Normally, the
background count under this condition is around 100–200 cpm.
6.12 β-counting
Since β-particles, unlike γ -rays (Fig. 2.3), have a continuous spectrum (Fig. 5.3C),
counting condition for such particles are different from that of γ -rays. In fact, there
is no hard and fast rule to find out the best operating condition, but the following
method can be adopted to find out the best counting condition for a sample containing
β-particles.
6.12.1 Counting Conditions for β-particles
An amplification factor in the amplifier is set to an arbitrarily convenient value to
start with. Then, the activity is measured for various applied EHT, and a graph is
plotted between the count rate and the EHT voltage (Fig. 6.7).
This procedure is followed to count radioactive sample (S) as well as background
activity (B) (i.e., counting without any radioactive material in the counter), separately (Fig. 6.7). The pulse height analyzer is used as a discriminator and its gate
is selected at a voltage, at which all X -rays peaks are cut off (i.e., voltage corresponding to position marked “C” in Fig. 6.4). This is normally achieved by setting
the discriminator value at 5–10 V. In Fig. 6.7, S is the usual type of curve obtained
with any sample containing β-emitter, and B is the activity due to the background
activity. Figure 6.7 shows that a certain potential activity due to the background is
greater than that of the source. One should not be alarmed by this observation. The
reasons for getting such a result is not yet fully understood. In Fig. 6.7, similar to an
109
window width such that it covers the area of the photopeak ( A or B), because
window width can set the lower and upper value from the photopeak value [ i.e.,
A
± (widow width)]. This method reduces background activity to about 20–40
cpm and can tolerate, to some extent, small fluctuations in the voltage change.
Nevertheless, it is advisable to use voltage stabilized units for this method also.
• Pulse height analyzer could be used as a discriminator bias where voltage in the discriminator is set at a value corresponding to the starting of the photopeak (Fig. 6.4).
Under this condition, all pulses greater than the set voltage would be recorded.
For example, if the discriminator bias voltage is fixed at potential corresponding
to the beginning of the photopeak, the counter will record all pulse greater than
this voltage to its upper limit (i.e., 100 V). The advantage of this method is that
this type of counting is not affected by any fluctuations in power supply as it did
while counting at the photopeak. Moreover, if the activity present in the sample
is very low to be detected effectively at the photopeak, then also this method is
preferred. But the background count under this condition would be much higher
than what one would get by counting the sample at the photopeak. Normally, the
background count under this condition is around 100–200 cpm.
6.12 β-counting
Since β-particles, unlike γ -rays (Fig. 2.3), have a continuous spectrum (Fig. 5.3C),
counting condition for such particles are different from that of γ -rays. In fact, there
is no hard and fast rule to find out the best operating condition, but the following
method can be adopted to find out the best counting condition for a sample containing
β-particles.
6.12.1 Counting Conditions for β-particles
An amplification factor in the amplifier is set to an arbitrarily convenient value to
start with. Then, the activity is measured for various applied EHT, and a graph is
plotted between the count rate and the EHT voltage (Fig. 6.7).
This procedure is followed to count radioactive sample (S) as well as background
activity (B) (i.e., counting without any radioactive material in the counter), separately (Fig. 6.7). The pulse height analyzer is used as a discriminator and its gate
is selected at a voltage, at which all X -rays peaks are cut off (i.e., voltage corresponding to position marked “C” in Fig. 6.4). This is normally achieved by setting
the discriminator value at 5–10 V. In Fig. 6.7, S is the usual type of curve obtained
with any sample containing β-emitter, and B is the activity due to the background
activity. Figure 6.7 shows that a certain potential activity due to the background is
greater than that of the source. One should not be alarmed by this observation. The
reasons for getting such a result is not yet fully understood. In Fig. 6.7, similar to an