114
6 Scintillation Counter
by a scintillation counter, majority of photons are generated due to lower energy βparticles. These low energetic photons are absorbed by the quencher more effectively,
compared to photons which have been produced by higher energy β-particles (i.e.,
by β-particles of energy near E max ).
Thus, addition of a small amount of quencher in the counting sample compresses
the β-spectrum such that the activity of lower β-particle is reduced more compared
to the high energy β-particles (Fig. 6.10). If activity of β-particle is recorded in two
channels of the pulse height analyzer, say in channel corresponding to energy A and
B (Fig. 6.10), the ratio of the activity in the two channels will change with the amount
of quencher present in the counting sample. The channels “A” and “B” of the pulse
height analyzer are selected first, by plotting β-spectrum of the radioactive isotope
to be counted, applying the method discussed earlier. In other words, spectrum is
measured in absence of the quencher. Two channels ( i.e., pulse height voltages)
“A” and “B” are selected by examination of this spectrum, such that, the activity
at one pulse height setting bias (A) is about 80% of that recorded at the pulse
height setting (B). Now for performing the experiment, pulse height analyzer is
used as a discriminator. Two values of the discriminating bias (or gate) are selected
to correspond to the potential at “A” and “B” position of the spectrum. Sample is
counted at these two values of discriminators separately. In some sophisticated liquid
scintillation counting systems, arrangements are made internally to give count rates
at two values of the pulse height voltages, so that activity in one channel is 80% of
the other channels.
Quenching correction can then be carried out in the following manner. A β-emitter
sample, of known activity, in absence of any quencher is counted to get the sample’s
detection efficiency by using the method discussed in the previous section (Fig. 6.9).
Let a radioactive sample (S) giving disintegration per minute (Y dpm) be recorded in
channel A giving an activity of x cpm. Then, its detection efficiency “ f ” in channel
A is given by
f =
x/cpm × 100
y/dpm
(6.1)
The activity of this sample (S) is also determined in channel B. After this counting,
a small amount of quencher is added to the sample (S), and its activity in channel A
and B are determined. In this fashion, the experiment is repeated for various amount
of the quencher added to the sample (S). Detection efficiency in channel A for each
sample is calculated from Eq. (6.1). Finally, a graph is plotted between the ratio of
activity recorded in channel A and B versus the detection efficiency in channel A
calculated from Eq. (6.1). This graph is expected to be almost linear for small amount
of added quencher (i.e., when the activity does not fall down due to the addition of
quencher) to less than 20–30% of the initial activity (Fig. 6.11).
Activity of the sample (whose amount of quencher present in the mixture is not
known) is now recorded in channels A and B. The ratio of activities in channels A
and B is calculated (assume it to be K 1 ). With the help of the previously calibrated
graph (Fig. 6.11), detection efficiency of the sample for the corresponding value of
6 Scintillation Counter
by a scintillation counter, majority of photons are generated due to lower energy βparticles. These low energetic photons are absorbed by the quencher more effectively,
compared to photons which have been produced by higher energy β-particles (i.e.,
by β-particles of energy near E max ).
Thus, addition of a small amount of quencher in the counting sample compresses
the β-spectrum such that the activity of lower β-particle is reduced more compared
to the high energy β-particles (Fig. 6.10). If activity of β-particle is recorded in two
channels of the pulse height analyzer, say in channel corresponding to energy A and
B (Fig. 6.10), the ratio of the activity in the two channels will change with the amount
of quencher present in the counting sample. The channels “A” and “B” of the pulse
height analyzer are selected first, by plotting β-spectrum of the radioactive isotope
to be counted, applying the method discussed earlier. In other words, spectrum is
measured in absence of the quencher. Two channels ( i.e., pulse height voltages)
“A” and “B” are selected by examination of this spectrum, such that, the activity
at one pulse height setting bias (A) is about 80% of that recorded at the pulse
height setting (B). Now for performing the experiment, pulse height analyzer is
used as a discriminator. Two values of the discriminating bias (or gate) are selected
to correspond to the potential at “A” and “B” position of the spectrum. Sample is
counted at these two values of discriminators separately. In some sophisticated liquid
scintillation counting systems, arrangements are made internally to give count rates
at two values of the pulse height voltages, so that activity in one channel is 80% of
the other channels.
Quenching correction can then be carried out in the following manner. A β-emitter
sample, of known activity, in absence of any quencher is counted to get the sample’s
detection efficiency by using the method discussed in the previous section (Fig. 6.9).
Let a radioactive sample (S) giving disintegration per minute (Y dpm) be recorded in
channel A giving an activity of x cpm. Then, its detection efficiency “ f ” in channel
A is given by
f =
x/cpm × 100
y/dpm
(6.1)
The activity of this sample (S) is also determined in channel B. After this counting,
a small amount of quencher is added to the sample (S), and its activity in channel A
and B are determined. In this fashion, the experiment is repeated for various amount
of the quencher added to the sample (S). Detection efficiency in channel A for each
sample is calculated from Eq. (6.1). Finally, a graph is plotted between the ratio of
activity recorded in channel A and B versus the detection efficiency in channel A
calculated from Eq. (6.1). This graph is expected to be almost linear for small amount
of added quencher (i.e., when the activity does not fall down due to the addition of
quencher) to less than 20–30% of the initial activity (Fig. 6.11).
Activity of the sample (whose amount of quencher present in the mixture is not
known) is now recorded in channels A and B. The ratio of activities in channels A
and B is calculated (assume it to be K 1 ). With the help of the previously calibrated
graph (Fig. 6.11), detection efficiency of the sample for the corresponding value of