142
8 Sample Preparation for Counting
Sample preparation is an important parameter to get an accurate activity of the
sample, specially when the activities of series of samples are to be compared from
each other. In radiochemical experiments, the radioactive sample can be either in
liquid, gas, or solid form. The sample in gas form can be counted directly by injecting
it into gas chamber of the counter (e.g., G.M. or proportional counter). Liquid sample
can be directly counted by a liquid G.M. counter or liquid scintillation counter (i.e.,
by using either an organic scintillator, or an organic phosphor or well type NaI
scintillator).
Solid sample, however, needs special attention, especially for α-source and weak
β-source. γ -rays have strong penetrating power, thus sample preparation does not
have much influence on efficiency of counting. However, selection of method, for
source preparation, largely depends upon the nature and energy of radiation of the
sample to be counted. For α-source and β-source, a thin homogeneous and stable
source is needed to prevent loss of activity due to self-absorption. This is important if specific activity of the sample of α-emitter or low energy β-emitter is to be
determined. To prepare a source for solid counting, of weak β-source or α-source,
it is necessary to make a thin and even source. Moreover, when activity present in a
series of samples are to be compared, thickness of each sample must be the same and
should be uniformly even throughout the source (Fig. 8.1A and not like Fig. 8.1B, C,
or D).
The sources, except for shown in Fig. 8.1A, cause loss of radiation due to selfabsorption of varied degree due to the variation in thickness of the source (Fig. 8.1B,
C, and D). Thus, the count rate for samples B, C, or D may lead to a wrong speculation.
In addition to making a sample of uniform thickness, magnitude of thickness of
the sample also plays an important role, especially when comparisons of activities
of various samples are to be made.
The following example would be able to explain the effect of variation in thickness
on the count rate. In this experiment, variation in the source thickness is been achieved
by adding various amount of BaCl 2 solution to a set of different test tubes containing
same amount of Na
35 SO 4 solution, BaCl 2 present in the test tube gets precipitated by
adding NaSO 4 solution which results, in giving Ba
35 SO 4 precipitate, with different
amount of BaSO 4 , but same amount of radioactive
35 S. The precipitate from each
test tube, after several washings with water, is transferred to source tray forming
an even surface (Fig. 8.1A) of BaSO 4 labeled with Sulfur-35. Since each sample
contains same amount of
35 S, one would expect the count rate of each sample to be
the same. Contrary, one initially observes a decrease in count rate with increase in
amount of BaSO 4 (Fig. 8.1E) after some amount of BaSO 4 the count rate becomes
independent of the amount of BaSO 4 . This thickness of the material is called as
saturation thickness. This effect can be hypothetically explained by dividing the
thickness of BaSO 4 by infinitesimally small thin layers. β-particle of each of these
infinitesimally thin layers will try to reach the counting system. As the number of layer
increases, absorption of β-particles starts to take place; some of them are absorbed
by the layers while others escape the layer to reach the counting system. Thus, it is
possible to imagine certain fixed number of layers of BaSO 4 which will always allow
a fixed number of β-particles to reach the counting system. A thickness lesser than
8 Sample Preparation for Counting
Sample preparation is an important parameter to get an accurate activity of the
sample, specially when the activities of series of samples are to be compared from
each other. In radiochemical experiments, the radioactive sample can be either in
liquid, gas, or solid form. The sample in gas form can be counted directly by injecting
it into gas chamber of the counter (e.g., G.M. or proportional counter). Liquid sample
can be directly counted by a liquid G.M. counter or liquid scintillation counter (i.e.,
by using either an organic scintillator, or an organic phosphor or well type NaI
scintillator).
Solid sample, however, needs special attention, especially for α-source and weak
β-source. γ -rays have strong penetrating power, thus sample preparation does not
have much influence on efficiency of counting. However, selection of method, for
source preparation, largely depends upon the nature and energy of radiation of the
sample to be counted. For α-source and β-source, a thin homogeneous and stable
source is needed to prevent loss of activity due to self-absorption. This is important if specific activity of the sample of α-emitter or low energy β-emitter is to be
determined. To prepare a source for solid counting, of weak β-source or α-source,
it is necessary to make a thin and even source. Moreover, when activity present in a
series of samples are to be compared, thickness of each sample must be the same and
should be uniformly even throughout the source (Fig. 8.1A and not like Fig. 8.1B, C,
or D).
The sources, except for shown in Fig. 8.1A, cause loss of radiation due to selfabsorption of varied degree due to the variation in thickness of the source (Fig. 8.1B,
C, and D). Thus, the count rate for samples B, C, or D may lead to a wrong speculation.
In addition to making a sample of uniform thickness, magnitude of thickness of
the sample also plays an important role, especially when comparisons of activities
of various samples are to be made.
The following example would be able to explain the effect of variation in thickness
on the count rate. In this experiment, variation in the source thickness is been achieved
by adding various amount of BaCl 2 solution to a set of different test tubes containing
same amount of Na
35 SO 4 solution, BaCl 2 present in the test tube gets precipitated by
adding NaSO 4 solution which results, in giving Ba
35 SO 4 precipitate, with different
amount of BaSO 4 , but same amount of radioactive
35 S. The precipitate from each
test tube, after several washings with water, is transferred to source tray forming
an even surface (Fig. 8.1A) of BaSO 4 labeled with Sulfur-35. Since each sample
contains same amount of
35 S, one would expect the count rate of each sample to be
the same. Contrary, one initially observes a decrease in count rate with increase in
amount of BaSO 4 (Fig. 8.1E) after some amount of BaSO 4 the count rate becomes
independent of the amount of BaSO 4 . This thickness of the material is called as
saturation thickness. This effect can be hypothetically explained by dividing the
thickness of BaSO 4 by infinitesimally small thin layers. β-particle of each of these
infinitesimally thin layers will try to reach the counting system. As the number of layer
increases, absorption of β-particles starts to take place; some of them are absorbed
by the layers while others escape the layer to reach the counting system. Thus, it is
possible to imagine certain fixed number of layers of BaSO 4 which will always allow
a fixed number of β-particles to reach the counting system. A thickness lesser than