80
5 Ionization Counters
f solvent D 1 = Activity loss in density D 1 =
(A 0 − A 1 )
A 0
f solvent D 2 = Activity loss in density D 2 =
(A 0 − A 2 )
A 0
(5.1)
These two factors can then be used for correcting the observed activity in the two
experimental solvents. The actual activity recorded in each solvent should then
be multiplied by the corresponding factor (i.e., f solvent D 1 or f solvent D 2 to get the
actual count rate (i.e., count rate with density correction). These corrected count
rates would be independent of the solvent density.
2. Effect of volume of the radioactive solution: The amount of liquid added to the
liquid G.M. counter should always be the same. The liquid G.M. counter filled
with liquid of different volumes gives different count rate, though each solution
may contain same amount of radioactive material. This can be shown by the following experiment. A radioactive sample KI labeled with
131 I (1.0 ml) is added
to a counter and its activity is recorded. Then, 1.0 ml of pure solvent is added
to this counter, and activity is recorded after the counter is shaken to mix the
solvent thoroughly. 1.0 ml of pure solvent is again added to this counter and corresponding activity is measured after shaking. This experiment is continued till
no further solvent could be added to the counter (i.e., its inner tube is filled with
the liquid). The volume of liquid which counter can accommodate is about 14.0
ml. The activity recorded for each volume of solvent is plotted against the volume
of solvent, added to the counter. A typical graph showing the variation in count
rate with volume of liquid is shown in Fig. 5.16.
It is observed (Fig. 5.16) that after 8.0–10.0 ml of dilution, activity remains almost
constant up to about 12–13 ml, but thereafter it shows a decrease in the count rate.
The decrease in the activity after addition of 13.0 ml of solvent is due to poor
geometry of counting. The radiations are lost due to self-absorption by solvent
present above the central anode and also due to escape of radiations from the
central compartment of the chamber, as the height of the solution is above the
central anodic compartment (Fig. 5.14A). When the liquid is filled up to the upper
part of the inner tube, a 2π -geometrical efficiency can be obtained, but when the
liquid level is above the inner tube, radiations traveling in horizontal direction and
upward direction escape the counter (i.e., away from the counter) hence are not
counted. Due to these reasons, the counting rate decreases after a certain volume
of liquid is added to the liquid G.M. counter. Hence, it is advisable to keep the
total volume of liquid added to the counter always same, especially when activity
of one sample is to be compared with the other.
3. Washing of liquid G.M. counter: It is necessary to emphasize that, in addition
to washing with water, liquid G.M. counter should also be washed with solvent
containing a non-radioactive isotope carrier. This process helps to decontaminate
the counter completely from the radioactive isotope. For example, washing the
counter with sodium bromide solution (0.01 M) when counting
82 Br isotope as
bromide ions removes traces of radioactive
82 Br isotope adsorbed on the inner
wall of the counter. The stable isotope must be in the same chemical state as that
5 Ionization Counters
f solvent D 1 = Activity loss in density D 1 =
(A 0 − A 1 )
A 0
f solvent D 2 = Activity loss in density D 2 =
(A 0 − A 2 )
A 0
(5.1)
These two factors can then be used for correcting the observed activity in the two
experimental solvents. The actual activity recorded in each solvent should then
be multiplied by the corresponding factor (i.e., f solvent D 1 or f solvent D 2 to get the
actual count rate (i.e., count rate with density correction). These corrected count
rates would be independent of the solvent density.
2. Effect of volume of the radioactive solution: The amount of liquid added to the
liquid G.M. counter should always be the same. The liquid G.M. counter filled
with liquid of different volumes gives different count rate, though each solution
may contain same amount of radioactive material. This can be shown by the following experiment. A radioactive sample KI labeled with
131 I (1.0 ml) is added
to a counter and its activity is recorded. Then, 1.0 ml of pure solvent is added
to this counter, and activity is recorded after the counter is shaken to mix the
solvent thoroughly. 1.0 ml of pure solvent is again added to this counter and corresponding activity is measured after shaking. This experiment is continued till
no further solvent could be added to the counter (i.e., its inner tube is filled with
the liquid). The volume of liquid which counter can accommodate is about 14.0
ml. The activity recorded for each volume of solvent is plotted against the volume
of solvent, added to the counter. A typical graph showing the variation in count
rate with volume of liquid is shown in Fig. 5.16.
It is observed (Fig. 5.16) that after 8.0–10.0 ml of dilution, activity remains almost
constant up to about 12–13 ml, but thereafter it shows a decrease in the count rate.
The decrease in the activity after addition of 13.0 ml of solvent is due to poor
geometry of counting. The radiations are lost due to self-absorption by solvent
present above the central anode and also due to escape of radiations from the
central compartment of the chamber, as the height of the solution is above the
central anodic compartment (Fig. 5.14A). When the liquid is filled up to the upper
part of the inner tube, a 2π -geometrical efficiency can be obtained, but when the
liquid level is above the inner tube, radiations traveling in horizontal direction and
upward direction escape the counter (i.e., away from the counter) hence are not
counted. Due to these reasons, the counting rate decreases after a certain volume
of liquid is added to the liquid G.M. counter. Hence, it is advisable to keep the
total volume of liquid added to the counter always same, especially when activity
of one sample is to be compared with the other.
3. Washing of liquid G.M. counter: It is necessary to emphasize that, in addition
to washing with water, liquid G.M. counter should also be washed with solvent
containing a non-radioactive isotope carrier. This process helps to decontaminate
the counter completely from the radioactive isotope. For example, washing the
counter with sodium bromide solution (0.01 M) when counting
82 Br isotope as
bromide ions removes traces of radioactive
82 Br isotope adsorbed on the inner
wall of the counter. The stable isotope must be in the same chemical state as that