144
8 Sample Preparation for Counting
one should experimentally check the saturation thickness, as discussed earlier. After
having obtained information about the saturation thickness, in all experiments one
should add enough non-radioactive material to the counting sample, before making
the counting source, so that all samples to be counted has thickness greater than
the saturation thickness. Under this counting condition, though the count rate measured at the saturation thickness is less, it becomes independent of thickness of the
sample (provided thickness is uniform like shown in Fig. 8.1A). Hence, when one
needs to study variation of activity in different solid samples, it is desirable to find
the saturation thickness and make all samples of thickness equal to the saturation
thickness.
8.2.1 Planchet Material
Solid sample source needs to be prepared on a material, which can be easily handled
and placed near the counter for counting. The material onto which the source is
prepared is called planchet. This is made of a circular shape to match the shape and
size of the counter’s window. Thickness of the material is kept such that it does not
twist easily and remains sturdy and flat. The type of material used for the planchet
is also important. This is normally made of aluminum, copper, glass, plastic, etc.
However, one has to take care that metal of the planchet does not react with the
solvent (being used for drying the materials).
Acids may etch aluminum, stainless steel, or copper. For such purpose, we either
use acid resistant metal. If a metal like aluminum is to be used, then such planchet is
regarded as dispensable. Glass or plastic planchet may sometimes be used for acidic
solution, or even platinum for some purpose. In order to keep the backscattering uniform, planchet made of same material should be used for all experiments, especially
when comparison of activities of a series of samples is desired.
8.2.2 Preparation of Solid Sample from Liquid Sample
Sometimes we may like to evaporate the solvent to get a solid substance for counting,
especially when the amount of radioactive material present in the sample is very low
to be precipitated. However, while drying a solution of sample it gives rise to nonhomogeneous solid sample source. Sample gets concentrated in form of big crystals
at the boundary of the original drop, resulting in a rather high and non-reproducible
sample, which gives rise to varied degree of self-absorption (Fig. 8.1D). This is
avoided by making surface of the sample holder (planchet) hydrophilic by addition
of wetting agent prior to dispersing the drop of radioactive solution. Care is taken,
however, to add minimum amount of wetting agent in order to avoid unnecessary
addition of solids (i.e., wetting agent) to the source. Addition of artificial centers
of crystallization to the radioactive liquid also improves the source quality. These
8 Sample Preparation for Counting
one should experimentally check the saturation thickness, as discussed earlier. After
having obtained information about the saturation thickness, in all experiments one
should add enough non-radioactive material to the counting sample, before making
the counting source, so that all samples to be counted has thickness greater than
the saturation thickness. Under this counting condition, though the count rate measured at the saturation thickness is less, it becomes independent of thickness of the
sample (provided thickness is uniform like shown in Fig. 8.1A). Hence, when one
needs to study variation of activity in different solid samples, it is desirable to find
the saturation thickness and make all samples of thickness equal to the saturation
thickness.
8.2.1 Planchet Material
Solid sample source needs to be prepared on a material, which can be easily handled
and placed near the counter for counting. The material onto which the source is
prepared is called planchet. This is made of a circular shape to match the shape and
size of the counter’s window. Thickness of the material is kept such that it does not
twist easily and remains sturdy and flat. The type of material used for the planchet
is also important. This is normally made of aluminum, copper, glass, plastic, etc.
However, one has to take care that metal of the planchet does not react with the
solvent (being used for drying the materials).
Acids may etch aluminum, stainless steel, or copper. For such purpose, we either
use acid resistant metal. If a metal like aluminum is to be used, then such planchet is
regarded as dispensable. Glass or plastic planchet may sometimes be used for acidic
solution, or even platinum for some purpose. In order to keep the backscattering uniform, planchet made of same material should be used for all experiments, especially
when comparison of activities of a series of samples is desired.
8.2.2 Preparation of Solid Sample from Liquid Sample
Sometimes we may like to evaporate the solvent to get a solid substance for counting,
especially when the amount of radioactive material present in the sample is very low
to be precipitated. However, while drying a solution of sample it gives rise to nonhomogeneous solid sample source. Sample gets concentrated in form of big crystals
at the boundary of the original drop, resulting in a rather high and non-reproducible
sample, which gives rise to varied degree of self-absorption (Fig. 8.1D). This is
avoided by making surface of the sample holder (planchet) hydrophilic by addition
of wetting agent prior to dispersing the drop of radioactive solution. Care is taken,
however, to add minimum amount of wetting agent in order to avoid unnecessary
addition of solids (i.e., wetting agent) to the source. Addition of artificial centers
of crystallization to the radioactive liquid also improves the source quality. These