140
8 Sample Preparation for Counting
agent. Two types of gelling agents: Cab-o-Sil and thixine are commonly used, as they
require no heating for forming gel. The gelling agent is blended into the scintillation
liquid and an aliquot is poured into a counting vial containing fine powder sample.
The vial is shaken to disperse powder evenly. The vial may need slight heating to
form a gel. This gel holds powder in suspension. This method ensures thorough
mixing of the radioactive substance with the scintillator. The vial with gel mixture
is counted for measuring its activity.
8.1.3 Counting by Spreading Sample over a Filter Paper
This is comparatively a recent technique, which offers high promise for substances
which are insoluble in toluene or other liquid scintillator bases. Either thinly spread
sample obtained by filtration through a Millipore filter paper or liquid sample soaked
filter paper is dried and placed directly into the counting vial. Liquid scintillator is
poured through the side of the vial without disturbing the sample. The vial is counted
by placing it over a photomultiplier tube. Filter paper on addition of liquid scintillator
becomes transparent to visible light, hence scintillations formed can be detected by
the photomultiplier tube.
Paper counting technique is also excellent for counting radioactive material separated over the paper-chromatograms. For this purpose, paper-chromatorgram is cut
into equal segments and each segment is inserted into the counting vial to which
liquid scintillator is added and then counted. Since the radioactive spot is insoluble
in the scintillator, the paper can be removed from vial and radioactive material can
be extracted from segment if required.
8.1.4 Deposition of Sample by Electrolysis
Radioactive samples for β- and α-counting are prepared frequently by the electrodeposition technique. This technique gives uniform and thin sample. As a result, losses
of activities due to self-absorption are avoided. This technique gives excellent result
with small quantity of radioactive nuclide, specially when it is too low to prepare
thin source by precipitation method.
The advantage of this technique is usage of simple apparatus and high yields in
short time. Furthermore, stable sources of high quality can be obtained. Electrodeposition is done on a thick electrode so that source remains flat during transfer for
counting. Moreover, care has to be taken such that the deposition occurs only on
one side of the electrode. This is achieved by insulating all sides of the electrode,
except for one side, including the wire that is used for soldering the electrode for
making the connection. The efficiency of electrodeposition can be determined by
measuring the ratio of activities deposited on the electrode and that remaining in the
aliquot. This method, for instance, has been applied for preparation of sources used
8 Sample Preparation for Counting
agent. Two types of gelling agents: Cab-o-Sil and thixine are commonly used, as they
require no heating for forming gel. The gelling agent is blended into the scintillation
liquid and an aliquot is poured into a counting vial containing fine powder sample.
The vial is shaken to disperse powder evenly. The vial may need slight heating to
form a gel. This gel holds powder in suspension. This method ensures thorough
mixing of the radioactive substance with the scintillator. The vial with gel mixture
is counted for measuring its activity.
8.1.3 Counting by Spreading Sample over a Filter Paper
This is comparatively a recent technique, which offers high promise for substances
which are insoluble in toluene or other liquid scintillator bases. Either thinly spread
sample obtained by filtration through a Millipore filter paper or liquid sample soaked
filter paper is dried and placed directly into the counting vial. Liquid scintillator is
poured through the side of the vial without disturbing the sample. The vial is counted
by placing it over a photomultiplier tube. Filter paper on addition of liquid scintillator
becomes transparent to visible light, hence scintillations formed can be detected by
the photomultiplier tube.
Paper counting technique is also excellent for counting radioactive material separated over the paper-chromatograms. For this purpose, paper-chromatorgram is cut
into equal segments and each segment is inserted into the counting vial to which
liquid scintillator is added and then counted. Since the radioactive spot is insoluble
in the scintillator, the paper can be removed from vial and radioactive material can
be extracted from segment if required.
8.1.4 Deposition of Sample by Electrolysis
Radioactive samples for β- and α-counting are prepared frequently by the electrodeposition technique. This technique gives uniform and thin sample. As a result, losses
of activities due to self-absorption are avoided. This technique gives excellent result
with small quantity of radioactive nuclide, specially when it is too low to prepare
thin source by precipitation method.
The advantage of this technique is usage of simple apparatus and high yields in
short time. Furthermore, stable sources of high quality can be obtained. Electrodeposition is done on a thick electrode so that source remains flat during transfer for
counting. Moreover, care has to be taken such that the deposition occurs only on
one side of the electrode. This is achieved by insulating all sides of the electrode,
except for one side, including the wire that is used for soldering the electrode for
making the connection. The efficiency of electrodeposition can be determined by
measuring the ratio of activities deposited on the electrode and that remaining in the
aliquot. This method, for instance, has been applied for preparation of sources used