104
6 Scintillation Counter
volume is made by a suitable solvent. However, users are expected to find out the best
composition by a trial and error method, keeping in mind that a minimum volume
of the liquid scintillator should preferably be used.
6.8 Gel Scintillator
Many times measurement of activity of
14 C or
3 H or any other low energetic βparticle present in an inorganic substance, which are insoluble in non-polar solvents
like toluene or xylene is required. Moreover, the substance may be insoluble in nonquenching solvents. Such materials, however, can be counted by a gel-type scintillator. This scintillator consists of a mixture of solid organic scintillator powder and
silica gel. The solid sample is thoroughly mixed with the gel scintillator. Sometimes,
it is better to put the container under ultrasonic vibrator for thorough mixing of
sample with the scintillator. The glass vial containing this mixture is heated to about
50
◦ –60
◦ C for few a minutes. During this process, the mixture forms a transparent gel
with solid radioactive sample dispersed homogeneously. On slow cooling (i.e., up to
room temperature), the container of the glass vial becomes almost a transparent solid
gel. By this process, we get the radioactive sample thoroughly dispersed in the solid
suspension of transparent silica gel. The glass vial is then put on the photomultiplier
tube, as described for NaI(Tl) system and the radioactivity is counted as discussed
earlier. Size and shape of the glass vial depend upon size of the photomultiplier tube.
Manufacturers of scintillator, however, keep on developing new kinds of scintillator, and it is thus always advisable to ask for details of all types of scintillator
manufactured by the various firms so that one can select the best-suited scintillator
for the experiment.
6.9 Filter Paper Soaked with Scintillator
One can also use low carbon content filter paper for counting purpose. The sample
(in slurry form) is homogeneously spread over a filter paper and connected to a small
vacuum system. The solution is then filtered through the filter paper and semidried
by sucking air for some time. This paper along with the thin film of solid radioactive
sample is carefully transferred to a counting vial, such that the filter paper sits on the
bottom of the vial with radioactive sample facing upwards. Liquid scintillator and
solvent are added from the side of the wall of the vial, taking care that the thin layer
of radioactive sample is not disturbed. Total volume of the liquid is maintained to
about 10–12 ml. Filter paper in the presence of liquid scintillator becomes transparent. Radiations emitted by the sample interact with the liquid scintillator producing
photons which interact with the CsI photocathode of the photomultiplier tube. For
this purpose, the vial is kept over the photomultiplier tube, with the help of sili-
6 Scintillation Counter
volume is made by a suitable solvent. However, users are expected to find out the best
composition by a trial and error method, keeping in mind that a minimum volume
of the liquid scintillator should preferably be used.
6.8 Gel Scintillator
Many times measurement of activity of
14 C or
3 H or any other low energetic βparticle present in an inorganic substance, which are insoluble in non-polar solvents
like toluene or xylene is required. Moreover, the substance may be insoluble in nonquenching solvents. Such materials, however, can be counted by a gel-type scintillator. This scintillator consists of a mixture of solid organic scintillator powder and
silica gel. The solid sample is thoroughly mixed with the gel scintillator. Sometimes,
it is better to put the container under ultrasonic vibrator for thorough mixing of
sample with the scintillator. The glass vial containing this mixture is heated to about
50
◦ –60
◦ C for few a minutes. During this process, the mixture forms a transparent gel
with solid radioactive sample dispersed homogeneously. On slow cooling (i.e., up to
room temperature), the container of the glass vial becomes almost a transparent solid
gel. By this process, we get the radioactive sample thoroughly dispersed in the solid
suspension of transparent silica gel. The glass vial is then put on the photomultiplier
tube, as described for NaI(Tl) system and the radioactivity is counted as discussed
earlier. Size and shape of the glass vial depend upon size of the photomultiplier tube.
Manufacturers of scintillator, however, keep on developing new kinds of scintillator, and it is thus always advisable to ask for details of all types of scintillator
manufactured by the various firms so that one can select the best-suited scintillator
for the experiment.
6.9 Filter Paper Soaked with Scintillator
One can also use low carbon content filter paper for counting purpose. The sample
(in slurry form) is homogeneously spread over a filter paper and connected to a small
vacuum system. The solution is then filtered through the filter paper and semidried
by sucking air for some time. This paper along with the thin film of solid radioactive
sample is carefully transferred to a counting vial, such that the filter paper sits on the
bottom of the vial with radioactive sample facing upwards. Liquid scintillator and
solvent are added from the side of the wall of the vial, taking care that the thin layer
of radioactive sample is not disturbed. Total volume of the liquid is maintained to
about 10–12 ml. Filter paper in the presence of liquid scintillator becomes transparent. Radiations emitted by the sample interact with the liquid scintillator producing
photons which interact with the CsI photocathode of the photomultiplier tube. For
this purpose, the vial is kept over the photomultiplier tube, with the help of sili-