102
6 Scintillation Counter
1. Types of organic scintillator
(a) p-terphenyl
(b) 1,2-phenyl-(4-biphenyl)-oxazole (PBO)
(c) 2-phenyl-5(4-biphenyl)-1,3,4-oxidiazole (PBD)
(d) 1-4-di(2-(5-phenyloxazolyte))-benzene (POPOP)
2. Types of solvents
(a) Xylene (b) Toluene
(c) Phenylcyclohexane (d) Shellsol-A
(e) Decaline (f) Dioxane
Xylene and toluene are the most common solvents used for counting samples dissolved in non-polar solvents. Dioxane is used for samples containing up to 30%
water. Other solvents are used when large volume of scintillator is to be used.
6.7 Precautions Liquid Scintillation
The radioactive material to be counted, should be soluble in the scintillator, for which
one has to use a suitable solvent. But one cannot use any solvent as there are some
solvents capable to absorb the photons (released by the scintillator), e.g., acetone,
halogenated solvents, alcohol, etc. One of the essential conditions for accurate measurement of radioactivity by organic scintillator is the presence of a homogeneous
and stable mixture of radioactive solution and the scintillator. Adsorption of radioactive materials to the wall of the container or precipitation of radioactive material
from liquid scintillator sample are the major problem with organic scintillator. These
factors lead to an appreciable error in the determination of the activity of the sample.
Moreover, the radioactive solution should be colorless, and should not absorb the
photons released by the scintillator. However, it is possible to make a correction for
the loss of count for absorption up to 1–10%.
6.7.1 Size of the Sample Holder
The volume of solution to be counted is also determined by the diameter of the photomultiplier tube. Minimum amount of liquid scintillator should be used. Usually, 1
ml scintillator, 1–2 ml of the radioactive sample, and 5–10 ml solvent are used for
liquid scintillation counting. Volume of liquid for counting depends upon geometrical arrangement of the photomultiplier tube. When the photomultiplier tube is kept
vertical (Fig. 6.3A, B) then the diameter of the bottom of the sample holder should
be 1/10th smaller than that of the photomultiplier tube (Fig. 6.3A).
Height of the liquid in the sample holder should be about 1 cm. Diameter of
the sample holder is kept smaller to minimize the amount of radiation escaping
6 Scintillation Counter
1. Types of organic scintillator
(a) p-terphenyl
(b) 1,2-phenyl-(4-biphenyl)-oxazole (PBO)
(c) 2-phenyl-5(4-biphenyl)-1,3,4-oxidiazole (PBD)
(d) 1-4-di(2-(5-phenyloxazolyte))-benzene (POPOP)
2. Types of solvents
(a) Xylene (b) Toluene
(c) Phenylcyclohexane (d) Shellsol-A
(e) Decaline (f) Dioxane
Xylene and toluene are the most common solvents used for counting samples dissolved in non-polar solvents. Dioxane is used for samples containing up to 30%
water. Other solvents are used when large volume of scintillator is to be used.
6.7 Precautions Liquid Scintillation
The radioactive material to be counted, should be soluble in the scintillator, for which
one has to use a suitable solvent. But one cannot use any solvent as there are some
solvents capable to absorb the photons (released by the scintillator), e.g., acetone,
halogenated solvents, alcohol, etc. One of the essential conditions for accurate measurement of radioactivity by organic scintillator is the presence of a homogeneous
and stable mixture of radioactive solution and the scintillator. Adsorption of radioactive materials to the wall of the container or precipitation of radioactive material
from liquid scintillator sample are the major problem with organic scintillator. These
factors lead to an appreciable error in the determination of the activity of the sample.
Moreover, the radioactive solution should be colorless, and should not absorb the
photons released by the scintillator. However, it is possible to make a correction for
the loss of count for absorption up to 1–10%.
6.7.1 Size of the Sample Holder
The volume of solution to be counted is also determined by the diameter of the photomultiplier tube. Minimum amount of liquid scintillator should be used. Usually, 1
ml scintillator, 1–2 ml of the radioactive sample, and 5–10 ml solvent are used for
liquid scintillation counting. Volume of liquid for counting depends upon geometrical arrangement of the photomultiplier tube. When the photomultiplier tube is kept
vertical (Fig. 6.3A, B) then the diameter of the bottom of the sample holder should
be 1/10th smaller than that of the photomultiplier tube (Fig. 6.3A).
Height of the liquid in the sample holder should be about 1 cm. Diameter of
the sample holder is kept smaller to minimize the amount of radiation escaping