8
General Features of Radioisotopic Methodology
oughly reviewed (Brandsome 1970; Neame and Homewood 1974; Faires and
Boswell 1981; Peng 1981).
Liquid scintillation counting is now the main method for the radioactivity measurements of (X- and [3-emitters, but it can also be used for the estimation of gramma sources. Its counting efficiency (K) for low-energy [3-emitters
is 10-15 times better than the most sensitive G-M end-window detectors,
which had been used in hydro biological laboratories for over 20 years. For
example, for tritium eH) it is about 0.2, for 14C and 35S_ about 0.6-0.8. The
highest efficiency of liquid scintillation counting can be achieved when the
sample is fully dissolved in the scintillation solution (scintillation cocktail).
Much research was invested to develop methods of solubilization of samples
(animal tissues, plant materials) in the most efficient kinds of scintillation cocktails (Horrocks 1974; Neame and Homewood 1974; Peng 1981). The high
counting efficiency of this method is also ensured radioactive particles being
unable to escape from the detector, as occurs when G-M detectors are used,
and by a drastic decrease in self-absorption of radioactive particles by the
material of the sample. The materials of the sample cause this quenching effect
also during liquid scintillation counting, but it is incomparably less than in dry
samples counted by G-M detectors.
Liquid scintillation counting is proportional. Its efficiency depends on the
energy of particles emitted and on the presence of impurities, including the
material of the sample itself, which quench the process of light production by
the emitted radioactive particles, decreasing their average energy. When the
labeled sample to be counted is dissolved in the scintillation cocktail or completely mixed with it, it represents the "internally counting" sample, which consists of radioactive material, a solvent, and scintillation fluors. The solvent
comprises the bulk of whole internally counting sample. For the preparation
of scintillation cocktails, most often derivatives of benzole, toluene, and the
aromatic ether dioxane are used as basic solvents. Toluene is more efficient in
energy excitation by [3-particles, but it is less miscible with water than dioxane.
Therefore the choice of solvent depends also on the amount of water in the
samples. Dry or slightly wet samples are counted usually in toluene cocktails,
while those containing more than 0.2-0.3 ml of water should be counted in a
dioxane cocktail. Some modern cocktails such as Aquasol or Instagel can
accept as much as 40% water by volume. Concerning the scintillation solutes,
as the primary fluor, PPO is used in a molar concentration of 10- 2 , and the secondary POPOP, at a concentration of 1O- 3 m. The transformation of energy
of primary [3-particles emitted by the radioactive sample into the quants of
fluorescent light energy appears roughly as follows. The [3-particle interacts
with several molecules of the solvent. The energy of these interactions is transferred to these molecules, exciting them to the singlet state. Then the excitation energy of these solvent molecules is transferred to the molecules of the
primary fluor, PPO. The excited molecules of the fluor return to their ground
state by emitting quanta of light in the visible or near ultraviolet wavelength.
Thus, the energy of the radioactive particle is transformed to the energy of
General Features of Radioisotopic Methodology
oughly reviewed (Brandsome 1970; Neame and Homewood 1974; Faires and
Boswell 1981; Peng 1981).
Liquid scintillation counting is now the main method for the radioactivity measurements of (X- and [3-emitters, but it can also be used for the estimation of gramma sources. Its counting efficiency (K) for low-energy [3-emitters
is 10-15 times better than the most sensitive G-M end-window detectors,
which had been used in hydro biological laboratories for over 20 years. For
example, for tritium eH) it is about 0.2, for 14C and 35S_ about 0.6-0.8. The
highest efficiency of liquid scintillation counting can be achieved when the
sample is fully dissolved in the scintillation solution (scintillation cocktail).
Much research was invested to develop methods of solubilization of samples
(animal tissues, plant materials) in the most efficient kinds of scintillation cocktails (Horrocks 1974; Neame and Homewood 1974; Peng 1981). The high
counting efficiency of this method is also ensured radioactive particles being
unable to escape from the detector, as occurs when G-M detectors are used,
and by a drastic decrease in self-absorption of radioactive particles by the
material of the sample. The materials of the sample cause this quenching effect
also during liquid scintillation counting, but it is incomparably less than in dry
samples counted by G-M detectors.
Liquid scintillation counting is proportional. Its efficiency depends on the
energy of particles emitted and on the presence of impurities, including the
material of the sample itself, which quench the process of light production by
the emitted radioactive particles, decreasing their average energy. When the
labeled sample to be counted is dissolved in the scintillation cocktail or completely mixed with it, it represents the "internally counting" sample, which consists of radioactive material, a solvent, and scintillation fluors. The solvent
comprises the bulk of whole internally counting sample. For the preparation
of scintillation cocktails, most often derivatives of benzole, toluene, and the
aromatic ether dioxane are used as basic solvents. Toluene is more efficient in
energy excitation by [3-particles, but it is less miscible with water than dioxane.
Therefore the choice of solvent depends also on the amount of water in the
samples. Dry or slightly wet samples are counted usually in toluene cocktails,
while those containing more than 0.2-0.3 ml of water should be counted in a
dioxane cocktail. Some modern cocktails such as Aquasol or Instagel can
accept as much as 40% water by volume. Concerning the scintillation solutes,
as the primary fluor, PPO is used in a molar concentration of 10- 2 , and the secondary POPOP, at a concentration of 1O- 3 m. The transformation of energy
of primary [3-particles emitted by the radioactive sample into the quants of
fluorescent light energy appears roughly as follows. The [3-particle interacts
with several molecules of the solvent. The energy of these interactions is transferred to these molecules, exciting them to the singlet state. Then the excitation energy of these solvent molecules is transferred to the molecules of the
primary fluor, PPO. The excited molecules of the fluor return to their ground
state by emitting quanta of light in the visible or near ultraviolet wavelength.
Thus, the energy of the radioactive particle is transformed to the energy of