6
General Features of Radioisotopic Methodology
try of the detector, and their radioactivity (Red) is counted. The corresponding
curve of self-absorption (Fig. 1.3) is used to find the Ran value by extrapolation of the curve to the ordinate, and then to find coefficients of selfabsorption, Q, for the points of curve which correspond to the various thicknesses of the samples. Then the curve for the Q-coefficients can be constructed
(as shown in Fig. 1.2), which could be used in practice to calculate Ran values,
thus correcting counting samples of various thickness for the quenching decrease of counting efficiency Ren = RjQ.
The influence of the sample material on counting efficiency K becomes
more complicated when the liquid scintillation method is used for radioactivity estimation. Besides, with the self-absorption, the molecules of the sample
material interact with the molecules of the fluors and influence the scintillation processes of energy transfer. Thus, they decrease the counting efficiency
of the scintillation process, quenching it. This problem will be discussed below
(see Sect. 1.2.2.3).
1.2 Measurement of Radioactivity
1.2.1 General Considerations
In any radiation detection method, the efficiency of radioactivity counting,
K, e.g., the ratio of the recorded counting rate to the absolute rate of radioactive disintegrations in the sample (see above) is greatest when the maximum
number of emitted particles may reach the detector and interact with it.
Losses in air occur when the emitted radioactive particles escape in directions
away from the detector, and are then captured by molecules of air on the
way to the detector, or interact with the material of the detector by some
mechanism not resulting in a count. The radioisotopes used in hydrobiological studies are mostly beta-emitters with a relatively low radiation energy of
except 32p, which is the emitter with high energy of radiation (see Table 1.1).
The ~-radiation consists of negatively charged electrons. They lose their energy
when passing through the matter more rapidly than y-rays of same energy and
have much shorter ranges. The absorption along their paths within the sample
or between the sample and the detector is most severe for low energy particles, emitted by the radioisotopes 14C, 3H, and 35S. To diminish these losses
of efficiency, the above-mentioned distances must be reduced as much as possible. This stipulation was best accomplished in the liquid scintillation counting methodology. But it became available for routine hydrobiological research
only in the early 1970s, while many basic isotopic methods in this field had
been developed in 1950-1960s with the use of end-window Geiger-Muller
(G-M) detectors, which had an efficiency maximum of 0.05 for 14C or about
0.1 for 35S. More efficient defectors like gas flow or scintillation crystal
G-M counters with efficiency for 14C-carbon 0.1-0.2 found no support
General Features of Radioisotopic Methodology
try of the detector, and their radioactivity (Red) is counted. The corresponding
curve of self-absorption (Fig. 1.3) is used to find the Ran value by extrapolation of the curve to the ordinate, and then to find coefficients of selfabsorption, Q, for the points of curve which correspond to the various thicknesses of the samples. Then the curve for the Q-coefficients can be constructed
(as shown in Fig. 1.2), which could be used in practice to calculate Ran values,
thus correcting counting samples of various thickness for the quenching decrease of counting efficiency Ren = RjQ.
The influence of the sample material on counting efficiency K becomes
more complicated when the liquid scintillation method is used for radioactivity estimation. Besides, with the self-absorption, the molecules of the sample
material interact with the molecules of the fluors and influence the scintillation processes of energy transfer. Thus, they decrease the counting efficiency
of the scintillation process, quenching it. This problem will be discussed below
(see Sect. 1.2.2.3).
1.2 Measurement of Radioactivity
1.2.1 General Considerations
In any radiation detection method, the efficiency of radioactivity counting,
K, e.g., the ratio of the recorded counting rate to the absolute rate of radioactive disintegrations in the sample (see above) is greatest when the maximum
number of emitted particles may reach the detector and interact with it.
Losses in air occur when the emitted radioactive particles escape in directions
away from the detector, and are then captured by molecules of air on the
way to the detector, or interact with the material of the detector by some
mechanism not resulting in a count. The radioisotopes used in hydrobiological studies are mostly beta-emitters with a relatively low radiation energy of
except 32p, which is the emitter with high energy of radiation (see Table 1.1).
The ~-radiation consists of negatively charged electrons. They lose their energy
when passing through the matter more rapidly than y-rays of same energy and
have much shorter ranges. The absorption along their paths within the sample
or between the sample and the detector is most severe for low energy particles, emitted by the radioisotopes 14C, 3H, and 35S. To diminish these losses
of efficiency, the above-mentioned distances must be reduced as much as possible. This stipulation was best accomplished in the liquid scintillation counting methodology. But it became available for routine hydrobiological research
only in the early 1970s, while many basic isotopic methods in this field had
been developed in 1950-1960s with the use of end-window Geiger-Muller
(G-M) detectors, which had an efficiency maximum of 0.05 for 14C or about
0.1 for 35S. More efficient defectors like gas flow or scintillation crystal
G-M counters with efficiency for 14C-carbon 0.1-0.2 found no support