Measurement of Radioactivity
15
The selection of a method to estimate and correct the counting efficiency
of radioactive samples (K) depends on the kinds of isotopes used and the
material of the sample. Recommendations will be given below for each given
hydro biological method.
Quench correction had been thoroughly reviewed (Neame and
Homewood 1':.174; Rogers and Moran, 1966; Gibson 1980). As measure of
quenching, the value of relative counting efficiency, K, is usually used: K =
R;lRo, if Ri is the counting rate in the sample as recorded by the scaler (in cpm
units), and Ro the counting rate measured in a practically non quenched
sample, which contains the same amount of radioactive isotope in a minimal
amount of matter. Therefore to correct for quenching counting samples which
are quasiequal in chemical composition and physical parameters, the following equation can be used: Ro = R;lK, if Ri is the cpm counting rates actually
recorded in them. To calculate the absolute counting rate Ra in the sample
in dpm units (e.g., in number of nuclear disintegrations) the efficiency of
the instrument under given counting conditions should also be known. This
may be estimated with the aid of a corresponding standard counting sample
with known absolute (dpm) radioactivity (Rsn). Then Ra = Ro (RsclRsJ, where
Rsc is the actual counting rate of the same standard recorded by the scaler in
cpm units
The coefficients of counting efficiency K can be estimated by an empirical quenching curve similar to the self-absorption curves shown in Fig. 1.6
or by an internal standard. The quenching curve describes the change of counting efficiency (K) in dependence the mass of radioactive material contained
in the counting samples. To construct such a curve for counting samples
of a tissue hydrolyzate, for example, varying volumes of nonradioactive
hydrolyzate dissolved in NCS + toluene with a known amount of dry material
per volume is added starting from zero to a series of about ten scintillation
K
0,75
0,5
0,25
w
o~--------------------------------------Fig. 1.6. General character of curves describing the dependence of counting efficiency
coefficients (K) upon the amount of material (W) in the liquid scintillation counting
samples
15
The selection of a method to estimate and correct the counting efficiency
of radioactive samples (K) depends on the kinds of isotopes used and the
material of the sample. Recommendations will be given below for each given
hydro biological method.
Quench correction had been thoroughly reviewed (Neame and
Homewood 1':.174; Rogers and Moran, 1966; Gibson 1980). As measure of
quenching, the value of relative counting efficiency, K, is usually used: K =
R;lRo, if Ri is the counting rate in the sample as recorded by the scaler (in cpm
units), and Ro the counting rate measured in a practically non quenched
sample, which contains the same amount of radioactive isotope in a minimal
amount of matter. Therefore to correct for quenching counting samples which
are quasiequal in chemical composition and physical parameters, the following equation can be used: Ro = R;lK, if Ri is the cpm counting rates actually
recorded in them. To calculate the absolute counting rate Ra in the sample
in dpm units (e.g., in number of nuclear disintegrations) the efficiency of
the instrument under given counting conditions should also be known. This
may be estimated with the aid of a corresponding standard counting sample
with known absolute (dpm) radioactivity (Rsn). Then Ra = Ro (RsclRsJ, where
Rsc is the actual counting rate of the same standard recorded by the scaler in
cpm units
The coefficients of counting efficiency K can be estimated by an empirical quenching curve similar to the self-absorption curves shown in Fig. 1.6
or by an internal standard. The quenching curve describes the change of counting efficiency (K) in dependence the mass of radioactive material contained
in the counting samples. To construct such a curve for counting samples
of a tissue hydrolyzate, for example, varying volumes of nonradioactive
hydrolyzate dissolved in NCS + toluene with a known amount of dry material
per volume is added starting from zero to a series of about ten scintillation
K
0,75
0,5
0,25
w
o~--------------------------------------Fig. 1.6. General character of curves describing the dependence of counting efficiency
coefficients (K) upon the amount of material (W) in the liquid scintillation counting
samples