Physical Background
5
grams or micromoles, and R; and Ra the radioactivity of the sample in cpm
or dpm.
The efficiency of counting K is directly connected with another important
feature of radioisotopic radiation - its consumption by the molecules of the
sample, the detector, and the intermediate air layer. This phenomenon of selfabsorptIOn radiation within the matter of the samples subjected to radioactivity counts is one of the main factors determining the counting efficiency K,
especially if radioactivity is measured with the aid of G-M counters which had
been used for counting dry preparations. The degree of self-absorption
(quenching) could be expressed in this case by the quenching coefficient Q:
Q = Rj Ren. where Red is the specific radioactivity of sample (cp mg- 1 ) by giving
its thickness (di, mgcm- 2 ), and Ren the same by its thickness approaching to
zero, in which the self-absorption is negligible. The latter value, R en , may be
practically found by extrapolation of the self-absorption curve to zero thickness (Fig. 1.3). The value of self-absorption coefficient Q depends on the characteristics of the isotope (mode of radiation, energy of particles) and on the
material of the labeled sample under examination. In practice, self-absorption
curves are constructed for each kind of isotope and each kind of sample. The
curve of self-absorption for the G-M counters can be constructed by counting
under standard conditions a series of samples with equal portions of radioactivity but with different thicknesses measured as mgcm- 2 . The exact equal portions radioactivity within the labeled samples with a minimal possible (but
known) weight, accessible its precise distribution, are mixed with the samples
of homogenous unlabeled material of the same kind having a different (but
known) weight. The samples are distributed at equal areas fit for the geomeRed
100 :III
Ran
75
50
25
w
o
4
12
16
20
24
Fig. 1.3. Self-absorption curve of 14C radiation in precipitates of BaC03 as measured
with the end-window G-M counter. Red Relative counting rate, % of zero thickness
counting Ran; W weight of BaC03 precipitates, mg cm- 2
5
grams or micromoles, and R; and Ra the radioactivity of the sample in cpm
or dpm.
The efficiency of counting K is directly connected with another important
feature of radioisotopic radiation - its consumption by the molecules of the
sample, the detector, and the intermediate air layer. This phenomenon of selfabsorptIOn radiation within the matter of the samples subjected to radioactivity counts is one of the main factors determining the counting efficiency K,
especially if radioactivity is measured with the aid of G-M counters which had
been used for counting dry preparations. The degree of self-absorption
(quenching) could be expressed in this case by the quenching coefficient Q:
Q = Rj Ren. where Red is the specific radioactivity of sample (cp mg- 1 ) by giving
its thickness (di, mgcm- 2 ), and Ren the same by its thickness approaching to
zero, in which the self-absorption is negligible. The latter value, R en , may be
practically found by extrapolation of the self-absorption curve to zero thickness (Fig. 1.3). The value of self-absorption coefficient Q depends on the characteristics of the isotope (mode of radiation, energy of particles) and on the
material of the labeled sample under examination. In practice, self-absorption
curves are constructed for each kind of isotope and each kind of sample. The
curve of self-absorption for the G-M counters can be constructed by counting
under standard conditions a series of samples with equal portions of radioactivity but with different thicknesses measured as mgcm- 2 . The exact equal portions radioactivity within the labeled samples with a minimal possible (but
known) weight, accessible its precise distribution, are mixed with the samples
of homogenous unlabeled material of the same kind having a different (but
known) weight. The samples are distributed at equal areas fit for the geomeRed
100 :III
Ran
75
50
25
w
o
4
12
16
20
24
Fig. 1.3. Self-absorption curve of 14C radiation in precipitates of BaC03 as measured
with the end-window G-M counter. Red Relative counting rate, % of zero thickness
counting Ran; W weight of BaC03 precipitates, mg cm- 2