Measurement of Radioactivity
13
appropriate value of K the standard error at the desired confidence level can
be calculated.
1.2.2.3 Quenching and Quench Correction
The quenching of photoresponse in counting samples is a major problem in
liquid scintillation radiometry (Brandsome 1970). The presence of molecules
of impurities and gases and molecules of the radioactive material itself inside
the counting sample decreases the efficiency coefficient of radioactivity counting K (Dobbs 1963). Great efforts have therefore been undertaken to evaluate this decrease in order to correct it and to calculate the "true" counting rate.
In a quenched liquid scintillation sample the spectrum of [3-emitter shifts
to the lower energy its end. This shift compresses the spectra of concurrent
[3-emitters, particularly those with a low E max, such as 14C and 3H. The quenching effect on energy spectra of these radioisotopes is shown in Fig. 1.4. Within
a heavily quenched counting sample virtually all the 14C spectrum resides into
the 3H counting channel (Fig. 1.5).
The quenching is caused first of all by loss of energy by the primary particles on their way through the medium. The molecules of impurities interacting with passing [3-particles intercept part of their excitation energy and emit
80 .....
full14C
..
Ri
I
60 I
.....
narrow 14C
..
- - • 2
...... "
- • 3
40
/
4
,
\
/
/
\
/
.
20
\
/
\
I,
..... _ _ •
,.; <"
\
"
H
0
100
200
300
400
500
600
700
800
Fig. 1.5. Effect of quenching on 14C and 3H radiation spectra. R; Radioactivity counted
per channel, 10 9 cpm; H log of the pulse heights; the radioisotopes: 1 3H (nonquenched);
2 3H (quenched); 3 14C (nonquenched); 4 14C (quenched)