156
9 Factors Affecting the Counting Efficiency
Fig. 9.7 Decay scheme of
Sodium-24
toelectric effect would be more with γ -rays of lower energy, intensity of photopeaks will appear in the order of 0.511 MeV > 1.02 MeV > 2.75 MeV. Due to
the high energies of β-particles or γ -rays, the sample can be counted either in liquid or solid form and unless very high accuracy of counting is required, liquid or
end-window G.M. counter can also be used for counting this isotope. Moreover,
while counting by G.M. counter, it would be difficult to isolate count rates due to
γ -rays (2.75 MeV) and β-particles (1.39 MeV). Therefore, though it would be easier
to detect its activity by a G.M. counter, it would be difficult to calculate absolute
activity due to interferences of these two radiations. Because these radiations get
added up in counting, thus making the observed count rate higher than the expected
from the decay of one nucleus of Sodium-24.
9.3.4 Strontium-90
Strontium-90 is a pure β-emitter (Fig. 9.8) with very long half-life (28.5 years).
Energy of β-particle is low (0.546 MeV). Hence, care is necessary for source preparation. It can be counted by the end-window G.M. counter or scintillation counter. However,
90 Sr undergoes a secular equilibrium with
90 Y. The equilibrium is established
within 2–3 d. After the establishment of secular equilibrium activity of Strontium-90,
it becomes equal to that of Yttrium-90. Hence, instead of measuring the activity of
Fig. 9.8 Decay scheme of
Strontium-90