9.3 Decay Scheme
157
Fig. 9.9 Decay scheme of
Cesium-137
90 Sr, one prefers to count the activity of
90 Y. Because,
90 Y has a strong β-particle
(2.27 MeV) with 99.9% abundance. Therefore, the counting efficiency for counting Yettrium-90 is higher. This can be counted by an end-window or liquid G.M.
counter. Source preparation would also require less attention to make a uniform
sample. Considering these advantages, Sr
90 isotope is preferred to be counted after
allowing the sample to achieve secular equilibrium. For this purpose, one carry a
normal experiment with Strontium-90 isotope, which is counted after 3 d.
9.3.5 Cesium-137
Cesium-137 isotope has a very long half-life (Fig. 9.9) but gives low energy βparticles with high abundance and γ -rays with 94% abundance. The efficiency of
counting, however, would certainly be higher with γ -rays, than β-particle (0.51
MeV), unless plastic phosphor scintillator or proportional counter is used. However,
if high efficiency is not essential, an ordinary G.M. counter would be sufficient for
this isotope.
Summary
In this chapter, we have learnt the importance of source preparation and loses of
activities due to several factors like self-absorption, backscattering etc. What
precautions are needed to minimize the effect of these factors are also discussed. Finally, utility of knowing the decay scheme of radioactive isotopes at
least for making a decision as to in which form the sample should be counted,
which radiation to be used, and by which counter the isotopes should be counted
is illustrated by discussing the decay schemes of few radioactive isotopes of
long- lived and short-lived type.
157
Fig. 9.9 Decay scheme of
Cesium-137
90 Sr, one prefers to count the activity of
90 Y. Because,
90 Y has a strong β-particle
(2.27 MeV) with 99.9% abundance. Therefore, the counting efficiency for counting Yettrium-90 is higher. This can be counted by an end-window or liquid G.M.
counter. Source preparation would also require less attention to make a uniform
sample. Considering these advantages, Sr
90 isotope is preferred to be counted after
allowing the sample to achieve secular equilibrium. For this purpose, one carry a
normal experiment with Strontium-90 isotope, which is counted after 3 d.
9.3.5 Cesium-137
Cesium-137 isotope has a very long half-life (Fig. 9.9) but gives low energy βparticles with high abundance and γ -rays with 94% abundance. The efficiency of
counting, however, would certainly be higher with γ -rays, than β-particle (0.51
MeV), unless plastic phosphor scintillator or proportional counter is used. However,
if high efficiency is not essential, an ordinary G.M. counter would be sufficient for
this isotope.
Summary
In this chapter, we have learnt the importance of source preparation and loses of
activities due to several factors like self-absorption, backscattering etc. What
precautions are needed to minimize the effect of these factors are also discussed. Finally, utility of knowing the decay scheme of radioactive isotopes at
least for making a decision as to in which form the sample should be counted,
which radiation to be used, and by which counter the isotopes should be counted
is illustrated by discussing the decay schemes of few radioactive isotopes of
long- lived and short-lived type.