The decay of 57Co proceeds by electron capture
(EC) to an excited state of 57Fe, which subsequently decays to the
ground state by emission of three gamma rays. As a rule of thumb, gamma rays of
lower energy have a greater likelihood of occurring. Thus the decay of the
136.5 keV level to the 14.41 keV level happens more often than the decay to the
ground state. Since gamma-1 is part of the cascade gamma-1-2, it has the same
likelihood.
Therefore the spectrum has three peaks, with the 14 and 122
keV peaks equal in height and somewhat higher than the 136 keV peak. Recorded
with a high-resolution Ge detector, the spectrum would look like this
(schematically):
For your prelab exercise you are to construct schematically
the gamma spectrum of the 133Ba decay. The decay scheme is shown in
the figure to the right. Note that 86 of 100 decays proceed through the 437 keV
level, with subsequent decays via the 384, 161, and 81 keV levels to the ground
state. The other 14 of 100 decays begin at the 384 keV level. Try to estimate
as best as you can the relative peak heights based on the simple example above.
In the lab we will be doing the opposite. We will have the gamma spectrum and try
to deduce the decay-scheme from this. However, understanding how the two are
connected this will be beneficial for this process.
