Page 83 - Radiochemistry and nuclear chemistry

72 Radiochemistry and Nuclear Chemistry

where we again have to consider the net atomic charge.
Part of the nuclear excitation energy is required to overcome the binding energy Ebe, of
the electron in its electronic orbital. 1 The remaining excitation energy is distributed
between the recoiling daughter nucleus and the ejected electron E e. The relationship is given
by the equation

0,- ebo = ed + eo (4.36)

The ejected electron, known as the conversion electron, normally originates from an inner
orbital, since their wave functions have greater overlap with the nucleus. It is to be noted
that the conversion electrons are mono-energetic. Inasmuch as the binding energies of the
atomic orbitals are different, the values of E e reflect the differences in the electronic
binding energies. In Figure 4.1 two sharp peaks are observed just beyond Ema x. The first
peak, designated as K, is due to conversion electrons originating in the K atomic shell,
while the peak labeled L is due to conversion electrons originating in the L atomic shell.
Both of these groups of conversion electrons arise from the decay of 137tuBa. Figure 4.5(f)
shows schematically the decay process of 137Cs --, 137Ba +/3- for the principal decay path;
IT is an abbreviation for isomeric transition. The decay of 137tuBa proceexls both by
emission of a 0.66 MeV -y-ray and by the competitive process of internal conversion. The
ratio between the number of conversion electrons and the number of 'y-rays emitted in this
competition is called the conversion coefficient. The amount of internal conversion is not
indicated in simplified decay schemes like Figure 4.5.
If we denote the conversion coefficient as ai, it is equal to the ratio of K-electrons ejected
(which we may denote with leK) to that of gamma quanta emitted (Iv):

a K = leK/l v (4.37)

Usually a K < 0.1. Also a K > 0% > OeM, etc. For 137mBa the ratio of K-electrons to
L-electrons emitted is about 5 while the value of o~ K is 0.094.
It can be shown that the energy of the recoiling nucleus (E d, eqn. (4.36)) is much smaller
than the kinetic energy of the ejected electron E e and may be ignored. The mathematical
expression to use is (4.32).
A note on terminology. Consider Figure 4.5(f). Though the 'y is emitted from an exited
state of 137Ba, in common language we nevertheless talks of "137Cs-'y" (of 0.662 MeV),
i.e. as if the 'y was emitted from the 137Cs nucleus. The quotation is to be interpreted as
"137Cs (decay through/3- followed by) 'y(-emission)".

4.6. Spontaneous fission

As the nuclear charge increases to large values, nuclei become more unstable. This is
reflected by decreasing half-lives for nuclei heavier than uranium. In 1940 K. Petrzak and

1 Electron-binding energies are tabulated in standard physics tables.