named after the parent. One refers to cobalt-60 g rays even though they are
emitted from the daughter nickel-60, which is created with two excited states
which promptly (within 10
712
s) de-excite in cascade (Figure 3.4(b)).
An important consequence of the fact that g rays are emitted from the
daughter nuclide relates to their multiplicity per decay. Primary decays, e.g.
those signalled by b particle emissions (Sections 3.3.1), have to obey the rule
of one b particle per decay. There is no such restriction on follow-on g rays.
Daughter nuclides are frequently created in numerous excited states which
are then de-excited by g rays, though de-excitations could also occur by a
competing process known as internal conversion (Section 3.6.2).
When the number of excited daughter states is large, the number of de-
excitations per state is invariably a small fraction of the decay rate of the
parent nuclide, on average rarely exceeding the equivalent of two g rays per
parent decay. This is readily veri®ed from decay scheme data. Multiple g ray
decay will be discussed in more detail in Section 3.7.3.
While the decay of large numbers of radionuclides result in several
daughter excitations, de-excited by g rays (see Figure 3.9(a) later), others are
followed by fewer than one g ray per decay of the parent. In Figure 3.4(c), the
number of g rays per decay of
137m
Ba is 90 g rays per 100 decays and there are
many much lower g ray fractions, e.g. the 9.85 g rays following 100
51
Cr
decays shown in Figure 3.11(a), about 1 gamma ray per eleven decays.
In Figure 3.4(c) the parent activity is
137
Cs with 95 out of 100 decays
feeding
137m
Ba, its isomeric daughter.
137m
Ba de-excites partly by g ray
emissions (~90%) and partly by conversion electron emissions (~10%), a
process which will be introduced in Section 3.6.2. Although the g rays
originate from
137m
Ba, they are known as
137
Cs g rays. Their intensity relative
to
137
Cs is ~0.956 ~0.90 & 0.85 or 85% (Figure 3.4(c), (d)).
The importance of adequate knowledge of relevant decay data of radio-
nuclides employed for applications cannot be overemphasised. Unless suf®-
cient data are known, notably the energies and intensities of the radiations of
interest and their half lives, applications are unlikely to yield satisfactory
results.
3.4.2 X rays
Electromagnetic radiations in the kiloelectronvolt range and higher comprise
X rays as well as g rays. Radiations are known as X rays if their energies
exceed about 0.1 keV but there is no upper limit. X rays are produced as
bremsstrahlung (Section 3.8.1), or as ¯uorescent X rays (Section 3.9.1). They
are not only used in medicine but also extensively for industrial purposes and
3.4 Properties of gamma rays and X rays 69