of interactions cause the observed spectrum to show broad rather than sharp
edges. The Compton edge is due to electrons dislodged when g rays are fully
backscattered within the detector and adjacent material (Figure 3.8(b)).
Further details about the formation of g ray spectra go beyond the scope of
this book, except as regards backscattered g rays (1508 to 1808 ) lead to pulses
in the so-called backscatter peak shown in Figure 3.4(d). This peak is small as
is the backscatter peak in Figure 3.6(f ) since these spectra were obtained in a
relatively scatter-free environment. However, these peaks can become domi-
nant in spectra due to strongly scattered radiations (Section 3.9.1).
The energies of the Compton edge (E
C
) or the backscatter peak (E
B
) are not
always easily identi®ed, especially so in multi gamma ray spectra. However, if
the position of one of them is known, that of the other can be calculated since
E
C
+ E
B
= E
g
, the energy of the interacting g ray (Figure 3.8(b)).
3.6 Electron capture (EC), gamma rays and conversion electrons
3.6.1 EC decays and their use as quasi-pure gamma ray emitters
Electron capture (EC) decays occur (Figure 3.11) when an atomic electron is
pulled into the nucleus of its atom where it can be pictured to turn a proton
into a neutron. EC decays are the inverse of b
7
decays. An EC decay is a
primary transformation, decreasing the Z number of the parent by one unit
(see Figure 3.11) without the emission of primary radiations (except for
neutrinos), though there are follow-on radiations emitted by the daughter
nuclide.
Between 80 and 90% of EC decays originate in the K shell of their atoms,
the shell nearest to the nucleus. The resultant vacancies are at once ®lled by
electrons from beyond the K shell. The energy liberated during these transfers
is emitted mainly either as KX rays or as K-Auger electrons (Figure 3.12).
Each electron transfer from an outer to an inner shell leads to the release of
binding energy which appears either as a ¯uorescent X ray (Section 3.9.1) or
as the kinetic energy of an Auger electron. The ratio of the intensities of
¯uorescent X rays to those of Auger electrons is known as the ¯uorescent
yield, denoted v, which is a function of the atomic number Z of the element
of interest (Section 3.9.3). For each element ¯uorescent X ray energies are
signi®cantly high for interactions in the K shell (v
K
), but for outer shells they
are commonly too low to be measured except by specialists. The dependence
of v
K
on Z is illustrated in Figure 3.12.
The decay energy liberated during the EC process is carried away by
ordinarily unobservable mono-energetic neutrinos, each neutrino carrying
3.6 Electron capture, g rays and conversion electrons 79