Biomedical Engineering Reference
In-Depth Information
2.3 Modes of Decay
The nucleus of an atom contains both protons and neutrons, which are col-
lectively known as nucleons. In a stable nucleus, the number of protons and
neutrons is such that the repulsive electrostatic force between the positively
charged protons is balanced by the very strong attractive nuclear forces which
act on all nucleons. It is possible to create unstable isotopes which have an ex-
cess number of protons using nuclear reactors or cyclotrons. These
proton-rich
(or
neutron-deficient
) isotopes can have two means of decay that will reduce
the excess positive charge on the nucleus: (1) electron capture and (2) positron
emission.
If the nucleus does not have sufficient energy to decay by positron emission
(to be described next), it will decay by
electron capture
, whereby the nucleus
captures one of the orbital electrons from the inner shells and combines this
with the proton to form a neutron, while the vacancy in the inner electron shell is
immediately filled by an electron from a higher energy shell, resulting in emission
of characteristics X-rays whose energies are carried off by the neutrino:
A
A
Z
−
1
Y
+
ν
Z
X
+
e
−
−→
(2.1)
where
Z
represents the atomic number of the atom
X
,
A
is the mass number,
e
−
is an electron, and
ν
is a neutrino, which has a very small mass and zero
charge. Electron capture occurs in heavier proton-rich nuclides with higher
likelihood due to the closer proximity of the inner (usually K or L) shell electrons
to the nucleus and the greater magnitude of the Coulombic attractive force
from the positive charges. The characteristics X-ray energy increases with the
mass number of the nuclides. For example, the decay of
125
I produces 27 keV
characteristics X-ray which is used for
in vitro
counting, whereas the decay of
201
Tl produces characteristics X-rays ranged from 68 to 80 keV which are used
in gamma-camera imaging.
The major radioactive decay mechanism for positron emitters used in PET
is
positron emission
, whereby a proton in the nucleus is transformed into a
neutron and a positron. The positron (
β
+
) has exactly the same mass and same
magnitude of charge as the electron except that the charge being carried is
positive. The nuclear equation for positron emission can be written as
A
A
Z
−
1
Y
+
β
+
+
ν
Z
X
−→
(2.2)
