Pharmaceutical Manufacturing Handbook: Production and

62 RADIOPHARMACEUTICAL MANUFACTURING
The stabilization process can proceed by several different processes, such as spontaneous
fi ssion, α - particle emission, β - particle emission, positron emission, γ - ray
emission, or electron capture. In all decay processes the mass, energy, and charge of
radionuclides must be conserved, and many nuclides can decay by a combination
of any of the above - mentioned processes.
Fission is the process in which a nucleus breaks down into two fragments (thus
leading to two different new nuclides) with an emission of two or three neutrons
and a lot of energy. Spontaneous fi ssion is a rare process that can only occur in heavy
nuclei. Fission can also be produced by bombardment of certain nuclides with high -
energy particles (such as neutrons) and is in fact the nuclear process used for the
production of energy in nuclear energy plants by bombardment of highly enriched
uranium with neutrons.
The usual decay process of heavy nuclei is α - particle emission. An α particle is
a helium ion containing two protons and two neutrons. Alpha particles are heavy
particles that have a very short range in matter due to their mass, and radiopharmaceuticals
labeled with α emitters are used only with therapeutic purposes. Their
clinical use is very limited, and they are mainly used for research purposes or in
early phase clinical studies.
Radioactive nuclides that are neutron rich disintegrate by β decay. A β − particle
is originated by the conversion of a neutron into a proton, along with the emission
of an antineutrino to conserve energy in the decay process. Beta - emitting radionuclides
are also used in radiopharmaceuticals for therapeutic purposes.
Positron decay occurs in proton - rich nuclei. In this case, the positron (or β + particle)
is originated by conversion of a proton into a neutron, along with the emission
of a neutrino to conserve the energy. Positrons are the antiparticle of electrons. In
a very fast process (10 − 12 s), emitted positrons collide with an electron of a nearby
atom and both particles disappear in a process called annihilation. The necessary
conservation of mass and energy accounts for the transformation of the mass of
both particles into energy, which is characteristically emitted in the form of two
511 - keV photons almost in opposite directions. Consequently, positron emitters are
used to label radiopharmaceuticals produced with diagnostic purposes by imaging.
Proton - rich nuclei can also decay by electron capture. In this process, an electron
from the innermost electron shell orbitals is captured into the nucleus and transforms
a proton into a neutron (and a neutrino is emitted for conservation of energy).
The vacancy created by the lost electron is fi lled by the transition of an electron
from a higher level orbital, and the energy difference between the intervening orbitals
is emitted as energy in the form of an X ray.
For any particular nucleus, several different energy states can be defi ned by
quantum mechanics. All the excited energy states above the ground state are referred
to as isomeric states and decay to the ground state by the so - called isomeric transition.
In β , positron, or electron - capture decay processes, the parent nucleus may
reach any of these isomeric states of the daughter nucleus. The energy difference
between the nuclear energy states can be emitted as γ rays. A particular situation
for isomeric transition is that in which the excited state is long lived and is then
called the metastable state.
Radioactive Decay Equations, Magnitudes, and Units Radioactive decay is a
random process, being impossible to tell which particular atom from a group of atoms
will decay at a specifi c moment. It is then only possible to talk about the average