Radioactive decay
There are several different radioactive processes that certain isotopes of
elements can undergo. To understand these processes it is necessary to
keep track of the specific isotopes involved. For the decay processes
each isotope is denoted by the mass number, which equals the number
of protons and neutrons, and the atomic number, which equals the
number of protons. For example, the carbon isotope^126 C has a mass
number of 12 and an atomic number of 6, whereas the uranium isotope
238
92 U has a mass number of 238 and an atomic number of 92. The differ-
ent types of radioactive decay can be classified according to the particles
that are emitted by the decay process. For example, αparticles con-
sisting of two protons and two neutrons, identified as^42 α, are emitted by
radium and uranium:
224
88 Ra →
220
86 Rn +
4
2 α
(19.5)
238
92 U →
234
90 Th +
4
2 α
As another example, a photon is emitted in the form of γrays from the
isotope^11950 Sn. In this case, there can be no change in the isotopic com-
position, only in the energetics of the nucleus. In electron capture, an
electron is captured by a nucleus and the atomic number can change along
with a release of energy.
Of interest for its use in PET are isotopes than undergo βdecay. Two
examples of this process are the decay of the carbon isotope^146 C into the
nitrogen isotope^147 N and the decay of^189 F into^188 O:
14
6 C →
14
7 N +− 1
(^0) β
(19.6)
18
9 F →
18
8 O ++ 1
(^0) β
Since the β decay process results in a change of the atomic number
by 1, there is an emission of a charged particle to conserve the overall
charge. For^146 C, the conversion into^147 N results in an increase in the atomic
number; hence, the decay is accompanied by the emission of an electron
to conserve charge. For^189 F, the decay into^188 O results in a decrease in
the atomic number; hence, charge conservation demands the emission
of a positively charged particle with the same mass as an electron, namely
a positron. As explained in Chapter 12, positrons are part of a whole
family of antiparticles that match all particles but with some opposite
properties. Positrons are the antimatter form of electrons, with a mass
identical to, but charge opposite to, an electron. Positrons are not stable
and will quickly combine with the corresponding particle to produce
energy. Whereas there are several isotopes that will emit positrons,
most have half-lives of only a few minutes and so are difficult to use in
imaging techniques. Even the commonly used isotope^189 F has a half-life