There are three forms of radioactivity (Table 35.1) arising from three main
types of nuclear decay:
|Table 35.1 Types of radioactivity and their properties
*Note that 1MeV = 1.6 × 10−13
**Distance at which radiation intensity is reduced to half.
- Alpha decay involves the loss of a particle equivalent to a helium nucleus.
Alpha (α) particles, being large and positively charged, do not penetrate
far in living tissue, but they do cause ionization damage and this makes
them generally unsuitable for tracer studies.
- Beta decay involves the loss or gain of an electron or its positive
counterpart, the positron. There are three sub-types:
- Negatron (β−) emission: loss of an electron from the nucleus when a
neutron transforms into a proton. Examples of negatron-emitting
isotopes are: 3H, 14C, 32P, 45Ca and 60Co.
- Positron (β−) emission: loss of a positron when a proton transforms
into a neutron. This only occurs when sufficient energy is available
from the transition and may involve the production of gamma rays
when the positron is later annihilated by collision with an electron.
- Electron capture (EC): when a proton 'captures' an electron and
transforms into a neutron. This may involve the production of x-rays
as electrons 'shuffle' about in the atom (as with 125I) and it
frequently involves electron emission.
- Internal transition involves the emission of electromagnetic radiation in
the form of gamma (γ) rays from a nucleus in a met astable state and
always follows initial alpha or beta decay. Emission of gamma radiation
leads to no further change in atomic number or mass.
Note from the above that more than one type of radiation may be emitted
when a radioisotope decays. The main radioisotopes used in chemistry and
their properties are listed in Table 35.2.
Each radioactive particle or ray carries energy, usually measured in
electron volts (eV). The particles or rays emitted by a particular radioisotope exhibit a range of energies, termed an energy spectrum, characterized by the
maximum energy of the radiation produced, Emax
|Table 35.2 Properties of selected isotopes
The energy spectrum of a particular radioisotope is relevant to the following:
- Safety: isotopes with the highest maximum energies will have the greatest
penetrating power, requiring appropriate shielding (Table 35.1).
- Detection: different instruments vary in their ability to detect isotopes
with different energies.
- Discrimination: some instruments can distinguish between isotopes, based
on the energy spectrum of the radiation produced.
The decay of an individual atom (a 'disintegration') occurs at random, but
that of a population of atoms occurs in a predictable manner. The
radioactivity decays exponentially, having a characteristic half-life (t½
is the time taken for the radioactivity to fall from a given value to half that
value (Fig. 35.1). The tuz values of different radioisotopes range from
fractions of a second to more than 1019
years (see also Table 35.2). If t½
is very short, as with 15
≈ 2min), then it is generally impractical to use the isotope in experiments because you would need to account for the decay during the experiment and counting period.
|Fig. 35. 1 Decay of a radioactive isotope with
time. The time taken for the radioactivity to
decline from × to 0.5× is the same as the time
taken for the radioactivity to decline from 0.5×
to 0.25×, and so on. This time is the half-life
(t½) of the isotope.
To calculate the fraction (f
) of the original radioactivity left after a
particular time (t
), use the following relationship:
|⇒ Equation [35.1]
||f = ex, where x = −0.693t/t½
Note that the same units must be used for t
in the above equation.