Radioactive decay

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 J.
**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:
  1. 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.
  2. 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.
  3. 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, E_max (Table 35.2).

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 (t_½). This 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 t_½ is very short, as with ¹⁵O (t_½ ≈ 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 and t_½ in the above equation.