Radioactivity

For any element there is a limited range in which the number of neutrons can be part of the nucleus and still be stable. If there are too few neutrons or too many neutrons in the nucleus, the atom is unstable. An unstable atom will try to become more stable by emitting energy in the form of radiation, and it is said to be radioactive.

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Radioactivity can be simply defined as that process in which unstable atoms attempt to stabilize themselves by emitting radiation.

Using the previous example of hydrogen, when the nucleus consists of two neutrons and one proton (i.e. the isotope tritium), the atom is unstable and therefore radioactive. The combination of one proton and three neutrons is so unstable, that for all practical purposes it does not exist.

2.2.1. Chart of radionuclides

All of the existing known isotopes (stable and unstable) can be shown on a chart such as that in Fig. 3. Each square represents one isotope (i.e. one combination of protons and neutrons).

On the left is shown the number of protons in the isotope and, along the bottom, the number of neutrons is shown. A curved line of stable isotopes can be seen by the shaded squares on the chart. This is called the line of stability. The chart itself is called the Chart of the Nuclides and is published with useful information about each isotope in its own square.

2.2.2. Radioactive decay and half-life

The further an isotope is distanced from the line of stability the more radioactive it is.

When an unstable (radioactive) atom emits radiation to become more stable, it is said to disintegrate or decay. Radioactive decay is an interesting process in that it has regular and predictable aspects as well as totally random aspects. The moment at which any one particular atom decays is random and cannot be predicted. However, the time in which, on average, half of a certain (large) number of atoms of a particular isotope will decay is regular, known and entirely predictable. This is somewhat analogous to the situation in which a large number of coins are placed in a tray all with the same side down. If they are thoroughly shaken up, half of the coins will have one side up and half will have the other side up on average. However, prior to shaking, it cannot be predicted which way up any particular coin will land.

Each kind of radioactive isotope has a specific known time period in which half of the atoms will decay. This is called the half-life. If the number of atoms of a particular radioactive isotope is plotted against time, a curve such as that shown in Fig. 4 is obtained.

2.2.3. Quantities and Units

There are special quantities to characterize radioactive material:

 The half-life is the time after which half of a given number of radioactive nuclei has decayed. The corresponding unit is usually given in years (a), days (d), hours (h), minutes (min) or seconds (s).

 The activity of a radioactive material gives the number of decays per unit of time. The corresponding unit is the becquerel (Bq). The conversion of activity into dose is treated in Section 3.1. One becquerel is equivalent to one atom decaying (or disintegrating) each second.

Due to the fact that some radioisotopes decay more rapidly than others, i.e. they have a shorter half-life, equal masses of different radioisotopes can have widely differing activities.

The activity per unit mass of a certain radionuclide is called the specific activity of that radionuclide, and is a constant. The corresponding unit that is most commonly used is becquerels per gram (Bq/g). In most cases, a certain radioactive material consists of a mixture of a radionuclide and inactive material. In these cases the term ‘specific activity’ is not 10

relevant. The activity per unit mass of such a mixture depends on the mixing ratios of radionuclide and inactive material and is called the activity concentration (also in becquerels per gram).

FIG. 3. Chart of radionuclides .

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FIG. 4. Radioactive decay curve

In some cases, the half-life of a radionuclide is extremely long. Examples are ⁴⁰K, ²³⁸U and ²³²Th, with half-lives of 1.3 × 10⁹, 4.5 × 10⁹, and 1.4 × 10¹⁰ years, respectively. These half-lives are comparable to the age of the world, and a large portion of such radionuclides have decayed only partially since its formation. They are therefore called primordial radionuclides or radionuclides of natural origin. They are ubiquitous in nature and are the cause of the terrestrial background radiation.

2.2.4. Decay chains

Many radionuclides loose their surplus of energy in a single step. The resulting nuclide is stable and therefore part of the line of stability. It is also possible that the radionuclide decays via distinct separate steps. In such cases the surplus of energy of the original radionuclide is lost in a chain of several decays, before the resulting nuclide becomes ‘at rest’

on the line of stability. An example is the decay of ⁹⁰Sr:

38 is called the parent radionuclide, with a half-life of 29.1 a;

90Y

39 is called the progeny radionuclide, with a half-life of 64 h;

90Zr

40 is stable.

2.2.4.1. Metastable radionuclides

A specific type of decay is the formation of metastable radionuclides. In this case, the progeny radionuclide has already a stable proton/neutron combination, but still has a surplus of energy. The metastable state releases its surplus of energy only by emitting gamma radiation, and therefore there is no change in the number of protons and neutrons. The decay

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process itself is again of a stochastic nature, and is characterized by a specific half-life. An

37 is the parent radionuclide, with a half-life of 4.6 h;

mKr

81

36 is the metastable progeny radionuclide, with a half-life of 13 s;

81Kr

36 is stable

2.2.4.2. The natural uranium and thorium decay series

There are also much longer decay chains. The radionuclides of natural origin ²³⁸U and

232Th decay via a series of progeny radionuclides before they reach their stable endpoints on the line of stability. The full radioactive decay chains of these radionuclides are shown in Tables 4 and 5. In situations where the progeny radionuclide is contained in the matrix of certain minerals throughout their existence, such as for instance in geological deposits, all progeny radionuclides have the same activity as the parent, because the latter has the longest half-life. This is called radioactive equilibrium. However, when the deposit is disturbed, by leaching or processing, the decay chain may also become disturbed, as the elements to which the radioactive progeny belong may behave differently during the disturbing process.

TABLE 4. URANIUM-238 DECAY SERIES

Radionuclide Half-life Major mode of decay

238U 4.468 × 10⁹ a Alpha

TABLE 5. THORIUM-232 DECAY SERIES

Radionuclide Half-life Major mode of decay

232Th 1.405 × 10¹⁰ a Alpha

228Ra 5.75 a Beta

228Ac 6.15 h Beta

228Th 1.912 a Alpha

224Ra 3.66 d Alpha

220Rn 55.6 s Alpha

216Po 0.145 s Alpha

212Pb 10.64 h Beta

212Bi 60.55 min Beta 64.06%

Alpha 35.94%

212Po 0.299 µs Alpha

208Tl 3.053 min Beta

208Pb Stable —