Which nuclei are radioactive

Any nucleus is formed from a combination of neutrons and protons known collectively as nucleons , with the naturally-occurring varieties ranging from one solitary proton hydrogen to a combined total of nucleons uranium. With so many particles in such a small volume, the description of the forces within the nucleus rapidly becomes exceedingly complicated. As a result, physicists often use simplified models, which provide similar results.

One particularly popular one is the comparison of the nucleus to an incompressible droplet of water. At first glance, the nuclear stability appears impossible; a nucleus contains only particles of positive charge protons and those with no charge at all neutrons would not interact.

As like charges repel, any nucleus should fall apart very quickly. It is important to realise that radioactive nuclei disintegrate: spontaneously; and randomly. Key points Some nuclei are unstable. They disintegrate, emitting radiation randomly, and spontaneously.

Such nuclei are described as radioactive. Their chemical properties linked to protons are almost identical, but their nuclear properties can be very different. Isotopes are commonly noted by their chemical symbol preceded by their atomic mass by exposing: 1 H, 4 He, 12 C, 16 O, U. The electrons are distributed over layers more or less distant from the nucleus. Peripheral electrons play a major role in chemical reactions.

In a molecule, atoms more or less share these peripheral electrons. An atom has the same number of protons and electrons: it is electrically neutral. If the two numbers differ, the atom is said to be ionized — but it retains the chemical nature determined by its protons.

In the nucleus, neutrons ensure the cohesion of protons that would otherwise repel each other, since they have the same electrical charge. If a nucleus has too many or too few neutrons, it is unstable , and will tend to reach, more or less quickly, the valley of stability, disintegrating according to one of the mechanisms described in the next chapter. This disintegration is accompanied by an emission of energy and radiation: this is called radioactivity.

Natural radioactivity is often opposed to artificial radioactivity: it is an abuse of language, there is only radioactivity, but it can come from nuclei naturally present on Earth and in particular in the human body which naturally contains potassium 40 K, and carbon 14 C or nuclei artificially manufactured in particle accelerators or nuclear reactors.

They can also be formed by the interaction of pre-existing nuclei with cosmic radiation particles, such as carbon 14 C. Finally, they may come from the decay of another radioactive nucleus, such as radium, which descends from uranium. Each radioactive isotope — also known as radionuclide — is characterized by the rate of its decay. The period is a statistical feature: it is impossible to predict when an individual radioactive nucleus will disintegrate, but if we start from one billion of these nuclei, we know with extreme precision when only million will remain.

And this characteristic period of a specific nucleus can range from a fraction of a millisecond to a few billion years… see Table 1. A radioactive atom tends to return to the valley of stability by emitting radiation. This can happen in different ways as shown in Figure 3. In doing so, the atom decreases by two atomic numbers and its atomic mass decreases by 4: plutonium Pu thus becomes uranium U and uranium U becomes thorium Th, for example. With unchanged atomic mass, the atom gains an atomic number.

Figure 3. Thus, after 16 successive disintegrations, uranium U becomes Pb lead, which is stable. The activity of a radioactive atom is the rate of its disintegration.

The becquerel is a very small unit: the natural radioactivity of the human body, mentioned in the previous chapter, is about Bq. Not enough to require wearing a lead coat to avoid irradiating your neighbours!

This is why the activity of a radioactive source is often expressed in giga billions or even terabecquerels thousand billion , which are GBq and TBq. The activity is a measure of rhythm, but not a measure of effect: if you are bombarded at the same rhythm with ping-pong balls or petanque balls, it will not have the same effect!

Hence the notion of dose. When radiation hits a material, it gradually loses all or part of its energy, which is absorbed by the material. The amount of energy absorbed by a given mass of matter is called the dose. These are particles with enough energy to knock electrons off atoms or molecules.

The degree of radioactivity depends on the fraction of unstable nuclei and how frequently those nuclei decay. The effect of radioactivity also depends on the type and energy of the particles produced during nuclear decay.

For example, neutrinos pass constantly through the Earth, while alpha particles are blocked by a sheet of paper. Radioactivity can cause damage in materials and in plant, animal, and human tissue.