As old as the world...
Natural radioactivity has been part of the Universe since its creation. It is found on Earth, within matter and even in living beings. The radiations emitted are invisible, but can be measured with high sensitivity and precision.
Invisible but perfectly measurable
| Since the beginning of the Universe, probably some fifteen thousand million years ago, radioactive atoms have been disintegrating. Most of them have disappeared, yielding stable atoms. However, some of them are still radioactive, sometimes for thousands of millions of years to come, pursuing series of transformations which should bring them to final stability. Others are created daily. This is why, since the dawn of time, the Earth, all living beings and everything around them, are radioactive. Natural radioactivity is also present inside the human body, as the food and water we absorb, the air we breathe contain naturally radioactive atoms. This radioactivity all around us can be measured, by means of specific instruments (radiation counters), with great precision, high sensitivity and good spatial resolution. | Ionising radiations A radiation is said to be ionising when it has enough energy to eject one or more electrons from the atoms or molecules in the irradiated medium. This is the case of a and b radiations, as well as of electromagnetic radiations such as g radiations, X-rays and some ultra-violet rays. Visible or infrared light are not, nor are microwaves or radio waves.
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Activity
When one nucleus is transformed into another nucleus by radioactive emission, it is said to disintegrate or decay. The activity of a radioactive body is the number of disintegrations of its atoms in one second. It is measured in becquerels. One becquerel corresponds to the disintegration of one atomic nucleus per second. This is a very small unit of measure. |
Radioactive half-life Activity, Time As unstable atoms are transformed, the radioactivity of a substance decreases. The time required for this activity to decrease by half is called half-life. This half-life is characteristic of each radioactive isotope. It may range from a few fractions of a second to several thousand million years, depending on the isotope. Nature thus provides several hundred radioactive isotopes which constitute as many calibrated clocks. Examples: Polonium-214 (0.164 second), Oxygen-15 (2 minutes), Iodine-131 (8 days), Cobalt-60 (5.3 years), Carbon-14 (5730 years), Plutonium-239 (24110 years), Uranium-238 (4.5 thousand million years). |
Natural or artificial
Artificial or man-made radioactivity is a phenomenon of the same type as natural radioactivity, but for which the emitting nuclei are produced in the laboratory or in reactors.
The discovery of artificial radioactivity
In January 1934, Irène Curie and Frédéric Joliot
discovered artificial radioactivity. By bombarding a sheet of
Aluminium-27 with a particles, they observed the creation of a
new radioactive isotope, or radioisotope,
Phosphorus-30. They received the Nobel Prize in Chemistry for this discovery.
This experiment showed that, by bombarding stable nuclei, it is possible to fabricate radioisotopes that do not exist in nature. Today, it is known how to create hundreds of artificial radioisotopes for a broad range of uses.
