The activity of a sample of radioactive matter is defined by the number of disintegrations taking place at its core at any given moment. The activity also represents the number of radiations emitted. One call therefore alpha, beta, gamma activity the numbers of alpha, beta, gamma rays that are emitted and are in proportions of the number of disintegrations. These activities are fundamental features of the radioactive sample and of the type of radiation emitted. They represent its ‘baseline radioactivity’. When the sample contains more than one element, the total activity is the sum of the individual activity values.
In the majority of cases, disintegrations leads to the emission of an alpha or beta ray. The numbers are then equal. If certain decays are accompanied by the emission of gamma rays, a gamma activity is introduced which is in proportion to the emission frequency of these gamma rays.
The activity of a radioelement varies inversely with its lifespan. The longer the half-life of a substance, the lower its activity. An analogy can be made with a soft-burning candle, whose lower flame burns for longer. This is why in uranium 238, which has a half-life of 4.5 billion years, only one nucleus out of 65 million will decay every century.
By contrast, a radioactive nucleus such as oxygen 15 (which is commonly used in medical scans) will disappear within minutes, in the same way that a firework will burn brightly and then vanish. A very small quantity of this radioisotope is all that is needed for significant levels of activity to be observed.
When one measures activity, one counts the number of atomic nuclei that are disintegrating. As the number of atoms present in even the smallest sample of matter is always enormous, the number of disintegrations per second will also be large, even if the proportion of radioactive atoms is low.
The large number of disintegrations allows sensitive detectors to observe infinitesimal doses of radioactive substances and makes extremely accurate measurements. It only takes a millionth of a billionth of a gram of caesium 137, for instance, to carry out proper sedimentation studies.
One defines the activity of a sample of radioactive material as the number of disintegrations taking place every second. The basic unit of activity is the becquerel (Bq) named after Henri Becquerel, the first man to discover radioactive radiation. A becquerel represents a decay rate of one disintegration per second. For historical reasons, activity values were measured in Curies (Ci) for many years : a Curie is the activity of a standard source of one gram of radium. Such a radium source has 37 billion decays every second, which is a very high value by comparison. As a result, milli- and microcuries were units that were more commonly used. The becquerel is, by contrast, a unit on the same scale as a nucleus: more practical but less realistic. One millionth of a Curie (1 microcurie) is equivalent to 37,000 becquerels (Bq)
The extreme smallness of the becquerel is rarely reminded when by the medias during accidents of radioactivity or nuclear accidents. The absence of this reminder generates unjustified fears.
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