The activity of a sample of matter which contains radioactive nuclei is one of the intrinsic properties of this radioactive source. It represents the number of decays occurring every second or, alternatively, the number of rays emitted. In this latter case, we talk about alpha, beta and gamma radiation. The activity of a source does not take into account the nature or the energy of the radiation, nor the effects such rays would have in different media.
The activities of different sources were for many years expressed in units known as curies (Ci). The radiations emitted by radioactive substances were measured by a comparison with a standard, the radiation emitted by radium, the radioactive element found by Marie Curie. One curie corresponded to the activity of one gram of radium, or 37 billion disintegrations per second. Given this very high value of one curie, millicuries or microcuries were more commonly used.
Physicists and engineers have since adopted a more logical unit – the Becquerel – which corresponds to a decay rate of one per second. The associated inconvenience, however, is that the Becquerel is a very small unit, adapted to the scales of the atom. Activities expressed in becquerels therefore lead to misleadingly large numbers, which can cause confusion among non-specialists. For example, the human body has an activity of 8000 Bq – a value which may seem high but is in reality very small. It corresponds a few microcuries.
Seeing activities written down in becquerels often gives the impression that such activities are exceptionally high and thus very dangerous. This is mainly due to the small value of a Becquerel, and substances are rarely that dangerous. The Becquerel is like the Deutschmark used by the Weimar republic in the 1920s when a wheelbarrow full of which was needed to buy a croissant.
In the rational and cartesian countries lof the western world, it was logical to define the basic unit of activity as one disintegration per second. But men of science could be awkward psychologists. The choice of the becquerel has revealed itself to be a public relations disaster. The tiniest levels of activity expressed in becquerels are represented by tremendously high numbers, which feed the insecurity of the public.
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