Radioactive Activity Doses

Becquerel or Bq : the unit of activity for a radioactive source

The Becquerel: a very small unit…:
The becquerel, the commonly-used unit of activity, corresponds to a decay rate of one nucleus per second. This unit on the scale of the atom is so small (and therefore ill-adapted to describe the activities of most radioactive substances we deal with) that multiples of becquerels are frequently used: kilo (thousands), mega (millions), giga (billions) and terabecquerels (thousands of billions), etc. The diagram above shows the range of activities different substances – from a litre of water up to the spent fuel extracted from the core of nuclear reactors – can have. Historic units such as curies and millicures have also been indicated.

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.

Evolution of the activity of caesium 137 in the human body
The activity of caesium 137 in the human body has been measured near the Mol laboratory in Northern Belgium for more than half a century. Despite the laboratory large distance from nuclear tests bomb sites and Chernobyl, the peaks of activity due to these tests and the Chernobyl accident (four times weaker) can be seen. In these two cases, the decay of caesium 137 in the human body takes place much more quickly than the natural decay, which has a half-life of 30 years. Though caesium is absorbed slowly by the soil, the rapid decay of its activity in the human body means that no soil decontamination (using, for instance, Prussian blue), is needed in Belgium.

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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