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Key scientific concepts

//Key scientific concepts

Measuring the invisible

Finding your way in the jungle of radiation units is quite a challenge.

All the « fathers » of radioactivity found new phenomena that had to be quantified, and the “inventors’” names were eventually given to most of the new units:

Röntgen (Rem), Becquerel (Bq), Sievert (Sv), Curie (Ci), Gray (Gy)

A photon (RPL technique) and neutron (trace detector) dosimeter by Marion Baumann, (IRSN / PRP-HOM / SDE / LDRI).

Despite the first concerns raised in 1904 by Charles Béclère and first hints towards safety culture, by the 1920s, some 400 doctors and physicists using ionizing radiation (then called X-Rays by Roentgen) died of radiation-related diseases. Marie Curie herself died in 1934 of aplastic anaemia, most probably developed from extended exposure to various radioactive elements.

A further unit, the rad, was named after...radioactivity.

These units can be categorized in two different systems: Since the beginning of the 20century, a conventional system based on Ci, Rad and Rem has been used and still is in the US (because it’s in the law). But today the most recognized worldwide is the SI (Système International, from the French) based on Bq, Sv and Gy.

What do these units stand for? In this case, everything is linked to dose, exposure and intake. Obviously, these concepts are interrelated.


The amount of radiation given off by a radioactive material (the radioactivity of a source,  whether it is alpha or beta particles, gamma rays, x-rays, or neutrons) is measured in Bq (or in Ci).



The radiation absorbed by a person (how much your cells absorb) is given in Gy.



For the biological risk of exposure to radiation (the harm done to your body), the measure used is Sv. And as the different particles (α,β, γ, X-rays) ionise differently, it is usually expressed as a “dose equivalent” in Sieverts (Sv).

Moreover, as the Sievert is quite a large unit, you are more likely to come across millisieverts (mSv) or microsieverts (μSv).

A good way to think of it is to imagine you are under heavy rain. The amount of rain falling is in Becquerels, the amount of rain hitting you is in Grays, and how wet you are is measured in Sieverts.

In situ measurement of soil radioactivity by gamma spectrometry in Gunma's Japanese prefecture. This measurement is carried out using a germanium spectrometer on a LEICA tripod. IRSN.


The rate of accumulation of radiation received by a persons expressed as dose rate or dose accumulated per unit time in Becquerel per hour (Bq/h). Thus, the dose received by a person is the dose rate multiplied by the duration of exposure.

The concentration of radioactivity in food, water, or air, is usually given in Bq.kg, Bq/l or Bq/m3.

To read more about radiation: http://www.unscear.org/unscear/en/faq.html

Some examples

2.4 mSv/year (0.27 μSv/h avg)

Human exposure to natural background radiation, global average[a]

24 mSv/year (2.7 μSv/h avg)

Natural background radiation at airline cruise altitude

80 μSv

average dose to people living within 16 km of the Three Mile Island nuclear power plant in 1979.

5 to 10 μSv

one set of dental X-rays

0.4 to 0.6 mSv

two-view mammogram

0.25 μSv

banana equivalent dose, an illustrative unit of radiation dose representing the measure of radiation from a typical banana

0.25 μSv

US legal limit on effective dose from a single airport security screening