Measuring radiation and
assessing
its effects
Gray: the measurement of the absorbed dose
Measuring radiation and
assessing
its effects
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Two
measurements are essential for radiation
protection : the measurement of the dose of radiation absorbed
by the body and the assessment of the risk associated with this
absorbed dose.
Two units were thus created: the Gray and the Sievert.
The Gray: the measurement of the absorbed dose
When it comes into contact with matter, inonising radiation collides with the atoms comprising it. During these interactions, it releases a part or all of its energy. The absorbed dose (expressed in Gray) is defined by the ratio of this released energy over the mass of the matter. A Gray corresponds to one Joule of energy released in one kilogram of matter.
The Sievert: the assessment of the biological risk
In order to have a single unit which expresses the risk of the occurrence of the stochastic effects associated
with all possible exposure situations, physicists developed an indicator known as the "effective dose",
a measurement using the sievert (Sv), named after the Swedish physicist who was one of the pioneers in protection
against ionising radiation. The effective dose is calculated from the dose (expressed in Gy) absorbed by the various
exposed tissues and organs, by applying weighting factors which take into account the radiation types (alpha, beta,
gamma, X, neutrons), the means of exposure (external or internal) and the specific sensitivity of the organs
or tissues (cf. tables). By definition, the effective dose, expressed in Sv, can only be used to
assess the risk of the appearance of stochastic effects in man, and cannot be used either for acute effects nor for the
effects on the fauna or flora.
Note: two sub-multiples of the sievert are very frequently used: the millisievert or thousandth of a sievert,
indicated as mSv; and the microsievert or millionth of a sievert, indicated as µSv.
Units of measurement
| Quantity measured | International system (SI) | Definition (SI) |
|---|---|---|
| Dose absorbed | GRAY (Gy) | 1 Gy: energy released by joule per kilogram of matter |
| Equivalent dose and effective dose | SIEVERT (Sv) | Sv: Gy multiplied by a weighting factor specific to each type of radiation and organ |
Radiation weighting factor
| photons (gamma, X) | 1 |
| electrons (beta) | 1 |
| neutrons | 5 to 20 |
| protons | 5 |
| Solar particles, heavy ions | 20 |
Weighting factors for organs and tissues
| Gonads | 0.20 |
| Bone marrow, colon, lungs, stomach | 0.12 |
| Bladder, breast, liver oesophagus, thyroid | 0.05 |
| Skin, bony surfaces | 0.01 |
| Other | 0.05 |
Means for measuring the radiation on board aircraft
Given the complex nature of cosmic radiation due to the variety of particles and their energy characteristics,
measuring the dose is not easy. Generally, detectors only provide correct readings for some of these particles.
To obtain a correct measurement, it is necessary to use a series of detectors or certain meters that "see"
all of the components of cosmic radiation. Unfortunately, the use of such devices is not suitable for normal
conditions on commercial flights.
Another approach would be to calculate the doses using relatively complex models. The elaboration of models is
difficult, since they must describe the interaction of cosmic particles and their contribution to the dose.
However, the calculation is very reliable if the models are validated using experimental data from the international
scientific community. A calculation is also much easier to use. The vast majority of specialists use models to
assess the doses of radiation occurring during flights. There are currently various calculation tools, validated
through actual measurements on board aircraft. The main problem is that these models do not allow an assessment
of the effect of an unpredictable solar flare. To get around this problem, the
(IRSN) includes, thereafter, the dose values provided by the Paris Observatory, into the S.I.E.V.E.R.T.
system used in France by airline companies.