Measuring radiation and                  assessing its effects

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.

	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

	photons (gamma, X)		1
	electrons (beta)		1
	neutrons		5 to 20
	protons		5
	Solar particles, heavy ions		20

	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.