What is
cosmic radiation ?
A galactic origin A solar origin
Radiation in
the earth's environment
Protection of the magnetosphere
Protection of the atmosphere
Sources of ground-level
ambient ionising radiation
What is
cosmic radiation ?
|
|
 |
 |
|
The origins of radiation |
||
At the end of the 18th century, Frenchman Charles de Coulomb noticed that a charged sphere, though suspended
by an insulating wire, would gradually discharge. One hundred years later, in 1900, British physicist Charles
Wilson, studying ionisation currents in the atmosphere using a gold-leaf electroscope, attempted to explain his
results through the existence of radiation coming from sources outside the atmosphere. After several years of
measurements at sea and travelling in balloons, he observed a net increase in ionisation with altitude: the
Earth was indeed receiving ionising radiation from the
heavens.
This cosmic radiation consists of particles travelling near the speed of light. It consists of two components,
the first of which is permanent and of galactic origin, while the other is more sporadic, depending on the sun's
activities.
A galactic origin
The permanent component of cosmic radiation comes from the galaxy. It consists of very highly charged particles ejected by the gigantic explosions of supernova, massive stars which have reached the end of their days. These particles are atoms which have been stripped of their electrons because of the temperatures within these giant stars. They are of different types, primarily hydrogen nuclei (protons) and helium nuclei (alpha particles), but also heavier nuclei such as iron and nickel. They travel at close to the speed of light.
Composition of the permanent cosmic radiation
| Particles | Rate |
|---|---|
| hydrogen nuclei (protons) | 85% |
| helium nuclei (alpha particles) | 12.5% |
| nuclei of heavier atoms | 1% |
| electrons | 1.5% |
Le rayonnement cosmique galactique est isotrope, c’est-à-dire qu’il est le même dans toutes les directions. Par conséquent, toute la surface de la Terre y est exposée en permanence.
Galactic cosmic radiation is isotropic, which means that it is the same in all directions. Consequently,
the Earth's entire surface is constantly exposed.
Part of the galactic radiation is deviated by the magnetic field carried along by solar wind.
The sun's atmosphere constantly releases a flow of particles which fills the inter-planetary space,
and which we refer to as solar wind. The characteristics, particularly the magnetic ones, of the solar
wind vary with the sun's activity and create the field which redirects cosmic radiation from the Earth.
As such, the galactic cosmetic radiation reaching the Earth is reduced during periods of high solar activity.
As we know that solar activity follows an 11-year cycle, it is possible to predict the exposure to galactic
radiation over several years.
Solar activity cycle compared with the intensity of cosmic radiation
A solar origin
The sun is the origin of the unpredictable component of cosmic radiation. It constantly ejects particles
with an intensity which varies according to an 11-year cycle; these particles are added to the galactic radiation.
However, these particles have less energy than those coming from the galaxy, and only a small number of them
reach the Earth.
Indeed, the sun's main contribution is still the unpredictable solar flares. Large solar flares release
particles for several hours, and these are more highly charged than during periods of normal activity;
therefore they reach the Earth in greater quantity. However, solar flares strong enough to eject a flow of
particles which can be detected on the ground or on a commercial aircraft are quite exceptional. There have
been only a few per year, at the most.
These eruptions take place during periods of maximum solar activity, as in 2000-2001. The sun's magnetic field
is particularly disturbed, which results in the appearance of many sunspots on its surface. The largest solar
flares originate in the most complex groups of sunspots. More frequent during peaks in the solar cycle, solar
flares can nevertheless occur at any time.
Unlike the stable radiation of galactic origin, the particles resulting from solar flares are not distributed evenly
across the surface of the Earth. In order to determine their effects, it is necessary to measure their intensity
and to make scientific calculations on a case by case basis.
Characteristics of extra-terrestrial radiation
| Cosmic rays | Solar particles |
|---|---|
| permanent | sporadic |
| highly charged particles | medium-energy particles |
| isotropic | < anisotropic |
Radiation in the earth's environment
Before reaching the ground, or the cruising altitude of aircraft, cosmic radiation is partially
stopped by the two "barriers" which protect the Earth: the planet's magnetic field
(which creates a region isolated from solar wind, called the "magnetosphere")
and the earth's atmosphere. It is this twofold protection which has allowed the development of life.
The particles which finally do reach the earth contribute to a minor degree of ambient
ionising radiation. However, their
relative share increases rapidly with altitude (mountains, planes, and spaceships).
Protection of the magnetosphere
Just like all electrically charged particles, the ions which are included in the cosmic radiation
are directed or deviated by magnetic fields, as in the hands of a compass. However, the Earth acts
as a huge magnet surrounded by a magnetic field, with lines of force which "enter"
by the North Pole, and eventually "exit" by the South Pole: this is what is known
as the magnetosphere.
If the cosmic particles possess energy which is greater than a certain threshold,
i.e. the magnetic cutoff energy,
they will cross through the magnetosphere and reach the upper layers of the atmosphere.
But if their energy is insufficient, they will have a tendency to follow the magnetic lines of force,
with which they more "easily", due to their lack of energy, succeed in reaching the poles.
It is the reason why the areas located near the poles receive
radiation in higher quantities than near the equator, which is better protected by the earth's magnetic field.
Protection of the atmosphere
Upon arriving in the upper layers of the earth's atmosphere, the ions interact
with the atoms which they encounter. From these collisions, new cascades of particles
are created. It is this secondary radiation, consisting of charged particles and neutrons,
which succeeds in reaching the ground, at least when the primary particle has sufficient energy.
Ground-level ambient ionising radiation
At ground-level, cosmic radiation only represents a small part (11%) of the
ionising radiation to which an individual
is commonly exposed.
Natural land-based sources expose each of us to an average total dose of 2.4 mSv per year
(source UNSCEAR), though with significant variations according to regions. The larger part of
the sources is a gaseous descendant of natural uranium, i.e. radon, which concentrates in enclosed
areas such as houses. There is also soil-based radiation, coming from surface rocks, granite in
particular, which contain radioactive elements such as uranium, dating from the formation of the
planet. The water and foods which we ingest also contain radioactive elements. Finally, there is
also the internal radiation, i.e. coming from within our own bodies, namely from the potassium 40
which is naturally present in our tissues.
In addition, the human activities using ionising
radiation contribute to an average annual exposure of 1.4 mSv, originating primarily with medical
activities (radio-diagnostic and radiation therapy). The "average" medical
exposure is obviously lacking in any individual significance:
it depends on the tests and treatments undergone by each person.
Sources of radioactivity as a percentage of the dose received
