If a Geiger counter is deliberately held in such a way that it does not point in the direction of a radiation source – for example, somewhere in the air – then it continues to tick anyway. Where does the radiation come from that the counter registers there?? It is mainly fed from three sources: From cosmic radiation , from natural radionuclides in the air (especially the decay products of the radioactive noble gas radon ), and finally from the natural radioactive substances in soil and rock.
In addition, there is the influence of man
In addition to this natural radiation, there are artificial sources of radiation, which expose practically everyone to a certain amount of radiation. This includes, for example, X-rays at the doctor’s office. This module first shows us what everything around us radiates – from stars to stones. Then comes a section on how much this radiation and that from man-made sources total, and whether you can protect yourself from it.
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The cosmic radiation
Cosmic radiation was discovered as early as 1912. Today we know that it comes mainly from supernova explosions and X-ray bursts around black holes, quasars, and neutron stars. And it is very penetrating: it has even been detected in 4.000 meters of ocean depth detected. The particles have energies up to 10 to the power of 20 eV, which is over a thousand times more than can be produced in terrestrial particle accelerators.
The primary radiation from space is composed mainly of protons (84 percent) and helium nuclei (12 percent); in addition, there is a small fraction of heavier nuclei. Most of these particles react with the gas atoms in the high atmosphere, producing new types of radiation. Neutrons, electrons, muons, pions. The cosmic radiation , which you measure on sea level, consists therefore practically completely of this secondary radiation.
At the beginning of the last century, the unexpected phenomenon of the conductivity of air was intensively studied. Soon it became clear that the radioactivity of uranium and other rocks, discovered shortly before by Henri Becquerel, had to be one of the causes. "Then this effect must be less on the top of the Eiffel Tower" thought Theodor Wulf, climbed up in 1910 with his measuring apparatus, but found no decrease of the effect. His bold thesis: extraterrestrial radiation .
The Austrian Viktor Hess wanted to know this more exactly. In several balloon ascents he investigated this cosmic radiation . The ascent at 17. April 1912 took place during a solar eclipse. But he found no decrease in the intensity of radiation in the process. So the sun could probably not be the cause, he assumed. The balloon flight at 7. August 1912 up to heights of 5000 m brought then finally the proof for the radiation from the cosmos.
It is true that the sun is also involved in cosmic rays. But if you think of solar radiation only in terms of visible or UV light, you’re wrong: Most of the energy leaves the sun in the form of gamma radiation and huge "particle showers" that keep erupting from the sun’s interior. Every eleven years, these eruptions are particularly strong. These are enormous amounts of protons, but of relatively low energy . A small part of it also hits the earth – more precisely: the magnetosphere of the earth. And it keeps these protons away from the earth, deflects them quasi like a mirror around the earth.
The SOHO satellite ("Solar and Heliospheric Observatory") of NASA and ESA has been continuously photographing the sun at several wavelengths for more than ten years. For a short trip to the SOHO website with some impressive pictures of solar flares just click on the links below. The images from the "LASCO"-cameras deserve special attention: They show the proton storms.
The magnetic space suit of the earth
"Have you ever heard of northern or aurora borealis or even observed one yourself during a vacation in Scandinavia??". A mysterious, flickering glow in the atmosphere, with large parts of the sky glowing fuzzily in all sorts of colors.
This glow is the visible evidence on earth for the part of the cosmic radiation coming from the sun: It is caused by protons of the solar wind, which fall into the atmosphere and excite air particles there. When the gas atoms return to their ground state, they emit light in the visible range – nitrogen rather blue-violet, oxygen green-red.
The particles from space not only have enough energy to make gas molecules glow, they can even break them up and create particles themselves.
This is how new isotopes are created, such as the radioactive tritium or the equally radioactive C-14, but also electrons, muons, mesons, neutrons and gamma quanta. Some of these "secondary" particles collide with air molecules on their way to the ground. Thus, for example, electrons, neutrons and gamma radiation are effectively shielded by the air envelope. Example neutrons: At 15 km altitude they still account for about half of the radiation dose, at ground level their contribution is vanishingly small.
But the bulk of the secondary radiation on the ground – about 90 percent – consists of muons, a very short-lived negatively or positively charged elementary particle with about 200 times the electron mass.
Radiation when flying
The higher you get, the bigger our radiation dose becomes due to cosmic rays . In 1.000 meters altitude it is 0.4 millisievert (mSv)/a, in 3.000 meters at 1.1 mSv/a.
In airplanes, which usually fly in a cruising altitude between 7 and 12 kilometers, it depends very much on the flight route how much radiation you get; if you fly near the equator, the radiation is much lower than near the poles because of the earth’s magnetic field. The table gives some examples of radiation exposure during flights (single flight).