Radiation is all around us. In, around, and above the world we live in. It is a natural energy force that surrounds us. It is a part of our natural world that has been here since the birth of our planet. All living creatures, from the beginning of time, have been, and are still being, exposed to ionizing radiation. Ionizing radiation is generated through nuclear reactions, nuclear decay, by very high temperature, or via acceleration of charged particles in electromagnetic fields.
Radiation on Earth and in Space
All living creatures, from the beginning of time, have been, and are still being, exposed to ionizing radiation. This radiation is not associated with any human activity. There are radioactive isotopes in our bodies, houses, air, water and in the soil. We all are also exposed to radiation from outer space.
Radiation on Earth – Natural Background Radiation
We divide all these natural radiation sources into three groups:
- Cosmic Radiation. Cosmic radiation refers to sources of radiation in the form of cosmic rays that come from the sun or from outer space. At ground level the muons, with energies mostly between 1 and 20 GeV, contribute about 75 % of the absorbed dose rate in free air. The remainder comes from electrons produced by the muons or present in the electromagnetic cascade. The annual cosmic ray dose at sea level is around 0.27 mSv (27 mrem). If you live at higher elevations or are a frequent airline passenger, this exposure can be significantly higher, since the atmosphere is thinner here. The effects of the earth’s magnetic field also determines the dose from cosmic radiation.
- Terrestrial Radiation. Terrestrial radiation refers to sources of radiation that are in the soil, water, and vegetation. The major isotopes of concern for terrestrial radiation are uranium and the decay products of uranium, such as thorium, radium, and radon. The average dose rate that originates from terrestrial nuclides (except radon exposure) is about 0.057 µGy/hr. The maximum values have been measured on monazite sand in Guarapari, Brazil (up to 50 µGy/hr and in Kerala, India (about 2 µGy/hr), and on rocks with a high radium concentration in Ramsar, Iran (from 1 to 10 µGy/hr). The average annual radiation dose to a person from radon is about 2 mSv/year and it may vary over many orders of magnitude from place to place. Radon is so important, that it is usually treated separately.
- Internal Radiation. In addition to the cosmic and terrestrial sources, all people also have radioactive potassium-40, carbon-14, lead-210, and other isotopes inside their bodies from birth. The concentration of potassium-40 is nearly stable in all persons at a level of about 55 Bq/kg (3850 Bq in total), which corresponds to the annual effective dose of 0.2 mSv. The annual dose from carbon-14 is estimated to be about 12 μSv/year.
Radiation in Space
Cosmic radiation refers to sources of radiation in the form of cosmic rays that come from the Sun or from outer space. The earth has always been bombarded by high-energy particles originating in outer space that generate secondary particle showers in the lower atmosphere. Charged particles (especially high-energy protons) from the sun and outer space interact with the earth’s atmosphere and magnetic field to produce a showerof radiation (i.e. air shower), typically beta and gamma radiation. If you live at higher elevations or are a frequent airline passenger, this exposure can be significantly higher, since the atmosphere is thinner here. The effects of the earth’s magnetic field also determines the dose from cosmic radiation.
Composition of Cosmic Radiation
The primary cosmic radiation consist of a mixture of high-energy protons (~87%), alpha particles (~11%), high-energy electrons (~1%) and a trace of heavier nuclei (~1%). The energy of these particles range between 108 eV and 1020 eV. A very small fraction are stable particles of antimatter, such as positrons or antiprotons. The precise nature of this remaining fraction is an area of active research.
Energy of Cosmic Rays
The energies of the most energetic ultra-high-energy cosmic rays (UHECRs) have been observed to approach 3 x 1020 eV, about 40 million times the energy of particles accelerated by the Large Hadron Collider. The origin of the high energy particles is from outer space. It is assumed that particles with an energy up to about 1015 eV are coming from our own galaxy, whereas those with the highest energies probably have an extragalactic origin.
Classification of Cosmic Radiation
Cosmic radiation can be divided into different types according to its origin. There are three main sources of such radiation:
- Solar Cosmic Radiation. Solar cosmic radiation refers to sources of radiation in the form of high-energy particles (predominantly protons) emitted by the sun, primarily in solar particle events (SPEs). Solar radiation is a significant radiation hazard to spacecraft and astronauts, also produces significant dose rates at high altitudes, but only the most energetic radiation contribute to doses at ground level. Note that, anyone who had been on the Moon’s surface during a particularly violent solar eruption in 2005 would have received a lethal dose.
- Galactic Cosmic Radiation. Galactic cosmic radiation, GCR, refers to sources of radiation in the form of high-energy particles originating outside the solar system, but generally from within our Milky Way galaxy. The nucleonic component then consist of a mixture of high-energy protons (~86%), alpha particles (~12%), and a trace of heavier nuclei (~1%). GCR are trapped by the galactic magnetic field, therefore they have probably been accelerated within the last few million years, and have traveled many times across the galaxy. Their acceleration mechanism is uncertain, but one of possible mechanisms is that the particle are accelerated by shock waves expanding from supernovas. The energy of these particles range between 108 eV and 1020 eV.
Radiation from Earth’s Radiation Belts (van Allen belts). Van Allen radiation belts are zones of high-energy particles (especially protons) trapped by earth’s magnetic field. Most of these high-energy particles originate from the solar wind, that were captured by and held around a planet by that earth’s magnetic field. The van Allen belt is formed like a torus above the equator. There are two van Allen radiation belts, an internal belt is centered at about 3,000 kilometers and an outer belt is centered at about 22,000 kilometers from the earth’s surface. It contains mainly energetic protons in the 10-100 MeV range. Spacecraft travelling beyond low Earth orbit enter the zone of radiation of the Van Allen belts. Beyond the belts, they face additional hazards from cosmic rays and solar particle events. A region between the inner and outer Van Allen belts lies at two to four Earth radii and is sometimes referred to as the “safe zone”.