Natural Radiation

Natural background radiation comes from three sources:

• Cosmic Radiation 

• Terrestrial Radiation 

• Internal Radiation

Cosmic Radiation:

• Cosmic radiation (C.R) comes from extremely energetic particles from the sun and stars that enter Earth’s atmosphere. 

• They are the nuclei of atoms, ranging from the lightest to the heaviest elements in the periodic table. C.R include high energy electrons, positrons, and other subatomic particles. 

• Some particles make it to the ground, while others interact with the atmosphere to create different types of radiation. 

• Radiation levels increase as you get closer to the source, so the amount of cosmic radiation generally increases with elevation. The higher the altitude, the higher the dose. That is why those living at high altitudes receive a higher annual radiation dose from C.R than someone living at sea level (altitude of 0 feet).

Cosmic rays come from various places, including the Sun, supernova explosions [a large explosion that takes place at the end of a heavy star’s life cycle (greater than mass of Sun ~15 times)],and extremely distant sources such as galaxies and quasars (Quasars: a massive and extremely remote celestial (outer space) object, emitting exceptionally large amounts of energy, and typically having a star-like image. It has been suggested that quasars contain massive black holes.) 

• Because of their high energy, they can be dangerous to people and to machines. On Earth we are mostly shielded from them by our planet's magnetic field and atmosphere. 

Types of cosmic rays:

1. Solar cosmic rays

 Solar cosmic rays have energies of ~107 to 1010 eV and come from the Sun. They have less energy than most other types of cosmic rays. Solar flares* and other explosions on the Sun make this type of cosmic ray.

 (*it occurs when magnetic energy that has built up in the solar atmosphere is suddenly released. Radiation is emitted across virtually the entire electromagnetic spectrum).

2. Galactic cosmic rays (related to galaxy)

 Galactic cosmic rays are charged particles with energies between ~1010 and 1015 eV. They have more energy than solar cosmic rays and come from supernova explosions and neutron stars** within our own Milky Way galaxy. (**a star four to eight times as massive as the Sun explode in a violent supernova, their outer layers can blow off leaving behind a small, dense core that continues to collapse due to gravity, this gravity bring protons and electrons closer i.e. combined and make neutrons yielding name as neutron star)

3. Extra galactic cosmic rays 

                    OR 

    Ultra High Energy Cosmic rays

     Ultra-high-energy cosmic ray (UHECR) have energy greater than 1018 eV, this energy is more than the galactic cosmic rays. They may come from quasars*** or from the nucleus of an active galaxy or may be they are made when galaxies crash into each other. Nobody knows for sure. Cosmic rays have been observed with energies from 109 eV to over 1020 eV. Over this range, the "flux" of cosmic rays (the number of arriving particles per unit area, per solid angle, per unit time) appears to follow a single power law ~E-3 Particles with intermediate energy levels, around what is called the "Knee" of the spectrum, are called Very High Energy cosmic rays and they occur with a frequency of one per square meter per year. For the highest energy cosmic rays, above 1016 eV or what is called the "Ankle," the rate of events falls to one per square kilometer per century.

Detection of Cosmic rays 

There are several ground-based methods of detecting cosmic rays currently in use. 

1. 1. Air Cherenkov telescope: It is designed to detect low-energy (<200 GeV) cosmic rays by means of analyzing their *Cherenkov radiation, which for cosmic rays are high energy photons emitted as they travel faster than the speed of light in their medium. 
2. 2. Another Cherenkov telescope uses water as a medium through which particles pass and produce Cherenkov radiation to make them detectable. 
3. 3. Another method was developed by Robert Fleischer, P. Buford Price, and Robert M. Walker for use in high-altitude balloons. In this method, sheets of clear plastic, like 0.25 mm Lexan poly-carbonate, are stacked together and exposed directly to cosmic rays in space or high altitude. The nuclear charge causes chemical bond breaking or ionization in the plastic. 
4. 4. A fourth method involves the use of cloud chambers (particle detector used for visualizing the passage of ionizing radiation) to detect the secondary muons (similar to electron -> same electric charge but with a much greater mass) created when a pion decays. Cloud chambers in particular can be built from widely available materials and can be constructed even in a high-school laboratory

Terrestrial Radiation (related to earth):

 Radioactive material found in:

 • Soil 

• Water 

• Vegetation

     Radioactive material is also found throughout nature. It is in the soil, water, and vegetation. Low levels of Uranium, Thorium, and their decay products are found everywhere. Some of these materials are ingested with food and water, while others, such as Radon, are inhaled. The dose from terrestrial sources also varies in different parts of the world.

The major isotopes of concern for terrestrial radiation are Uranium and the decay products of Uranium, such as Thorium, Radium, and Radon.

According to the IAEA (International Atomic Energy Agency), soil typically contains four natural radioisotopes: 40K, 226Ra, 238U, and 232Th.

 In one kilogram of soil, the potassium-40 amounts to an average 370 Bq of radiation, with a typical range of 100–700 Bq; the others each contribute some 25 Bq, with typical ranges of 10–50 Bq (7– 50 Bq for the 232Th).

Internal Radiation: 

• Potassium-40 

• Carbon-14 

• Lead-210 

    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 variation in dose from one person to another is not as great as the variation in dose from cosmic and terrestrial sources. The average annual dose to a person from internal radioactive material is about 40 millirems/year.