Wednesday, February 6, 2008



What is Radiation?

Atoms, the building blocks of the universe, are held together by natural forces. Tremendous amounts of energy are stored in the center of the atom, the nucleus. Although most nuclei are stable, some are unstable. Unstable nuclei, made up of combinations of protons and neutrons that cannot maintain balance, spontaneously change to reach a more stable form through a process known as radioactive decay. During the decay process (that is, the atom changing to reach a more stable form), some of the atom’s natural energy is released. The released energy, known as radiation, can be in the form of particles or waves.
The unstable atoms that undergo radioactive decay are known as radioisotopes or radionuclides. Some unstable isotopes of radium, radon, uranium, and thorium, exist naturally in the Earth. Other radionuclides are continually being made naturally (by cosmic rays) or by human activities, such as the splitting of atoms in a nuclear reactor.

Terrestrial radiations:
Terrestrial sources contribute about 84 per cent of the total radiation from natural sources to which an individual is exposed. This dose is about 2 mSv per year and includes overall (indoor as well as outdoor) radiation dose. It is an accumulated effect arising from a number of materials around such as rocks, soil, water, air, food and even from within the human body. The sources of terrestrial radiation are certain elements which are radioactive and some radionuclides created at the time of formation of the earth. These radionuclides, while giving out radiation, decay in the form of different chains to result in products known as ‘daughter elements’. At least, there are four well-known radioactive series or chains of elements, one starting with uranium-238, a second with thorium-232, a third with uranium-235 and fourth with plutonium-241.
The earth’s crust contains a very large number of radioactive substances in different geographical regions of the world. A survey of twenty three countries having more than half of the world’s population has shown that exposure to such external radiation is nearly the same. These studies suggested that 95 per cent of the world’s population gets an average annual dose of 0.4 mSv due to outdoor terrestrial radiation. Such areas are in Brazil, Sweden, France, and India. Guarapari, Meaipe, and Pocos de Caldas in Brazil, and coastal areas of Kerala and Tamil Nadu in India have thorium-rich monazite sands which provide much higher doses than the average mentioned above. Similarly, there are active regions rich in granite, in France, with a high Uranium and Thorium content and alum shale, in Sweden, with high uranium and radium concentrations which also provide larger doses of radiation.
Uranium may be found in:
· zircon
· urananite
· monazite
· apatite
· sphene
Thorium in the environment
Thorium is surprisingly abundant in the Earth's crust, being almost as abundant as lead and three times more abundant than uranium. It is found in small amounts in most rocks and soils. Granite contains up to 80 ppm of thorium. Because thorium oxide is highly insoluble, very little of this element circulates through the environment. Thorium occurs naturally as the minerals thorite, uranothorite, thorianite, it is a major component of monazite and it is present in significant amounts in the minerals zircon, titanite, gadolinite and betafite
What is radium?
Radium is a naturally occurring silvery-white radioactive metal that can exist in several forms called isotopes. Radium is formed when uranium and thorium break down in the environment. Uranium and thorium are found in small amounts in most rocks and soil. Two of the main radium isotopes found in the environment are radium-226 and radium-228.
What happens to radium when it enters the environment?
· Radium is constantly being produced by the radioactive decay of uranium and thorium.
· Radium is present at very low levels in rocks and soil and may strongly attach to those materials.
· Radium may also be found in air.
· High concentrations are found in water in some areas of the country.
· Uranium mining results in higher levels of radium in water near uranium mines.
· Radium in the soil may be absorbed by plants.
· It may concentrate in fish and other aquatic organisms.

It is worthwhile to mention here that, in general, igneous rocks such as granites are more radioactive than the sedimentary rocks. However, highly radioactive shales and phosphates rocks are the exceptions in the category of sedimentary rocks.
We are affected by terrestrial radiation even inside our house. The single most important and all-pervasive source of internal terrestrial radiation in some countries is the odourless gas, radon. It enters buildings mostly from the ground by the process of soil convection or soil diffusion through the underlying or surrounding soils. If the ventilation is inadequate, its concentration in the house may be hundred or even thousand times higher than outside.
Radon is a terrestrial source of radiation that is of particular concern because, although on average it is very rare, this intensely radioactive element can be found in high concentrations in many areas of the world, where it represents a significant health hazard. Radon is a decay product of uranium, which is relatively common in the earth's crust, but generally concentrated in ore-bearing rocks scattered around the worldThe widespread construction of well insulated and sealed homes in the northern industrialized world has led to radon becoming the primary source of background radiation in some localities in northern North America and Europe.
Building materials, such as stone and concrete release higher doses of radiation because of higher concentrations of uranium, thorium and their daughter nuclides, mainly radon gas. In addition to this, large groups of population also get high doses of radiation through drinking water, containing high concentrations of the elements, radium-226 and radon-222. Some of the springs in Austria, Iran, Italy and India as well and several wells in the USA (particularly in Texas and Argonne areas) have a high radioactive content.
Among the internal sources of radiation are potassium-40, lead-210 and polonium-210 which enter our body through our food. Besides these, very minute quantities of uranium, radium, carbon-14 and tritium are already present in our body. While the quantity of potassium is controlled and remains the same in the body, the levels of lead-210 and polonium-210 are affected through the dietary intake. For example, the concentration of these nuclides in sea-food is five times higher in Japan than that in the USA. Similarly, in the northern hemisphere, people who depend for their food mainly on reindeer or caribou meat, show a significantly higher intake of these radionuclides. This is because these animals graze on lichens which are concentrated with lead and polonium. These two radionuclides are also found in tobacco. Among other foods, the radium content in nuts and cereals is higher than that in milk, or meat.
Potassium-40 is a naturally occurring nucleid. It was formed together with the other elements during the creation of the earth. Due to its long half life of 1.28 billion years it is still present on earth. K-40 is the only radioactive isotope of potassium and is present in an amount of 0.0119% in this natural element. Potassium as well as K-40 are present in most terrestrial and biological substances, for ex. it is a macronutrient for plants. The body of a 70 kg person contains ca. 140 g of potassium and thus an activity of 4000 Bq K-40. Due to its presence in almost all foods this nucleid accounts for the greatest proportion of the naturally occurring radiation load through ingestion among people.
Potassium 40 is found in:
· potassium feldspar (orthoclase)
· muscovite
· amphibole
· glauconite (greensand; found in some sedimentary rocks; rare)
What Is the Primary Health Effect?
Potassium-40 can present both an external and an internal health hazard. While in the body, potassium-40 poses a health hazard from both the beta particles and gamma rays. Potassium-40 behaves the same as ordinary potassium, both in the environment and within the human body – it is an essential element for both. Hence, what is taken in is readily absorbed into the bloodstream and distributed throughout the body, with homeostatic controls regulating how much is retained or cleared. The health hazard of potassium-40 is associated with cell damage caused by the ionizing radiation that results from radioactive decay, with the general potential for subsequent cancer induction.
Thus it may be concluded that the radioactive content entering into our body depends upon the type of food we take and the environment in which it is grown.
Jain, H.C. 1994. Radiation and Man. National Book Trust, India.
Wagner, T. 1994. In our backyard. John Wiley & Sons,Inc. New York.

Dr.Nitish Priyadarshi
Post a Comment