Sunday, December 27, 2009

Biological effect of Radiation

Biological effect of Radiation

When radiation passes through matter, it deposits energy in the material concerned. Alpha and beta particles, being electrically charged, deposit energy through electrical interactions with electrons in the material. Gamma rays and X rays lose energy in a variety of ways, but each involves liberating atomic (orbiting) electrons, which then deposit energy in interactions with other electrons.

Neutrons also lose energy in various ways, the most important being through collisions with nuclei that contain protons. The protons are then set in motion and, being charged, they again deposit energy through electrical interactions. So in all cases, the radiation ultimately produces electrical interactions in the material.

In some cases, an electron in the material may receive enough energy to escape from an atom leaving the atom or molecule thus formed positively charged.

The process by which a neutral atom or molecule becomes charged is called ionization and the resulting entity an ion. Once removed from an atom, an electron may in turn ionize other atoms or molecules. Any radiation that causes ionization - either directly, as with alpha and beta particles or indirectly as with gamma rays, X rays, and neutrons - is known as ionizing radiation. Charged particles passing through atoms may also give energy to the atomic electrons without actually removing them; this process is called excitation.

Each time a charged particle ionizes or excites an atom, it loses energy until it no longer has enough energy to interact; the final result of these energy losses is a minute rise in the temperature of the material of which the atom is a part . In this way, all the energy deposited in biological tissues by ionizing radiation is eventually dissipated as heat through increased vibrations of the atomic and molecular structures.

The basic unit of biological tissue is the cell, which has a control center called the nucleus. The nucleus of a cell is an intricate structure and not to be confused with the nucleus of an atom. About 80 per cent of the cell consists of water, the other 20 per cent being complex biological compounds. When ionizing radiation passes though cellular tissue, it produces charged water molecules. These break up into entities called free radicals, such as the free hydroxyl radical (OH), which is composed of an oxygen atom and a hydrogen atom. Free radicals are highly reactive chemically and can alter important molecules in the cell.

One Particularly important molecule is deoxyribonucleic acid, DNA , found mainly in the nucleus of the cell. DNA controls the structure and function of the cell and passes on copies of itself: its molecules are large and the structures that carry them, chromosomes, are visible through the microscope. We still do not fully understand all the ways in which radiation damages cells, but many involve changes to the DNA .

There are two ways in which this can happen. Radiation may ionize a DNA molecule lead­ing directly to a chemical change, or the DNA may be changed indi­rectly when it interacts with a free hydroxyl radical produced in the water of the cell by the radiation. In either case, the chemical change cause a harmful biological effect leading to the development of cancers or inherited genetic defects.

A most important property of the various types of ionizing radiation is their ability to penetrate matter. The depth of penetration for a particular type of radiation increases with its energy, but varies from one type of radiation to another for the same amount of energy. With charged particles such as alpha and beta particles, the depth of penetration also depends on the mass of the particle and its charge. For equal energies, a beta particle will penetrate to a much greater depth than an alpha particle. Alpha particles can scarcely penetrate the dead, outer layer of human skin; consequently, radionuclides that emit them are not hazardous unless they are taken into the body through breathing or eating or through a skin wound. Beta particles penetrate about a centimetre of tissue, so radionuclides that emit them' are hazardous to superficial tissues, but not to internal organs unless they too are taken into the body. For indi­rectly ionizing radiation, such as gamma rays and neutrons, the degree of penetration depends on the nature of their interactions with tissue. Gamma rays can pass through the body, so radionuclides that emit them may be hazardous whether on the outside or the inside. X rays and neutrons can also pass through the body.

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