Radioactive Substances

Radioactive Substances |Units of Radioactivity & Its Effect

What are radioactive substances?

Radioactive substance produces radiation and is harmful to the environment. There are two important areas of radiation ecology:

1. effects of radiation on organisms in the biosphere, and
2. the fate of radioactive substances released into the environment.

Radionuclides become dispersed or diluted when they are released in the environment but they may also become concentrated in living organisms during food chain transfers. This is called biomagnification. The ratio of a radionuclide in the organism to that in the environment is called concentration factor.

Radiation permeates the universe, the solar system, and the earth. The surface of the planet would be bombarded with radiation from the sun so intense that life may be in danger if there were no atmosphere to surround the earth. The atmosphere absorbs most of the solar radiation, including the one that would be lethal to life. Some of the incoming radiation is trapped by earth’s magnetic field. The far-reaching portion of the atmosphere-magnetosphere, contains an area of high-energy radiation called the Van Allen region. This region extends from about 500 miles above the earth at the equator to an altitude of about 40,000 miles. The sun pumps a steady stream of visible light, ultraviolet, infrared, gamma rays and other forms of radiation into space. Some of the radiation that strikes the earth’s atmosphere penetrates to the biosphere. Cosmic rays, comes from outer space at high velocity, strike the earth continuously and penetrate deeply into the surface.

Naturally occurring radioactive elements are present in the rocks, water and air and in all living organisms. The composite of all forms of natural radiation that we are exposed to is called background radiation. However besides natural, there are man-made radiations as well.

Radionuclides

The smallest unit of an element is called the atom. Each atom has proton (positively charged), neutron (uncharged) and electron (negatively charged). Atom of each element has characteristic numbers of protons, neutrons and electrons. Most elements, however, in nature contain atoms that are not exactly like the predominate form. These atoms have different number of neutrons. These different forms of the same element having different mass number are called isotopes. Some isotopes of common elements are stable under ordinary conditions. Others have various degrees of instability, and some of them disintegrate with the emission of radiations of one kind or another. Radioactive isotopes are isotopes that give off ionising radiation. Since the radiations are highly energetic (as X-rays) and these tend to split substances, including living matter or ions, they are called ionising radiation. The term isotope has been used loosely and the appropriate general term for a particular kind of atom is nuclide. The term used for a radioactive nuclide is radionuclide.

Half-life

The half-life of a radionuclide refers to its period of radioactivity and has a bearing on the length of time that a radioactive substance might remain active as environmental pollutant.

Units of Radioactivity

Curie

It was named after Marie Curie. Each radionuclide decays with the emission of radiation at a fixed and measurable rate. Curie is the unit used to express the rate of nuclear disintegration or intensity of radioactivity of a particular sample of material. One curie is equal to 37 billion nuclear disintegration per second. The term curie is also used to express the quantity of a nuclide having one curie of radioactivity. For example, 1gm of radium has a radioactivity of one curie.

Roentgen

It was named after Wilhelm Roentgen. It is the unit for radiation by X-rays and gamma rays. One roentgen is the amount of radiation that ionizes one atom in about 10 billion atoms of air or the energy that will produce 2,083 billion ion pairs in 1 cm³ of air under standard conditions.

Rad

Rad is as unit of radiation used for doses of alpha and beta particles, other fast neutrons and other kind of radiations. It is the radiation dose. It is the amount of radiation that gives energy per gram of substance (tissue). With X-rays, roentgen is equal to an energy absorption of 84 ergs per gram. Thus, practically, the rad is nearly equivalent of the roentgen.

Rem

Rem is the unit of radiation that gives an indication of biological damage. It is an estimate of the amount of radiation of any type which produces the same biological injury in man as that resulting from the absorption of a given amount of X-Ray radiation or gamma radiation    However, International System of Units has changed the radiation units, roentgen, rad and rem by new SI units of Coulomb Kg^-1,Gray (Gy) Kg^-1 and Joule Kg^-1 respectively.

Kinds of Radiation

Two major types of radiation or ionizing radiation are released by nuclear disintegration or man-made devices: electromagnetic radiations and particulate radiations.

1. Electromagnetic radiations

Physical properties of electromagnetic radiation are similar to that of light. These include a broad-spectrum of energy. These include ultraviolet rays, X-rays, gamma rays, infra-red rays, radio waves, visible light rays. The near UV extends from visible violet, about 390 nm to about 170 nm, whereas the far UV roughly 100 nm. X-rays include a broad spectrum of wave lengths, from about 100 nm to less than 0.00001 nm. An average X-ray has a wavelength of about 0.1 nm. Gamma rays are similar to X-rays. Their wavelength overlap but these rays are usually of shorter wavelength and they generally have high energy and has more penetrating power than X-rays.

2. Particulate radiations

They consist of the particles ejected from atoms at high speed and often with tremendous energy. Particulate radiations have electron, proton or neutron.

Whether the radiation from nuclear disintegration is electromagnetic or particulate, the emission is so energetic and powerful that they can cause huge damage to living tissues. This type of radiations includes:

Beta particles- These are high-speed electrons, emitted by many radionuclides. They ionize any substance with which they collide in their path. Examples: C¹⁴, P³², H³ etc.

Alpha particles- These are fast-moving particles, containing 2 protons and 2 neutrons. They are positively charged and do not have electrons. These are less penetrating than X-rays, gamma rays and beta-particles. They can be stopped by a sheet of paper.

Proton particles- They have similar effect to that of alpha particle.

Cosmic rays- They are energetic particles from the sun and outer space. They strike the earth at high velocity, some penetrate several thousand feet into solid rock. Many of the cosmic ray particles are charged atomic nuclei called primaries. Additional types of cosmic ray are called secondaries. The cosmic radiation is about 40 milli-rem per year, at sea level. As per rough estimate, this value increases at the rate of 1 with increase in altitude of every 100 feet.

With the result of survey, it was found that people living at higher elevations are likely to be exposed to greater cosmic radiations. Their intensity in the biosphere is very low, yet they are a great hazard in space travel. At about 20 km cosmic radiation becomes very intense. A commercial pilot receives about 300 m rad/year of cosmic radiations.

Sources of Radiation Exposure

Man is exposed to different sources of radiation. These are either natural, or man-made.

Different forms of these sources are as follows:

1. Natural sources

These include: (i) cosmic rays (ii) environment (rocks, water, air), (iii) living organisms. Radionuclides of radium, thorium, uranium and isotopes of potassium (K^-40) and carbon (C^-14) are very common in soil, rock, air and water. Marine sediments generally have higher concentrations of radionuclides. On an average, man receives about 50 m rad/year from terrestrial radiations and it may be as high as 2,000 m rad/year in areas where uranium-containing rocks exist, as in Kerala.

Radiations from atmosphere are also common. For instance, radioactive gases like Thoron and radon are present in air though with low values of roughly 2 m rad/year.

Man is also exposed to internal radiations from radioactive substances in the body tissues. For instance, uranium, thorium and isotopes of potassium, strontium and carbon exist in small amounts in the body. Internal radiation values vary from 25 to 75 m rad/year.

2. Man-made sources

These include X-ray machines (diagnostic and radio therapeutic),  radioactive fallout (nuclear tests), nuclear reactor wastes, industrial, medical and research uses of radioactive materials, and some other sources like television.

Biological Effects of Radiations

Radioactive substances are among the most toxic substances known. Radium is 25,000 times more lethal than Arsenic. Biological importance of radiation became known in 1895 when Wilhelm Roentgen placed his hand between X-ray tube and fluorescent screen. The bones cast a deeper shadow than the flesh. The following year, the French physicist, Antonie Becquerel discovered natural radioactivity in a Uranium compound-potassium Uranyl Sulphate, a discovery for which he was awarded with the Nobel Prize with the Curies in 1903. The most tragic early evidence of the potency of radiation toxicity was the death of Marie Curie, while working with her husband, Pierre Curie. She died of leukemia due to radiation exposure.

Ionizing radiations bring about more dangerous effects than other toxicants. Their effects may continue in subsequent generations. They bring about following two types of undesirable effects in organisms:

1. Somatic effects

These are the direct results of action of radiation on the body cells and tissues. Radiologists, uranium mine workers and painters of radium dials suffer most. More evidence of degree and kind of damage from radiation came from studies of the Nagasaki and Hiroshima survivors. The somatic effects may be immediate or delayed.

High radiation exposures have much acute toxicity and can kill animal quickly. A dose of 400 to 500 roentgen on whole body is fatal in about 50% cases of man, and 600-700 in practically every case. The victim declines in vitality and dies from anemia, infection and Haemorrhage.

Parts of body differ in sensitivity. The most sensitive tissues from acute doses are intestines, lymph nodes, spleen and bone marrow. The radiation destroys the body’s immune response. The effects of low-penetrating radiation are less severe than the penetrating ones.

In delayed effects the patient may survive for months or years. Delayed effects of radiation include eye cataracts, leukemia, malignant tumors, cardiovascular disorders, premature ageing and reduced life span. Diagnostic X-ray exposure of pregnant women may increase the risk of cancer in child.

2. Genetic effects

Both, background natural and man-made radiations bring about genetic effects. Studies  Drosophila shows that mutation rates go very high due to radiation exposures. Background radiation differs in different part of Earth. Most genetic effects are brought about by man-made radiation. Most important of these are exposure during medicare, and exposure from nuclear power plants. People in industry, research and medicine using radionuclides are exposed more than others. The greatest damage is an dividing cells chiefly the gonads. The effects include mutation, or Lethal effect on egg or embryo. The intensity of radiation affects the rate of mutation. Generally higher animals are more susceptible to genetic damage than lower animals as insects. Genetic effects also occur in plants.

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