Tuesday, March 2, 2010

न्यूयार्क के पुरातत्वविद प्रो. मर्विन मिलर ने ताज के यमुना की तरफ़ के दरवाजे की लकड़ी की कार्बन डेटिंग के आधार पर 1985 में यह सिद्ध किया कियह दरवाजा सन् 1359 के आसपास अर्थात् शाहजहाँ के काल से लगभग 300 वर्ष पुराना है... 

Thursday, October 15, 2009


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Naturally occurring radioactive substances have high nucleon number. It is possible to make artificial radioactive substances by bombarding lighter nuclides with - particles, protons or neutrons. The radioactive substances produced in this manner are known as radioisotopes.

A nuclide is any species of atom of which each atom has an identical proton number and also an identical nucleon number. Different nuclides, which have the same proton number (but different nucleon numbers) are called isotopes (isotopic nuclides). The first radioisotope was an unstable isotope of phosphorus. It was produced in 1934 by bombarding aluminium with - particles, i.e.,

Phosphorus - 30 was produced, together with a neutron. Notice that on each side of the equation the sum of the nucleon number is 31 and the sum of the proton number is 15. Phosphorus - 30 decays by ejecting a positron and has a half-life of about 3 minutes.

The positron has not been mentioned before because it does not occurring natural radioactivity. It has a mass equal to that of the electron, and a positive electron. It is denoted by

When magnesium is bombarded by neutrons a radioisotope of sodium is formed. The reaction is

The sodium decays with the emission of a - particle.


The important point is that it is now possible to produce any radioisotope. Most of those produced have short half-life periods. This is very important because the activity and hence the danger from radioactive emissions does not last very long.

Uses of Radioactive Isotopes

All isotopes of a substance have the same chemical properties and behave in an identical manner. The advantage of a radioisotope is that its position can be detected very easily by the radiation which it emits. It has wide applications in various fields like:

In Medicine

Radio isotopes are used in detection of diseases and also in radio therapy:

  • The rays from radium is used in the treatment of skin diseases.
  • Radiation from Co60 ( - rays) is used to diagnose and treat thyroid disorders.
  • Radio iodine (I131) is used to diagnose and treat thyroid disorders.
  • Radio phosphorus (P32) is used in the treatment of leukaemia and tumours.
  • Radio sodium (Na24) in the form NaCl is used to study circulation of blood.

In Agriculture

  • Radioactive phosphorus (P32) is used in the study of metabolism of plants.
  • Radioactive sulphur (S35) helps to study advantages and disadvantages of


  • Pests and insects on crops can be killed by - radiations.,
  • - rays are used for preservation of milk, potatoes etc.
  • Yield of crops like carrot, root, apples, grapes can be increased by irradiation with radioisotopes.

In Industry

  • In is used in the manufacture of paper, plastic and metal sheets. (Used to control the thickness of the sheets.)
  • Radioisotopes can be used to estimate the amount of wear in bearings.
  • Leaks in pipes may be traced by introducing a small quantity of radioisotopes into the fluid in the pipe.
  • It is also used to detect the cracks in the welding, casting etc.

Natural Radioactive Isotopes - Radiocarbon and Carbon 14 Dating

There are a small number of radioisotopes of low proton number, which occur naturally. They are produced by bombardment by radiation from outer space (cosmic rays). The most well known of these is radioactive carbon-14, which is produced when nitrogen is bombarded by neutrons.

Carbon-14 decays with the emission of a

b - particle, and reverts to nitrogen

In nature, radiocarbon is formed when high energy atomic particles called cosmic rays break down the atoms in the atmosphere into electrons, protons, neutrons and other particles. Some of the neutrons strike the nuclei of nitrogen atoms in the atmosphere get converted into radiocarbon atoms.

Carbon-14 has a long half-life of about 5600 years. It is reasonable to assume that equilibrium has been reached between the rate at which carbon - 14 forms in the atmosphere and the rate at which it decays, and that the amount of it in the atmosphere is constant. When plants photosynthesise they take in CO2 from the air. The carbon 14 atoms in these molecules slowly decay to Nitrogen. Human beings and other animals take in radiocarbon chiefly from the food provided by plants. Thus, all living things contain radiocarbon. Plants are utilised as cotton or linen; or might form coal. But whatever happens the C - 14 in it gradually decays. When the plant or animal dies fresh carbon is no longer taken in and the C - 14, which is present, decays. Thus, the length of time a specimen has been dead may be determined by the activity of the C - 14, which remains in it. Carbon - dating has therefore become an important tool for archaeologists and anthropologists.

Radiocarbon or C -14, is a radioactive isotope of carbon. It is used to determine the age of fossils and other ancient organic matters.

So by finding out how much carbon 14 is there in an object, we can approximate the age of the sample. The less the C - 14 compared with carbon 12, the older the sample is. C - 14 dating is widely used in Archaeology to determine the age of archaeological samples like tools, ornaments, paintings, furniture etc.

Carbon Dating

The age of fossils of plant or animal origin can be determined by carbon dating technique developed by Willard Libby in 1949. The radioactive isotope of carbon (6C14) is used to determine the date at which an animal or plant had died.

Carbon Dating

Carbon Dating

The method of measuring the age of archaeological materials that contain matter of living origin using the radioactive isotope 6C14 is called carbon dating. It is continuously formed in the upper strata of the atmosphere by the action of neutrons in the cosmic rays on 7N14.

By photosynthesis, plants take up CO2 from the atmosphere which contains small amount of radio isotope 6C14. It is used by plants to build carbohydrates which are then consumed by living animals.

Because of the natural plant - animal carbon cycle, an equilibrium will be set up and all living matter will contain a constant equilibrium concentration of C - 14, if the intensity of cosmic rays reaching the earth remains constant over a long period of time.

Once the plant or animal dies, the process of incorporation of 6C14 stops and 6C14 already present begins to decay.

(t1/2 = 5770 years)

Thus, by knowing the equilibrium concentration of 6C14 in a living plant and the concentration of 6C14 in a dead piece of organic matter at a particular time, the age of the material can be determined.

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Food and Agriculture

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Insect Control
Food Treatment and Preservation
Quarantining and Exportation
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Agricultural Tracers

Tracers like those used in medicine are also used in agriculture to study plants and their intake of fertilisers. The usage of tracers allows scientists and farmers to optimise the use of fertilising and weedkilling chemicals. Optimisation of these chemicals is desirable because it saves money, and reduces chemical pollution. When fertilisers are used in overly excessive amounts, the excess will run off and pollute rivers nearby, as well as possibly seeping through to the water table underground and polluting the water supply. To prevent this, studies are conducted to find out the optimal amount of chemical required, with fertilisers and weedkillers often tagged by nitrogen-15 or phosphorus-32 radioisotopes. These radioisotopes are analysed in the crops to see how much of the original chemical was actually consumed by the plants, compared to how much was given.

The ionising radiation from radioisotopes is also used to produce crops that are more drought and disease resistant, as well as crops with increased yield or shorter growing time. This practice has been in place for several decades, and has helped feed some third-world countries. The collection of crops that have been modified with radiation include wheat, sorghum, bananas and beans.

Insect Control

About 10% of the world's crops are destroyed by insects. In efforts to control insect plagues, authorities often release sterile laboratory-raised insects into the wild. These insects are made sterile using ionising radiation - they are irradiated with this radiation before they hatch. Female insects that mate with sterile male insects do not reproduce, and the population of the insect pests can be quickly curbed as a consequence. This technique of releasing sterile insects into the wild, called the sterile insect technique (SIT), is commonly used in protecting agricultural industries in many countries around the world.

The technique is considered to be safer and better than conventional chemical insecticides. Insects can develop resistance against these chemicals, and there are health concerns about crops treated with them.

The largest application of this technique so far was conducted in Mexico against Mediterranean fruit-fly and screwworm in 1981. It was highly successful, and over the next 10 years the eradication program yielded about US$3 billion in economic benefits to the country.

SIT is in use in several countries, with support from the UN Food and Agriculture Organisation (FAO) and the International Atomic Energy Agency (IAEA). Australia is a large producer of many fruits and sterilises up to 25 million fruit fly pupae per week.

Food Treatment and Preservation

Ionising radiation is used as an alternative to chemicals in the treatment and preservation of foods. A French scientist first discovered that radiation could be used to prolong food shelf life in the 1920s and it became more widely used in World War II. Today, astronauts often eat radiation-preserved food while on space missions.

In meats and other foods of animal origin, irradiation destroys the bacteria that causes spoilage as well as diseases and illneses such as salmonella poisoning. This allows for a more safer food supply, and meats that can be stored for longer before spoilage. Additionally, irradiation also inhibit tubers that cause fruits and vegetables to ripen. The result is fresh fruits and vegetables that can be stored for longer before ripening.

The irradiation technique is particularly important when exporting to countries with tropical climates, where foods can be spoiled easily due to the warm temperatures.

Irradiation of food is carried out using accelerated electrons (beta radiation), and ionising radiation from sources such as the radioisotopes cobalt-60 and cesium-137. X-rays are also sometimes used. None of these sources of radiation used have enough energy to make the exposed foods radioactive.

Radiation dose (kilograys, kGy)Purpose
"low" up to 1 kGyinhibits fruit and vegetable ripening
controls some bacteria in meats
controls insects in grains
"medium" 1-10destroys bacteria in meat including salmonella, shigella, campylobacter and yersinia
inhibits mold growth on fruit
"high" more than 10 kGydestroys insects and bacteria in spices
sterilises food to the same extent achieved by high heat
the above table shows the typical doses of radiation used for food treatment

Inside the food treatment plant there is a conveyor belt or similar system that transports the food to the radiation source, so that workers do not have to move close to the radiation. The source is packaged in a pencil like device, about 1cm in diametre. The room where irradiation takes place is shielded by concrete walls to prevent radiation from escaping into the environment, although the radiation risk is considerably much less than that from a nuclear reactor. Where gamma radiation is used from a radioisotope source, the radioisotope is stored in a pool of water while not in use, to also help prevent radiation from escaping. However, the plant is in many ways similar to any other - refrigeration is still important. No process can make food completely spoil-proof.

Food treatment plants of this kind are monitored closely by government health and occupational safety authorities to ensure safe working conditions for employees, as well as safety to any nearby residents.

Food irradiation is a well-tested process. Scientists have performed numerous decades of research, and it has been shown that irradiation will not cause significant chemical changes in foods that may affect human health, nor will it cause losses that may affect the nutritional content of food. (Chemical residues left behind by irradiation are in concentrations equivilant to about 3 drops in a swimming pool. Chemical-based preservatives and treatments usually leave more residues.) Taste is usually unaffected. The World Health Organisation and food safety authorities in many countries have approved irradiation as a safe method of food treatment and preservation.

Radiation-treated food is still not very widely used today. Despite the scientific evidence and approvals, many activist organisations claim that irradiation is unsafe and exploit the lack of public awareness and concerns about food safety and nuclear issues. Some even say that irradiation is a way that governments can utilise nuclear wastes left over from weapons testing or power generation. (However, the wastes left cannot be used in food processing because they do not provide the right type of ionising radiation.) Consequently, these scare tactics deter the public and some food producers are reluctant to use irradiation for fear of consumer boycotts. However, a recent survey conducted in mid-1998 by the Food Marketing Institute (a United States organisation) revealed that less than one percent of all those surveyed identified irradiation as a concern. Most said that spoilage and microbial hazards were of great concern - they very problem that irradiation addresses. Another study by an academic revealed that about 99% of consumers were willing to buy irradiated food after they were shown scientific data and irradiated food samples. This compared to 50% before shown this data.

Irradiation poses less of a risk to human health than many chemical treatments that are used today, which include the addition of chemical preservatives. The use of radiation is sometimes favoured to using chemical preservatives, because no allergic side-effect results. It is also better than heat-sterilisation because irradiation does not destroy nutrients and vitamins, whereas heat treatment does.

Irradiation is inexpensive - typical costs are about 1-20 cents per kilogram of food irradiated.

About 40 countries worldwide allow irradiation of foods. Depending on the country, irradiated foods may need to be labelled.

Quarantine and Exportation

Ionising radiation is used to rid goods of parasites and bugs before they are exported out of a country. The radiation kills these parasites that may be quarantine hazards in other countries.

The technique is used in Australia to clear primary produce materials such as raw wool and wood for export. It is also used worldwide in transporting archival and historical documents. This is beneficial in that any microorganisms existing in the paper that cause paper deterioration are destroyed.

A radionuclide is an atom with an unstable nucleus, which is a nucleus characterized by excess energy which is available to be imparted either to a newly-created radiation particle within the nucleus, or else to an atomic electron (see internal conversion) . The radionuclide, in this process, undergoes radioactive decay, and emits a gamma ray(s) and/or subatomic particles. These particles constitute ionizing radiation. Radionuclides may occur naturally, but can also be artificially produced.

Radionuclides are often referred to by chemists and physicists as radioactive isotopes or radioisotopes, and play an important part in the technologies that provide us with food, water and good health. However, they can also constitute real or perceived dangers.




Naturally occurring radionuclides fall into three categories: primordial radionuclides, secondary radionuclides and cosmogenic radionuclides. Primordial radionuclides originate mainly from the interiors of stars and, like uranium andthorium, are still present because their half-lives are so long that they have not yet completely decayed. Secondary radionuclides are radiogenic isotopes derived from the decay of primordial radionuclides. They have shorter half-lives than primordial radionuclides. Cosmogenic isotopes, such as carbon-14, are present because they are continually being formed in the atmosphere due to cosmic rays.

Artificially produced radionuclides can be produced by nuclear reactors, particle accelerators or by radionuclide generators:

  • Radioisotopes produced with nuclear reactors exploit the high flux of neutrons present. The neutrons activate elements placed within the reactor. A typical product from a nuclear reactor is thallium-201 and iridium-192. The elements that have a large propensity to take up the neutrons in the reactor have a high Barnes Number.
  • Particle accelerators such as cyclotrons accelerate particles to bombard a target to produce radionuclides. Cyclotrons accelerate protons at a target to produce positron emitting radioisotopes e.g. fluorine-18.
  • Radionuclide generators contain a parent isotope that decays to produce a radioisotope. The parent is usually produced in a nuclear reactor. A typical example is the technetium-99m generator used in nuclear medicine. The parent produced in the reactor is molybdenum-99.
  • Radionuclides are produced as an unavoidable side effect of nuclear and thermonuclear explosions.

Trace radionuclides are those that occur in tiny amounts in nature either due to inherent rarity, or to half-lives that are significantly shorter than the age of the Earth. Synthetic isotopes are inherently not naturally occurring on Earth, but can be created by nuclear reactions.


Radionuclides are used in two major ways: for their chemical properties and as sources of radiation. Radionuclides of familiar elements such as carbon can serve as tracers because they are chemically very similar to the non-radioactive nuclides, so most chemical, biological, and ecological processes treat them in a near identical way. One can then examine the result with a radiation detector, such as a geiger counter, to determine where the provided atoms ended up. For example, one might culture plants in an environment in which the carbon dioxide contained radioactive carbon; then the parts of the plant that had laid down atmospheric carbon would be radioactive.

In nuclear medicine, radioisotopes are used for diagnosis, treatment, and research. Radioactive chemical tracers emitting gamma rays or positrons can provide diagnostic information about a person's internal anatomy and the functioning of specific organs. This is used in some forms of tomography: single photon emission computed tomography and positron emission tomography scanning.

Radioisotopes are also a promising method of treatment in hemopoietic forms of tumors, while the success for treatment of solid tumors has been limited so far. More powerful gamma sources sterilise syringes and other medical equipment. About one in two people in Western countries are likely to experience the benefits of nuclear medicine in their lifetime.

In biochemistry and genetics, radionuclides label molecules and allow tracing chemical and physiological processes occurring in living organisms, such as DNA replication or amino acid transport.

In food preservation, radiation is used to stop the sprouting of root crops after harvesting, to kill parasites and pests, and to control the ripening of stored fruit and vegetables.

In agriculture and animal husbandry, radionuclides also play an important role. They produce high intake of crops, disease and weather resistant varieties of crops, to study how fertilisers and insecticides work, and to improve the production and health of domestic animals.

Industrially, and in mining, radionuclides examine welds, to detect leaks, to study the rate of wear, erosion and corrosion of metals, and for on-stream analysis of a wide range of minerals and fuels.

Most household smoke detectors contain the radionuclide americium formed in nuclear reactors, saving many lives.

Radionuclides trace and analyze pollutants, to study the movement of surface water, and to measure water runoffs from rain and snow, as well as the flow rates of streams and rivers. Natural radionuclides are used in geology,archaeology, and paleontology to measure ages of rocks, minerals, and fossil materials.


If radionuclides are released into the environment, through accident, poor disposal, or other means, they can potentially cause harmful effects of radioactive contamination. They can also cause damage if they are excessively used during treatment or in other ways applied to living beings. This is called radiation poisoning. Radionuclides can also cause malfunction of some electrical devices.