RADIOACTIVITY
Radioactivity is the spontaneous emission of radiation by certain unstable atomic nuclei. This phenomenon was first discovered by Henri Becquerel in 1896 while experimenting with uranium salts. Later, it was extensively studied by Marie Curie and Pierre Curie, who discovered radioactive elements like polonium and radium. Radioactivity occurs naturally in some elements and can also be artificially induced in laboratories. The atoms of radioactive elements have unstable nuclei, which disintegrate or decay over time to become more stable, releasing energy in the form of particles or electromagnetic waves during the process.
There are three main types of radioactive emissions: alpha (α), beta (β), and gamma (γ) radiation. Alpha particles are heavy and positively charged, beta particles are lighter and negatively charged, while gamma rays are electromagnetic waves with no mass or charge but high penetration power. Each type of radiation behaves differently and has different effects on matter and biological tissues. The unit of radioactivity is the becquerel (Bq), which represents one disintegration per second, while older units like curie (Ci) are also sometimes used.
Radioactivity has a wide range of applications. In the medical field, radioactive isotopes are used in cancer treatment (radiotherapy) and diagnostic imaging. In archaeology and geology, radiometric dating techniques like carbon dating and uranium dating use radioactive decay to estimate the age of fossils and rocks. In nuclear power, radioactive materials like uranium-235 are used as fuel in reactors to produce energy. However, exposure to high levels of radiation can be dangerous, causing radiation sickness or even cancer, which is why safety and shielding are essential in handling radioactive materials.
Alpha (α) Rays:
- These rays consist of positively chargedhelium nuclei (He++). They have +2 unitcharge and 4 u mass.
- They have low penetrating power butvery high ionising power and kinetic energy.
- An α-emission reduces the atomic massby 4 and atomic number by 2, thus, the New nuclei formed occupy a position twoplaces left to the parent nuclei in the periodic table.
Beta (β) Rays:
- These rays consist of negatively charged electrons (-1e0) and have -1 unit charge and zero mass.
- These are more dangerous than α-rays.
- These have high penetrating power as Compared to a-rays.
- A β - emission increased the atomic number by one with no change in atomic mass, thus, the new nuclei obtained.
Pauli Exclusion Principle:
- It is the quantum mechanical principlewhich states that no two identicalfermions (particles with half-integerspin) may occupy the same quantumstate simultaneously.
- Occupy a position one place right to theparent nuclei in the periodic table.
Gamma(γ) Rays:
- These are electromagnetic radiation andhave very high penetrating power.
- These have low ionising power and kinetic energy.
- Their emission does not affect theposition of nuclei in the periodic table.
Half-Life Period:
- It is the time in which a radioactive substance remains half of its original amount.
[t1/2=0.693λ]
Nuclear Fission:
- It is a process in which a heavy nucleusis broken down into two or more lighter fragments.
- It is usually accompanied with the emission of neutrons and large amount of energy. It is used in nuclear reactorand atom bomb.
Atom Bomb:
- It is based on uncontrolled nuclear fission. It contains 235U or 239Pu as fuel.
Nuclear Reactor:
- It is a device that is used to produceelectricity and permits a controlled chainnuclear fission.
- It contains fuels e.g.,92U235, moderator (e.g., graphite and heavy water, D2O) toslow down neutrons and control rodsmade up of boron steel or cadmium) toabsorb neutrons.
- It may also contain liquid sodium as coolant.
Nuclear Fusion:
- It is a process which involves fusion of two or more lighter nuclei to give heavier nuclei.
- It occurs only at extremely high temperature (> 10° K), so also called thermonuclear reactions.
- It is used in hydrogen bomb. Energy of Sun is also a result of a series of nuclear fusion reactions
Hydrogen Bomb:
- It contains a mixture of deuterium oxide (D2O) and tritium Oxide (T2O) in a space surrounding an ordinary atom bomb.
Radiocarbon Dating:
Radiocarbon dating, also known as Carbon-14 dating, is a scientific method used to determine the age of ancient organic materials. This technique was developed by American scientist Willard Libby in 1949, for which he was later awarded the Nobel Prize in Chemistry. Radiocarbon dating is based on the principle of radioactive decay of the isotope Carbon-14 (C-14), which is a weakly radioactive isotope of carbon present in all living organisms. While an organism is alive, it continuously absorbs Carbon-14 from the atmosphere through carbon dioxide. However, when the organism dies, it stops absorbing C-14, and the amount present in its body begins to decay at a predictable rate.
The half-life of Carbon-14 is approximately 5730 years, meaning that after this time, half of the original C-14 will have decayed into nitrogen-14. By measuring the remaining amount of Carbon-14 in a sample and comparing it with the atmospheric levels, scientists can estimate how long ago the organism died. This method is extremely useful in archaeology, geology, and environmental science for dating organic materials such as wood, charcoal, bones, cloth, paper, and other biological remains. However, radiocarbon dating is not suitable for inorganic materials like rocks, metals, or pottery unless they contain some trace organic residue.
The effective dating range of radiocarbon dating is up to about 50,000 years. Beyond this period, the remaining Carbon-14 is so minimal that accurate dating becomes unreliable. The method has played a crucial role in understanding the timeline of human civilization, ancient ecosystems, and climate changes. It is important to note that radiocarbon dating requires careful sample preparation and is influenced by environmental factors, contamination, and calibration with dendrochronology (tree-ring dating) for more accurate results. In summary, radiocarbon dating remains one of the most important and widely used techniques for dating ancient organic materials in both historical and scientific research.
Uranium Dating:
Uranium dating, also known as Uranium-series dating or Uranium-lead (U-Pb) dating, is a radiometric dating method used to determine the age of rocks, minerals, and archaeological materials. This technique is based on the radioactive decay of Uranium isotopes (U-238 and U-235) into stable lead isotopes (Pb-206 and Pb-207) over a long period of time. Uranium is a naturally occurring element found in various minerals, especially in igneous rocks. As uranium decays at a known and constant rate, scientists can measure the ratio of parent uranium to daughter lead isotopes in a sample and calculate the time that has passed since the rock solidified.
Uranium-238 has a half-life of approximately 4.5 billion years, while Uranium-235 has a half-life of about 704 million years, making uranium dating one of the most reliable techniques for dating some of the oldest materials on Earth. It is especially used for dating zircon crystals in granite and other ancient geological formations. Unlike carbon dating, which is effective only for organic remains up to 50,000 years old, uranium dating can be used to date materials millions to billions of years old. It is thus a vital tool in understanding the geological time scale, the age of the Earth, and the formation of continental crust.
This method is also used in archaeology to date calcium carbonate materials, such as cave deposits, stalactites, and corals, using Uranium-Thorium dating (a variation of uranium dating). For competitive exams, it is important to remember that uranium dating is used for dating inorganic materials, particularly rocks and minerals, and not suitable for organic fossils. Candidates should also note that uranium-lead dating is considered one of the most accurate and precise radiometric dating techniques used in modern science. This method has played a significant role in establishing the age of the Earth, which is approximately 4.54 billion years.
Uses of Radioisotopes:
- Iodine-131 is employed to study the structure and activity of thyroid gland. It is also used in internal radiation therapy for the treatment of thyroid disease.
- lodine-123 is used in brain imaging.
- Cobalt-60 is used in external radiation therapy for the treatment of cancer.
- Sodium-24 is injected along with salt solution to trace the flow of blood.
- Phosphorus-32 is used for leukemia therapy
- Carbon-14 is used to study the kinetics of photosynthesis.
