CHEMISTRY: RADIOACTIVITY

CHEMISTRY: RADIOACTIVITY

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About radioactivity

• It is the process by which an unstable atomic nucleus spontaneously decays (loses energy) by emitting ionizing particles and radiation
• This decay results in the atom of one type (parent nuclide) transforming into an atom of a different type (daughter nuclide)
• Eg: Carbon-14 emits radiation and transforms into nitrogen-14
• The SI unit of radioactivity is Becquerel (Bq). Another commonly used unit is the Curie (Ci)
• Radioactivity of a material is quantified by its half life. This is the time taken for a given amount of a radioactive material to decay to half its initial value
• Radiation can be measured using scintillation counters and Geiger counters

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History of radioactivity research

• Radioactivity was first discovered by French scientist Henri Becquerel in 1896
• Research in radioactivity of uranium led Marie Curie to isolate a new element Polonium and to separate Radium from Barium
• The dangers of radioactivity was discovered by Nikola Tesla in 1896, when he intentionally subjected his fingers to X-rays
• Henri Joseph Muller was awarded the Nobel Prize in Physiology or Medicine in 1946 for his discovery (in 1927) of the harmful genetic effects of radiation

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Transmutation of elements

• Isotopes: they are atoms of an element with the same atomic number but different mass number (eg uranium-238 and uranium-235)
• Isobars: elements with same mass number but different atomic number. Usually occurs when a radioactive nucleus loses a beta particle (eg. Thorium-234 and palladium-234)
• Isotones: radioactive nuclei that contain the same number of neutrons (eg. Radium-226 and Actium-227)
• Isomers: are different excitation states of nuclei. The higher-energy (unstable) element undergoes isomeric transition to form the less energetic variant without change in atomic or mass number

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Types of radioactive decay

Alpha rays can be stopped by a sheet of paper, beta rays by aluminium shielding, while gamma rays can only be reduced by a thick layer of lead

• Radioactive radiation can be split into three types of beams
• Alpha rays: they are helium particles that carry a positive charge. They have low energy and can be stopped by a sheet of paper
• Beta rays: they are streams of electrons and carry negative charge. They have higher energy than alpha rays
• Gamma rays: they are high energy rays (like X-rays) that carry no electrical charge

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Radioactivity and the Big Bang theory

• According to the Big Bang theory stable isotopes of the lightest elements (H, He, Li, Be, B) were formed immediately after the Big Bang
• Radioactive (unstable) isotopes of these light elements have long since decayed, and isotopes of elements heavier than boron were not produce at all in the Big Bang
• Thus, the radioactive materials currently in the universe were formed later and are relatively young compared to the age of the universe
• These radioactive nuclei were formed in nucleosynthesis in stars and during interactions between stable isotopes and energetic particles
• For instance, carbon-14 is constantly produced in the earth’s upper atmosphere due to interactions between cosmic rays and nitrogen

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Applications of radioactivity

• Radioisotopic labeling: used to track the passage of a chemical through the human body. Some common radio isotopes used for labeling are
  • Tritium: used to label proteins, nucleic acids
  • Sodium-22 and Sodium-36: ion transporters
  • Sulphur-35: proteins and nucleic acids
  • Phosporous-32 and Phosphorous-33: nucleotides (like DNA)
  • Iodine-125: thyroxine
  • Carbon-14 is not used for radioactive labeling due to its long half life (5730 years)
• Random number generators: based on the premise that radioactive decay is truly random
• Radiometric dating: used to date materials based on a comparison between observed abundance of radioactive isotopes and its decay products, using known decay rates. The most common methods of radiometric dating are
  • Carbon dating: when organic matter grows, it traps carbon-14. The age of the organic matter can be estimated by measuring the amount carbon-14 remaining in the body. Used for dating material up to 60,000 years old
  • Potassium-argon dating: used in geochronology and archeology, especially for dating volcanic material. Used for samples older than a few thousand years
  • Uranium-lead: one of the oldest and most refined radiometric dating techniques. Used in geochronology to estimate material from 1 million to 4.5 billion years old. A variant, the lead-lead dating scheme was used by American scientist Clair Cameron Patterson to estimate the age of the earth (4.55 billion years) in 1953

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Radioactive therapy

• Used for palliative and therapeutic treatment
• Common applications include treatment of thyroid eye disease, heterotopic ossification, trigeminal neuralgia
• In low doses, it is used for cancer treatment. However, in large doses, it can cause cancer
• Total body irradiation is used to prepare the body to receive a bone marrow transplant

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Radiation poisoning

• It is a form of damage to organ tissue due to excessive exposure to ionizing radiation
• Caused by exposure to large doses of radiation in short periods of time, or by exposure to small doses over long periods
• Increases the probability of contracting other diseases like cancers, tumours and genetic damage
• Common symptoms are nausea and vomiting
• Common occurrances of radiation poisoning include nuclear warfare, nuclear reactor accidents, spaceflight (exposure to cosmic rays), ingestion and inhalation of radioactive compounds (such as strontium in cow’s milk)
  • In Nov 2006, Russian dissident died due to suspected deliberate ingestion of Polonium-210

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JOHAR