Day 56 | RAS Mains 2025 Answer Writing | 90 Days

• A. H. Becquerel discovered radioactivity in 1896 purely by accident
• Radioactivity is the spontaneous emission of radiation from the nucleus of an unstable atomic nucleus in order to achieve a more stable configuration. This emission can take the form of alpha particles (helium nuclei), beta particles (electrons or positrons), or gamma rays (high-energy photons).
  • Becquerel is the SI unit for measurement of radioactivity. It is defined as the number of disintegration per second (d.p.s.)

1 Becquerel (Bq) = 1 dps

• The time during which the number of atoms in a radioactive sample reduces to half of the initial number is called the half life period.The principle was first discovered in 1907 by Ernest Rutherford. 

​​

where , 

1. t_½ is the half-life of the decaying quantity,
2. τ  is mean lifetime of the decaying quantity (time at which both N and R(decay rate) have been reduced to 1/e of their initial values.)
3. λ is the decay constant of the decaying quantity.

• Thus, half-life of any radioactive substance is inversely proportional to its decay constant and is a characteristic property of the radioactive nucleus. The half-life of 14 C (radioactive carbon) is 5730 years. 

      b. Give examples of each reaction: Alpha decay, Beta decay, and Gamma decay. 

        Alpha decay: 

Beta-minus (β-) decay: electron is emitted

In beta-minus decay, a neutron transforms into a proton within the nucleus

Beta-plus (β+) decay:  positron is emitted

whereas in beta-plus decay, a proton transforms into neutron (inside the nucleus) via

​represent antineutrino and neutrino, respectively; 

Gamma Decay :

Cobalt-60 (Co-60) undergoes gamma decay to form Nickel-60 (Ni-60) by emitting a gamma ray (high-energy photon).

(Gamma decay

Most radionuclides after an alpha decay or a beta decay leave the daughter nucleus in an excited state. The daughter nucleus reaches the ground state by a single transition or sometimes by successive transitions by emitting one or more gamma rays.)

      c. Write about the various applications of radioactivity in different fields.

         Medical Uses

Type of Radiation Therapy	Example
External Radiation Therapy	Cobalt-60 emits x-rays externally to treat various cancers. cesium-137
Internal Radiation Therapy	Iodine-131, a β and γ-emitter, is administered to treat hyperthyroidism by concentrating in the thyroid gland.
Brachytherapy or Seed Implantation	Titanium capsules containing γ-emitters like iodine-125, palladium-103, or cesium-131 are implanted to treat prostate cancer.
Tracers In medical imaging	Positron emission tomography (PET): like carbon-11 and fluorine-18 incorporated in a suitable compound like glucose allow following the metabolic path of the compound.Phosphorus-32 is useful in the identification of malignant tumors because cancerous cells tend to accumulate phosphates more than normal cells do.

Energy:

• Nuclear Power: Radioactive isotopes, such as uranium-235 and plutonium-239, are used as fuel in nuclear reactors to generate electricity through controlled nuclear fission reactions]

Industry:

• Industrial Radiography: Gamma radiation is used to inspect the internal structure of heavy machinery and detect flaws or imperfections, such as air bubbles.
• Sterilisation: Radioactive sources, such as gamma irradiators, are utilized to sterilize medical instruments, pharmaceuticals, and food products, ensuring safety and preventing contamination.
• Thickness Measurement: Radioisotopes are used to measure the thickness or density of materials like metal and plastic sheets in industrial processes.

In GeologyRadiocarbon dating : radioactive carbon-14 is used in radiocarbon dating to determine the age of archaeological artifacts and geological samples by measuring the decay of carbon-14 isotopes relative to stable carbon isotopes.