8.1 Discovery of Radioactivity
Radioactivity is the spontaneous decay process of an unstable atomic nucleus by emitting radioactive radiation to become a more stable nucleus.
Key Historical Pioneers
- Wilhelm Röntgen: Discovered X-rays in 1895.
- Henri Becquerel: Discovered radioactivity in 1896 using uranium compounds that blackened photographic plates in the dark.
- Marie and Pierre Curie: Isolated radioactive elements polonium and radium from pitchblende. Marie Curie coined the term radioactivity.
Units of Radioactivity
Radioactivity measures the rate of decay (disintegrations per second):
- Becquerel (Bq): $1\text{ Bq} = 1\text{ decay per second}$.
- Curie (Ci): $1\text{ Ci} = 3.7 \times 10^{10}\text{ decays per second } (3.7 \times 10^{10}\text{ Bq})$.
Half-Life ($T_{1/2}$)
The half-life is the time taken for the number of unstable nuclei (or activity) of a radioactive sample to decay to half of its initial value.
8.2 Atom and Nucleus
An atom consists of a central nucleus containing protons (positive) and neutrons (neutral), surrounded by orbiting electrons (negative).
Ion Formation
- Neutral Atom: Equal number of protons and electrons ($\text{Net charge} = 0$).
- Positive Ion (Cation): Formed when an atom loses one or more electrons ($\text{Protons} > \text{Electrons}$).
- Negative Ion (Anion): Formed when an atom gains one or more electrons ($\text{Electrons} > \text{Protons}$).
8.3 Ionising Radiation and Non-Ionising Radiation
Radiation is divided into ionising and non-ionising radiation based on its energy level and ability to produce ions in matter.
Comparison of Radiation Types
- Ionising Radiation: High-energy radiation capable of knocking electrons out of atoms, creating ions. Examples: Alpha ($\alpha$), Beta ($\beta$), Gamma ($\gamma$), X-rays.
- Non-Ionising Radiation: Low-energy radiation incapable of knocking out electrons. Examples: Radio waves, Microwaves, Infrared, Visible light, Ultraviolet.
Properties of Radioactive Rays
| Property | Alpha ($\alpha$) | Beta ($\beta$) | Gamma ($\gamma$) |
|---|---|---|---|
|
Nature | Helium nucleus ($^4_2\text{He}$) | High-speed electron ($^0_{-1}e$) | Electromagnetic wave |
|
Charge | Positive (+2) | Negative (-1) | Neutral (0) |
|
Ionising Power | Very High | Moderate | Low |
|
Penetrating Power | Low (Stopped by a piece of paper) | Moderate (Stopped by thin aluminum sheet) | Very High (Stopped by thick lead or concrete) |
|
Deflection in Electric Field | Deflected towards negative plate | Deflected towards positive plate | Not deflected |
Sources of Ionising Radiation in the Environment
- Natural Background Radiation: Cosmic rays, terrestrial radiation from rocks and soil (radon gas), radioactive isotopes inside the body (Potassium-40).
- Man-made Radiation: Nuclear fallouts, medical X-rays, radioisotope diagnosis, luminous watches.
- Background Radiation Unit: Dose measured in Sievert (Sv) or microSievert per hour ($\mu\text{Sv/h}$). Safe level is $< 0.2\,\mu\text{Sv/h}$.
8.4 Uses of Radioactive Radiation
Radioisotopes emit radioactive rays that have critical applications across multiple industries:
Key Applications
- Medicine: Cobalt-60 (cancer radiotherapy), Iodine-131 (thyroid gland diagnosis and treatment), Sodium-24 (blood circulation tracking).
- Agriculture: Phosphorus-32 (monitors fertilizer absorption in plants), gamma rays (sterilizes pests).
- Industry: Beta rays (regulates paper or sheet metal thickness), Gamma rays (detects internal pipe leaks and weld cracks).
- Archeology: Carbon-14 dating (determines age of organic fossils up to 50,000 years old).
Safety & Handling Precautions
- Store radioactive materials in thick lead containers.
- Handle radioactive sources using robotic arms or long tongs, never directly with hands.
- Wear protective suits and film badges (dosimeters) to monitor radiation exposure levels.
- Display official radioactive warning hazard symbols near storage facilities.