Maarg Manthan · Topic 13.8

Radiation and Radioactivity

Science & Technology › Nuclear Technology and Energy · Topic 13.8

Radioactivity is the spontaneous breakdown of unstable atomic nuclei, which gives out energy and particles, and radiation is the energy that travels from a source as waves or particles. Henri Becquerel discovered radioactivity in 1896 in uranium salts, and Marie and Pierre Curie then isolated polonium and radium. The three main types of radiation from nuclei are alpha particles, which are helium nuclei and are stopped by paper, beta particles, which are electrons or positrons and are stopped by thin aluminium, and gamma rays, which are electromagnetic waves and need thick lead or concrete to stop them. Radiation is ionising, meaning it can knock electrons from atoms, as in X-rays and gamma rays, or non-ionising, as in radio waves and visible light. The half-life is the time in which half of a radioactive sample decays.

5 min read · UPSC Prelims · Science & Technology · Nuclear Technology and Energy

At a Glance

Discovered by Becquerel, 1896
Radiation types Alpha, beta, gamma
Most penetrating Gamma rays
Activity unit Becquerel
Dose unit Sievert
Exam link UPSC Prelims, Science & Technology: Nuclear Technology and Energy

Where Does It Fit?

This topic follows the account of nuclear institutions. It covers the basic science of radiation and radioactivity which is the base for the following topics on applications, waste and nuclear disasters. Previous topic: Department of Atomic Energy and Bhabha Atomic Research Centre (BARC). Next topic: Radiation Technologies and Their Applications.

What Are the Types of Radiation?

Type	Features
Alpha particle	Two protons and two neutrons, same as a helium nucleus. Heavy, positive, travels a few centimetres in air and is stopped by paper or skin. Dangerous only if inhaled or swallowed
Beta particle	A fast electron (beta minus) or positron (beta plus). Passes through paper but is stopped by a few millimetres of aluminium
Gamma ray	High energy electromagnetic radiation with no mass or charge. Needs thick lead or concrete to reduce. Used in cancer therapy and sterilisation
X-rays	Similar to gamma rays but produced by electrons hitting a target, as in medical X-ray machines
Neutron radiation	Neutrons from fission, which pass through many materials and are stopped by water or boron
Ionising radiation	Alpha, beta, gamma, X-rays and neutrons, with enough energy to ionise atoms and damage cells
Non-ionising radiation	Radio waves, microwaves, infrared and visible light, which lack energy to ionise

Which Terms and Units Matter?

Half-life: the time in which half of the atoms in a sample decay. Iodine-131 has about 8 days, cobalt-60 about 5.3 years, carbon-14 about 5,730 years and uranium-238 about 4.5 billion years
Activity: the number of decays per second, measured in becquerel, where one becquerel is one decay per second. An older unit is the curie
Absorbed dose: energy absorbed per kilogram of tissue, measured in gray
Equivalent or effective dose: takes into account the type of radiation and the organ, measured in sievert
Background radiation: natural radiation from cosmic rays, rocks, soil, radon and the human body, about 2 to 3 millisievert per year on average worldwide
Exposure: the older unit is the roentgen for X-rays
Isotopes: atoms of the same element with different numbers of neutrons; radioactive ones are radioisotopes
Decay chain: a radioactive element such as uranium decays through a series of daughter products to a stable lead isotope

What Else Should You Remember?

Discoverers: Becquerel found radioactivity in 1896, Marie Curie coined the word radioactivity and discovered polonium and radium in 1898 with Pierre Curie
Nobel Prizes: Becquerel and the Curies shared the 1903 Nobel Prize in Physics, and Marie Curie won the 1911 Nobel Prize in Chemistry
Rutherford: named alpha and beta rays and explained radioactive decay
Health effects: high doses cause radiation sickness and cancer risk, while very low doses are hard to link to effects
Protection: time, distance and shielding reduce exposure
Radon: a radioactive gas from uranium in rocks and soil, a main source of natural exposure indoors
Natural radioactivity in India: thorium-rich monazite sands in coastal Kerala give high background radiation in some areas, such as Chavara
Detecting radiation: Geiger-Muller counters, scintillation detectors and film badges
Dose limits: AERB limits are 20 mSv per year for workers (5-year average) and 1 mSv per year for the public, as in Atomic Energy Regulatory Board (AERB) and Safety Standards in Nuclear Plants
Radiation sources: used in medicine and industry, as in Radiation Technologies and Their Applications
Natural and artificial: radiation from natural sources and from human activities such as medical imaging and nuclear plants
Cosmic rays: high energy particles from space which add to the natural dose and increase at high altitude

Why Does It Matter for the Exam?

Penetration order: alpha less than beta less than gamma
Units: becquerel, gray, sievert
Half-life examples: I-131, Co-60, C-14
Current affairs link: radiation accidents and medical isotopes
Common errors: calling gamma rays particles or saying alpha rays are the most penetrating

As an analytical point, radiation is both useful and hazardous. Clear understanding of types, doses and protection allows safe use in medicine and industry and prevents panic about low-level exposure.

Why Is This Topic Important?

Types Alpha, beta, gamma
Units Bq, Gy, Sv
Half-life Calculations and examples

What Came Next?

The next topic covers radiation technologies and their applications. Next topic: Radiation Technologies and Their Applications.

Key People and Terms

Alpha particle Helium nucleus
Beta particle Electron or positron
Gamma ray High energy photon
Half-life Time to halve
Becquerel One decay per second
Sievert Effective dose unit

Exam Corner

Points to Remember

Radioactivity discovered: 1896, Becquerel.
Penetration: gamma greatest, alpha least.
Co-60 half-life: about 5.3 years.
Unit of activity: becquerel; dose: sievert.
Worker limit: 20 mSv per year.

Do Not Confuse With

Becquerel and sievert Decays per second versus biological dose
Gray and sievert Absorbed dose versus effective dose
Gamma rays and X-rays From nuclei versus from electrons

Memory Hook

Paper stops alpha, aluminium stops beta, lead for gamma; Becquerel counts, Sievert tells the harm.

Mains Angle

Questions ask for types, units and half-lives. These points help.

Science Types and units
Effects Dose and protection
Uses Medicine and industry

A Question You May Face

An original practice question, not a past paper question.

“Explain the different types of radiation, the units used to measure radioactivity and dose, and the principles of radiation protection.”

How to Answer

Types Alpha, beta, gamma.
Units Bq, Gy, Sv.
Protection Time, distance, shield.
Limits AERB doses.

GS Relevance

Prelims: types of radiation, penetration, half-life, units, ionising versus non-ionising. GS Paper 3: science and technology, nuclear science.

Frequently Asked Questions

What are alpha, beta and gamma rays?

Alpha rays are streams of helium nuclei, beta rays are fast electrons or positrons, and gamma rays are high energy electromagnetic waves. Alpha particles are stopped by paper, beta particles by thin aluminium, and gamma rays need thick lead or concrete, so gamma rays have the greatest penetrating power and alpha rays the least.

What is half-life?

Half-life is the time taken for half of the atoms in a radioactive sample to decay. It is fixed for each isotope and is not changed by heat or chemical state. For example, iodine-131 has a half-life of about 8 days, cobalt-60 about 5.3 years and uranium-238 about 4.5 billion years.

What is the difference between becquerel and sievert?

The becquerel measures the activity of a radioactive source, where one becquerel is one nuclear decay per second. The sievert measures the biological effect of radiation on the human body, taking into account the type of radiation and the sensitivity of tissues. The gray measures only the energy absorbed per kilogram.

What is the difference between ionising and non-ionising radiation?

Ionising radiation, such as alpha, beta, gamma rays, X-rays and neutrons, has enough energy to remove electrons from atoms and damage living cells. Non-ionising radiation, such as radio waves, microwaves, infrared and visible light, does not have enough energy to ionise atoms, though intense non-ionising radiation can still heat tissue.

What is background radiation?

Background radiation is the radiation that is always present from natural sources, such as cosmic rays from space, radioactive elements in rocks and soil, radon gas and the human body itself. It gives an average dose of about 2 to 3 millisievert per year worldwide, and is higher in some regions with thorium rich sands.

PYQ Practice — Statement Analysis

1 Gamma rays have greater penetrating power than alpha particles.

True

They need thick lead to stop.

2 The unit of radioactivity is the sievert.

False

It is the becquerel.

3 Radioactivity was discovered by Henri Becquerel in 1896.