Which one of the following is the unit of activity of a radioactive source?

1. Fundamental SI Units and Derived Units (basic)

To understand the universe—from the massive galaxies down to the tiny atoms we will study in this module—we need a standard language of measurement. This language is the International System of Units (SI). At its foundation are seven Base Units, which are independent physical quantities like length (meter), mass (kilogram), and time. For instance, the standard unit of time is the second (s), always written in lowercase and singular form Science, Class VII (NCERT 2025), Measurement of Time and Motion, p.111. These base units are the building blocks for everything else. Most other units we use in physics are Derived Units. These are created by mathematically combining base units. For example, when we combine mass, length, and time, we get the unit for Force, which is the newton (N) Science, Class VIII (NCERT 2025), Exploring Forces, p.65. Similarly, volume (like cm³) is derived from length, and resistivity (Ω m) is a derived property of materials Science, Class X (NCERT 2025), Electricity, p.178. In our upcoming study of nuclear physics, we will encounter the Becquerel (Bq), a derived unit that measures radioactivity as one disintegration per second (1/s). When writing these units, precision matters. Following standard conventions, unit names like 'newton' or 'second' begin with a lowercase letter, but their symbols are capitalized if named after a person (e.g., 'N' for Newton, 'Bq' for Becquerel) and lowercase otherwise (e.g., 's' for second) Science, Class VII (NCERT 2025), Measurement of Time and Motion, p.111.

Type of Unit	Definition	Examples
Base Unit	Fundamental, independent units.	Meter (m), Kilogram (kg), Second (s), Ampere (A)
Derived Unit	Formed by combining base units.	Newton (N), Joule (J), Watt (W), Becquerel (Bq)

Sources: Science, Class VII (NCERT 2025), Measurement of Time and Motion, p.111; Science, Class VIII (NCERT 2025), Exploring Forces, p.65; Science, Class X (NCERT 2025), Electricity, p.178; Science, Class VIII (NCERT 2025), The Amazing World of Solutes, Solvents, and Solutions, p.146

2. The Atomic Nucleus and Radioactivity (intermediate)

To understand radioactivity, we must first look at the heart of the atom: the nucleus. Most atoms around us are stable, but some possess an 'uncomfortable' nucleus—one that is either too heavy or has an unstable ratio of protons to neutrons. To reach a more stable state, these nuclei undergo spontaneous disintegration, a process we call radioactivity Environment, Shankar IAS Academy, Environmental Pollution, p.82. During this breakdown, the atom sheds excess energy by emitting three primary types of radiation:

• Alpha (α) particles: Essentially helium nuclei consisting of two protons and two neutrons.
• Beta (β) particles: High-energy, high-speed electrons or positrons.
• Gamma (γ) rays: Short-wave electromagnetic waves that carry no mass but immense energy Environment, Shankar IAS Academy, Environmental Pollution, p.84.

How do we measure this activity? In the scientific world, we use the Becquerel (Bq). Named after Henri Becquerel, the discoverer of radioactivity, one Becquerel represents exactly one nuclear disintegration per second. While you might encounter the Curie (Ci) in older texts or specific industrial contexts, the Becquerel is the standard International System (SI) unit for expressing the strength of a radioactive source. Every radioactive substance decays at its own unique, constant rate known as its half-life—the time required for half of the radioactive atoms in a sample to decay Environment, Shankar IAS Academy, Environmental Pollution, p.83. This is why some isotopes, like those from mining or nuclear power plants, remain dangerous for thousands of years, while others vanish in seconds Environment, Shankar IAS Academy, Environmental Pollution, p.79.

Sources: Environment, Shankar IAS Academy, Environmental Pollution, p.82; Environment, Shankar IAS Academy, Environmental Pollution, p.83; Environment, Shankar IAS Academy, Environmental Pollution, p.79

3. Nuclear Reactions: Fission and Fusion (intermediate)

To understand nuclear reactions, we must first look at the binding energy that holds an atom's nucleus together. Unlike chemical reactions, which involve the exchange of electrons, nuclear reactions involve changes in the nucleus itself, releasing millions of times more energy. This energy release is governed by Einstein’s famous equation, E = mc², where a tiny amount of lost mass is converted into a massive amount of energy.

Nuclear Fission occurs when a heavy, unstable nucleus (like Uranium-235 or Plutonium-239) is struck by a neutron and splits into two or more lighter nuclei. This process releases additional neutrons, which can go on to split more atoms, creating a self-sustaining chain reaction. This is the principle behind nuclear power plants and the fission devices tested by India during the 1974 and 1998 Pokhran tests A Brief History of Modern India (Spectrum), After Nehru..., p.754. To manage this safely, strict protocols for waste disposal and accident prevention are essential, as radiation damage has no simple cure Environment (Shankar IAS), Environmental Pollution, p.83.

Nuclear Fusion is the opposite process: two light nuclei (typically isotopes of Hydrogen like Deuterium and Tritium) combine to form a heavier nucleus (Helium). While fusion releases even more energy than fission and produces less radioactive waste, it requires extreme temperature and pressure to overcome the electrostatic repulsion between nuclei. These conditions exist in the cores of stars, but notably, the Earth's interior is not massive enough to generate the pressure required for natural fusion to occur Physical Geography by PMF IAS, Earths Interior, p.59. Human-made fusion was demonstrated by India in the 1998 "Operation Shakti" tests A Brief History of Modern India (Spectrum), After Nehru..., p.754.

Feature	Nuclear Fission	Nuclear Fusion
Process	Splitting a heavy nucleus into lighter ones.	Fusing light nuclei into a heavier one.
Fuel	Uranium, Plutonium, Thorium Majid Hussain, Distribution of Resources, p.40.	Isotopes of Hydrogen (Deuterium, Tritium).
Conditions	Requires critical mass and neutron bombardment.	Requires millions of degrees of temperature.
Energy Yield	High.	Extremely High (3-4 times fission).

India’s strategic approach to these technologies is defined by its Nuclear Doctrine, which emphasizes a "No First Use" policy and maintaining a "credible minimum deterrent" Indian Polity (Laxmikanth), Foreign Policy, p.611. Furthermore, India is a pioneer in researching Thorium as a future fuel source, as seen in the Kakrapara-1 reactor, utilizing the vast monazite sands found on its shores Environment and Ecology (Majid Hussain), Distribution of World Natural Resources, p.40.

Sources: A Brief History of Modern India (Spectrum), After Nehru..., p.754; Physical Geography by PMF IAS, Earths Interior, p.59; Indian Polity (Laxmikanth), Foreign Policy, p.611; Environment (Shankar IAS), Environmental Pollution, p.83; Environment and Ecology (Majid Hussain), Distribution of World Natural Resources, p.40

4. Applications of Isotopes in Science & Medicine (exam-level)

To understand how isotopes serve humanity, we first need a way to measure them. In physics, the strength of a radioactive source is known as its activity, which refers to the frequency of radioactive disintegrations occurring per unit of time. The standard International System of Units (SI) unit for this is the Becquerel (Bq), named after Henri Becquerel. One Becquerel is defined as one radioactive decay per second. While you may also encounter the Curie (Ci) in older texts or specific medical contexts, the Bq is the official scientific standard for expressing source strength.

In the realm of science, isotopes act as 'atomic clocks.' Radiocarbon dating, specifically using the unstable isotope Carbon-14 (¹⁴C), allows archaeologists to determine the age of organic remains. For instance, in the Keeladi excavations in Tamil Nadu, scientists used Accelerator Mass Spectrometry (AMS) dating on carbon samples to trace the site's history back to 580 BCE History, class XI (Tamilnadu state board 2024 ed.), Evolution of Society in South India, p.70. Because the half-life of ¹⁴C is known, measuring the remaining activity in a sample reveals how long ago the organism (like a tree or animal) ceased its intake of carbon from the atmosphere.

In medicine, isotopes are indispensable tools for both diagnosis and therapy. Diagnostic isotopes, often called 'tracers,' are used to create images of internal organs. For example, Technetium-99m is the most widely used medical isotope for imaging the skeleton and heart muscle. For treatment, high-energy isotopes like Cobalt-60 are used in radiotherapy to destroy cancer cells by damaging their DNA.

Field	Isotope / Concept	Primary Application
Archaeology	Carbon-14 (¹⁴C)	Determining the age of organic artifacts (Dating).
Medicine (Diagnostic)	Iodine-131	Assessing thyroid function and treating hyperthyroidism.
Medicine (Therapy)	Cobalt-60	Gamma-ray therapy for localized cancer treatment.
Industry	Americium-241	Used in smoke detectors for safety.

Sources: History, class XI (Tamilnadu state board 2024 ed.), Evolution of Society in South India, p.70

5. Units of Light, Magnetism, and Electricity (intermediate)

In our journey through physics, understanding physical quantities is only half the battle; we must also master the units used to measure them. These units allow us to quantify everything from the brightness of a lamp to the invisible pull of a magnet or the decay of an atom.

Let’s start with Light. We categorize objects as luminous if they emit their own light, like the Sun or a glowing filament (Science-Class VII . NCERT(Revised ed 2025), Light: Shadows and Reflections, p.154). When this light falls on a surface, we measure the intensity of that "illumination" using a unit called the Lux. While the total light emitted by a source is measured in lumens, the Lux tells us how much light is actually hitting a specific area—crucial for knowing if a study desk is bright enough for reading.

When we move to Magnetism and Electricity, the units become more specialized. An electric current flowing through a wire creates a magnetic field (Science, class X (NCERT 2025 ed.), Magnetic Effects of Electric Current, p.207). The strength of this magnetic field (technically called magnetic flux density) is measured in Tesla (T). In the realm of electricity, while we measure resistance in Ohms, we measure conductance—the ease with which current flows—in Siemens (S). Essentially, Siemens is the mathematical reciprocal of the Ohm.

Finally, since this is part of your path in nuclear physics, we must address the strength of a radioactive source. Radioactivity is the frequency at which unstable atoms decay. The standard SI unit for this is the Becquerel (Bq), named after Henri Becquerel. One Becquerel is defined as exactly one atomic disintegration per second. While you might see the term Curie in older texts or medicine, the Becquerel is the modern scientific standard.

Field	Quantity	SI Unit
Light	Illuminance	Lux (lx)
Magnetism	Magnetic Flux Density	Tesla (T)
Electricity	Electrical Conductance	Siemens (S)
Nuclear Physics	Radioactive Activity	Becquerel (Bq)

Sources: Science-Class VII . NCERT(Revised ed 2025), Light: Shadows and Reflections, p.154; Science, class X (NCERT 2025 ed.), Magnetic Effects of Electric Current, p.207; Science, class X (NCERT 2025 ed.), Magnetic Effects of Electric Current, p.202

6. Measuring Radioactivity and Radiation Dose (exam-level)

When we talk about measuring radiation, we are actually looking at two distinct things: the strength of the source (how many atoms are popping per second) and the impact on the target (how much energy is absorbed by a person or object). Understanding these units is vital for UPSC, as they appear frequently in topics ranging from nuclear energy to disaster management.

The fundamental unit for the activity of a radioactive source is the Becquerel (Bq). Named after Henri Becquerel, the co-discoverer of radioactivity, one Becquerel is defined as one radioactive decay (or disintegration) per second. While you might also encounter the Curie (Ci)—a much larger unit based on the activity of one gram of radium—the Becquerel is the standard SI unit used globally to describe how "active" a sample is.

However, knowing how fast a source is decaying doesn't tell us the whole story. We also need to measure the Radiation Dose. This is where units like the Gray (Gy) and Sievert (Sv) come in. While the Gray measures the physical energy deposited in a material, the Sievert is the most critical unit for human health, as it adjusts that energy based on the biological damage caused to human tissue. To keep things clear, remember that Bq measures the "source," Gy measures the "absorption," and Sv measures the "biological risk."

Unit	What it Measures	Analogy
Becquerel (Bq)	Source Activity (Decays per second)	The number of sparks flying off a fire.
Gray (Gy)	Absorbed Dose (Energy per kg)	How much heat your clothes actually take in.
Sievert (Sv)	Effective Dose (Biological Impact)	How likely that heat is to cause a burn on your skin.

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental principles of nuclear physics and scientific measurements, this question serves as a direct application of your knowledge regarding radioactive decay. The 'activity' of a radioactive source is essentially a measure of its intensity—specifically, how many atomic nuclei disintegrate every second. By connecting the concept of decay rates to the standard International System of Units (SI), we can see that the building blocks of measurement units are designed to honor the pioneers of the field.

To arrive at the correct answer, think back to the discovery of radioactivity. The unit is named after Henri Becquerel, who first observed this phenomenon. One decay per second is defined as one Becquerel (Bq). While you may encounter the 'Curie' (Ci) in various contexts, as highlighted in REMM Radiation Measurement Systems, the Becquerel is the official SI unit. Therefore, when the question asks for the standard unit of activity, your reasoning should immediately point toward the scientist who started it all.

UPSC frequently uses "distractor" units from unrelated chapters of physics to test your precision. For instance, Lux is used to measure illuminance in optics, while Tesla and Siemens are staples of electromagnetism, representing magnetic flux density and electrical conductance respectively. Identifying these as 'mismatched' units is a vital exam strategy; by recognizing that three options belong to light and electricity, you can confidently isolate Becquerel as the only term belonging to the domain of nuclear activity.