Radioactivity A Level Comprehensive
Notes
1. Introduction to Radioactivity
Radioactivity refers to the spontaneous disintegration of unstable atomic nuclei, resulting in the
emission of radiation in the form of particles or electromagnetic waves. It is a nuclear process,
meaning it occurs within the nucleus of an atom and not due to chemical reactions.
The study of radioactivity is an essential part of A Level Physics and Chemistry, particularly in topics
concerning atomic structure, nuclear reactions, energy, and medical applications.
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2. Discovery and Historical Background
Henri Becquerel (1896): Discovered natural radioactivity while investigating phosphorescence in
uranium salts.
Marie and Pierre Curie: Identified two new radioactive elements, polonium and radium.
The term radioactivity was coined by the Curies.
These discoveries marked the beginning of nuclear physics and modern atomic theory.
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3. Structure of the Atom and Isotopes
Atoms consist of protons, neutrons, and electrons.
The nucleus contains protons and neutrons (nucleons).
Isotopes are atoms of the same element with the same number of protons but different numbers of
neutrons.
Some isotopes are stable, while others are unstable and undergo radioactive decay.
Nuclear Notation
Example:
23892U
238 = mass number (A) = protons + neutrons
92 = atomic number (Z) = number of protons
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4. Types of Radiation
4.1 Alpha Radiation ()
Consists of 2 protons and 2 neutrons (Helium nucleus).
Mass number decreases by 4; atomic number decreases by 2.
Weak penetrating power: stopped by paper or skin.
Strong ionising ability.
Example:
23892U 23490Th + 42He
4.2 Beta Radiation ()
Beta-minus (): Emission of an electron when a neutron transforms into a proton.
Increases atomic number by 1.
Example:
146C 147N + + anti-neutrino
Beta-plus (+): Emission of a positron when a proton transforms into a neutron.
Decreases atomic number by 1.
4.3 Gamma Radiation ()
Electromagnetic radiation emitted from an excited nucleus.
No change in mass or atomic number.
Highly penetrating; reduced by thick lead or concrete.
Least ionising among the three.
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5. Properties and Comparison Table
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6. Nuclear Equations
Radioactive decay is represented using nuclear equations, where both mass number and atomic
number are conserved.
Example Alpha Decay: 22688Ra 22286Rn + 42He
Example Beta-minus Decay: 32Li 33Be + + e
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7. Half-life (T12)
The half-life is the time taken for half of the nuclei in a radioactive sample to decay.
Exponential decay: The number of undecayed nuclei (N) at time t is given by:
N(t) = N0 e^(-t)
Where:
N0 = initial number of nuclei
= decay constant
t = time
Relation between half-life and decay constant:
T12 = ln(2) /
Graphical representation: A decay curve shows exponential decrease of N over time.
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8. Activity (A)
Activity is the number of disintegrations per second.
Measured in becquerels (Bq).
A=N
Where A = activity, = decay constant, N = number of undecayed nuclei.
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9. Radioactive Decay Series
Some elements undergo a series of decays until they reach a stable isotope. For example,
uranium-238 decays through multiple steps to become lead-206.
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10. Detection of Radiation
10.1 Geiger-Muller (GM) Tube
Detects ionising radiation.
Produces a click or pulse per detection.
Measures activity.
10.2 Photographic Film
Darkens when exposed to radiation.
Used in film badges for workers in radioactive environments.
10.3 Cloud Chamber
Shows visible tracks of charged particles.
Different types of radiation produce different track patterns.
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11. Dangers of Radiation
Biological Effects
Ionisation can damage or kill cells.
Causes mutations, cancer, burns, or radiation sickness.
Deterministic effects: Severity increases with dose.
Stochastic effects: Probability increases with dose.
Safety Measures
Use of lead shielding.
Minimizing exposure time.
Keeping distance.
Radiation monitors and badges.
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12. Applications of Radioactivity
In Medicine
Radiotherapy: Treats cancer using gamma rays (e.g., cobalt-60).
Medical Imaging: Uses tracers (e.g., technetium-99m in PET scans).
In Industry
Thickness Control: Radioactive sources measure material thickness.
Leak Detection: Tracers locate pipeline leaks.
In Archaeology
Carbon Dating (Carbon-14): Determines age of ancient organic materials.
Half-life of carbon-14 = 5730 years.
In Power Generation
Nuclear Reactors: Use controlled fission reactions to generate electricity.
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13. Nuclear Fission and Fusion (Related Topics)
Though not strictly part of radioactivity, nuclear fission and fusion are related nuclear reactions:
Fission
Splitting of heavy nuclei (e.g., uranium-235) into lighter nuclei and release of energy.
Produces radioactive waste.
Fusion
Combining light nuclei (e.g., hydrogen) to form heavier ones.
Powers the sun; clean and efficient, but not yet fully practical.
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14. Sample A Level Questions
1. Define half-life and derive its mathematical expression.
2. Explain the difference between alpha, beta, and gamma radiation in terms of composition and
penetration.
3. Calculate the remaining activity of a sample after 3 half-lives.
4. Describe the working of a Geiger-Muller tube.
5. Explain how carbon dating works and its limitations.
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15. Summary
Radioactivity is a nuclear process involving unstable nuclei.
Three types of emissions: alpha (), beta (), and gamma ().
Key concepts: half-life, decay constant, activity.
Detection methods include GM tubes, cloud chambers, and photographic film.
While useful in medicine and industry, radiation poses health risks and requires safety precautions.
Understanding radioactivity is fundamental in many advanced areas of physics, chemistry, and
modern technology.
