✨ Welcome to the World of Radioactivity! ✨

Hello future physicists! This chapter, Radioactivity, might sound a bit spooky, but it's one of the most exciting and fundamental topics in modern science. You will learn about how some atoms are naturally unstable and how they release energy and particles, a process that happens constantly all around us.
Don't worry if this seems tricky at first—we will break down these tiny atomic events into simple steps using clear analogies. Let's get started on understanding the power within the nucleus!

1. Reviewing the Atom and Isotopes

The Structure of the Atom (Quick Review)

Remember that everything is made of atoms, and every atom has two main parts:

  • Nucleus: The dense, central core containing Protons (positive charge) and Neutrons (no charge).
  • Electrons: Orbiting the nucleus (negative charge).
The Proton Number (Atomic Number, Z) defines the element. The Nucleon Number (Mass Number, A) is the total number of protons and neutrons.

What Makes an Isotope?

An Isotope is an atom of the same element (meaning it has the same number of protons) but a different number of neutrons.
Example: Carbon-12 and Carbon-14 are isotopes of Carbon.

Key Concept: Radioactivity happens when the nucleus of an atom is unstable. Usually, this instability is caused by an imbalance in the number of protons and neutrons (often too many neutrons).

Takeaway: Radioactive materials use unstable isotopes whose nuclei are ready to change!

2. The Nature of Radioactive Decay

What is Radioactivity?

Radioactivity (or Radioactive Decay) is the process where an unstable atomic nucleus spontaneously emits radiation (particles or energy) to become more stable.

The Two Golden Rules of Decay

  1. It is Spontaneous: We cannot predict *when* a specific nucleus will decay. It happens randomly and is not affected by external conditions like temperature, pressure, or chemical bonding. (Think of popping popcorn – you know some will pop, but you don't know exactly which kernel or when!)
  2. It is Random: Decay is a matter of chance. While we can predict how long it takes for a large number of nuclei to decay (using half-life, discussed later), we cannot predict the decay of individual atoms.

Quick Review: Unstable nuclei decay randomly and spontaneously.

3. The Three Types of Radiation

When an unstable nucleus decays, it can emit one of three main types of radiation: Alpha, Beta, or Gamma.

A. Alpha (\(\alpha\)) Radiation

What it is: An alpha particle is essentially the nucleus of a Helium atom.
Structure: 2 Protons and 2 Neutrons.
Charge: +2 (Positive).
Mass: Heavy (High mass).

Step-by-Step Alpha Decay:

The nucleus gets rid of 4 nucleons (2 protons + 2 neutrons). This changes the element!

Formula Example (Uranium-238 to Thorium-234):
\[{}^{238}_{92}\text{U} \rightarrow {}^{234}_{90}\text{Th} + {}^{4}_{2}\alpha\]

B. Beta (\(\beta\)) Radiation

What it is: A fast-moving electron, emitted from the nucleus.
Wait, where does the electron come from? Inside the nucleus, a neutron changes into a proton and an electron. The electron is instantly fired out as a beta particle, and the new proton stays behind.
Charge: -1 (Negative).
Mass: Very light (Negligible mass).

Step-by-Step Beta Decay:

The total mass number (A) stays the same, but the proton number (Z) increases by 1. The nucleus changes into a new element.

Formula Example (Carbon-14 to Nitrogen-14):
\[{}^{14}_{6}\text{C} \rightarrow {}^{14}_{7}\text{N} + {}^{0}_{-1}\beta\]

C. Gamma (\(\gamma\)) Radiation

What it is: A high-energy electromagnetic wave (part of the EM spectrum).
Structure: Energy wave (Photons).
Charge: 0 (Neutral).
Mass: 0 (Massless).
Crucial Point: Gamma radiation is often emitted after alpha or beta decay, when the nucleus settles down from an "excited" state. It does not change the element or the mass number.

Comparison of Penetration and Ionisation Power

These three types of radiation have dramatically different properties.

Penetration Power (How far can they travel?)
  • Alpha (\(\alpha\)): Very low penetration. Stopped easily by a sheet of paper, skin, or a few centimetres of air. (Analogy: A gentle puff of air.)
  • Beta (\(\beta\)): Medium penetration. Can pass through paper but is stopped by a thin sheet of aluminium (a few millimetres).
  • Gamma (\(\gamma\)): Very high penetration. Requires thick lead or metres of concrete to significantly reduce its intensity. (Analogy: A laser beam.)
Ionisation Power (How much damage can they do?)

Ionisation is the process of knocking electrons off atoms, turning them into ions. This is how radiation damages living cells.
Rule: The heavier and slower the particle, the better it is at colliding and causing ionisation.

  • Alpha (\(\alpha\)): Highest ionising power (heavy and charged).
  • Beta (\(\beta\)): Medium ionising power.
  • Gamma (\(\gamma\)): Lowest ionising power (pure energy, difficult to interact with matter).

Memory Aid: A I L (Alpha, Ionising, Lowest Penetration)

4. Detecting and Measuring Radiation

What is Background Radiation?

Radiation is everywhere! Background Radiation is the low level of ionising radiation that is constantly present in the environment.

Sources of Background Radiation:

  • Natural Sources:
    • Radon Gas: Produced by rocks and soil (the biggest natural source).
    • Rocks and Soil: Containing trace amounts of radioactive elements.
    • Cosmic Rays: High-energy radiation from space (increases at high altitude).
  • Man-made Sources:
    • Medical uses (X-rays, gamma scans).
    • Nuclear industry/fallout (minor source).

The Geiger-Müller (GM) Tube

The most common way to detect and measure radioactivity is using a Geiger-Müller (GM) tube (or counter).

How it works (Simply): The radiation enters the tube and causes ionisation of the gas inside. This ionisation creates a small electrical pulse that the counter registers as a "click."

Units: The activity (the rate of decay) of a sample is measured in Becquerels (Bq), where 1 Bq means one decay per second.

Key Takeaway: Always measure the background radiation *before* measuring a radioactive source and subtract it from the final reading to get the accurate reading of the source itself.

5. Half-Life: The Rate of Decay

What is Half-Life?

Since decay is random, we cannot know when one atom will decay. However, we can measure how long it takes for *half* of a large sample of nuclei to decay.

The Half-life (\(T_{1/2}\)) is the time taken for:

  1. Half of the radioactive nuclei in a sample to decay, OR
  2. The count rate (activity) of the sample to halve.

Crucial Fact: Every radioactive isotope has a fixed, unique half-life, which can range from fractions of a second to billions of years.

Understanding Half-Life through Calculation

Half-life is always an exponential decay. We use a step-by-step approach to solve problems.

Analogy: Imagine you start with 100 grams of material. After one half-life, you have 50 grams left. After the second half-life, you halve 50 to get 25 grams, and so on.

Step-by-Step Example:

A radioactive source has an initial activity of 800 Bq and a half-life of 2 days. What is its activity after 6 days?

  1. Determine the number of half-lives: \(6 \text{ days} / 2 \text{ days per half-life} = 3 \text{ half-lives}\).
  2. Track the activity reduction:
    • Start: 800 Bq
    • After 1 half-life (2 days): \(800 / 2 = 400 \text{ Bq}\)
    • After 2 half-lives (4 days): \(400 / 2 = 200 \text{ Bq}\)
    • After 3 half-lives (6 days): \(200 / 2 = 100 \text{ Bq}\)
Answer: The activity after 6 days is 100 Bq.

Interpreting Half-Life Graphs

If you plot activity (Bq) against time, the curve will always look like a steep, downward slope.

How to find the half-life from a graph:

  1. Start at the initial activity (e.g., 1600 Bq).
  2. Find half of that initial activity (800 Bq).
  3. Draw a line across from 800 Bq until you hit the curve.
  4. Draw a line straight down to the time axis. This time value is the half-life.

6. Uses, Dangers, and Safety

Uses of Radioactivity

Radioactive materials have many beneficial uses, often relying on their specific half-lives and penetration powers.

  • Medical Tracers (Diagnosis): Short half-life isotopes (so they decay quickly) are injected into the body. Their radiation (usually gamma, because it penetrates the body easily) is detected externally to check organ function.
  • Treating Cancer (Radiotherapy): High-energy gamma rays are directed at cancerous tumours to kill the harmful cells. This uses strong sources with careful shielding.
  • Sterilisation: Gamma rays can sterilise medical instruments without using heat, as the radiation kills bacteria and viruses.
  • Thickness Gauging (Industry): A source (often beta or gamma) is placed on one side of a material (like paper or metal), and a detector on the other. If the material gets too thick, less radiation passes through, automatically adjusting the machinery.
  • Carbon Dating: Using the half-life of Carbon-14 (very long half-life) to determine the age of ancient organic materials.

Dangers and Safety Precautions (Handling Radioactive Sources)

Ionising radiation can damage living tissue, leading to radiation sickness or cancer. We must take precautions to minimise exposure.

Safety Precautions:
  1. Shielding: Use protective barriers (lead sheets, thick concrete walls). Alpha sources only need paper, Beta needs aluminium, and Gamma needs lead/concrete.
  2. Distance: Keep the radioactive source as far away as possible (intensity decreases rapidly with distance).
  3. Time: Minimise the time spent near the source.
  4. Handling: Always use tongs or remote handling tools, never handle sources directly.
  5. Storage: Store sources in a locked, lead-lined container when not in use.

Did You Know? The total dose of radiation received is measured in Sieverts (Sv). It is important to keep exposure "As Low As Reasonably Achievable" (ALARA).

Irradiation vs. Contamination

These two terms are often confused!

  • Irradiation: Being exposed to radiation from an external source. Once the source is removed, the person stops being irradiated. (Think of an X-ray scan.)
  • Contamination: When radioactive material lands on or enters an object (or person), making the object itself radioactive. This is much more dangerous as the exposure continues until the contaminant is removed. (Think of radioactive dust settling on skin.)

Rule: Contamination is generally more harmful than irradiation because the source is now *inside* or *on* the body, causing ionisation constantly.

Final Thoughts

You have successfully tackled one of the most powerful and fascinating chapters in Physics! Remember the characteristics of Alpha, Beta, and Gamma, master the concept of Half-life, and you will be well on your way to acing this topic. Keep practicing those half-life calculations—they are often examined! Great work!