Welcome to Nuclear Physics: Understanding the Heart of Matter!

Hello future scientists! Nuclear physics sounds intimidating, but it’s actually one of the most fascinating topics in science. It deals with the tiny, dense core of the atom—the nucleus—and how it behaves.


In this chapter, we will uncover the secrets of unstable nuclei, understand why they decay, and explore the powerful and often dangerous radiation they emit. Don't worry if this seems tricky at first; we will break down the concepts using simple analogies and clear steps!


Why is this important? Nuclear physics underpins medical treatments, provides vital power sources, and helps us understand the age of our Earth.


1. Reviewing Atomic Structure and Isotopes

To understand nuclear physics, we first need to quickly recall the structure of the atom:

  • The nucleus contains protons (positive charge, mass = 1) and neutrons (no charge, mass = 1).
  • Electrons orbit the nucleus (negative charge, negligible mass).
Key Definitions

Mass Number (\(A\)): The total number of protons and neutrons in the nucleus.

Atomic Number (\(Z\)): The number of protons (this determines the element).


Isotopes:

An isotope is an atom of the same element (meaning they have the same number of protons, or the same Z) but a different number of neutrons (meaning they have a different mass number, or A).

Analogy: Think of isotopes as siblings. They share the same parents (the element/protons) but can look slightly different (different number of neutrons/mass).

Example: Carbon-12 and Carbon-14. Both have 6 protons (Z=6), but Carbon-12 has 6 neutrons, while Carbon-14 has 8 neutrons. Carbon-14 is an unstable isotope.


Quick Review: Isotopes are atoms of the same element with different masses.

2. Radioactivity and Unstable Nuclei

Not all isotopes are stable. Some nuclei are too large or have too many neutrons, making them unstable (or radioactive).

Radioactive Decay: This is the process where an unstable nucleus spontaneously breaks down, spitting out particles and energy to try and achieve a more stable configuration.

  • This process is entirely random and spontaneous—we cannot predict exactly when a single atom will decay.
Background Radiation

We are constantly exposed to radiation, known as Background Radiation. This comes from many sources:

  1. Natural Sources:
    • Cosmic rays (high-energy particles from space).
    • Rocks and soil (especially granite, which contains radioactive elements like uranium).
    • Radon gas (a radioactive gas that seeps out of the ground).
    • Food and drink (tiny amounts of natural radioactive isotopes).
  2. Man-made Sources:
    • Medical uses (X-rays, gamma scans).
    • Nuclear industry (waste products).

3. The Three Types of Nuclear Radiation

When an unstable nucleus decays, it typically emits one of three types of radiation: Alpha, Beta, or Gamma.

Alpha (\(\alpha\)) Radiation
  • Identity: A Helium nucleus (2 protons and 2 neutrons).
  • Charge: +2 (positive).
  • Penetration Power: Very low. They are large and slow.
  • Stopped by: A sheet of paper, a few centimetres of air, or skin.
  • Ionisation Power: Very high. Because they are large and highly charged, they easily knock electrons off other atoms, causing damage.
Beta (\(\beta\)) Radiation
  • Identity: A fast-moving electron. (Wait, where does the electron come from? A neutron in the nucleus turns into a proton, spitting out an electron in the process!)
  • Charge: -1 (negative).
  • Penetration Power: Medium. They travel faster than alpha particles.
  • Stopped by: A few millimetres of aluminium.
  • Ionisation Power: Medium. They cause less damage than alpha particles but more than gamma rays.
Gamma (\(\gamma\)) Radiation
  • Identity: High-energy electromagnetic wave (like X-rays, but higher energy). They are pure energy.
  • Charge: 0 (neutral).
  • Penetration Power: Very high. They travel at the speed of light and can pass through most materials.
  • Stopped by: Thick lead or several metres of concrete.
  • Ionisation Power: Very low. They usually pass straight through tissue without interaction.

Memory Trick (Penetration):

Think APB: Alpha is stopped by Paper, Beta is stopped by Aluminium.

Key Takeaway: Alpha is great for ionising but poor at penetrating. Gamma is great at penetrating but poor at ionising.

4. Measuring Decay: Half-Life (\(T_{1/2}\))

Since we cannot predict when a single atom will decay, we measure the overall speed of decay using a concept called Half-life.

Activity: The rate at which a source decays is called its activity, measured in Becquerels (Bq). 1 Bq means 1 decay per second.


Half-life (\(T_{1/2}\)):

The half-life is the time taken for the activity (or the number of undecayed radioactive nuclei) of a sample to halve.

Analogy: Imagine you have 16 slices of radioactive pizza. If the half-life is 1 hour:

  1. Start: 16 slices.
  2. After 1 hour (1 half-life): 16 / 2 = 8 slices remain.
  3. After 2 hours (2 half-lives): 8 / 2 = 4 slices remain.
  4. After 3 hours (3 half-lives): 4 / 2 = 2 slices remain.

Why is half-life important?

Half-lives vary hugely—from fractions of a second to billions of years. This determines how dangerous a radioactive source is and how long it needs to be safely stored.

  • Sources used in medicine often have short half-lives so they decay quickly inside the patient.
  • Sources used for dating rocks (like Uranium) have very long half-lives.
Working with Half-life Calculations (Step-by-Step)

Example Question: A radioactive source has an initial activity of 800 Bq and a half-life of 5 days. What is its activity after 15 days?

Step 1: Determine how many half-lives have passed.

Total time / Half-life = 15 days / 5 days = 3 half-lives.

Step 2: Halve the activity for each half-life.

Start: 800 Bq

1st half-life (5 days): 800 / 2 = 400 Bq

2nd half-life (10 days): 400 / 2 = 200 Bq

3rd half-life (15 days): 200 / 2 = 100 Bq

Answer: The activity after 15 days will be 100 Bq.

Common Mistake to Avoid: Don't calculate the half-life from the number of decayed atoms, calculate it from the number of remaining undecayed atoms (or remaining activity).

5. Uses, Dangers, and Safety of Radiation

Uses of Radiation

Despite the dangers, controlled radiation is incredibly useful:

  1. Medical Tracers: A small amount of a radioactive isotope (often a gamma emitter with a short half-life) is injected into the body. The radiation can then be detected externally to observe how organs are functioning (e.g., checking blood flow or thyroid gland activity).
  2. Sterilisation: Strong gamma emitters are used to sterilise surgical instruments in hospitals. Gamma rays kill microbes without generating high temperatures that might damage the equipment (unlike heat sterilisation).
  3. Thickness Gauging (Industry):
    • Beta sources are used to monitor the thickness of thin sheets (like paper or aluminium foil). If the sheet gets too thick, less beta radiation passes through to the detector.
    • Gamma sources are used for thick materials (like steel) since gamma rays are highly penetrating.
  4. Smoke Detectors: They use an alpha source. Alpha particles ionise the air, allowing a small current to flow. If smoke enters the chamber, it absorbs the alpha particles, the current drops, and the alarm sounds.
Dangers of Radiation

Radiation is dangerous because of its ionising power. When alpha, beta, or gamma radiation strikes human cells:

  • It can break chemical bonds within molecules, including DNA.
  • This can lead to cell mutation, uncontrolled cell division (cancer), or cell death.
  • The damage depends on the type of radiation, the dose received, and the time exposed.

Did you know? While alpha particles are the most ionising, they are only highly dangerous if the source is inside the body (e.g., swallowed or inhaled), as the body’s skin cannot protect the internal organs.

Safety Precautions

Handling radioactive sources requires strict safety measures:

  1. Time: Minimise the time spent near the source (reduce exposure time).
  2. Distance: Use long tongs and keep the source far away whenever possible, as radiation intensity decreases rapidly with distance (inverse square law).
  3. Shielding: Use appropriate shielding materials (e.g., lead screens for gamma sources, thick aluminum for beta sources, or simply keeping alpha sources in their container).
  4. Monitoring: Workers wear dosimeters (film badges) to measure the total radiation dose received.

Final Key Takeaway: Nuclear physics is about balancing powerful energy and critical applications with strict safety measures to protect against ionising radiation.