An atom consists of three main particles:

• Protons: Positively charged particles found in the nucleus. The number of protons (atomic number, Z) identifies an element.
• Neutrons: Neutral particles also found in the nucleus. Together with protons, they make up the mass number (A = Z + N, where N is the number of neutrons).
• Electrons: Negatively charged particles orbiting the nucleus in energy levels. In a neutral atom, the number of electrons equals the number of protons.

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. For example, carbon-12 and carbon-14 are isotopes of carbon.

The nucleus contains protons and neutrons (collectively called nucleons). Because protons carry positive charges, they repel each other. However, a powerful force called the strong nuclear force overcomes this repulsion and holds the nucleus together.

• Binding energy is the energy required to separate a nucleus into its individual nucleons. The greater the binding energy, the more stable the nucleus.
• Stable atoms have sufficient binding energy to keep the nucleus permanently intact.
• Unstable atoms have insufficient binding energy and are radioactive, meaning they will spontaneously emit radiation to become more stable.

Radioactivity is the spontaneous process by which an unstable atomic nucleus loses energy by emitting ionizing radiation. This process transforms the original (parent) nucleus into a different (daughter) nucleus.

Radioactivity can be:

• Natural: Occurs in naturally occurring unstable elements like uranium and radium
• Artificial: Produced when stable elements are bombarded with particles (such as neutrons) in nuclear reactions

When radioactive decay occurs, the nucleus emits one or more types of radiation:

Alpha Particles (α)

• Consists of 2 protons and 2 neutrons (helium nucleus)
• Positively charged (+2)
• Low penetration power — stopped by a sheet of paper or human skin
• High ionization power — can cause severe damage if ingested

Beta Particles (β)

• High-energy, high-speed electrons emitted from the nucleus
• Negatively charged (-1)
• Greater penetration power than alpha — can pass through skin but stopped by aluminum foil
• Lower ionization power than alpha

Gamma Rays (γ)

• High-energy electromagnetic radiation (no charge, no mass)
• Highest penetration power — requires thick lead or concrete to stop
• Lowest ionization power but carries enormous energy

A Geiger counter (GM tube) is a common device used to detect radiation. When radiation enters the tube, it ionizes the gas inside, creating electrical pulses that are counted and displayed as a reading. The clicking sound from a Geiger counter indicates the presence of radiation.

Other detection methods include:

• Cloud chambers (show tracks of radiation particles)
• Spark counters
• Photographic film

Half-life is the time required for half of the radioactive atoms in a sample to decay. After one half-life, the count rate falls to half its original value.

Example: If a radioactive sample has an initial count rate of 800 counts per minute and a half-life of 30 minutes:

• After 30 minutes: 800 ÷ 2 = 400 counts/min
• After 60 minutes: 400 ÷ 2 = 200 counts/min
• After 90 minutes: 200 ÷ 2 = 100 counts/min

Nuclear Fission

Fission occurs when a heavy nucleus splits into smaller fragments. When a heavy nucleus (like uranium-235) absorbs a neutron, it becomes unstable and splits, releasing:

• Two or more smaller nuclei
• More neutrons
• Large amounts of energy

This process is used in nuclear power plants to generate electricity.

Nuclear Fusion

Fusion occurs when two light nuclei combine (fuse) to form a heavier nucleus, releasing enormous energy. This is the process that powers the Sun.

Key difference: Fission splits heavy nuclei; fusion combines light nuclei.

Thermionic emission is the process by which electrons escape from a heated metal surface when given enough thermal energy. When a metal is heated to high temperatures, electrons gain sufficient kinetic energy to overcome the work function (the energy needed to escape the metal surface).

This principle is applied in:

• Cathode ray tubes (old television screens)
• Electron guns
• Vacuum tubes

Question: A sample of radioactive material has an initial activity of 1600 Bq and a half-life of 4 hours. What will be its activity after 12 hours?

Solution:

Number of half-lives = 12 hours ÷ 4 hours = 3 half-lives

After 1 half-life: 1600 × ½ = 800 Bq After 2 half-lives: 800 × ½ = 400 Bq After 3 half-lives: 400 × ½ = 200 Bq

Therefore, the activity after 12 hours will be 200 Bq.

In Tanzania, radioactive materials are used in medicine — hospitals such as Bugando Medical Centre in Mwanza and Muhimbili National Hospital in Dar es Salaam use radioactive isotopes for diagnosing and treating diseases like cancer. Understanding radioactivity helps health professionals safely handle these materials and protects patients and staff from harmful radiation exposure. Additionally, radiation detection equipment (Geiger counters) is used at airports and border points to check for illegal trafficking of radioactive materials, keeping Tanzania safe.