ICSE Class 10 Physics Modern Physics: Radioactivity Introduction

Ever wondered why old watches still tick after decades? That tiny glow inside comes from a natural source of energy called radioactivity.

Radioactivity is the spontaneous release of tiny particles or energy from an unstable atom. Think of a wobbling Jenga tower that eventually drops a block – the atom ‘drops’ something to become stable.

What is Radioactivity?

Atoms are made of a nucleus (center) and electrons swirling around. The nucleus contains protons (positive charge) and neutrons (no charge). When the balance of forces inside the nucleus is off, the atom becomes unstable. An unstable atom tries to fix itself by emitting something – that emission is radioactivity.

Types of Radioactive Decay

There are three main ways an unstable nucleus can calm down. Each type spits out a different particle or wave.

Alpha Decay

An alpha particle is just a helium nucleus – two protons and two neutrons stuck together. Imagine a heavy truck losing a small car; the truck gets lighter and moves faster. Alpha particles are heavy and carry a +2 charge, so they don’t travel far – a sheet of paper can stop them.

Beta Decay

Beta decay comes in two flavors. In beta-minus decay, a neutron turns into a proton and shoots out an electron (a tiny negatively‑charged particle). It’s like swapping a brick for a marble and tossing the marble out. In beta-plus decay, a proton becomes a neutron and a positron (the electron’s positively‑charged twin) flies away. Beta particles are lighter than alpha particles, so they penetrate further – a thin piece of aluminum can block them.

Gamma Decay

After an alpha or beta step, the new nucleus may still be “excited” – like a stretched rubber band. It releases excess energy as a gamma ray, which is just high‑energy electromagnetic radiation (think X‑ray but stronger). Gamma rays have no mass or charge, so they zip right through most materials; you need thick lead or concrete to tame them.

Comparison of Decay Types

Simple Decay Process Flowchart

Worked Example: Half‑Life Calculation

Suppose a sample contains 8×10^6 atoms of a radionuclide with a half‑life of 3 years. How many atoms remain after 9 years?

• One half‑life (3 yr) cuts the number in half: 8×10^6 → 4×10^6.
• Two half‑lives (6 yr) halve it again: 4×10^6 → 2×10^6.
• Three half‑lives (9 yr) halve once more: 2×10^6 → 1×10^6.

So after 9 years, only one million atoms are left. Remember the rule: after n half‑lives, the remaining fraction is (1/2)^n.

Key Points to Remember

• Radioactivity = spontaneous emission from an unstable nucleus.
• Alpha = heavy, +2 charge, stopped by paper.
• Beta‑minus = electron, medium penetration, stopped by aluminium.
• Beta‑plus = positron, similar to beta‑minus.
• Gamma = high‑energy photon, needs dense shielding.
• Half‑life tells how long it takes for half the atoms to decay.

📝 Likely Exam Questions

1. Define radioactivity and give one everyday example.
   Radioactivity is the spontaneous emission of particles or energy from an unstable atom. Example: the glow in old watch dials comes from radium.
2. Write the three types of radioactive decay and mention one characteristic of each.
   Alpha – heavy +2 charge, stopped by paper.
   Beta‑minus – electron emitted, stopped by aluminium.
   Gamma – high‑energy photon, requires lead.
3. A sample of a radionuclide has a half‑life of 5 years. How many half‑lives pass in 20 years and what fraction of the original nuclei remains?
   20 yr ÷ 5 yr = 4 half‑lives. Remaining fraction = (1/2)^4 = 1/16.
4. Why is a lead shield used in X‑ray rooms?
   Lead is dense and stops gamma‑type radiation, which has high penetration power.
5. Explain, with a short analogy, why an alpha particle cannot travel far.
   An alpha particle is like a heavy bowling ball trying to roll through a garden of leaves; it quickly loses energy and stops.