Radioactive Decay and Nuclear Radiation | Physics

📖 In-Depth Theory

Radioactive Decay

Some atomic nuclei are UNSTABLE — they spontaneously emit radiation to become more stable.

This is RADIOACTIVE DECAY — a RANDOM process (cannot predict exactly when any nucleus will decay).

ACTIVITY: the rate at which a source decays — measured in BECQUEREL (Bq).

1 Bq = 1 decay per second.

COUNT RATE: decays recorded per second by a detector (e.g. Geiger-Müller tube).

Three main types of nuclear radiation:

ALPHA (α) — helium nucleus (2 protons + 2 neutrons)

BETA (β) — fast electron from the nucleus

GAMMA (γ) — high-energy electromagnetic wave

Properties of Alpha, Beta and Gamma

ALPHA (α):

Composition: 2 protons + 2 neutrons (helium-4 nucleus, ⁴₂He)

Charge: +2

Mass: 4 amu

Range in air: a few centimetres

Penetration: stopped by a few cm of air, or paper, or skin

Ionisation: STRONGLY ionising — causes most damage to nearby cells

BETA (β):

Composition: fast-moving electron (⁰₋₁e)

Charge: −1

Mass: negligible

Range in air: a few metres

Penetration: stopped by a few mm of aluminium

Ionisation: moderately ionising

GAMMA (γ):

Composition: high-energy electromagnetic wave (photon)

Charge: 0

Mass: 0

Range in air: effectively unlimited

Penetration: reduced by several cm of lead or metres of concrete

Ionisation: weakly ionising per unit path

DETECTION: Geiger-Müller tube connected to a counter.

Uses and Dangers of Nuclear Radiation

USES:

ALPHA — smoke detectors: alpha source ionises air → current flows → alarm triggers when smoke absorbs alpha and current drops.

BETA — paper thickness monitoring: beta passes through paper; more absorbed = thicker paper → adjust rollers.

GAMMA — medical imaging (gamma cameras), cancer treatment (radiotherapy), sterilising medical equipment, food irradiation.

GAMMA/BETA — industrial thickness gauges, pipeline fault detection.

DANGERS:

All ionising radiation damages living cells by ionising molecules in DNA → mutations → cancer.

HIGH DOSE → cell death → radiation sickness.

ALPHA: most dangerous INSIDE the body (highly ionising, can't escape). Safe outside the body (stopped by skin).

GAMMA: most dangerous OUTSIDE the body (penetrates to internal organs). Less ionising per path length.

BETA: intermediate — penetrates skin, absorbed by soft tissue.

PROTECTION:

Distance — inverse square law applies (intensity decreases with distance).

Shielding — appropriate materials (paper for alpha, aluminium for beta, lead for gamma).

Time — minimise exposure duration.

Monitoring — dosimeters worn by radiation workers.

⚠️ Common Mistake

Alpha is the MOST ionising but LEAST penetrating. Gamma is the LEAST ionising per path length but MOST penetrating. These are often confused. Alpha is most dangerous INSIDE the body; gamma is most dangerous OUTSIDE the body.

📌 Key Note

α: helium nucleus (+2), stopped by paper, most ionising. β: fast electron (−1), stopped by aluminium, moderate. γ: EM wave (0 charge), needs lead/concrete, least ionising per path. Activity in Bq. Decay is random. Uses: smoke detectors (α), thickness gauges (β), cancer treatment/sterilisation (γ).

🎯 Matching Activity — Radiation Properties

Match each radiation type to its composition, penetration and use. — drag the symbols on the right to match the component names on the left.

Alpha (α)

Drop here

Beta (β)

Drop here

Gamma (γ)

Drop here

Most ionising

Drop here

Most penetrating

Drop here

Fast electron — stopped by aluminium — used in paper thickness monitoring

Gamma radiation — passes through most materials, needs lead/concrete shielding

EM wave — needs lead/concrete — used in cancer radiotherapy and sterilisation

Alpha radiation — causes most ion pairs per cm of path

Helium nucleus — stopped by paper — used in smoke detectors

🎯 Test Yourself

Question 1 of 2

1. Why is alpha radiation the most dangerous type when a source is inside the body?

2. A Geiger counter measures 340 Bq from a radioactive source. What does this mean?

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⚡ Radioactive Decay and Nuclear Radiation

Radioactive Decay

Properties of Alpha, Beta and Gamma

Uses and Dangers of Nuclear Radiation

Mr. Badmus AI