Radioactivity

O Level / IGCSE Physics · Nuclear physics. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

Test this topic free All O Level / IGCSE Physics topics

The idea

Some nuclei are unstable. They change by giving out alpha particles, beta particles or gamma radiation. This decay is spontaneous and random: nothing you do to the sample changes it, and you cannot say which nucleus will decay next.

A Geiger-Müller tube and counter measure the count rate. Some counts always come from background radiation (radon gas, rocks and buildings, food and drink, cosmic rays), so subtract the background to get the corrected count rate.

The half-life is the time taken for half the nuclei of an isotope in a sample to decay. In fission a large nucleus such as uranium-235 absorbs a neutron and splits, releasing energy and more neutrons. In fusion two small nuclei join, which is how stars release energy.

Rules to remember

Alpha (α): 2 protons + 2 neutrons, charge 2+, most ionising, stopped by paper; pulled towards a negative plate
Beta (β): fast electron from the nucleus, charge 1−, stopped by a few millimetres of aluminium; pulled towards a positive plate and deflected more than alpha
Gamma (γ): electromagnetic wave, no charge, least ionising, most penetrating (reduced by thick lead); not deflected by electric or magnetic fields
Alpha decay: nucleon number goes down by 4, proton number goes down by 2
Beta decay: nucleon number stays the same, proton number goes up by 1. Gamma emission changes neither
corrected count rate = measured count rate − background count rate
After 1, 2, 3 half-lives the fraction left is 1/2, 1/4, 1/8

Common mistake

Halving the reading on the counter without taking off the background first. Subtract the background, do the half-life steps on the corrected count rate, then add the background back if the question asks what the detector reads.

Worked example

A detector near a source reads 340 counts/min. Twenty-four days later it reads 60 counts/min. The background count rate is 20 counts/min. What is the half-life of the source?

Corrected count rate at the start = 340 − 20 = 320 counts/min.
Corrected count rate after 24 days = 60 − 20 = 40 counts/min.
Halve until you reach 40: 320 → 160 → 80 → 40, which is 3 half-lives.
Half-life = 24 ÷ 3 = 8 days.

Answer: 8 days

Practice questions

Try each one, then open the answer.

1. Which is the largest source of background radiation for most people?

A
nuclear power stations
B
radon gas from rocks and soil
C
medical X-rays
D
cosmic rays only

Show answer

Answer: B. Radon gas from the ground is the biggest contributor for most people, followed by rocks and buildings, food and drink, cosmic rays and medical uses.

2. In nuclear fission, a uranium-235 nucleus

A
joins with another nucleus to form a heavier one
B
absorbs a neutron and splits into two smaller nuclei, releasing neutrons and energy
C
emits an alpha particle and becomes stable
D
loses an electron

Show answer

Answer: B. Fission: a heavy nucleus absorbs a neutron, splits into daughter nuclei and releases more neutrons, which can cause a chain reaction, plus energy.

3. Alpha, beta and gamma radiation pass between two oppositely charged parallel plates. Which radiation is deflected the most, and towards which plate?

A
alpha, towards the negative plate
B
gamma, towards the positive plate
C
beta, towards the negative plate
D
beta, towards the positive plate

Show answer

Answer: D. Beta particles are negative electrons with a very small mass, so they are deflected most, towards the positive plate. Alpha particles are deflected much less, towards the negative plate, and gamma rays are not deflected.

Read the full notes

Keep going

← The nuclear model of the atom Earth and the Solar System →All rules on one page Stuck? Ask a question