Ch 4 · 5.2.2 The Three Types of Emission · O Level Physics 5054
Topic 4 of 8 in Nuclear Physics — O Level Physics 5054 — 45 lessons.
Introduction
Unstable nuclei give out three kinds of radiation, named after the first three letters of the Greek alphabet: alpha, beta and gamma. They differ in what they are, how far they travel and how much damage they do. (Syllabus section 5.2.2.)
Key ideas — Spontaneous and random
The emission of radiation from a nucleus is:
🧑🏫 Teacher's explanation
Imagine you are standing in a crowded room where people are throwing tennis balls randomly. You cannot predict exactly when a ball will be thrown (spontaneous) or which direction it will fly (random in direction). Even if you change the temperature of the room or how loud the music is, the balls keep flying out at unpredictable times and angles.
📖 Definitions
🔍 Worked example — Easy
A sample of a radioactive isotope is placed in a vacuum. If the temperature of the sample is increased by 50 ^circC, how does the rate of decay change?
🔍 Worked example — Standard
A single nucleus of a radioactive isotope is about to decay. Can we predict the exact time and the exact direction in which the emitted α-particle will travel?
🔍 Worked example — Exam-style
A detector is placed 10 cm from a radioactive source. If the source is rotated 90^° to the side, the number of particles detected per minute changes from 500 to 450. Explain why this change occurred based on the properties of emission.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State two characteristics of the emission of these particles. [2] (b) Explain why the rate of emission does not change when the sample is heated. [1] [3 marks]
Key ideas — What each radiation is
🧑🏫 Teacher's explanation
Imagine a crowded, chaotic nightclub where people are suddenly being pushed out of the doors. You can't predict exactly who will leave or which door they will exit through, and they don't all leave at once. This is exactly how a nucleus behaves during radioactive decay: it is a spontaneous and random process. The nucleus is unstable and 'spits out' particles to try and become stable.
📖 Definitions
🔍 Worked example — Easy
A radioactive source emits α, β, and γ radiation. Arrange them in order of increasing penetrating power.
🔍 Worked example — Standard
An α-particle is deflected in a uniform electric field. If the particle has a charge of +2e and a mass of 4u, and a β-particle has a charge of -1e and a mass of 1/2000u, which particle will experience a greater force in the same electric field?
🔍 Worked example — Exam-style
A nucleus emits a particle that is observed to be deflected by a magnetic field. The particle is deflected towards the positive plate in an electric field. Identify the particle type and its charge.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Ionising and penetrating
Ionising effect: how strongly the radiation knocks electrons off atoms it passes. Penetrating power: how far it travels and what it can pass through.
🧑🏫 Teacher's explanation
Imagine you are throwing objects through a crowded room to see what happens. An alpha particle is like throwing a heavy bowling ball; it crashes into everything (high ionising power), knocking people over immediately, but it can't get very far before it loses all its momentum. A beta particle is like throwing a tennis ball; it might hit a few people, but it travels much further than the bowling ball. A gamma ray is like a laser beam; it passes through most people without hitting them at all, traveling a huge distance before it finally hits something (low ionising power, high penetration).
📖 Definitions
🔍 Worked example — Easy
A source emits alpha particles. Which of the following is true regarding its ionising power and penetrating power compared to beta particles? (a) Higher ionising, lower penetration (b) Lower ionising, higher penetration (c) Higher ionising, higher penetration
🔍 Worked example — Standard
A technician is using a Geiger-Müller tube to detect radiation. They place a thin sheet of paper between the source and the tube. The count rate drops significantly. Which type of radiation is being blocked?
🔍 Worked example — Exam-style
An unknown radiation source is placed behind a 5mm sheet of aluminium. The detector still records a high count rate. However, when a 2cm thick block of lead is placed in the path, the count rate drops to near zero. Identify the radiation.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State which type of radiation has the highest ionising effect. (b) A thin sheet of aluminium is placed between the source and a detector. The β radiation is absorbed, but the γ radiation is not. Explain why γ radiation is more penetrating than β radiation. [3] [3 marks]
Key ideas — Deflection in electric and magnetic fields
Charged particles are deflected by electric and magnetic fields; uncharged radiation is not.
🧑🏫 Teacher's explanation
Imagine a busy airport terminal. An α-particle is like a heavy, slow-moving freight train; it's hard to turn and has massive momentum. A β-particle is like a tiny, fast-moving scooter; it can zip around corners very easily because it has very little mass. A γ-ray is like a beam of light; it has no mass and no charge, so it just zips straight through without being affected by the 'traffic' of electric or magnetic forces.
📖 Definitions
🔍 Worked example — Easy
A beam of radiation passes through a pair of parallel charged plates. If the beam shows no deflection, what type of radiation is it most likely to be?
🔍 Worked example — Standard
An α-particle and a β-particle enter a uniform electric field. Which particle will show a greater deflection from its original path?
🔍 Worked example — Exam-style
A beam of β-particles is directed into a magnetic field. If the β-particles are electrons, in which direction will they curve relative to the direction of their motion if the magnetic force is directed upwards?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
A source emits radiation. A GM tube reads 250 counts/min with nothing in between, 248 counts/min with paper in between, and 30 counts/min with 5 mm of aluminium in between. The background is 30 counts/min. Which radiations does the source emit?
Common mistakes
Practice
Answers: (1) Two protons and two neutrons (a helium nucleus). (2) Gamma; it is only reduced significantly by thick lead or concrete. (3) Alpha. (4) α is deflected slightly towards the negative plate; β is deflected strongly towards the positive plate; γ goes straight through. (5) We cannot predict when a particular nucleus will decay or in which direction the radiation will go.