AQA GCSE Physics 4.4 Β· Year 10 Β· Foundation & Higher
Radioactive Decay
Unstable nuclei; random and spontaneous decay; alpha, beta and gamma emissions overview
Learning Objectives
π¬ Explain what makes a nucleus unstable and why it decays
π² Describe what is meant by random and spontaneous radioactive decay
βοΈ Identify the three main types of nuclear radiation: alpha, beta and gamma
π State the properties of each radiation type including charge, mass and penetration
π Explain what happens to the nucleus during each type of decay
π‘οΈ Describe the ionising power and penetrating ability of each radiation type
βοΈ The Nucleus and Instability
Every atom has a central nucleus made up of protons and neutrons (collectively called nucleons). Protons carry a positive charge, while neutrons are neutral. Electrons orbit the nucleus in shells but are not part of the nucleus itself.
Nucleon number (A) β the total number of protons and neutrons in a nucleus (also called mass number). Proton number (Z) β the number of protons in a nucleus (also called atomic number). This defines which element the atom is.
In stable nuclei, the strong nuclear force holds protons and neutrons together tightly. However, when a nucleus has too many neutrons, too many protons, or is simply too large, the balance between the strong nuclear force and electrostatic repulsion breaks down. The nucleus becomes unstable.
An unstable nucleus has too much energy. To become more stable, it emits radiation in a process called radioactive decay. The original atom is called the parent nucleus and the new atom formed after decay is called the daughter nucleus.
π‘ Radioactivity is a property of the nucleus β it is not affected by chemical bonding, temperature, pressure or any external factor.
Symbol
Meaning
Unit / Value
A
Nucleon (mass) number
No unit (integer)
Z
Proton (atomic) number
No unit (integer)
N
Neutron number
N = A β Z
π² Random and Spontaneous Decay
Two key words describe how radioactive decay happens. Students often lose marks by not explaining these precisely β make sure you know both!
Spontaneous β decay happens without any external trigger or cause. Nothing you do to the material (heating it, putting it under pressure, exposing it to chemicals) can make it decay faster or slower. It happens on its own.
Random β it is impossible to predict which particular nucleus will decay next, or when it will decay. Each nucleus has the same probability of decaying per unit time, but we cannot know in advance which one will go next.
This randomness can be observed with a GeigerβMΓΌller tube and counter: the count rate fluctuates unpredictably from moment to moment. Even if you watched a single atom for a year, you could not say exactly when it would decay β it might decay in the next second or not for a million years.
Despite individual decays being random, when we have a very large number of atoms (as in any real sample β on the order of 10Β²Β³ atoms per mole), we can predict statistically how many will decay on average per second. This average decay rate is called activity, measured in becquerels (Bq), where 1 Bq = 1 decay per second.
Activity (A) is measured in becquerels (Bq)
1 Bq = 1 nuclear decay per second
π‘ Think of it like popcorn popping β you can't predict which kernel pops next, but you can estimate roughly how many will pop per minute from a full bag.
π΄ Alpha (Ξ±) Decay
An alpha particle consists of 2 protons and 2 neutrons β it is identical to a helium-4 nucleus. When a heavy, unstable nucleus emits an alpha particle, the nucleon number decreases by 4 and the proton number decreases by 2.
ᴬπX β ᴬβ»β΄πββY + β΄βHe
Example: Uranium-238 decays by alpha emission to form Thorium-234:
Β²Β³βΈββU β Β²Β³β΄ββTh + β΄βHe
Property
Alpha (Ξ±)
Composition
2 protons + 2 neutrons (helium nucleus)
Charge
+2
Mass (relative)
4 (heavy)
Penetrating power
Low β stopped by a few cm of air or a sheet of paper
Ionising power
Very high β strongly ionises matter it passes through
Speed
Relatively slow (~5% speed of light)
π‘ Alpha particles are the most ionising but least penetrating of the three radiations. Their large charge and mass means they interact strongly with surrounding atoms, losing energy quickly.
π΅ Beta-minus (Ξ²β») Decay
A beta-minus particle is a fast-moving electron emitted from the nucleus. Wait β there are no electrons in the nucleus! So where does it come from? A neutron converts into a proton, releasing an electron and a very light particle called an antineutrino (which you don't need to worry about at GCSE).
neutron β proton + electron (Ξ²β») + antineutrino
ᴬπX β ᴬπββY + β°ββe
Notice that the nucleon number (A) stays the same β a neutron has become a proton, so the total number of nucleons is unchanged. The proton number increases by 1.
Example: Carbon-14 decays to Nitrogen-14:
ΒΉβ΄βC β ΒΉβ΄βN + β°ββe
Property
Beta-minus (Ξ²β»)
Composition
High-speed electron
Charge
β1
Mass (relative)
Very small (~1/2000 of a proton)
Penetrating power
Medium β stopped by a few mm of aluminium
Ionising power
Medium
Speed
Fast (~90% speed of light)
π‘ Beta decay occurs when a nucleus has too many neutrons compared to protons. Converting a neutron to a proton makes the nucleus more stable.
π Gamma (Ξ³) Radiation
Gamma radiation is very high-frequency electromagnetic radiation β it is a photon (a packet of energy), not a particle. Gamma rays are emitted from the nucleus when it still has excess energy after an alpha or beta decay. The nucleus drops to a lower energy state by emitting a gamma ray.
Gamma emission does NOT change A or Z
The nucleus loses energy but no nucleons are lost
Because no particles are ejected, the nucleon number and proton number both remain the same β only the energy state of the nucleus changes.
Property
Gamma (Ξ³)
Composition
Electromagnetic wave / photon
Charge
0 (neutral)
Mass
0
Penetrating power
Very high β requires several cm of lead or metres of concrete to reduce significantly
Ionising power
Low β weakly ionising
Speed
Speed of light (3 Γ 10βΈ m/s)
π‘ Remember the inverse relationship: alpha is most ionising, least penetrating; gamma is least ionising, most penetrating. Beta sits in between.
Summary Comparison
Radiation
Symbol
Charge
Stopped by
Ionising power
Alpha
β΄βHe or Ξ±
+2
Paper / few cm air
Highest
Beta
β°ββe or Ξ²
β1
Few mm aluminium
Medium
Gamma
Ξ³
0
Several cm lead
Lowest
βοΈ Example 1: Radium-226 (Β²Β²βΆββRa) undergoes alpha decay. Write the nuclear equation and identify the daughter nucleus.
1 Write down what alpha decay means: the nucleus loses an alpha particle, which has nucleon number 4 and proton number 2.
2 Calculate the daughter's nucleon number: A = 226 β 4 = 222
3 Calculate the daughter's proton number: Z = 88 β 2 = 86
4 Use the periodic table: element 86 is Radon (Rn).
5 Write the full balanced nuclear equation, checking numbers balance on both sides.
βοΈ Example 3: A student detects radiation from a source using different absorbers. With no absorber the count rate is 850 Bq. With a sheet of paper it drops to 210 Bq. With 3 mm of aluminium it drops to 205 Bq. With 5 cm of lead it drops to 12 Bq (background = 10 Bq). What types of radiation is the source emitting?
2 Paper stopped: 840 β 200 = 640 Bq. Paper stops alpha radiation. So alpha radiation IS present.
3 Aluminium stopped: 200 β 195 = 5 Bq. This tiny extra reduction means aluminium barely reduced the count. Beta radiation is stopped by aluminium β the small change suggests very little or no significant beta present. (The difference is within uncertainty β we conclude beta is NOT significantly present.)
4 Lead stopped: 195 β 2 = 193 Bq. This large reduction by lead shows gamma radiation IS present (gamma needs thick lead to be absorbed).
The source emits alpha (Ξ±) and gamma (Ξ³) radiation.
Alpha was stopped by paper (640 Bq reduction).
Gamma was significantly reduced by the lead absorber (193 Bq reduction).
Always subtract background radiation before drawing conclusions.
βοΈ Example 4: Explain what is meant by saying radioactive decay is (a) spontaneous and (b) random.
1 This is a 4-mark explain question. Plan: define each term clearly with context about the nucleus.
2 For spontaneous: think about what external factors could affect it β the answer is none.
3 For random: think about what we can and cannot predict about individual nuclei.
4 Give a clear, complete sentence for each β avoid vague language like "it just happens".
(a) Spontaneous: Radioactive decay is spontaneous because it occurs without any external trigger or stimulus. No external conditions β such as temperature, pressure, or chemical environment β can cause or prevent a nucleus from decaying. The decay happens on its own, driven only by the internal instability of the nucleus.
(b) Random: Radioactive decay is random because it is impossible to predict which particular nucleus will decay next, or exactly when any given nucleus will decay. Each nucleus has the same probability of decaying per unit time, but there is no way to know in advance which one will go first. The fluctuating count rate on a Geiger counter demonstrates this randomness.
Question 1: Which of the following correctly describes an alpha particle?
Question 2: A nucleus undergoes beta-minus decay. What happens to the proton number (Z)?
Question 3: Which type of radiation is stopped by a few centimetres of air or a thin sheet of paper?
Question 4: Thorium-234 (Β²Β³β΄ββTh) undergoes beta-minus decay. What is the proton number (Z) of the daughter nucleus?
Question 5: What is the meaning of "spontaneous" when describing radioactive decay?
Challenge 1 (4 marks): Polonium-210 (Β²ΒΉβ°ββPo) decays by alpha emission. Write the full balanced nuclear equation for this decay, identifying the daughter element. Show clearly that nucleon and proton numbers are conserved.
Step 1: Alpha particle = β΄βHe. Daughter nucleon number = 210 β 4 = 206. Daughter proton number = 84 β 2 = 82. Step 2: Element 82 = Lead (Pb). Nuclear equation: Β²ΒΉβ°ββPo β Β²β°βΆββPb + β΄βHe Check nucleon numbers: 210 = 206 + 4 β Check proton numbers: 84 = 82 + 2 β [4 marks: 1 for correct daughter A, 1 for correct daughter Z, 1 for identifying Pb, 1 for correctly balanced equation]
Challenge 2 (5 marks): A radioactive source is tested with a Geiger counter. Background radiation is measured as 15 Bq. The results with different absorbers are shown below. Determine which types of radiation the source emits, explaining your reasoning at each step.
No absorber: 980 Bq | After paper: 320 Bq | After 5 mm aluminium: 45 Bq | After 10 cm lead: 18 Bq
Step 1 β Subtract background (15 Bq) from all readings:
No absorber: 965 Bq | After paper: 305 Bq | After Al: 30 Bq | After Pb: 3 Bq
Step 2 β Alpha? Paper reduced count by 965 β 305 = 660 Bq. This large drop shows paper is stopping a type of radiation β alpha particles are stopped by paper. β Alpha IS present.
Step 3 β Beta? Aluminium reduced count by 305 β 30 = 275 Bq. Aluminium stops beta particles (beta gets through paper but not aluminium). This large additional drop shows β Beta IS present.
Step 4 β Gamma? After aluminium, 30 Bq remains (above background of ~0 after subtraction). Lead reduced this to 3 Bq β a drop of 27 Bq. Lead is needed to stop gamma. β Gamma IS present.
Conclusion: The source emits alpha, beta and gamma radiation. [5 marks: 1 for subtracting background; 1 for identifying alpha + reasoning; 1 for identifying beta + reasoning; 1 for identifying gamma + reasoning; 1 for clear overall conclusion]
Challenge 3 (6 marks): A nucleus of uranium-235 (Β²Β³β΅ββU) undergoes alpha decay to form element X, which then immediately undergoes beta-minus decay to form element Y. (a) Identify element X, giving its nucleon number, proton number and name. (b) Identify element Y, giving its nucleon number, proton number and name. (c) Write both nuclear equations.
(a) First decay β Alpha:
Daughter X: A = 235 β 4 = 231, Z = 92 β 2 = 90
Element 90 = Thorium (Th)
X = Thorium-231 (Β²Β³ΒΉββTh)
Equation: Β²Β³β΅ββU β Β²Β³ΒΉββTh + β΄βHe β
(b) Second decay β Beta-minus from Β²Β³ΒΉββTh:
Daughter Y: A = 231 β 0 = 231, Z = 90 + 1 = 91
Element 91 = Protactinium (Pa)
Y = Protactinium-231 (Β²Β³ΒΉββPa)
Equation: Β²Β³ΒΉββTh β Β²Β³ΒΉββPa + β°ββe β
[6 marks: 1 for each correct A and Z for X; 1 for naming X; 1 for each correct A and Z for Y; 1 for naming Y; 1 for both equations correctly written and balanced β award as appropriate]
Challenge 4 β Extended Answer (6 marks): A student says: "I could stop a radioactive source from decaying by cooling it down to β200 Β°C." Evaluate this claim. In your answer, explain the nature of radioactive decay and the properties of the three main types of radiation.
The student is incorrect.
Spontaneous decay: Radioactive decay is spontaneous β it occurs without any external trigger. Temperature, pressure, chemical state, and other physical conditions have no effect on whether or when a nucleus decays. Cooling the material does not change the instability of the nucleus itself, so decay will continue at the same rate.
Random decay: Decay is also random β you cannot predict which nucleus will decay or when. No external action, including cooling, can select or prevent which nuclei decay.
The three radiations:
β’ Alpha (Ξ±): emitted when a nucleus is too large/heavy; consists of 2p + 2n; highly ionising, stopped by paper.
β’ Beta (Ξ²): emitted when a nucleus has too many neutrons; a fast electron; stopped by aluminium; medium ionising power.
β’ Gamma (Ξ³): electromagnetic radiation emitted after alpha/beta decay; no mass or charge; very penetrating (needs lead to absorb); least ionising.
Conclusion: The student cannot stop decay by cooling. The instability lies within the nucleus and is unaffected by any external physical change.
[6 marks: 2 for explaining spontaneous + link to temperature claim; 1 for explaining random; 1 mark each for correct description of at least 2 radiation types with properties; 1 for clear evaluative conclusion]