Simulation of Chadwick’s Neutron discovery
Suggested class activity — “What does the data tell us?”
Let me check our past conversations for any relevant context first.
Chadwick’s Neutron Discovery — Simulation Description
The simulation recreates James Chadwick’s 1932 experiment that proved the existence of the neutron. It runs left to right across three stages:
A Po-210 source fires gold alpha particles (He²⁺) — shown with a solid ionisation trail because they are charged particles. These strike the beryllium target, triggering the nuclear reaction ⁹Be + α → ¹²C + n°. A flash marks the collision and a neutron is released. The neutron travels as a faint dashed line — this is physically accurate: neutrons carry no charge, so they produce no ionisation and leave no track. When the neutron enters the paraffin wax, it collides elastically with a hydrogen nucleus and ejects a proton (shown in green with a solid ionisation trail). The proton is then detected by the ionisation chamber on the right. Counters track α particles fired, neutrons produced, and protons ejected. Students can fire single particles, a burst of three, or set it to auto.
AQA Curriculum Context
This experiment sits just outside the core AQA GCSE Physics specification — it is not examined directly, but AQA’s own guidance notes it as useful historical context for the development of the nuclear model. Specifically, it supports:
- 4.4.1 — Atoms and isotopes: understanding that the nucleus contains protons and neutrons (Chadwick confirmed the neutron’s existence)
- 4.4.3 — Radioactive decay: alpha particles as helium nuclei; the nature of ionising radiation and penetration
- 4.4.2 — The development of the model of the atom: how experimental evidence changed scientific understanding — Chadwick is a clear example of evidence leading to a revision of the nuclear model (pre-1932, the nucleus was thought to contain only protons and electrons)
Suggested Classroom Activity — “Chadwick’s Problem” (20 min, KS4)
Best used as: a hook lesson at the start of the radioactivity topic, or as enrichment for higher-attaining students.
Starter — 3 min: Display this question: “Before 1932, scientists knew the helium nucleus had a mass of 4 but only 2 protons. Where did the rest of the mass come from?” Take two or three verbal answers. Do not resolve it — tell students Chadwick spent years on this exact problem.
Main task — 12 min: Students open the simulation and work through these questions in their books:
- Fire a single alpha particle. What happens when it hits the beryllium? What is produced? Write the nuclear equation shown in the legend.
- Watch the neutron’s trail carefully. How does it look different from the alpha particle and the proton? Why — what property of the neutron causes this?
- Switch to Burst mode. Fire several times. Does every neutron eject a proton from the wax? What does this suggest about detection?
- The detector on the right only lights up when a proton arrives. Why could Chadwick not detect the neutron directly? What did he have to measure instead?
Discussion — 3 min: “Chadwick could not see the neutron. He could only see what it did to something else. Is this still valid science?” This links directly to the AQA theme of how scientific models develop from indirect evidence — the same argument students will use when discussing the Geiger-Marsden (Rutherford) experiment.
Exit card — 2 min: Students write one sentence completing this: “Chadwick proved the neutron existed by… because…”
Expected answer: He bombarded beryllium with alpha particles, detected high-energy protons ejected from paraffin wax, and calculated that only an uncharged particle with a mass similar to a proton could transfer that much momentum — ruling out gamma rays (the rival hypothesis).