# Explain why a minimum mass of fissile material, known as the critical mass, is required to sustain a fission chain reaction.

> IB DP Physics Standard Level (2023 syllabus) — E.4 Fission · Explain · 4 marks

> In a nuclear fission chain reaction, each fission event releases neutrons that may go on to induce further fission events. Whether the reaction is self-sustaining depends critically on the quantity of fissile material present.

## Mark scheme (4 marks)

1. Each fission event releases (fast) neutrons that must be absorbed by other fissile nuclei to continue the chain reaction.
2. Neutrons can escape from the surface of the fissile material without causing further fission, and the proportion that escape depends on the surface-area-to-volume ratio of the material.
3. As the mass increases, the surface-area-to-volume ratio decreases, so a smaller fraction of neutrons escape and more are available to cause fission.
4. The critical mass is the minimum mass at which, on average, exactly one neutron per fission event causes a further fission, making the chain reaction self-sustaining (multiplication factor k = 1).

## Key terms

- [critical mass](https://www.gradenine.co.uk/glossary/critical-mass)
- [chain reaction](https://www.gradenine.co.uk/glossary/chain-reaction)

## Related

- [Revision notes for IB DP Physics Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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