AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Spontaneous fission is rare. Usually, for fission to occur the unstable…
Full specification wording
Nuclear fission: Spontaneous fission is rare. Usually, for fission to occur the unstable nucleus must first absorb a neutron.
A uranium-235 nucleus can split entirely on its own, but spontaneous fission is rare.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 2 of 9
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A uranium-235 nucleus can split entirely on its own, but spontaneous fission is rare. For fission to happen at a useful rate, the unstable nucleus must first absorb a neutron. Absorbing the extra neutron leaves the nucleus highly unstable, causing it to split into two smaller nuclei, releasing two or three further neutrons and gamma rays. These released neutrons can be absorbed by other large nuclei, creating a chain reaction. Note the word absorb: the neutron is taken into the nucleus rather than bouncing off it. A lump of uranium does not simply explode; this is because it lacks a critical mass to sustain an uncontrolled chain reaction, not merely because spontaneous fission is rare.
What earns the marks
- Stating that spontaneous fission is rare.
- Explaining that the unstable nucleus must first absorb a neutron before it splits.
- A diagram showing the neutron arriving at and being absorbed by the large nucleus.
- At least one neutron from that fission being absorbed by a further large nucleus to sustain a chain reaction.
Common mistakes and how to put them right
Drawing or describing a neutron that bounces off the nucleus rather than being absorbed.
Saying a proton, an electron or an alpha particle triggers the fission.
Claiming a lump of uranium does not explode solely because spontaneous fission is rare, ignoring the requirement of a critical mass.
Exam tips
- Use the verb 'absorbs' rather than 'hits', because it is the absorption that the mark scheme credits.
- On a diagram, draw the incoming neutron touching the nucleus, with an arrow showing it going in.
Try the lesson
The start of the Nuclear fission: Spontaneous fission is rare. Usually, for fission to occur the unstable nucleus must first absorb a neutron. lesson, made from checked teaching notes for this specification point.
Have a go first
Fission of uranium-235 can occur spontaneously, but this is a very rare event. What is usually required to induce the fission of a large, unstable nucleus?
Listen
A uranium-235 nucleus can split entirely on its own, but spontaneous fission is rare. For fission to happen at a useful rate, the unstable nucleus must first absorb a neutron. Absorbing the extra neutron leaves the nucleus highly unstable, causing it to split into two smaller nuclei, releasing two or three further neutrons and gamma rays. These released neutrons can be absorbed by other large nuclei, creating a chain reaction. Note the word absorb: the neutron is taken into the nucleus rather than bouncing off it. A lump of uranium does not simply explode; this is because it lacks a critical mass to sustain an uncontrolled chain reaction, not merely because spontaneous fission is rare. A strong answer earns marks for these points. stating that spontaneous fission is rare. explaining that the unstable nucleus must first absorb a neutron before it splits. a diagram showing the neutron arriving at and being absorbed by the large nucleus. at least one neutron from that fission being absorbed by a further large nucleus to sustain a chain reaction. Exam tip: Use the verb 'absorbs' rather than 'hits', because it is the absorption that the mark scheme credits.
See it
Key words
Say what each word means before you turn the card.
spontaneous fission
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neutron absorption
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chain reaction
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See every definition
- spontaneous fission
- A uranium-235 nucleus can split entirely on its own, but this is rare.
- neutron absorption
- The neutron is taken into the nucleus rather than bouncing off it, leaving the nucleus highly unstable.
- chain reaction
- Released neutrons can be absorbed by other large nuclei, sustaining this process.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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