Background radiation, uses, fission and fusion

Physics · Atomic structure. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All Physics topics

The idea

Background radiation is around us all the time. Natural sources include rocks (radon gas) and cosmic rays from space. Man-made sources include medical X-rays, fallout from weapons testing and nuclear accidents. Radiation dose is measured in sieverts (Sv).

A source with a short half-life is very active at first but its activity falls quickly. A source with a long half-life stays hazardous for a very long time. In medicine, tracers let doctors explore internal organs, and radiation is used to destroy unwanted tissue.

Fission is the splitting of a large nucleus. Fusion is the joining of two light nuclei. In both, a small amount of mass is converted into energy.

Rules to remember

1 sievert (Sv) = 1000 millisieverts (mSv).
Medical tracer: a gamma emitter (it passes out of the body) with a half-life of a few hours.
Radiotherapy: gamma beams are aimed from several directions so the tumour gets a high dose and healthy tissue a lower dose.
Fission: a uranium-235 nucleus absorbs a neutron, then splits into two smaller nuclei of roughly equal size, plus two or three neutrons and gamma rays.
Chain reaction: the neutrons released go on to cause more fissions.
Control rods absorb neutrons to control the rate of fission in a reactor.
Fusion: two light nuclei join to form a heavier nucleus; some mass is converted into energy.

Common mistake

Students mix up fission and fusion. Fission splits a large nucleus; fusion joins small ones. Remember also that fission in a reactor starts when the nucleus absorbs a neutron, not a proton.

Worked example

In a reactor, each fission of uranium-235 releases three neutrons. For the reactor to give out energy at a steady rate, how many of these neutrons should go on to cause another fission?

If all three caused a fission, the number of fissions would triple each time and the reaction would run out of control.
If none caused a fission, the chain reaction would stop.
For a steady rate, each fission must lead to exactly one more fission.
The other two neutrons must not cause a fission: they are absorbed, for example by the control rods, or escape.

Answer: One neutron from each fission; the others are absorbed (for example by the control rods) or escape.

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Keep going

← Radioactive decay, half-life and contamination Scalars, vectors, weight and resultant forces →Take a Physics test