Background Radiation (Triple Physics Only)

Background radiation is around us all of the time. It comes from: 

  • natural sources such as rocks and cosmic rays from space 
  • man-made sources such as the fallout from nuclear weapons testing  and nuclear accidents. 

The level of background radiation and radiation dose may be affected by  occupation and/or location. Radiographers will typically have a higher exposure than accountants. The rocks are naturally more radioactive income areas which will increase exposure. Living at high altitude will increase exposure because there is less atmosphere to absorb radioactive cosmic rays from space.

Radiation dose is measured in sieverts (Sv). 1000 millisieverts (mSv) = 1 sievert (Sv)

Radioactive isotopes have a very wide range of half-life values. For some uses a long half live is desirable (eg the alpha source in a smoke detector). For some uses a short half life is desirable (eg for radioactive tracers used in medical diagnosis)

Nuclear radiations are used in medicine for the: 

  • Exploration of internal organs; using radioactive tracers which are ingested/injected and can be detected outside the body
  • Control or destruction of unwanted tissue; using gamma radiation for radiotherapy to destroy tumours

people are able to compare a radiation risk / dose / hazard to the radiation dose from (eating) bananas

  • food / drink
  • rocks / building materials
  • cosmic rays / rays from space
  • nuclear power / coal power stations or nuclear waste
  • nuclear accidents
  • nuclear weapons testing

Fission & Fusion (Triple Physics Only)

Nuclear fission is the splitting of a large and unstable nucleus (eg  uranium or plutonium). 

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Nuclear Fission

Spontaneous fission is rare.

Usually, for fission to occur the unstable  nucleus must first absorb a slow moving neutron

The nucleus undergoing fission splits into two smaller nuclei, roughly  equal in size, and emits two or three neutrons plus gamma rays.

Energy  is released by the fission reaction. 

All of the fission products have kinetic energy

The neutrons may go on to start a chain reaction, where each neutron released during the fission process goes on to cause fission in another large nucleus.

The chain reaction is controlled in a nuclear reactor to control the energy  released. The explosion caused by a nuclear weapon is caused by an  uncontrolled chain reaction. 

A chain reaction
  • neutron absorbed by a uranium nucleus
  • nucleus splits into two parts
  • and (2/3) neutrons (are released)
  • and gamma rays (are emitted)

Nuclear fusion is the joining of two light nuclei to form a heavier nucleus. 

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Nuclear Fusion in the Sun

In this process some of the mass may be converted into the energy of  radiation.

Fusion needs very high temperature and pressure to happen.

  • lighter nuclei join to form heavier nuclei
  • some of the mass (of the nuclei) is converted to energy (of radiation)