The uses of radioactive isotopes depends on their penetrating power and the value of their half-life (see later).

• Because alpha particles are easily stopped, an alpha source is used in some smoke detectors. A sealed alpha source of Americium-241 (half-life 458 years, producing constant signal) sends a stream of alpha particles to a sensor across an air gap. Any smoke present will block the alpha particles and change the sensor signal, this change in signal triggers the alarm. Beta and gamma radiation would be of no use because the smoke particles would not stop them, no change in signal, no alarm triggered!

• Most Beta particles are stopped by a few mm or cm of solid materials. The thicker the layer the more beta radiation is absorbed. A beta source is placed on one side of a sheet of material. A detector (e.g. a Geiger counter) is put on the other side and can monitor how much radiation gets through. The signal size depends on thickness of the sheet and it gets smaller as the sheet gets thicker. Therefore the signal can be used to monitor the sheet thickness. The half-life must be quite long so that change in the signal does not result from rapid decay.
• This idea is used to control production lines of paper, plastic or steel sheeting. Before the sheet material passes through 'flattening' rollers, it passes between a beta source and detector. The detector signal is checked against that for a preset thickness. If the signal is too big the sheet is too thin and the rollers are moved apart to thicken the sheet. If the signal is too small the sheet is too thick and the rollers are moved closer together.

• Gamma radiation is highly penetrating and so gamma sources are used where the radiation must be detected after passing through an appreciable thickness of material. This is used in various tracer situations and usually the half-life should be relatively short to avoid any health hazards.
• A gamma emitting tracer can be added to the flow of water in a pipe and the outside of the pipes monitored with a Geiger counter. Any leaks would be detected by an increase in radiation reading. The flow of water in underground streams can be followed in a similar way.
• Radiotherapy: It seems ironic that the very radiation which causes cancer, can also be used to treat it. A beam of gamma radiation is directed onto the tumor site to kill the cancer cells. Unfortunately the radiation passes through the 'good' tissue too and kills or damages 'good' cells. Modern techniques use multiple rotating gamma sources that are focused on to the tumor. This means the surrounding 'good cells' are less frequently hit and minimises potential harmful side-effects on the rest of the body (e.g. sickness or other mutations). Radiotherapy also avoids the need for intrusive surgery which has its own risk factors. The gamma emitters used have relatively long half-lives to give the instrument a good working life.
• Gamma radiation can be used in a non-destructive way to test the structure of a material.
  • In a sense it is an alternative to X-ray photography for more dense materials e.g.
  • It is used test the structure and quality of pipe welds.
    • A gamma source is placed inside the pipe and photographic paper wrapped around the weld.
    • If there is any gap or flaw in the weld, more gamma radiation gets through and shows up as increased exposure on the 'gamma-ray picture'.
    • Its better to find out the fault now, rather than later when it fractures, and has to be 'dug up' or retrieved from the bottom of the sea!
• Because gamma radiation is so deadly and penetrating it can be used to sterilise surgical equipment or packaged food:
  • The radiation is deadly for bacteria even in the most microscopic pockets of apparently smooth and shiny stainless steel of surgical instruments.
  • It is very convenient for 'convenience' food!. After cooking and sealing in a plastic packet, you don't need to reopen to complete the sterilization to give it a long shelf-life!
  Technium-99 is a gamma emitter (half-life 6 hours) and is used in medicine as a tracer.
  • In medical applications, in a suitable chemical form, the radioisotope is injected into the body and its 'movement' can be followed. Time is allowed for the radioactive tracer to spread and its progress tracked with a detector outside the body.
  • The patient can be placed next to a 'detection screen' that shows where the radioactive tracer is.
  • The effective function of organs like the liver and digestion system can be checked.
  • Similarly, a patient can breathe in air with a gaseous gamma emitter in it, and the effectiveness and structure of the lungs can be checked.
  • The half-life should be long enough to allow good detection BUT NOT too long to be dangerous to the body over a period of time.
  • Beta sources can be used, though not as penetrating as gamma. 
  • Alpha sources are too readily absorbed to show up with a Geiger counter and so are not suitable for these 'tracer' applications.