K326 / K327 / 2027
Radioactivity overview

Topic 5 of 5

Uses and hazards

Radioactivity is useful when its emissions can be detected or transfer energy where needed. Those same interactions can damage living tissue.

Alpha is strongly ionising with low penetration; gamma is highly penetrating. Beta lies between them in the usual comparison. Half-life describes how rapidly a source's activity falls, not the range of its radiation.

Follow the source, material and detector

A gamma-emitting medical tracer

Radioactive material inside the body emits radiation that can reach a detector outside. The detected radiation gives information about the tracer's distribution.

The source is inside, and radiation travels outwards towards the detector. This differs from an external X-ray source sending radiation through the body. Suitable penetration is needed for the emitted signal to reach the detector.

A beta thickness gauge

A source and detector sit on opposite sides of a moving sheet. Under matched material and geometry conditions, a thicker sheet absorbs more, giving a lower detected rate. A thinner sheet gives greater transmission.

The chosen radiation must be partly transmitted through the relevant sheet. Alpha may be absorbed too completely, while very penetrating radiation may show too little change. Beta is useful for suitable paper, plastic or light-metal sheets, but is not the correct choice for every material and thickness.

Keep the source output and detector conditions accounted for: a falling count rate caused by source decay or changed alignment would not, by itself, show that the sheet became thicker.

Gamma radiation in cancer treatment

Ionisation can damage or kill tumour cells. It can also damage healthy cells, so the useful effect requires controlled exposure. Gamma rays do not automatically recognise and target cancer cells.

Follow the radiation and locate the radioactive material

Filled brown dots mark radioactive material. Teal arrows show selected radiation paths; blue arrows show sheet movement. These schematic paths and thicknesses are not numerical transmission measurements.

Thinner sheet: a larger detector rate

Thinner sheet: a larger detector rateThe same beta source is on the left and detector on the right, with a relatively thin sheet between them. Five selected incident radiation paths are illustrated: four continue to the detector and one ends inside the sheet to represent absorption. The drawing is qualitative, not a measured four-fifths transmission. A separate blue arrow below the sheet shows its downward movement. Under matched material, source, detector and geometry conditions, the thinner sheet gives the larger corrected detector count rate.BetasourceSheetDetectorSheetmotion

Thicker sheet: a smaller detector rate

Thicker sheet: a smaller detector rateThe source and detector remain in the same places, but the intervening sheet of the same material is thicker. Of the same five illustrated incident paths, two continue to the detector and three end within the sheet. This represents greater absorption with some radiation still transmitted; the arrow count is not a measured efficiency or universal thickness law. The blue downward arrow again means sheet movement. Other relevant conditions are held fixed.BetasourceSheetDetectorSheetmotion

Compare the same sheet material with the source, detector and other relevant conditions fixed. The useful comparison has partial transmission: some radiation is absorbed and some reaches the detector.

A tracer is an internal source

Gamma radiation travels from an internal tracer to an external detectorA rounded outline labelled body contains four filled brown marks representing radioactive tracer material. Three selected teal radiation arrows start inside the body and point outwards to a detector on the right. The material remains inside; the arrows represent emitted gamma radiation rather than transport of the tracer. Some gamma radiation reaches the external detector and can provide information about the source distribution. The paths do not show all emissions or guarantee that every emitted photon is detected.RadioactivetracerDetectorBody

The source is inside and the detector is outside. Some emitted gamma radiation travels out to the detector.

Irradiation: radiation reaches the object

External irradiation without transferring radioactive materialAn external source on the left contains filled brown marks for radioactive material. Teal radiation arrows reach a separate object on the right. No radioactive material marks are placed on or inside that object. The depicted event is irradiation: radiation reaches the object while the source material remains outside it. Exposure by itself does not establish contamination.ExternalsourceObjectSource material remains outside

Contamination: radioactive material is present

Radioactive material on or inside an objectThe object now has filled brown marks both on its upper surface and inside it. Grey label leaders identify those locations; they are not movement arrows. Teal arrows start at the radioactive material and represent emitted radiation. This is contamination because source material itself is present on or in the object. No source-handling procedure or route of transfer is illustrated.ObjectOn the surfaceInsideBrown dots represent source material

Irradiation concerns radiation reaching an object. Contamination concerns radioactive material on or inside it. The two descriptions refer to different things and can occur together.

A transmission gauge compares radiation arriving through a sheet. An internal tracer emits towards an external detector. The exposure examples distinguish radiation reaching an object from radioactive material being present on or inside it.

Distinguish irradiation from contamination

Irradiation or exposure
Radiation reaches an object or person. Exposure to an external source does not, by itself, mean radioactive source material has been transferred.
Radioactive contamination
Radioactive material is present where it is not wanted, such as on a surface or inside a body. It can continue emitting from that location.

Ionisation can damage cells or DNA, kill cells or increase cancer risk. The consequences depend on the radiation, amount and duration of exposure, and where its energy is absorbed.

Typical alpha particles have little penetration from outside, but alpha-emitting material inside the body can damage nearby cells. There may be no protective outer layer between that source and the tissue. Gamma can reach tissue from outside. Penetrating power alone does not rank every exposure's hazard.

Activity in Bq gives the source's decay rate. It does not directly give the energy absorbed by a particular person. Equal activities can produce different exposures because the emissions, shielding, separation and source locations can differ.

Match protection to the route of exposure

  • Reduce exposure time: less time exposed can reduce the energy received under otherwise unchanged conditions.
  • Increase separation: greater distance from an external source can reduce the radiation reaching a person.
  • Use suitable shielding: the material and thickness must suit the emission and conditions. Reduced transmission does not mean the source has stopped being radioactive.
  • Contain the source material: containment addresses transfer and spread of radioactive material, rather than only shielding radiation arriving from outside.
Optional check Typical alpha particles are stopped by paper or the outer layer of intact skin. Why can an alpha-emitting material inside the body still be hazardous?
Typical alpha particles are stopped by paper or the outer layer of intact skin. Why can an alpha-emitting material inside the body still be hazardous?