Solutions: Radioactivity Questions


(a) Which of the following forms of ionizing radiation produces the greatest number of ion pairs per mm in air? (Tick the correct answer)

α particles
β particles  
γ rays  

[1 mark]


(i) Complete the table below showing the typical maximum range in air for α and β particles.

Radiation Type
Typical maximum range in air /m


For alpha particles the examiners allowed 0.03 m to 0.07 m

For beta particles they allowed 0.20 m to 3.0 m

If a range of values was given the examiner was instructed to use the larger value.

A specific number was required so you could not just say 'just a few cm'.

[2 marks]

(ii) γ rays have a range of at least 1 km in air.

However, a γ ray detector placed 0.5 m from a γ ray source detects a noticeably smaller count-rate as it is moved a few centimetres further away from the source.

Explain this observation.

The gamma rays spread out radially from the source.

Therefore the intensity of radiation decreses with the square of the distance from the source - it follows the inverse square law. This means that the further away from the source the detector is placed the lower the intendity of radiation it will register - doubling the distance will reduce the intensity to a quarter of the value - tripling it will reduce the intensity to a ninth etc..

Any contradiction loses the mark. For example, 'it follows inverse square law so intensity falls exponentially' would get zero marks even though you mentioned the inverse square law as you incorrectly thought the level would drop exponentially.....

[1 mark]

(c) Following an accident, a room is contaminated with dust containing americium (which is an α-emitter). Explain the most hazardous aspect of the presence of this dust to an unprotected human entering the room.

The dust may be ingested (or 'taken into the body' or 'breathed in') causing molecules in human tissue (or cells) to be mutated (or made cancerous or killed or damaged) by ionisation

Alpha emitting dust 'on the skin' is not the really dangerous prospect - the alpha particles would be absorbed by the dead skin cells before the poses a threat to newly forming cells. Your answer needs to show you understand that alpha emitters are dangerous inside the body.

The second mark was awarded for the idea of damage from ionisation.

[2 marks]

(Total 6 marks)