IGCSE Physics 0625: Ionising Power, Penetrating Power and Radiation Safety

A student has learned that alpha particles are stopped by paper, beta particles are stopped by a few millimetres of aluminium, and gamma rays need several centimetres of lead to be absorbed significantly. She makes the following claim:

"Because gamma radiation passes through paper, aluminium and even several centimetres of lead more easily than alpha or beta radiation, it must interact least with the materials it passes through, and for that same reason, gamma radiation must also be the safest of the three to be near."

(a) State the order of alpha, beta and gamma radiation from the most ionising to the least ionising. [1]

(b) Explain the relationship between the ionising power of a type of radiation and its penetrating power. [2]

(c) Discuss whether the student's claim that gamma radiation is "the safest of the three to be near" is fully justified. In your answer, refer to both the ionising power and the penetrating power of alpha, beta and gamma radiation. [2]

Part (a): Order of ionising power

From most ionising to least ionising:

$\text{alpha} > \text{beta} > \text{gamma}$

Alpha particles are the most strongly ionising, beta particles are intermediate, and gamma rays are the least ionising of the three.

Part (b): Ionising power and penetrating power are linked inversely

A radiation’s ionising power depends on how often and how strongly it interacts with the atoms it passes:

A strongly ionising radiation (such as an alpha particle) interacts with atoms very frequently, knocking electrons off them and transferring its energy over a very short distance. Because it loses all its energy so quickly, it is absorbed by a thin layer of material. It has low penetrating power.
A weakly ionising radiation (such as a gamma ray) interacts with atoms only occasionally, so it can travel much further through a material before enough energy has been transferred to absorb it. It has high penetrating power.

So the more strongly ionising a type of radiation is, the less penetrating it is, and vice versa: ionising power and penetrating power vary in opposite directions for alpha, beta and gamma radiation.

Part (c): Is gamma radiation really “the safest”?

The student’s claim captures only part of the picture, so it is not fully justified:

Supporting the claim: Gamma radiation is the least ionising of the three, so per interaction it does the least damage to any single atom or cell, and because it is so penetrating, most of it passes straight through a person rather than being absorbed by the first few millimetres of tissue.
Against the claim: Precisely because gamma radiation is so penetrating, a gamma source outside the body can still deliver radiation deep inside it, ionising cells in internal organs from a distance. Something alpha radiation cannot do, since it is completely stopped by the skin or even a sheet of paper.
Alpha radiation is, in that sense, “safer” as an external hazard. However, if an alpha-emitting source is inhaled or swallowed, its intense, short-range ionisation is deposited directly into living tissue at very close range, which can cause serious localised damage. A hazard gamma radiation of similar activity would not produce in the same way, since it spreads its (weaker) ionisation over a much longer path.

So whether gamma is “safest” depends on whether the source is outside the body (where gamma’s high penetration is a hazard and alpha’s low penetration is not) or inside the body (where alpha’s high ionising power becomes the greater hazard). The student’s claim is too simple to be fully correct.

Final answers

(a) Most ionising to least ionising: alpha, beta, gamma.
(b) Ionising power and penetrating power vary inversely. More ionising means less penetrating, and vice versa.
(c) The claim is not fully justified: gamma is a greater hazard from outside the body due to its penetration, while alpha is a greater hazard if taken inside the body due to its strong, short-range ionisation.