Which of the following arrangements of radiations is in the order of decreasing penetrating power?

Analyze the question: You are asked to arrange alpha (α), beta (β) and gamma (γ) radiations from the most penetrating to the least penetrating.

Concept: Penetrating power depends on how strongly the radiation interacts with (ionizes) matter along its path.

• Alpha (α): a helium nucleus (2 protons and 2 neutrons). It is heavy and has a +2 charge, so it ionizes strongly and loses energy quickly. It is stopped by a sheet of paper or a few cm of air.
• Beta (β): a fast electron, which is light and has a -1 charge. It ionizes less strongly than alpha. It passes through paper but is stopped by a few mm of aluminium.
• Gamma (γ): a high-energy electromagnetic wave with no charge and no mass. It ionizes weakly, so it travels far. It is only reduced significantly by thick lead or concrete.

Solution:

• Most penetrating: γ
• Middle: β
• Least penetrating: α

So the order of decreasing penetrating power is γ > β > α.

The ionizing power is the reverse: α > β > γ. Option A is therefore the order of decreasing ionizing power, which is a common trap.

Checking the options:

• A. α, β, γ: this is increasing penetrating power, so it is wrong.
• B. α, γ, β: wrong.
• C. γ, β, α: correct.
• D. β, γ, α: wrong, because gamma penetrates more than beta.

Exam tip: Remember "Good Boys Always" (γ, β, α) for decreasing penetration. Also remember that penetration and ionization are opposites. The more strongly a radiation ionizes, the less it penetrates.