In nuclear physics and radiation safety, which type of ionising radiation listed here has the lowest penetrating ability and can be stopped by something as thin as a sheet of paper or human skin?

Introduction / Context:
When studying radioactivity, it is important to understand not only the types of radiation emitted but also how deeply they can penetrate materials. This affects how radiation is shielded and how hazardous it is when outside or inside the body. The three common types of ionising radiation are alpha, beta and gamma radiation. This question asks you to identify which of these has the lowest penetrating power.

Given Data / Assumptions:

• Alpha particles (α) are helium nuclei consisting of two protons and two neutrons.
• Beta particles (β) are high-energy electrons or positrons.
• Gamma rays (γ) are high-energy electromagnetic waves.
• We consider penetration through simple shielding materials such as paper, clothing, metal and concrete.

Concept / Approach:
Alpha particles are relatively heavy and carry a +2 charge. Because of their mass and charge, they interact strongly with matter and lose energy quickly, giving them very low penetration. They can be stopped by just a few centimetres of air, a sheet of paper or the outer dead layer of human skin. Beta particles are lighter and more penetrating; they can travel further in air and may require plastic, glass or thin metal sheets for shielding. Gamma rays, being high-energy photons, are highly penetrating and require thick layers of dense materials such as lead or concrete for effective shielding. Therefore, alpha radiation clearly has the lowest penetrating ability among the three.

Step-by-Step Solution:
Step 1: Recall the physical nature of each radiation type: alpha (heavy charged particles), beta (light charged particles), gamma (uncharged photons).Step 2: Recognise that heavier, highly charged particles tend to interact strongly with matter and lose energy quickly.Step 3: Remember that alpha particles can be stopped by a sheet of paper or even the outer layer of skin.Step 4: Recall that beta particles can penetrate paper but are often stopped by a few millimetres of aluminium or similar materials.Step 5: Recall that gamma rays are the most penetrating and need thick lead or concrete shields.Step 6: Conclude that alpha radiation has the lowest penetrating ability.

Verification / Alternative check:
Demonstrations in nuclear physics labs often show how a Geiger counter count rate changes when absorbers are placed between a radioactive source and the detector. Alpha sources show a sharp drop in count rate when a sheet of paper or a few centimetres of air are introduced. Beta sources require denser materials like plastic or aluminium to significantly reduce the count. Gamma sources are only weakly affected by thin shields and require much thicker barriers. These experiments confirm the relative penetration abilities.

Why Other Options Are Wrong:
Beta radiation has greater penetration than alpha radiation and can pass through paper and skin, though it is still less penetrating than gamma radiation. Gamma radiation is the most penetrating of the three and can pass through the human body and thick materials; it clearly does not have the lowest penetrating ability. The statement that all of the above have the same penetration is incorrect and ignores well-established differences in their interaction with matter.

Common Pitfalls:
Students sometimes assume that the most dangerous radiation must also be the most penetrating, leading to confusion. While gamma rays are very penetrating and hazardous externally, alpha particles can be extremely dangerous if inhaled or ingested despite their low penetration because they deposit a lot of energy in a small volume. To avoid confusion, separate the concepts of external penetration and internal biological hazard. For this question, the focus is on penetrating ability through materials, not overall danger.

Final Answer:
Alpha radiation has the lowest penetrating ability and can be stopped by materials as thin as paper or skin.