Question

Of the following radiations, the one most easily stopped by air is:
A. $\alpha $ -rays
B. $\beta $ -rays
C. $\gamma $ -rays
D. $X$ -rays

Answer 1

Hint: Penetration power of a particle or radiation is defined as its power to pass through a substance. In general radioactive nuclides undergo decay to give alpha, beta, gamma, X-rays. All this rays have different energy, different characteristics and different penetration powers. The ray which have lowest penetration power can be easily stopped by air.

Complete Step by Step Solution:
$\alpha $ -rays consist of$\alpha $ particles. The alpha particles are actually helium atoms. Helium atoms have a mass number of $4$ and atomic number of $2$. Alpha particles make stable nuclei. Due to high positive charge alpha particles are highly ionising and penetrate through a matter less effectively. Thus they can be stopped by air most effectively.

$\beta $ -rays consist of beta particles. The beta particles are actually high energetic electrons. Due to its smaller size than alpha particles it has less ionising power and can penetrate through a matter. Thus they can be stopped by air more effectively than alpha particles.

$\gamma $ -rays are not material particles. It is a high energetic electromagnetic radiation consisting of a packet of energy. It is emitted due to radioactive decay of radioactive nuclide. Thus it has the highest penetrating power.
$X-$ ray has higher energy. It has the highest penetrating power. Thus these can be stopped by air least effectively.
Thus the correct option is A.

Note: The penetration power into a matter or nucleus is highest in case of $X$ -ray then gamma ray followed by beta and alpha particle or rays. Thus the order of penetration power is as follows: $X>\gamma >\beta >\alpha $ ray.