Question

Which of the following represents the relative penetrating power of the three types of radioactive emission in decreasing order?

A. $\beta >\alpha >\gamma$
B. $\beta >\gamma >\alpha$
C. $\gamma >\alpha >\beta$
D. $\gamma >\beta >\alpha$

Answer 1

Hint: Penetrating power of radioactive emissions such as alpha, beta, and gamma rays depend on the polarizing power and mass of these rays. Alpha and beta rays are high-speed charged particles, whereas gamma rays are electromagnetic waves.

Complete step-by-step answer:
Let’s start by understanding the properties of each type of ray, and decide the penetrating power of these rays.

Alpha rays are a stream of Helium nuclei. As we already know, ‘He’ atom consists of two photons, two neutrons, and two electrons. So, we can write Alpha particles as,
${}_{2}^{4}H{{e}^{+2}}$

Hence, Alpha ray has the highest mass in comparison to the other two and it also has the highest charge. As a result, the polarising power of Alpha particles is maximum. More polarising power means less penetration. So, Alpha particles have the lowest penetration.

Beta particles are a stream of negatively charged electrons. Beta particles have less weight and charge in comparison to Alpha particles. The symbol is the same as that of electrons.
${{e}^{-1}}$
It also doesn’t have a large penetrating power. But it is certainly more than Alpha particles.
Gamma rays are electromagnetic rays, and it is considered to be one of the most harmful types of radiation. This is deemed to be harmful because of its ability to penetrate any material and travel a large distance.
Hence, we can say that the penetrating order of the radioactive emissions is -
$\gamma >\beta >\alpha$

So, the correct option is (D).
Note:
Although it is not necessary to know the penetrating power of these radioactive emissions, you should know these facts for better understanding.
Gamma rays can travel up to 1.6km through any material.
Beta rays can travel through flesh. However, a thin piece of metal can stop the beta particles.
Alpha particles can hardly travel 10 to 18 cm before losing power. It can be completely stopped by a piece of paper.