Question

The relative penetrating power of $\alpha ,\beta ,\gamma$ and neutron ($n$) follows the order:
A. $\alpha >\beta >\gamma >n$
B. $n>\gamma >\beta >\alpha$
C. $\beta >\alpha >n>\gamma$
D. None of these

Answer 1

Hint: The penetrating power of any particle depends on the mass of the particle. The greater the mass of the particle will be, the lesser the penetrating power. Also, the greater the mass of the particle, the greater will be the ionization power except neutron which is a neutral particle.

Complete answer:
The mass of the alpha particle is around 4 times greater than the mass of a neutron. The mass of the alpha particle is around 8000 times greater than the mass of a beta particle. The gamma particle has a weight of zero atomic mass unit. With these comparisons, we can easily state that the order of the mass of all the particles is:
$\alpha >n>\beta >\gamma$
Now, let us understand the relation between the mass of the particle and penetrating power. The penetrating power is the tendency of any particle to penetrate through a substance or barrier or obstacle. The greater the mass of the particle, the slower its velocity will be and the slower it can penetrate the substance. This means that an alpha particle being the heaviest has the least penetrating power and highest ionization power. In terms of mass, the next particle that should have a penetrating power less than the alpha particle is the neutron particle. But, as it does not carry any charge, it will not ionize the substance. After the neutron particle comes the beta particle which has higher penetrating power than both the alpha and neutron particle. At last, comes the gamma particle which is the lightest particle among all of them and has the highest penetrating power.
Hence, the correct order of penetrating power should be:
$\gamma >\beta >n>\alpha$

So, the correct answer is OptionD.

Note:
The gamma, beta and alpha particles are the product of radioactive decay of any substance. Based on their penetrating power, gamma particles are the most dangerous and can even cause permanent damage to the human body. Thus, the radioactive substances constantly keep on radiating these particles until their decay life ends.