ICSE Class 10 Answered

For safety, the radiations emitted by the radioactive sample should not come out of the sphere. -particles have a less penetrating power and therefore, the walls of the sphere easily stop them.  -particles will not be stopped by the walls and there will no absorption of -particles inside the sphere as the air has been withdrawn from it.  The number of -particles reaching per unit area at the surface of sphere will depend on the radius of sphere. This number will be reduced to one-fourth, if the radius is doubled.  So some safety is obtained if the sphere is made large.  -radiations are also not absorbed by the walls.  Thus, for more safety, the sphere must have lead walls, it must be of large radius and it should not be evacuated.  The air will help in absorbing the radiations.