Discovery of Radioactivity:

            In 1895 Henri Becquerel discovered this phenomenon of radioactivity. He found that certain elements like uranium, radium, thorium etc emit radiations that cause fogging of photographic plates.

What is radioactivity ?

“Emission of nuclear radiations from a substance as a result of nuclear fission is called radioactivity.”

Types of Radioactivity:

Radioactivity is of two types:

(1)  Natural Radioactivity:

            Certain elements like uranium, radium, thorium show this phenomenon. Usually elements having atomic number above 83 show natural radioactivity.

“Emission of nuclear radiations from a substance as a result of spontaneous nuclear fission is called natural radioactivity.”

A natural radioactive substance is placed in lead block for safety measurements. When a beam of these radiations is made to pass through electric field it splits into three beams i.e alpha (α), beta (β) and gamma (γ) rays.

The rays which bent towards –ve plate are +ve rays and are called α-rays, the one which bent towards +ve plate are – ve rays and are called β-ray still other which passed un-deflected are neutral rays and are called γ-rays.

α – Rays:

(i)         They have mass 4 a.m.u and e/m ratio 4.81 x 10⁴ C/kg which is that of helium divalent cation (He⁺²) or  helium nucleus. So α-ray particle is He⁺² ion or helium nucleus.

(ii)        They deflect towards – ve plate in electric field, so they are + ve rays.          

(iii)       As α-rays deflect less in electric field as compared to β-rays so they are heavy particles.

(iv)       Their velocity is 1/10 th of the velocity of light.

(v)        Being relatively larger particles their penetration power is low. They can only penetrate in 1-2 mm thick metal sheet or can pass through a few centimeters of gas at atmospheric pressure.

(vi)       Being heavy particles they are good ionizer of gases.

(vii)      They cause fogging of photographic plate.

(viii)     They produce fluorescence on striking with fluorescent screen.

β – Rays:

(i)         They have almost Zero mass and e/m ratio 1.76 x 10¹¹ C/kg which is that of electron. So β-ray particles are fast moving electrons.

(ii)        They deflect towards + ve plate in electric field, so they are – ve rays.

(iii)       As β-rays deflect more in electric field as compared to α-rays so they are lighter particles.

(iv)       Their velocity is almost equal to that of velocity of light.

(v)        Being relatively smaller particles their penetration power is high. Their penetration power is 100 times more than that of α-rays.

(vi)       Being lighter particles their power of ionizing gas molecules is less than that of α-ray particles.

(vii)      They cause fogging of photographic plate.

(viii)     They produce fluorescence on striking with fluorescent screen.

γ – Rays:

(i)         They are not material particles and are just electromagnetic waves.

(ii)        They are not deflected by electric field so they are neutral rays.

(iii)       They travel with velocity of light.

(iv)       Due to non-material nature their penetration power is much higher.

(v)        They are very poor ionizer of gas molecules.

(vi)       Their wave length is shorter than that of X-rays but energy is more.

(2)        Artificial Radioactivity:

“Emission of nuclear radiations from a substance as a result of nuclear fission caused by bombardment of certain particles like α-particles is called artificial radioactivity.”

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