Radioactivity, particles & Cause

Class XII - Physics

Radioactivity:

The radioactive substances emit radiations spontaneously and do not require favorable condition. This behavior is unaffected by temperature, pressure, electric & magnetic field. This radiation is called radioactive radiation and this process is known as radioactivity. The substances which emit such radiations are known as radioactive substances. Examples are uranium, polonium, radium, thorium etc.

α, β and γ Particles :-

Experiments were done to determine the nature of radiations where the particles were subjected to charged plates i.e. +ve and –ve plates. It was found that some of the particles were attracted by –ve plate which were called α- particles and some were attracted by +ve plate which were called β-particles and some passed straight away, those particles were called γ-rays.

Cause of radioactivity:-

In a nucleus of a atom, two types of forces are acting there. One is attractive strong nuclear force between proton and neutron and repulsive force between the protons.

                If the attractive force is greater than repulsive force, the nucleus is stable otherwise it's unstable.

                If this repulsive force is greater that attractive force, the nucleus is unstable such unstable nucleus emits radioactive radiations. Hence, unstable nucleus is the main cause of radioactivity. A nucleus can be unstable even in its ground state. It attains a state of greater stability by emitting an α- particle or β- particle.

Decay Constant (λ):

From the law of radioactive disintegration,

dN/dt α  - N

dN/dt α  -λN

λ         =  -(dN/dt)/N

Hence, the decay constant is defined as the ratio of the rate of decay per unit atom present.

Half life (T_1/2):-

The time in which the number of atoms present in a substance disintegrates to it's half is called half life.

We know,

      N = N₀e^-λt

or, N₀/2 = N₀e^-λt

or, e^λt = 2

or, log e^λt = log 2

or, λt = 0.693

So, t = 0.693/λ

Average life or Mean life :-

Mean life = (Sum of life of all the atoms)/(Total number of atoms)

The average life of a radioactive substance is equal to the sum of total life of the atoms divided by the total number of atoms in the element.

Units of radioactivity :-

Curie (Ci) :-

1 Ci = 3.7 × 10¹⁰ disintegration per second

Rutherford (rd) :-

1rd = 10⁶ disintegration per second

Becquerel (Bq):-

1 Bq = 1 disintegration per second

Interelation

1 Ci = 3.7 × 10¹⁰ Bq = 3.7 × 10⁴ rd