12th physics. NCERT notes

Radioactivity was discovered by A.H. Becquerel 1896 accidently.

Radioactivity was a nuclear phenomenon in which an unstable nucleus undergoes decay. This is referred to as radioactive decay. Three types of radioactive decay occur in nature:

  1. α-decay in which a helium nucleus is emitted;
  2. β-decay in which electrons or positrons (particles with the same mass as electrons, but with a charge exactly opposite to that of electron) are emitted;
  3. γ-decay in which high energy (hundreds of keV or more) photons are emitted.

Law of Radioactive Decay:

In any radioactive sample, which undergoes α, β or γ-decay, it is found that the number of nuclei undergoing the decay per unit time is proportional to the total number of nuclei in the sample. If N is the number of nuclei in the sample and ∆N undergo decay in time ∆t then

where λ is called the radioactive decay constant or disintegration constant.

Also, N = N0 e-λt , here N0 is the number of radioactive nuclei present initially.

Radioactive Decay Constant:

As, N = N0 e-λt , now, if we put t = 1/λ , we have, N = N0 e-1

The radioactive decay constant can may be defined as the reciprocal of the time during which the number atoms is a radioactive substance reduces to 36.8% of their initial number.

Half Life: Radioactive Substance

It is the time interval in which the mass of a radioactive substance or the number of its atoms is reduced to half of its initial value.

Relation between Half Life and Decay constant

Half life (T1/2) and decay constant (λ) are related as, T1/2 = 0.693/λ

Activity of Radioactive Constant

Rate of disintegration or count rate of a sample of radioactive material is called activity and is directly proportional to the number of atoms of left undecayed in the sample

Activity A, |dN/dt| = λN

Mean Life (or Average Life) of a Radioactive Substance

It is the average of the lives of all the atoms in a radioactive substance is called the ‘mean life’ or ‘average life’ of that substance.

The mean life (T) of a radioactive substance is equal to reciprocal of decay constant.

It means, T = 1/λ. Also, T = 1.443 T1/2

How useful was this post?

Click on a star to rate it!

Average rating / 5. Vote count:

No votes so far! Be the first to rate this post.

As you found this post useful...

Follow us on social media!

We are sorry that this post was not useful for you!

Let us improve this post!

Leave a Reply