# CBSE Class 12th Physics Notes: Nuclei (Part – II)

Find CBSE Class 12th Physics Notes on Nuclei. These NCERT based notes are important for coming CBSE class 12th board exam 2017.

Jan 31, 2017 15:00 IST

CBSE chapter wise notes based on chapter 13, Nuclei of Class 12 Physics NCERT textbook are available in this article. These notes are continuation of CBSE Class 12th Physics Notes: Nuclei (Part – I)

In part I, we have studied about following topics: Atomic Masses, Discovery of Neutron, Basic Properties of Neutron, Composition of Nucleus, Size of the Nucleus, Nuclear Density, Mass Defect, Binding Energy, Binding Energy per Nucleon, Binding Energy Curve and its Features etc.

Now, in part II we will study about the topics given below

 Radioactivity Alpha Particles and Alpha Decay Beta Particles and Beta Decay Gamma radiations and Gamma decay Law of Radioactive Decay Radioactive Decay Constant Half Life: Radioactive Substance Relation between Half Life and Decay constant Activity of Radioactive Constant Mean Life (or Average Life) of  a Radioactive Substance Units of Radioactive Decay Nuclear Fission Nuclear Fusion NCERT Solutions for Class 12 Physics

The notes are given below:

Radioactivity was discovered by A. H. Becquerel (1986) accidently.

Radioactivity is defined as the spontaneous and continuous disintegration of a nucleus of a heavy element on its own with the emission of certain type of radiations is known as natural radioactivity.

Alpha Particles and Alpha Decay

Alpha particles are helium nuclei of nuclear origin. It carries 2 unit positive charge and its mass is about four times the mass of hydrogen atom.

Phenomenon of emission of an α particles from a nucleus is called alpha decay

Example: 92U23890Th234 + 2He4 + Q

Here, Q = (mXmYmHe) c2

Generally:

ZXAz ‒ 2YA ‒ 4 + 2He4 + Q

Beta Particles and Beta Decay

Beta particles are fast moving electrons of nuclear origin. A nucleus that decays spontaneously by emitting an electron or a positron is said to undergo beta decay.

In βdecay, an electron an electron is emitted by the nucleus.

Example: 15P3216S32 + e + antineutrino

In β+ decay, a positron is emitted by the nucleus.

Example: 11Na22e + Neutrino

In beta-minus decay, a neutron transforms into a proton within the nucleus according to, np + e + antineutrino

Whereas in beta-plus decay, a proton transforms into neutron (inside the nucleus) by, pp + e+ + neutrino

The above processes show that mass number A of a nuclide undergoing beta decay does not change.

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There are energy levels in a nucleus, just like there are energy levels in atoms. When a nucleus is in an excited state, it can make a transition to a lower energy state by the emission of electromagnetic radiation. As the energy differences between levels in a nucleus are of the order of MeV, the photons emitted by the nuclei have MeV energies and are called gamma rays.

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 ΔN undergo decay in time Δt then,

Nt) ∝ N or (ΔNt) = λ N

Where, λ is known as radioactive decay constant or disintegration constant.

Also, N = No eλt, here No is the number of radioactive nuclei present initially.

As, N = No eλt, now, if we put t = 1/λ, we have, N = No e‒1

N = No/e

N = No/2.718

N =0.368 No

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

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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

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| = λ N

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 (τ) of a radioactive substance is equal to reciprocal of decay constant.

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

Becquerel (Bq) = 1 disintegration per second

Rutherford (rd) = 106 disintegration per second

Curie (Ci) = 3.7 × 1010 disintegration per second

Nuclear Fission

It is the process in which a heavy nucleus splits up into two nuclei of nearly comparable masses.

Example: 92U235 + 1n092U23636Ba141 + 36Kr92 + 31n0 +Energy

Nuclear Fusion

It is the process in which two or more small nuclei fuse together to form a single heavy nucleus.

The mass of the single heavy nucleus formed is less than the total initial mass of the mass of the parent nuclei.

This difference in mass appears in the form of energy (as per, E = mc2).

Example: 1H2 + 1H22He4 + Enormous amount of energy

Enormous amount of energy produced by the sun is due to the phenomenon of nuclear fusion.

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