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Atomic & Nuclear Physics

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Atomic & Nuclear Physics

Atomic Physics

E. Rutherford is known as the father of nuclear physics.
An atom is the smallest part of matter that takes part in chemical reactions. Atoms of the same element are similar in mass, size, and characteristics.
An atom consists of three fundamental particles: an electron, a proton, and a neutron. All the protons and neutrons are present in the central core of an atom called the nucleus. Electrons revolve around the nucleus.
In an atom, electrons and protons are equal in number and have equal and opposite charge. Hence atom is neutral.

Properties of Fundamental Particles

Particle	Mass (kg)	Charge (Coulomb)	Discoverer
Proton	1.672 X 10^-27	$+1.6 \times 10^{-19}$	Rutherford
Neutron	1.675 X 10^-27	0	Chadwick
Electron	9.108 X 10^-31	$-1.6 \times 10^{-19}$	J.J. Thomson

Note: Proton was discovered by Goldstein and named by Rutherford.

Till today, several subatomic particles have been discovered. Some of them are as follows.

Particle	Mass (kg)	Charge (Coulomb)	Discoverer	Discovery
Positron	9.108 X 10^-31	$+1.6 \times 10^{-19}$	Anderson	Antiparticle of an electron
Neutrino	0	0	Pauli
Pi-meson	274 times the mass of an electron	Positive and negative both	Yakawa	Unstable
Photon	0	0		Velocity equal to that of light

Cathode Rays:

If the gas pressure in a discharge tube is $10^{-2}$ to $10^{-3}$ mm of Hg and a potential difference of $10^4$ volts is applied between the electrodes, then a beam of electrons emerges from the cathode, which is called cathode rays. Hence, cathode rays are a beam of high-energy electrons. The cathode is an electrode with a negative charge.

Properties of cathode rays:

Cathode rays are invisible and travel in a straight line.
These rays carry negative charge and travel from the cathode to the anode.
These rays emerge perpendicular to the cathode surface and are not affected by the position of the anode.
Cathode rays travel with a very high velocity (1/ 10^ththe velocity of light).
These rays are deflected by electric and magnetic fields.
These rays can ionise gases.
These rays heat the material on which they fall.
They can produce chemical changes and thus affect a photographic plate.
These rays can penetrate through thin metal foils.
The source of the EMF used in the production of cathode rays is an induction coil.
When they strike a target of heavy metals such as tungsten, they produce X-rays.
The nature of cathode rays is independent of the nature of the cathode and the gas in the discharge tube.

Positive or Canal rays:

If a perforated cathode is used in a discharge tube, it is observed that a new type of rays is produced from the anode moving towards the cathode and passes through the holes of the cathode.

These rays are positively charged and are called positive rays, or canal rays, or anode rays. These rays were discovered by Goldstein.

Properties of Canal rays:

The positive rays consist of positively charged particles.
These rays travel in a straight line.
These rays can exert pressure and thus possess kinetic energy.
These rays are deflected by electric and magnetic fields.
These rays are capable of producing physical and chemical changes.
These rays can produce ionisation in gases.

Radioactivity

Radioactivity is the sending out of harmful radiation or particles, caused when atomic nuclei break up spontaneously.

Radioactivity was discovered by Henry Becquerel, for which he received the Nobel Prize in Physics jointly with Madame Curie and Pierre Curie.

The nucleus having 83 or more protons is unstable.
They emit α,β, and γ particles and become stable.

The elements of such a nucleus are called radioactive elements, and the phenomenon of emission of α, β, and γ particles is called radioactivity.

β rays are fast-moving electrons. In the nucleus, an electron is created due to the conversion of a neutron into a proton.
γ-rays are electromagnetic waves. 7-rays are emitted after the emission of α and β rays.
Robert Pierre and his wife, Madame Curie, discovered a new radioactive element, radium.
The rays emitted by

S.	Stable nucleus	Unstable nucleus
1.	Low atomic number	High atomic number.
2.	Low mass number	High mass number
3.	Nucleus of small size	Nucleus of a bigger size
4.	$\frac{n}{p}= 1$	$\frac{n}{p} > 1$

Properties of α,β, and γ particles

Properties	α	β	γ
Origin	Nucleus	Nucleus	Nucleus
Nature	Positively charged	Negatively charged	Neutral
Composition	${He^{4}$	$_1e^0$	Photon
Mass ‘'	$6.4 \times 10^{-27}$ Kg	$9.1 \times 10^{-31}$ Kg	zero
Charge	+2e	-e	zero
Chemical effect	Affects the photo graphic plate	Affects the photographic plate	Affects the photo graphic plate
Effect of electric and magnetic fields	Deflected	Deflected	No effect
Penetrating power	Minimum	In between the other two	Maximum
Ionising Power	Minimum	In between the other two	Maximum
Velocity	Between $1.4 \times 10^{7}$ m/s to $2.2 \times 10^{7}$ m/s	1% to 99% of the velocity of light	$3 \times 10^{8}$ m/s

With the emission α -α-particle, the atomic number is decreased by 2, and the mass number is decreased by 4.
With the emission of a β -particle atomic number is increased by one, and the mass number does not change.
The effect on the mass number and atomic number with the emission of α,β, and γ rays is decided by the Group-displacement law or Soddy-Fajan Law.
Radioactivity is detected by a Geiger-Muller counter.
The time in which half nuclei of the element decay is called half half-life of the radioactive substance.

Cloud chamber

A cloud chamber is used to detect the presence and kinetic energy of radioactive particles.

It was discovered by C.R.T. Wilson.
Radioactive carbon-14 is used to measure the age of fossils and plants (Carbon dating).
In this method, age is decided by measuring the ratio of $_6{C^{12}}$ $_6{C^{14}}$

Nuclear Fission and Fusion

A nucleus is represented as $_z{X^{A}}$ where Z is the number of protons (called atomic number), A is the sum of the number of protons (Z) and the number of neutrons (N), and is called mass number.

Number of neutrons N = A - Z For example, $__{92}{U^{238}}$ it has 92 protons, 238 - 92 = 146 neutrons, and 238 nucleons (protons + neutrons).

Isotopes—Atoms having the same atomic number but different mass numbers are isotopes.
Isobars—Atoms having the same mass number but different atomic numbers are isobars.
Isotones—Elements having the same number of neutrons but different atomic numbers are isotones.

Nuclear Fission:

The nuclear reaction in which a heavy nucleus splits into two nuclei of nearly equal mass is nuclear fission.

The energy released in the nuclear fission is called nuclear energy.
Nuclear fission was first demonstrated by Strassmann and O. Hahn.
They found that when ${U^{235}}$ the nucleus is excited by the capture of a neutron, it splits into two nuclei ${Ba^{142}}$ & ${K^{92}}$ .

Chain Reaction:

When a uranium atom is bombarded with slow neutrons, fission takes place. With the fission of each uranium nucleus, on average, 3 neutrons and a large energy are released.

These neutrons cause further fission.
A chain of fission of the uranium nucleus starts, which continues till the whole of the uranium is exhausted.
This is called a chain reaction.

A chain reaction is of the following two types-

1. Uncontrolled Chain Reaction:

In each fission reaction, three more neutrons are produced.
These three neutrons may cause the fission of three other ${U^{235}}$ nuclei, producing 9 neutrons and so on.
As a result, the number of neutrons increases till the whole of the fissionable material is consumed.
This chain reaction is called an uncontrolled or explosive chain reaction.
This reaction proceeds very quickly, and a huge amount of energy is liberated in a short time.

Atom bomb: atomic bomb is based on nuclear fission. ${U^{235}}$ and Pu 239 ${Pu^{239}}$ is used as a fissionable material. This bomb was first used by the USA against Japan in the Second World War (6th August 1945 at Hiroshima & 9th August 1945 at Nagasaki).

2. Controlled Chain Reaction:

A fission chain reaction that proceeds slowly without any explosion and in which the energy released can be controlled is known as a controlled reaction.
Actually, in this situation, only one of the neutrons produced in each fission can cause further fission.
In a controlled chain reaction, the rate of reaction remains constant.

Nuclear Reactor or Atomic Pile:

A nuclear reactor is an arrangement in which a controlled nuclear fission reaction takes place.
The first nuclear reactor was established at Chicago University under the supervision of Prof. Fermi.

There are several components of a nuclear reactor, which are as follows:

Fissionable Fuel: ${U^{235}}$ or ${U^{239}}$ is used.
Moderator: decreases the energy of neutrons so that they can be further used for the fission reaction. Heavy water and graphite are used as moderators.
Control rod: Rods of cadmium or boron are used to absorb the excess neutrons produced in the fission of the uranium nucleus so that the chain reaction continues to be controlled.
Coolant: A large amount of heat is produced during fission. Coolant absorbs that heat and prevents an excessive rise in temperature. The coolant may be water, heavy water ( $D_2O$ deuterium $(_1H^2)$ is an isotope of hydrogen), or a gas like He or $CO_2$ .

Uses of a nuclear reactor

To produce electrical energy from the energy released during fission.
To produce different isotopes which can be used in medical, physical, and agricultural science.

Fast Breeder Reactor: A nuclear reactor that can produce more fissionable fuel than it consumes is called a fast breeder reactor.

Nuclear Fusion:

When two or more light nuclei combine to form a heavier nucleus, tremendous energy is released.

This phenomenon is called nuclear fusion.
A typical example of nuclear fission is

$_1H^2 + _1H^3 \rightarrow _2He^4 + _on^1 + 17.6 Mev$

The energy released by the sun and other stars is by nuclear fusion.
For nuclear fusion, a temperature of the order of $10^8$ K is required.

Hydrogen bomb:

The hydrogen bomb was made by American scientists in 1952.
This is based on nuclear fusion.
It is 1000 times more powerful than an atomic bomb.

Mass Energy Relation:

In 1905, Einstein established a relation between mass and energy on the basis of the special theory of relativity.

According to this relation, mass can be converted into energy and vice versa, according to the relation $\textbf{E} = \textbf{mc}^\textbf{2}$ where c is the velocity of light and E is the energy equivalent of mass m.

Albert Einstein was an American scientist. He was born in Germany. He was given the Nobel Prize in Physics in 1921.
The sun is continuously emitting energy. Earth is continuously receiving $4 \times 10^^{26}$ joules of energy per second from the sun.
As a result mass of the sun is decreasing at the rate of approximately $4 \times 10^^{9}$ kg per second.
But the mass of the sun is so large that it is estimated that the sun will continuously supply energy for the next $10^{9}$ years.