Nuclear & Atomic Physics

Nuclear & Atomic Physics

Atomic Physics

• E. Rutherford, Father of Nuclear Physics

• E. Rutherford is known as the father of nuclear physics.

Structure of an Atom

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 charges. Hence, the atom is neutral.

Properties of Fundamental Particles

Note: The proton was discovered by Goldstein and named by Rutherford.

Other Subatomic Particles

Cathode Rays

If the gas pressure in a discharge tube is to mm of Hg and a potential difference of volts is applied between the electrodes, 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:

• Invisible and travels in a straight line.

• Carry a negative charge and travel from the cathode to the anode.

• Emerge perpendicular to the cathode surface and are not affected by the position of the anode.

• Travel with a very high velocity (1/10th the velocity of light).

• Deflected by electric and magnetic fields.

• Can ionize gases.

• Can heat the material on which they fall.

• It can produce chemical changes and thus affect a photographic plate.

• It can penetrate 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 passing through the holes of the cathode.

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

Properties of Canal Rays:

• Consists of positively charged particles.

• Travel in a straight line.

• It can exert pressure and thus possess kinetic energy.

• They are deflected by electric and magnetic fields.

• It can produce physical and chemical changes.

• It can produce ionization in gases.

Radioactivity

Definition:

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

Discovery:

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

• A nucleus having 83 or more protons is unstable and emits α, β, and γ particles to 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. γ-rays are emitted after the emission of α and β rays.

Discovery of Radium:

• Robert Pierre and his wife, Madame Curie, discovered a new radioactive element, radium.

Properties of α, β, and γ Particles

Emission of Particles and Their Effects on Atomic and Mass Numbers

• Emission of an α-particle: The atomic number is decreased by 2, and the mass number is decreased by 4.

• Emission of β-particle: The atomic number is increased by 1, 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 Detection

Radioactivity is detected by a Geiger-Muller counter.

Half-Life of Radioactive Substance

• The time in which half of the nuclei of the element decay is called the 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.

Carbon Dating

• Carbon-14 is used to measure the age of fossils and plants (Carbon dating). In this method, age is determined by measuring the ratio of 6C¹² to 6C¹⁴.

Nuclear Fission and Fusion

Nucleus Representation

A nucleus is represented as ZAX_{Z}^{A} \text{X}ZA​X, where:

• ZZZ is the number of protons (called atomic number)

• AAA is the sum of the number of protons (ZZZ) and the number of neutrons (NNN), and is called the mass number.

The number of neutrons N=A−ZN = A - A-ZN=A-Z.

For example, Uranium-238 (92238U_{92}^{238} \text{U}92238​U) has 92 protons, 238 - 92 = 146 neutrons, and 238 nucleons (protons + neutrons).

Isotopes, Isobars, and Isotones

• Isotopes: Atoms having the same atomic number but different mass numbers are called isotopes.

• Isobars: Atoms having the same mass number but different atomic numbers are called isobars.

• Isotones: Elements having the same number of neutrons but different atomic numbers are called isotones.

Nuclear Fission

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

• The energy released in nuclear fission is called nuclear energy.

• Nuclear fission was first demonstrated by Strassmann and O. Hahn. They found that when a uranium nucleus is excited by the capture of a neutron, it splits into two smaller nuclei, Barium (Ba) and Krypton (Kr).

Chain Reaction

A chain reaction occurs when the fission of one uranium atom releases neutrons, which then cause further fission, continuing until the entire uranium is consumed.

There are two types of chain reactions:

Uncontrolled Chain Reaction:

Atom bomb: The atomic bomb is based on nuclear fission.

Uranium-235 (92235U_{92}^{235} \text{U}92235​U) and Plutonium-239 (94239Pu_{94}^{239} \text{Pu}94239​Pu) are used as fissionable materials. This bomb was first used by the USA against Japan in World War II (6th August 1945 at Hiroshima and 9th August 1945 at Nagasaki).

• In each fission reaction, three more neutrons are produced.

• These three neutrons may cause the fission of three other nuclei, producing 9 neutrons, and so on.

• As a result, the number of neutrons increases rapidly, consuming the fissionable material.

• This reaction proceeds very quickly, releasing a huge amount of energy in a short time.

Controlled Chain Reaction:

• In this type, the fission chain reaction proceeds slowly without any explosion.

• Only one of the neutrons produced in each fission causes further fission.

• The rate of reaction remains constant and can be controlled.

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.

Components of a Nuclear Reactor:

1. Fissionable Fuel: Uranium-235 (92235U_{92}^{235} \text{U}92235​U) or Plutonium-239 (94239Pu_{94}^{239} \text{Pu}94239​Pu) is used as fuel.

2. Moderator: It decreases the energy of neutrons so that they can be further used for fission. Heavy water and graphite are commonly used as moderators.

3. Control Rod: Rods made of cadmium or boron are used to absorb the excess neutrons produced during fission to control the reaction.

4. Coolant: A large amount of heat is produced during fission. The coolant absorbs this heat and prevents an excessive rise in temperature. The coolant may be water, heavy water (deuterium is an isotope of hydrogen), or a gas like helium (He) or carbon dioxide (CO₂).

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 fast breeder reactor is a type of nuclear reactor that can produce more fissionable fuel than it consumes.

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 fusion is the process occurring in the sun.

• The energy released by the sun and other stars is a result of nuclear fusion.

• For nuclear fusion to occur, a temperature of the order of 10⁶ K is required.

Hydrogen Bomb

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

Mass-Energy Relation

In 1905, Albert Einstein established a relation between mass and energy based on his special theory of relativity.

The relation is given by:

• E=mc2E = mc^2E=mc2

where:

• EEE is the energy equivalent of mass mmm

• C is the velocity of light.

This equation shows that mass can be converted into energy and vice versa.

Energy from the Sun

• The sun continuously emits energy. The Earth receives approximately 3.86 x 10²⁶ joules of energy per second from the sun.

• As a result, the mass of the sun is decreasing at a rate of approximately 4.2 million tons per second.

However, the mass of the sun is so large that it is estimated that the sun will continue supplying energy for the next 5 billion years.

Final Thoughts

Nuclear fission and fusion are two powerful processes that release energy at the atomic level. Nuclear fission occurs when a heavy nucleus, like uranium, splits into two smaller nuclei, releasing energy and neutrons that trigger further reactions—a process vital in nuclear reactors and atomic bombs.

On the other hand, nuclear fusion, the process that powers the sun, combines light nuclei to form heavier ones, releasing massive amounts of energy. Both processes involve complex physics and are used for energy production, but nuclear fusion requires extreme temperatures and is still under research for sustainable use.

Understanding these reactions helps us harness atomic energy for various applications, from electricity generation to medical isotope production.