Course No.

PHY- 2251 (B), Physics – IV, Mathematics Discipline, Khulna University

Chapter 9
Nuclear Physics

Md. Shohel Parvez

Lecturer,
Physics Discipline, Khulna University, Khulna
What is Nucleus?
The central and most important part of an object, movement, or
group, forming the basis for its activity and growth.
Or, in language of physics, the positively charged central core of an
atom, consisting of protons and neutrons and containing nearly all its
mass.
Proton
Nucleus
Neutron

The size (diameter) of the nucleus is between 1.6 fm (10−15 𝑚) (for a

proton in light hydrogen) to about 15 fm (for the heaviest atoms, such as
uranium). These sizes are much smaller than the size of the atom itself by
a factor of about 23,000 (uranium) to about 145,000 (hydrogen).
The nucleus has most of the mass of an atom, though it is only a very
small part of it. Almost all of the mass in an atom is made up from the
protons and neutrons in the nucleus with a very small contribution from
the orbiting electrons.

General Properties of Nucleus:

1. Size
The nucleus is about 10,000 times smaller than the atom. The
1
empirical formula for the nuclear radius is 𝑅 = 𝑟0 𝐴 3

where A is the mass number and 𝑟0 =1.2×10−15 m=1.2Fm

2. Charge
The nuclei consist of protons and neutrons. Protons are positively
charged and neutrons are neutral. So the nuclei are positively charged
The charged of nuclei is positive.
3. Mass
Sine the nucleus consists of protons and neutrons, the mass of the
nucleus is the sum of masses of nucleons.
𝐴𝑠𝑠𝑢𝑚𝑒𝑑 𝑚𝑎𝑠𝑠 = 𝑍𝑚𝑝 + 𝑁𝑚𝑛
Where Z is the atomic number, N is the neutron number 𝑚𝑝 is the
mass of proton and 𝑚𝑛 is the mass of neutron.
4. Nuclear density
The nuclear density is defined by ρ.
3𝑚𝑁 𝐴 17 𝑘𝑔𝑚−3
ρ= = 2.3 × 10
4𝜋𝑟0 3𝐴
The density of nucleus is very high and is independent with A.
5. Nuclear spin and magnetic moment
Protons and neutrons are in continuous motion in discrete quantized
orbits. The nucleons have internal angular momentum called spin. As
a result, they possess magnetic momentum associated with their
orbital motion and spin.
Binding energy:
Binding energy is the energy required to disassemble a whole system
into separate parts. It is known the sum of separate parts has typically a
higher potential energy than a bound system, therefore the bound system
is more stable. A creation of bound system is often accompanied by
subsequent energy release. We usually distinguish the binding energy
according to these levels:
At atomic level the atomic binding energy of the atom derives from
electromagnetic interaction of electrons in the atomic cloud and nucleons
(protons) in the nucleus. The atomic binding energy is the energy
required to disassemble an atom into free electrons and a nucleus. This is
more commonly known as ionization energy.
 At molecular level the molecular binding energy of the molecule
derives from bond-dissociation energy of atoms in a chemical bond.
At nuclear level the nuclear binding energy is the energy required to
disassemble (to overcome the strong nuclear force) a nucleus of an atom
into its component parts (protons and neutrons).
 It is found that the mass of nucleus is less than the sum of the masses
of the constituent particles in the free state. According to Einstein, the
decrease in mass is due to the release of energy when the particles
combine to form a nucleus. The energy released is given by the relation
𝐸 = 𝑚𝑐 2 , where m is the decrease in mass and c the velocity of light.
The amount of energy represents the binding energy of the nucleus. If
the binding energy is large, the nucleus is stable.
If the nucleus is to be broken into a constituent particles, an energy
equal to or more than the binding energy must be supplied to the nucleus.
Let, lithium whose A=7 and Z=3. so its nucleus contains three
protons and 4 neutrons.
The mass of the lithium nucleus = 7.016005 amu.
 The mass of three protons = 3 × 1.007277 𝑎𝑚𝑢
= 3.021831 𝑎𝑚𝑢
The mass of four neutrons = 4 × 1.008665 𝑎𝑚𝑢
= 4.03466 𝑎𝑚𝑢
Total initial mass of the three protons and four neutrons is
= 3.021831 + 4.03466
= 7.056491 𝑎𝑚𝑢
As the mass of lithium nucleus is less than the total initial mass of 3
protons and 4 neutrons. So decrease in mass
= 7.056491 − 7.016005 = 0.040486 amu
This shows that in the formation of lithium nucleus the mass has
decreased by 0.040486 amu and decrease in mass due to the release of
 Energy in the process.
1 𝑎𝑚𝑢 = 931 𝑀𝑒𝑉
Binding energy of lithium nucleus = 0.040486 × 931
= 37.7 𝑀𝑒𝑉
In order to break the lithium nucleus, an energy more than 37.7 MeV
must be supplied. Therefore, the lithium nucleus is very stable in
structure. In any attempt to break the nucleus, energy must be supplied to
some external source.
In the case of lithium, there are 7 particles . Hence the binding energy
37.7
per nucleon = = 5.4 𝑀𝑒𝑉.
7
Radioactivity:
The property possessed by some elements (such as uranium) or
isotopes (such as carbon 14) of spontaneously emitting energetic
particles (such as electrons or alpha particles) by the disintegration of
their atomic nuclei; also :the rays emitted.
When an elements (most cases heavier elements) or an isotopes
became unstable then it emits various particles or rays or radiation and
become a new particle.
The system of emitting particles or rays from
a radioactive elements are called radioactivity.
The most common emitted particles are alpha
and beta and rays are gamma and others.
Fission:
In nuclear physics and nuclear chemistry, nuclear fission is either a
nuclear reaction or a radioactive decay process in which the nucleus of
an atom splits into smaller parts (lighter nuclei). The fission process
often produces free neutrons and gamma photons, and releases a very
large amount of energy even by the energetic standards of radioactive
decay.
Nuclear fission of heavy elements was discovered on December 17,
1938 by German Otto Hahn and his assistant Fritz Strassmann, and
explained theoretically in January 1939 by Lise Meitner and her nephew
Otto Robert Frisch.
Fission is a form of nuclear transmutation because the resulting
fragments are not the same element as the original atom.
 The two nuclei produced are most often of comparable but slightly
different sizes, typically with a mass ratio of products of about 3 to 2, for
common fissile isotopes. Most fissions are binary fissions (producing
two charged fragments), but occasionally (2 to 4 times per 1000 events),
three positively charged fragments are produced, in a ternary fission. The
smallest of these fragments in ternary processes ranges in size from a
proton to an argon nucleus.
Fusion:
The process or result of joining two or more things together to form a
single entity.
In case of physics fusion is the process of combing two or more light
nuclei to from a heavier nuclei by releasing a large amount of energy.
Nuclear Reactor:
A nuclear reactor, formerly known as an atomic pile, is a device used
to initiate and control a sustained nuclear chain reaction. Nuclear reactors
are used at nuclear power plants for electricity generation and in
propulsion of ships.
In another words, A nuclear reactor is a system that contains and
controls sustained nuclear chain reactions.
Or, Nuclear reactor, any of a class of devices that can initiate and
control a self-sustaining series of nuclear fissions. Nuclear reactors are
used as research tools, as systems for producing radioactive and medical
isotopes for imaging and cancer treatment, moving aircraft carriers and
submarines and most prominently as energy sources for nuclear power
Components of Nuclear Reactor:
1. Fuel
2. Moderator
3. Control rods
4. Coolant
5. Pressure vessel or pressure tubes
6. Steam generator
7. Containment

Types of Reactors:
I. Pressurized water reactor
II. Boiling water reactor
III. Canada Deuterium-Uranium Reactors (CANDU)
IV. Sodium Cooled Fast Reactor
V. Molten Salt Reactor
VI. High Temperature Gas Cooled Reactor
Etc.
Cosmic Rays:
Cosmic rays are high-energy radiation, mainly originating outside the
Solar System and even from distant galaxies.
Or, Cosmic rays are atom fragments that rain down on the Earth from
outside of the solar system. They blaze at the speed of light and have
been blamed for electronics problems in satellites and other machinery.
Composed primarily of high-energy protons and atomic nuclei, they
are of mysterious origin. Data from the Fermi Space Telescope (2013)
have been interpreted as evidence that a significant fraction of primary
cosmic rays originate from the supernova explosions of stars. Active
galactic nuclei probably also produce cosmic rays.