Nuclear Physics

Nuclear physics is the field of Physics that studies atomic nuclei. In other words, nuclear physics deals with the components and structure of the nucleus. Nuclear reaction comprises the merging of nuclei, radioactive decay, fusion, fission, and the break-up of a nucleus.

Nuclear Physics

Nuclear Physics and Atomic Physics

  • The simple difference between nuclear physics and atomic physics is that nuclear physics deals with the nucleus while atomic physics deals with an entire atom. But isn’t the nucleus a part of the atom? Then why do we have two separate branches?
  • Atomic physics deals with the properties of an atom as a whole, mainly due to its electronic configuration. Of course, the nucleus is a part of this but only in terms of its overall contribution.
  • Nuclear physics, on the other hand, deals exclusively with nuclei, their structures, properties, reactions, and interactions. Nuclear physics Atoms make up all the matter in the universe.
  • By understanding the concepts of quarks and gluons, the forces related to nuclear physics are understandable. 
  • The application of nuclear physics is largely in the field of power generation using nuclear energy. Once the force holding the nucleus was understood, we started splitting and fusing neutrons.
  • The energy evolved in this process can be used in the splitting of the nucleus to generate energy in Nuclear Fission and fusing two neutrons to generate energy is Nuclear Fusion.

Radius of Nucleus

‘R’ represents the radius of nucleus.

\(\large R = R_{o}\;A^{\frac{1}{3}}\)


  • Ro is the proportionality constant
  • A is the mass number of the element

Total Number of Protons & Neutrons in a Nucleus

The mass number (A), also known as the nucleon number, is the total number of neutrons and protons in a nucleus.

A = Z + N


  • N is the neutron number
  • A is the mass number
  • Z is the proton number

Mass Defect

Mass defect takes place when some mass is lost during the nuclei generation.

\(\large \Delta m = Z\,m_{p}+(A-Z)m_{n}-M\)


  • M is the mass of the nucleus
  • \(\Delta m\) is the difference between the mass of the nucleons and mass of the nucleus
  • \(m_{p}\) is the Mass of the Proton
  • \(m_{n}\) is the mass of the Neutron

Packing Fraction

Packing fraction is defined as Mass defect per nucleon.

Packing fraction (f) = Mass defect per nucleons

\(\large Packing\;fraction (f) = \frac{\left [ Zm_{p}+(A-Z)m_{n}-A \right ]}{A}\)

Use of Nuclear Physics

  • The continuous research in the field of nuclear physics has helped us find various other uses. For example, we now have nuclear medicine, nuclear weapons and have even found its uses in geology and archaeology in terms of carbon dating.
  • Atomic Physics An atom is made of a dense nucleus having neutrons and protons at the core surrounded by orbiting electrons as per the configuration. The centre is positively charged and the surrounding cloud of electrons carries a negative charge. As a whole the atoms in consideration can either be neutral or carry a charge (in this case we call them ions).
  • In many places, you may have seen that atomic energy is the source of energy production in terms of nuclear fission and fusion. Let this not confuse you as they both are often used and associated. The cutting difference between the two has already been mentioned at the beginning of this article.
  • Atomic physics concerns itself with the entire atom and how the electronic configuration of electrons can change. When an atom loses an electron, it becomes positively charged (cations) and when it gains an electron it becomes negatively charged (anions).

Law of Radioactivity

  • Radioactivity occurs due to the decay of the nucleus.
  • External parameters such as temperature and pressure do not affect the rate of decay.
  • Radioactivity is based on the law of conservation of charge.
  • The daughter nuclei will have unique physical and chemical properties (that is different from parent nuclei).
  • The decay rate of any radioactive material is directly proportional to the number of atoms present at that instant.
  • α, β, and γ rays are followed during the radioactivity.

Units of Radioactivity

There are two units of radioactivity and they are:

  • Curie (Ci): If the radioactive substance decay at the rate of 3.7×1010 decays per second, then the unit used is Curie.
  • Rutherford (rd): If the radioactive substance decay at the rate of 106 decays per second, then the unit is Rutherford.

Nature of Nuclear Force

Nuclear physics is based on the forces known as nuclear force. The nature of nuclear force is given as:

  • Nuclear forces are attractive in nature.
  • These forces are independent of charges.
  • The range of nuclear forces is short.
  • As the distance between two nucleons reduces, the nuclear force becomes weak between them.
  • The nuclear force is dependent on the spin.

Solved Examples of Nuclear Physics

Example 1:

Consider a radioactive material which is undergoing fusion and resulting in a mass defect of 2 kg. Calculate the amount of energy released from the material.


Mass defect, m = 2 kg

Velocity of light, c = 3 х 108 m/s

Using Einstein’s equation,

E = mc2

E = 2 х (3 х 108)2

E = 1.8 х 1017 J

Example 2: 

Determine the number of neutrons in the final daughter nucleus if \(U_{92}^{235}\) undergoes one α-decay and β-decay.


When the given substance undergoes the α-decay and β-decay, let these resulting nucleus be represented by X and Y.

\(U_{92}^{235}\xrightarrow[]{\alpha \;decay}X_{90}^{231}+He_{2}^{4}\)

\(X_{90}^{231}\xrightarrow[]{\beta \;decay}Y_{91}^{231}+e_{-1}^{0}\)


Number of neutrons = mass number – atomic number = 231 – 91 = 140

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Frequently Asked Questions-FAQs

Who discovered natural radioactivity?

Natural radioactivity was discovered by Henri Becquerel.

What is the function of control rods in a nuclear reactor?

The function of control rods in a nuclear reactor is it is used for controlling the number of neutrons in the chain reaction. Cadmium or boron are used as the control rods.

Mention two applications of radio isotopes in the field of agriculture.

  • Radio isotopes are used for killing insects and parasites and in the prevention of the agricultural wastes.
  • It is used for enhancing the storage time of vegetables such as onions, potatoes, and grams.

What is the chronological order of nuclear reactor, discovery of radium, artificial radioactivity, and radioactivity depending on their discovery.

The correct chronological order according to the discovery is as follows:
Radioactivity, discovery of radium, artificial radioactivity, and nuclear reactor.

What are the units of radiactivity?

Following are the units of radioactivity:

  • Curie
  • Becquerel
  • Roentgen

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