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Nuclear Reactions

when the world Exist Till when the world exist? All the matter in the universe today is only about a billionth of the amount of matter that existed during the very early universe, and it is that remaining amount of particles that now make the galaxies, stars, planets and all living things on Earth, including our own bodies. Soon after the Big Bang, when the universe was extremely dense and hot, equal amounts of matter and antimatter were created from the available energy. The obvious question is, where did the antimatter go? What happens if all the matter interacts with the antimatter? Will it lead to the end of the world? Let’s seek the guidance of this topic to get the answers.

Learning objectives

After completing the topic, the student will be able to:

  • Discuss the phenomenon of nuclear reaction and the types of nuclear reactions and its applications in everyday science scenarios.
  • Explore Einstein’s Mass–Energy relation and its application in nuclear fission reactions.
  • Investigate what are nuclear reactions and explore the applications of controlled chain reactions in nuclear reactors.
  • Discuss, probe and examine the working of a nuclear reactor with all its components and underline its application in producing electricity.
  • Explore the interiors of stars and examine different cycle of reactions that occur at the core of the star to reach the supernova.
  • Differentiate between fission and fusion reactions with their applications in specific science scenarios.
  • Discuss and debate the radiation hazards due to different types of nuclear radiations that are harmful to the mankind.
self-sustaining nuclear chain First self–sustaining nuclear chain reaction Painting depicting the moment at which the first self–sustaining nuclear chain reaction occurred, at 3.36 pm on December 2, 1942. The pile was built by a team led by the Italian physicist Enrico Fermi in a squash court under the stands of Chicago University’s Stagg Field, and consisted of a complex lattice of bricks of graphite and “slugs” of uranium. At bottom centre is George Weil, operating one of the control rods. At top right is the ‘liquid control squad’, armed with buckets of a solution of a cadmium salt.

A reaction in which suitable high–energy particles colliding with a stable nucleus change it into another nucleus is called a nuclear reaction. Such collisions change the identity or properties of a nucleus. Nuclear reactions cause the contents of the nucleus to change and the element gets converted into another element. Such a conversion of one element into another by bombarding the nucleus by high–energy particles is called transmutation. Induced nuclear transmutations can be used to produce isotopes that are not found naturally.

A nuclear reaction involves the bombardment of a target nucleus by some high energy incident particles and leads to a product nucleus and an outgoing particle (product particle). The neutrons that participate in nuclear reactions can have kinetic energies that cover a wide range and those that have a kinetic energy of about 0.04 eV or less are called thermal neutrons.

Like radioactive decay, all nuclear reactions obey the laws of conservation of mass, charge, momentum and energy. The total electric charge of the nucleons and the total number of nucleons is conserved during an induced nuclear reaction. The fact that these quantities are conserved makes it possible to identify the nucleus produced in a reaction.

Nuclear chain reaction Nuclear fission reaction Nuclear fission reaction, computer artwork. At left is a neutron (blue) about to collide with an uranium-235 nucleus (grey). Upon collision the neutron combines with the nucleus to form uranium-236. This is a highly unstable element that spontaneously undergoes nuclear fission (splitting), producing a vast amount of energy (seen here as a blue and green explosion), the elements barium-141, krypton-92 and three neutrons. This is the reaction that occurred in the atomic bombs dropped by the Americans on the Japanese cities of Hiroshima and Nagasaki during World War II.
Nuclear fission

When a neutron collides with a nucleus, it may simply scatter of the nucleus, exchanging some energy of motion with it or get absorbed by the nucleus. When neutron is absorbed a new compound nucleus is formed.

Thus when a neutron is absorbed by U–235, the compound nucleus U–236 is formed with. The new compound nucleus will be unstable due to its excited state as it absorbs the binding energy of the neutron. It may simply emit a gamma ray and return to its ground state. It may expel a neutron, a proton or an alpha particle; or if the nucleus is very excited, it may even spill out more than one neutron or proton.

For a neutron absorbed by a heavy nucleus, there is another distinct mode of decay possible; the excited compound nucleus splits into roughly equal fragments–nuclei of intermediate mass number. This decay mode is nuclear fission–the process basic to nuclear reactors and atomic bombs.

The dynamics of nuclear fission can be understood in a simple way. Heavy nucleus is like a charged liquid drop with a near spherical shape. When it absorbs a neutron, some energy is added to it which gives rise to surface oscillations that stretch the drop further away from the spherical shape. If the added energy is sufficient, the original drop will split into two smaller drops which mean that the heavy nucleus has undergone fission.

Thus when a nucleus absorbs neutron, a variety of reactions is possible. An experimental measure of probability for each variety of reaction is called cross section of the reaction. The greater the cross section, the greater the likelihood of that reaction taking place.

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