Annihilation

Physics is divided into different branches, and particle physics is one such branch. In simple terms, it is the branch of physics that deals with the study of nature, properties, and constituents of the particles that make up matter. Let us learn about two concepts under this branch of study.

Pair Production and Annihilation

Pair production is the process in which photons are split into subatomic particles and antiparticles; one is positively charged, and the other is negatively charged. On the other hand, annihilation is the reverse process of pair production.

Particle annihilation is the radiation produced by the amalgamation of a particle and its antiparticle. Generally, annihilating particles have the same mass, but their charges are opposite in nature. Particles annihilate to release energy in the form of photons. Annihilation obeys the conservation of energy and conservation of momentum. Collisions of different subatomic particles and antiparticles produce several distinct particles in the end-stage.

Thibaud and Joliot were attributed for the discovery of particle annihilation, and several other scientists have worked on the same concept.

Examples of Annihilation

Electron-Positron Annihilation

The first example of annihilation is electron-positron annihilation. In the process of annihilation between an electron and a positron, the positron combines with the electron to form positronium, which is a quasi-atom. Here, an electron is a subatomic particle that is negatively charged, which either occurs freely or is bound to the nucleus of the atom. Whereas the positron is the antiparticle of the electron and is positively charged.

Therefore, it is the opposite of the electron. The positron has the same mass 9.109 × 10^-31 kg, same electric charge (1.602 × 10^-19C), and same spin (1/2) as that of the electron.

Thus, in simple terms, particle annihilation is the process in which the collision of a subatomic particle and an antiparticle takes place. This then disappears to release energy i.e. photons which have electromagnetic energy. The annihilation of an electron and a positron releases two photons (2 gamma rays).

They are emitted at an angle of nearly 180° to one another. Each photon will have an energy of 0.511 MeV (million electron volts). Thus, the sum of the energies of two photons (2 gamma rays) amounts to 1.022 MeV. This total energy is equivalent to the rest mass of the two particles and disappears to release energy. The conservation of energy and momentum takes place here.

Proton-Antiproton Annihilation

Another example of annihilation is proton-antiproton annihilation. When a proton annihilates with the antiproton, it initially produces gluons. The gluons, along with the remains of quark-antiquark, lead to the production of mesons. The mesons produced are unstable unless they integrate with other particles. Finally, they decay to produce photons (electromagnetic energy), electrons, positrons, and neutrinos. The energy value of the released particles accounts for nearly 2 GeV (Giga electron Volt). A similar process occurs on the annihilation of an antinuleon with an atomic nucleus.

Theories of Annihilation

The annihilation of particles can be depicted by the Feynman diagram. It is the pictorial representation of mathematical expressions of the interaction of subatomic particles. It was put in place by Richard Feynman, an American physicist, in the year 1948. His diagram makes us understand the complex process of the interaction of subatomic particles in a very simple way.

In the Feynman diagram, the electron-positron particle annihilation is represented as follows: an electron (e^–) and a positron (e⁺) annihilate to produce photons (γ) which later becomes a quark-antiquark pair [quark(q) antiquark(q^–) ]. Thereafter, the antiquark radiates gluon (g). The later theories explain annihilation with respect to empirical observations by fitting mathematical models to it.

Applications of Annihilation

• One of the novel applications of positron annihilation is positron annihilation spectrography.
• The concept is used in various imaging processes such as Single Photon Emission Computed Tomography (SPECT), Positron Emission Tomography (PET), gamma camera, etc.
• Positron annihilation spectrography is used in the study of defects, such as crystallographic defects in metals.

Conclusion

We learnt about the branch of physics known as particle physics. Under this branch, we learnt about two concepts: pair production and annihilation. Pair production and annihilation are opposite processes. Pair production is the process in which photons are split into subatomic particles and antiparticles, where one is positively charged, and the other is negatively charged. Particle annihilation is the radiation produced by the amalgamation of a particle and its antiparticle. Though annihilating particles have the same mass, their charges are opposite in nature. Two examples of annihilation were discussed: electron-positron annihilation and proton-antiproton annihilation.