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Pair Production

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Pair production is the formation of elementary particle and its anti particles from a photon. During this process, radiant energy is directly converted to matter.Pair production happens when a photon of particular energy incident on an atomic nucleus, a pair of an elementary particle and its anti-particle produced. The details about this process is described in the given section.

Definition

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The third interaction of photons matter matter and the most important at high energies is pair production. The intense electric field near the nucleus can cause the photon to decay into an electron and a positron. The threshold energy for this process is 2mc2. The nucleus must be there to satisfy conservation of momentum, but it acquires very little recoil energy. Pair production may also take place near an atomic electron. The threshold in this case is 4mc2 and the recoil electron acquires significant kinetic energy. In a track sensitive detector this would appear as a triplet of tracks.

Pair Production

Process

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A common process in which a photon creates matter is called pair production. In this process, an electron and a positron are simultaneously produced, while the photon disappears. In order for pair production to occur, energy, momentum and charge must all be conserved during the process. It is impossible for a photon to produce a single electron because the photon has zero charge and charge would not be conserved in the process.The energy of an electron is mc2 = 0.51MeV, the minimum energy required for pair production is 1.02MeV.

Pair Production in Vacuum

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Pair production can't occur in a vacuum, but can only take place in the presence of a massive particle such as an atomic nucleus. The massive particle must participate in the interaction in order that energy and momentum be conserved simultaneously.

Pair Production and Annihilation

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In general pair production is the generation of elementary particle and its anti particles like electron and positron. Normally it will take place if a photon is interact with the nucleus. The detailed process is described in the above section.Pair annihilation is a process in which an electron-positron pair produces two photons- the inverse of pair production.

Pair Production and Annihilation
Given figure is one of the examples of pair annihilation in which an electron and positron initially at rest combine with each other, disappear and create two photons. Because the initial momentum of the pair is zero, it is impossible to produce a single photon. Momentum can be conserved only if two photons moving in opposite directions, both with the same energy and magnitude of momentum are produced.