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Physics Lesson 21.1.3 - Electron-Positron Pair
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Welcome to our Physics lesson on Electron-Positron Pair, this is the third lesson of our suite of physics lessons covering the topic of Elementary Particles, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
Electron-Positron Pair
Positron does not exist in the structure of common matter; the electron-positron pair appears only during the collision with matter of charged particles or high-energy gamma rays. This process is known as "pair production". It must be noted here that pair production is not an exclusive process of electron-positron only; it applies in all matter-antimatter pairs of elementary particles. During this process, electric charge must be conserved and an amount of energy E = 2 me · c2 sufficient to overcome the rest energy of the two particles. Hence, the minimum energy needed for pair production process must be
= 2 ∙ (9.1 × 10-31 kg) ∙ (3 × 108 m/s)2
= 1.64 × 10-13 J
The process of pair production is shown in the figure below.
Schematically, the process of electron-positron pair production is written as:
The reverse process may also occur. Thus, when an electron enters in contact with a positron, a gamma particle is produced. More specifically, if a positron encounters an electron on its way, this electron-positron pair is transformed into a pair of gamma quants according the reaction
The energy of this pair of gamma quants is not less than 2 me · c2, This radiation propagates in opposite direction to the movement of original particles. Thus, when a particle and an antiparticle collide with each other, they are annihilated, emitting energy. This process is (not so rightfully) called "pair annihilation".
The above fact is a demonstration that besides the mutual transformation of particles from matter to matter (i.e. within the family of material objects), a matter to energy or vice-versa transformation is also possible. More specifically, matter is transformed in EM (gamma) radiation and vice-versa.
We can find identify the positron as elementary particle during the explosion of unstable particles as well. A nucleus that contains too many neutrons emit an electron e- during a beta minus decay to become more stable. In this way, a neutron converts to proton, increasing the atomic number Z by 1 while the number of neutrons N decreases by 1. On the other hand, if an atomic nucleus is abundant in protons but lacks the sufficient number of neutrons to be stable, releases a positron e+, transforming in this way a proton into neutron. Such nuclei cannot be found in nature but they can be produced artificially in nuclear reactors.
Example 1
Calculate the rest mass of positron in MeV/c2 if the minimum energy needed to produce a positron is 1.02 MeV.
Solution 1
From theory, we know that electron and positron have the same rest mass denoted by me and the minimum energy needed for the electron-positron pair to produce is given by
Rearranging to isolate me, we obtain for the rest mass of positron:
= 1.02 MeV/2 ∙ c2
= 0.51 MeV/c2
You have reached the end of Physics lesson 21.1.3 Electron-Positron Pair. There are 4 lessons in this physics tutorial covering Elementary Particles, you can access all the lessons from this tutorial below.
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