AQA A-Level Physics/Particles and Anti-particles/Creation and annihilation 2

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Creation and Annihilation

This is part of topic 1.4 in the AQA Physics A syllabus. The main of the topic covers about Particles and antiparticles. And how there is an antiparticle for every particle in the universe. The theory of antimatter was first predicted by Paul Dirac in 1928 and has since been proved. However creation and annihilation in basic forms is either, when a photon creates a particle and an antiparticle or when a particle and an antiparticle collide creating a photon.

Creation

There are in fact many different types of creation; however in this syllabus there is one mentioned specifically, Pair Production. In Pair Production a photon creates a particle and a corresponding antiparticle, and vanishes in the process. For a particle and antiparticle, each of rest energy E0, we can calculate the minimum energy and maximum frequency fmin that the photon must have to produce this particle – antiparticle pair. Remember, c is the speed of light in a vacuum (3.0 x 10^8ms^-1).

Minimum energy of photon needed = hfmin = 2 E0

For example, the electron has a rest energy of 0.511 MeV. Therefore, for pair production of an electron and a positron from a photon:

The minimum energy = 2 x 0.511 MeV = 1.022 MeV = 1.64 x10^-13 J

A photon with less energy could not therefore create a positron and an electron.

Annihilation

Annihilation occurs when a particle and a corresponding antiparticle meet and their mass is converted into radiation energy. Two photons are produced in the process (as a single photon only would take away momentum which isn’t allowed, as no outside forces act). Therefore, the minimum energy of each photon, hfmin is given by equating the energy of the two photons, 2hfmin, to the rest energy of the positron and of the electron (i.e. 2hfmin = 2E0, where E0 is the rest energy of the electron.)

Minimum energy of each photon produced, hfmin = E0