2.1.3 Particles, antiparticles and photons

Particles and Their Antiparticles

Every fundamental particle has a corresponding antiparticle. An antiparticle has the same mass and rest energy as its particle counterpart but differs in certain other properties, such as charge. For instance:

• The positron is the antiparticle of the electron.
• The electron antineutrino is the antiparticle of the electron neutrino.

Here is a summary of some particles and their properties:

Particle	Mass (kg)	Rest energy (MeV)	Charge (C)
Electron $(e^-)$	$9.11 \times 10^{-31}$	0.511	$-1.6 \times 10^{-19}$
Positron $( e^+ )$	$9.11 \times 10^{-31}$	0.511	$+1.6 \times 10^{-19}$
Electron neutrino $(\nu_e )$	0	0	0
Electron antineutrino $( \bar{\nu}_e )$	0	0	0

Photons and Electromagnetic Radiation

Photons are particles of electromagnetic radiation that carry energy but have no mass. The energy of a photon is directly related to the frequency of the radiation it represents. The relationship is given by the equation:

$$E = hf = \frac{hc}{\lambda}$$

Where:

• $E$ is the photon energy,
• $h$ is Planck's constant, approximately 6.63 × 10^-34 J s,
• $f$ is the frequency of the electromagnetic wave,
• $\lambda$ is the wavelength of the radiation,
• $c$ is the speed of light in a vacuum, approximately 3.00 × 10^8 m/s.

Annihilation

Annihilation occurs when a particle collides with its corresponding antiparticle. In this interaction:

• Both the particle and the antiparticle are converted into energy.

• The energy is released in the form of two gamma photons moving in opposite directions, which conserves momentum. A practical application of annihilation is in Positron Emission Tomography (PET) scanning:

• A positron-emitting radioisotope is introduced into the body.

• The emitted positrons encounter electrons in the body and annihilate.

• This process releases gamma photons, which are detected to create a 3D image of the internal body structure, assisting in medical diagnosis.

Pair Production

Pair production is the process by which a high-energy photon is converted into a particle-antiparticle pair. For pair production to occur:

• The photon must have energy greater than the combined rest energy of the particle and antiparticle it produces.
• Excess energy (beyond the rest energy of the particles) is converted into kinetic energy of the created particles. An example of pair production is when a gamma photon near a nucleus transforms into an electron and a positron. This phenomenon demonstrates the principle of energy-mass equivalence, where energy can transform into mass and vice versa.