6.4NuclearCore

Fundamental Particles and Antiparticles

The continuous range of energies seen in beta decay electrons was a genuine puzzle until physicists proposed an entirely new, almost undetectable particle to balance the books. That particle — the neutrino — is one of a whole family of mirror-image "antiparticles" that populate the subatomic world alongside ordinary matter.

40 min Video by Science Shorts Module 6.4 Nuclear and particle physics

Particle Physics — A-level PhysicsWatch the full walkthrough before the notes below.

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What you'll be able to do

State the properties of the proton, neutron and electron, and their specific charge
Explain that every particle has a corresponding antiparticle
Describe the properties of the neutrino and antineutrino
Use the photon energy equation and describe annihilation and pair production

Protons, neutrons and electrons

An atom’s nucleus contains protons (charge $+ e$ ) and neutrons (uncharged), surrounded by electrons (charge $- e$ ). $Specific charge$ (charge ÷ mass) reveals a striking asymmetry: despite equal charge magnitude, the electron’s specific charge is about 1836 times the proton’s, since it is about 1836 times lighter.

1Use

specific charge = \frac{Q}{m}

= \frac{1.60 \times 1 0 ^{- 19}}{1.67 \times 1 0 ^{- 27}}

Answer

9.58 \times 1 0^{7}

C kg⁻¹

Antiparticles

Every particle has a corresponding antiparticle of equal mass but opposite charge (and opposite other properties, like baryon/lepton number). The positron is the electron’s antiparticle.

Tip — When a particle meets its antiparticle, both are destroyed and their mass-energy converts into photons — annihilation. The reverse, pair production, sees a photon convert into a particle-antiparticle pair.

The neutrino and antineutrino

Beta-minus decay electrons show a continuous range of energies, not a single fixed value — inconsistent with a simple two-body decay. Pauli proposed a near-massless, uncharged particle sharing the energy: the antineutrino, emitted alongside the electron.

Photons, annihilation and pair production

A photon carries energy $E = h f$ . When a particle and antiparticle annihilate at rest, their combined rest energy converts into two photons (not one, so momentum is conserved).

E_{photon} \geq 2 m_{e} c^{2} = 2 \times 8.19 \times 1 0^{- 14} J

Threshold photon energy for electron-positron pair production.

1Use

E \geq 2 m_{e} c^{2}

E = 2 \times 9.11 \times 1 0^{- 31} \times (3.00 \times 1 0^{8})^{2}

Answer

E \approx 1.64 \times 1 0^{- 13}

Tip — Pair production always needs a nearby nucleus to absorb recoil momentum — a lone photon in empty space can never spontaneously produce a pair.

Equation recap

specific charge = \frac{Q}{m}

Specific charge.

E = h f

Photon energy.

E_{photon} \geq 2 m_{X} c^{2}

Pair production threshold.

Common mistakes to avoid

Believing annihilation produces just one photon.

At least two photons are needed to conserve momentum.

Confusing the neutrino with the neutron.

The neutrino has almost zero mass and no charge; the neutron has a mass close to the proton’s — unrelated particles.

Key takeaways

Protons, neutrons and electrons make up atoms; specific charge (Q/m) reveals the electron’s far smaller mass.
Every particle has an antiparticle of equal mass, opposite charge.
The (anti)neutrino conserves energy, momentum and lepton number in beta decay.
Annihilation converts a particle-antiparticle pair into (at least) two photons; pair production is the reverse.

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Fundamental Particles and Antiparticles

40 min Video by Science Shorts Module 6.4 Nuclear and particle physics