Particles And Antiparticles

Question 1

Antimatter

Answer 1

When antimatter and matter particles (meet, they destroy each other and radiation is released. We make use of this effect in a positron emitting tomography (PET) hospital scanner. The P in PET stands for the positron, which is the antiparticle of the electron. When a PE T scanner is used for a brain scan, a positron-emitting isotope is administered to the patient and some of it reaches the brain via the blood system. Each positron emitted travels no further than a few millimetres before it meets an electron and they annihilate each other.
Two gamma photons, produced as a result, are sensed by detectors linked to computers. Gradually, an image is built up from the detector signals of where the positron-emitting nuclei are inside the brain.
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Question 2

Position emition

Answer 2

Positron emission takes place when a proton changes into a neutron in an unstable nucleus with too many protons. The positron (symbol or B*) is the antiparticle of the electron, so it carries a positive charge.
In addition, a neutrino (symbol v), which is uncharged, is emitted.

Question 3

Positron emitting isotopes

Answer 3

Positron-emitting isotopes do not occur naturally. They are manufactured by placing a stable isotope, in liquid or solid form, in the path of a beam of protons. Some of the nuclei in the substance absorb extra protons and become unstable positron-emitters.

Question 4

Diracs theory of antiparticles

Answer 4

Dirac’s theory of antiparticles predicted that for every type of particle there is a corresponding antiparticle that:
• annihilates the particle and itself if they meet, converting their total mass into photons
• has exactly the same rest mass as the particle
• has exactly opposite charge to the particle if the particle has a charge.

Question 5

Annihilation

Answer 5

Annihilation occurs when a particle and a corresponding antiparticle meet and their mass is converted into radiation energy. Two photons are produced in this process (as a single photon cannot ensure a total momentum of zero after the collision).

Question 6

Pair production

Answer 6

In pair production, a photon creates a partice and a corresponding antiparticle, and vanishes in the process.