Physics and Engineering of Radiation Detection

1.6. General Properties and Sources of Particles and Waves 45

Endpoint Energy

Electron Energy

Electron Yield

Figure 1.6.9: Typicalβ-particle energy

spectrum in aβ-decay.

1.6.C Positrons

A positron is the anti-particle of an electron. It has all the properties of an electron
except for the polarity of the electrical charge, which is positive. There a positron can
simply be considered an electron having positive unit electrical charge. Whenever
an electron and a positron come close to each other, they annihilate each other and
produce energy in the form of photons.

e+e+→ 2 γ (1.6.6)

Basic Properties of Positrons

Rest mass = 9. 11 × 10 −^31 kg=0. 511 MeV/c^2

Electrical charge = +1. 602 × 10 −^19 C

Internalstructure : Believedtohavenointernalstructure.

Positrons have been shown to be extremely effective tools in a variety of fields.
Most notably their utility in particle physics research has led to far reaching discov-
eries made at particle accelerators, such as LEP at CERN. In medical imaging, they
are employed in the so called Positron Emission Tomography or simply PET.

C.1 SourcesofPositrons

Particle Accelerators

In particle accelerators, positrons are produced through the process of pair pro-
duction. In this process a photon interacting with the electromagnetic field of a
heavy charge creates an electron and a positron.
The positrons thus created are then guided by electric and magnetic fields to form
a narrow beam for later collision with other particles. At the Large Electron-Positron
Collider (LEP), the positrons are collided with electrons at very high energies, which