1.6. General Properties and Sources of Particles and Waves 31
structure and are therefore regarded as elementary particles. There are 6 leptons
in Standard Model: electron, tau, muon, and their respective neutrinos. Although
these particles are not known to be composed of other particles, but there are the-
oretical models and empirical evidences that suggest that they might have internal
structures.
As a comprehensive discussion on elementary particles is out of the scope of this
book therefore we will end this discussion here and refer the curious readers to
introductory texts on particle physics (17; 16) for detailed explanations.
In order to understand the interaction mechanisms of different particles it is
necessary to first familiarize ourselves with their properties. In the following we will
survey some of the important particles with respect to the field of radiation detection
and discuss their properties. The way these particles interact with matter will be
discussed in the next chapter. We will also discuss some of the most important
man-made sources that emit these particles.
1.6.A Photons..............................
A photon represents one quantum of electromagnetic energy and is treated as a
fundamental particle in the Standard Model of particle physics. In this model the
photon is assumed to have no rest mass (although it is never at rest!). When the
photon is traveling in a medium, it slows down due to interaction with the medium
and acquires an effective mass. In vacuum, however, it is considered to be massless.
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Not a well defined boundaryphoton energy (eV)Figure 1.6.1: Electromagnetic spectrum.Photons do not correspond to only visible light. In fact, light spans a very
narrow region of their full energy spectrum (see Fig.1.6.1). Also, what we see as
visible light, is not really considered to be composed of individual photons, rather
it is a superposition of a number of photons. Photons are involved in all types of