2 Chapter 1. Properties and Sources of Radiation
modern physics, one can associate a wavelength to every particle whether it car-
ries a mass or not. This implies that a particle having mass can act as a wave
and take part in the formation of interference and diffraction patterns. On the other
hand, light, which can be well described by its wave character, comprises of photons,
which are particles having no rest mass. Hence we can conclude that one should not
characterize the radiation based on its particle and wave properties.
Let us have a look at the third category we mentioned above: hazardous and non-
hazardous. There are particles that pass through our bodies in large numbers every
second (such as neutrinos from the Sun) but do not cause any observable damage.
Still, there is a possibility that some of these particles would cause mutations in our
body cells, whichcouldultimately lead to cancer^1. On the other hand there are
particles, such as neutrons, that are known to be extremely hazardous to the body
but no one can ever be absolutely certain that a neutron entering our body would
in deed cause any harm. In fact, due to background radiation around us, our bodies
get small doses of neutrons all the time and still majority of us do not experience
any adverse effects.
Based on the above arguments it is safe to say that the categorization of radiation
found in the literature on these basis should not be considered to represent individual
particles. What this really means is that if we have a very large number of a certain
kind of particles, there is a high probability that most of them would behave in the
manner characteristic of their categorization. Long exposure from a highly intense
beam of neutrons would most definitely cause skin burns and most probably cancer
but it would be wrong to assume that a single neutron would do the same.
The words probability and chance were mentioned in the preceding paragraphs.
What does particle interaction have to do with chance? Well, the theoretical foun-
dations of particle interaction isquantum mechanics, which quantifies the variables
related to particle motion, such as momentum, energy, and position, in probabilistic
terms. We talk about theprobabilityof a particle being present at a specific place
at a specific time. Nothing is absolute in quantum mechanics. We will learn more
about this when we study the concept ofcross sectionin the next chapter.
1.2 WavesorParticles?............................
If we think about light without any prior knowledge, we would assume it to be
composed of waves that are continuously emitted from a source (such as a light
bulb). In fact, this was the dominant perception amongst scientists until the start
of the 20th century. During those days a major problem of theoretical physics
had started boggling the minds of the physicists. They had found it impossible
to explain the dependence of energy radiated by a black body (a heated cavity)
on the wavelength of emitted radiation if they considered light to have continuous
wave characteristics. This mystery was solved by Max Planck who developed a
theory in which light waves were not continuous but quantized and propagated in
small wave packets. This wave packet was later called aphoton. This theory and the
corresponding mathematical model were extremely successful in explaining the black
body spectrum. The concept was further confirmed by Einstein when he explained
(^1) This is a purely hypothetical situation and no data exists that could verify this assertion. The argument
is based on the interaction mechanisms of particles.