66 Chapter 2. Interaction of Radiation with Matter
mechanisms, most of which are now well understood through tools like quantum
mechanics, quantum electrodynamics, and quantum chromodynamics. However,
keeping in view the scope of this book, in this chapter we will not be looking at
the theoretical foundations of particle interactions that are based on these theo-
ries, rather spend some time on understanding the gross properties that have been
empirically and experimentally determined.
Before we go on to look at these mechanisms it is worthwhile to spend some time
on understanding some terminologies we will be needing in the following sections.
2.1.A InverseSquareLaw........................
The strength of radiation can be characterized by its flux, which is generally defined
as the number of particles passing through a unit area per unit time. Irrespective
of the type of source, this flux decreases as one moves away from the source. This
decrease in flux depends on the type of source and the type of radiation. For example,
laser light, which is highly collimated, does not suffer much degradation in flux with
distance. On the other hand, the flux from radioactive sources decreases rapidly as
the distance from it increases. The inverse square law, which is based on geometric
considerations alone, characterizes this change. It states that the radiation flux is
inversely proportional to the square of the distance from thepointsource, that is
Φ∝1
r^2, (2.1.1)
whereris the distance between the source and the point where flux is to be calcu-
lated. This law is the consequence of the isotropic nature of a point source because
such a source is expected to radiate equally in all directions. Since the flux is a
measure of the amount of radiation passing through an area therefore it should vary
according to how the area varies with distance from the source. Now, we know that
the surface area around a point is given by 4πr^2 , which means that the area varies as
r^2. Hence we conclude that the flux, which represents the amount of radiation pass-
ing through aunit areais proportional to the inverse of the square of the distance
or as 1/r^2.
As stated earlier, this law applies to only point sources. Now, a perfectly point
source does not exist in nature and is extremely difficult, if not impossible, to man-
ufacture. However this does not mean that this law can not be applied to practical
systems. The reason is that the notion of apoint sourceis relative to the distance
from the source. For example, a disk source can be be considered a point source if
one is at a considerable distance from its center. If this consideration is taken into
account then this law can be applied to most sources.
Another very important thing to consider is that the medium through which the
radiation travels should neither be scattering nor absorbing. It is understandable
that, if there is considerable scattering and absorption, the flux would change more
rapidly thanr^2. The inverse square law is therefore applied only in vacuum or low
pressure gaseous environments, such as air under atmospheric conditions. In liquids
and solids the scattering and absorption are not negligible and therefore this law
does not hold.
Inverse square law plays an important role in radiation protection, as it sets a
minimum distance a radiation worker must retain from a source to minimize the
possibility of radiation damage.