Physics and Engineering of Radiation Detection

1.5. Sources of Radiation 27

dependence due to the energy dependence of the activation cross section. However,
to get an estimate, one can use a cross section averaged over the whole energy
spectrum of the incident radiation. In that case, theaverageactivation rate is given
by
Ract=VΦσact, (1.4.4)

whereV is the total volume of the sample, Φ is the radiation flux, andσactis the
spectrum-averaged activation cross section.
The behavior of equation 1.4.3 is graphically depicted in Fig.1.4.1. Since the
decay rate depends on the number of activated atoms, the number of atoms available
for decay increases with time. The exponential increase in the activity eventually
reaches an asymptotic value equal to the activation rate.

A 0 =Ract

Activity

Time

Figure 1.4.1: Buildup of activ-

ity in a sample undergoing ac-

tivation through constant irra-

diation. The activity eventually

reaches an asymptotic value ap-

proximately equal to the activa-

tion rateRact.

1.5 SourcesofRadiation

Radiation sources can be broadly divided into two categories: natural and man-
made.

1.5.A NaturalSources..........................

There are three types of natural sources of radiation: cosmic, terrestrial, and inter-
nal. Exposure from most of these sources is very minimal and therefore does not
cause any measurable damage to our bodies. However, as we will see later in this
section, there are some potentially hazardous materials, such as radon in our sur-
roundings, which in deed are a cause of concern since they are capable to delivering
high integrated doses.

A.1 CosmicRadiationSources...................

The outer space is filled with radiation that comes from a variety of sources such
as burning (for example, our Sun) and exploding (for example, Supernovae) stars.
These bodies produce immense amounts of radiation, some of which reach earth.