  In Radiation Physics classes, point sources are typically used, for which it
is relatively easy to describe the signal obtained by a radiation detector,
such as the NaI(Tl) scintillation detector. The use of large extended radiation
sources is generally avoided due to the mathematical complexity that their
description may involve. However, the use of Monte Carlo simulation methods
allows this limitation to be overcome. Potassium chloride, containing the 40K
isotope, is an ideal candidate for carrying out this type of experiment. The
source activity is obtained through the detection of the 1460.8 keV
gamma-photon emitted in the 40K decay. In the first experiment, a cylindrical
container is used, placing the NaI(Tl) detector in its center and filling the
remaining space with potassium chloride. In a second, more complex case, a
large radioactive source consisting of a container filled with a mixture of
sand and potassium chloride, with the NaI(Tl) detector placed in the center of
the mixture, is used. In this case, the mass of potassium chloride is
approximately 1/5 of the sand mass. In both experiments, the detection
efficiency is obtained by Monte Carlo simulation. A careful analysis of the
experimental data allows to obtain a good agreement between the measured and
calculated value of the activity.
