The difference between the experimental and theoretical data may result from
systematic errors that may occur because of deviations from a good narrow beam geometry.
The exponential law has been confirmed to hold for pure bulk aluminum slabs at
0.662 MeV, 1.173 MeV and 1.332 MeV, as shown in Figure 4.1, which again verifies the
narrow beam geometry at the same time. This figure also illustrates the transmission
increases (attenuation decreases) as the photon energy increases, which is in agreement with
theory.