|In the gamma-ray photon experiment we considered on the previous page, the gamma
rays are collimated to
a narrow beam before they impinge on the absorber. This is called a 'good geometry'
situation. Real measurements are often carried out under different conditions in which the
gamma-rays are not collimated.
If we insert an absorber between the source and
the detector, some of the transmitted gamma-rays still travel directly from the source to the
detector. As well as these, some gamma-rays which would not otherwise reach the detector may
be Compton scattered in the absorber, such that they then reach the detector. This can
increase the signal at the detector. This phenomenon is referred to as build-up.
the simulation to illustrate the situation where the gamma-ray photons
are not collimated.
The situation for non-collimated beams is handled by the
the buildup factor.
The absorption of gamma-rays still follows an exponential relationship with the
thickness of the absorber, but it is modified by the buildup factor.
As a rough guide, the build up factors for thick slabs tend to be about equal to the
thickness of the absorber, measured in units of mean free path of the incident
This is provided the detector responds to a broad range of gamma energies.