Figure 4. Energy spectrum taken with a Nal detector and a multichannel
analyzer for a dual-energy beam of gamma radiation from 241Am and 13aCs.
The peak on the left represents the 60 KeV primary energy for 241Am and
the peak on the right is the 662 KeV energy for 137 Cs.


same size and exactly in line. On the source side, good collimation is
necessary to provide less divergence for the beam of gamma rays leaving
the source. On the detector side, good collimation minimizes the effect
of gamma scattering by providing a smaller selected area through which
gamma rays can reach the detector without interactions.
 The ratio between the maximum size of the opening and the length of
the collimator is very important because it will alter the possibility for
gamma rays to reach the detector after one or more Compton or Raleigh
scattering events. The probability for a Compton or Raleigh scattering
event to occur is a function of the photon energy and occurs for angles
close to or less than 180 (24). A good procedure for testing collima-
tion involves a comparison between theoretical and experimental mass
attenuation coefficients for a known material, such as water. Experi-
mental values of u will usually be smaller than the theoretical value, and
a high experimental value close to the theoretical one will indicate good
collimation. Groenevelt et al. (40) performed an extensive study on
collimation of 60 KeV gamma rays.
 Figure 4 shows the energy spectrum of a collimated beam of gamma
 photons obtained in the apparatus described by Mansell et al. (58) with
 24'Am and ''Cs. This photograph illustrates the effectiveness of lead