[ RadSafe ] scatter spectrum

[Mccormick, Luke I]

We are analysing a 6 MeV LINAC imaging system. As an imaging subject we
used a forklift. I took spectral readings of the scattered radiation and
was very surprised to see definite peaks and little else. For example, at
90 degrees from the direction of the beam the scatter spectrum, in line
with the forklift, analysed using a 2x2 NaI, has 2 distinct peaks one
centroid at 124 keV with the actual highest point at 103 keV  going from 30
keV to 313 keV and another peak at 1028 keV with the actual high point at
1023 keV going from 979 to 1094 keV.  the linac rolled from 10' away from
my detector to 50' away from my detector. In the next measurment, rolling
the other direction. but with my detector positioned parallel to the beam
but in line with the LINAC I had 3 distinct peaks; Centroid 117 keV highest
point 96.7 keV, new centroid at 551 keV highest point at 555 keV going from
443 to 629 keV, and a centroid at 1017 with highest point 1016 going from
992 to 1047 keV. I am not sure I understand why the scatter spectrum would
look like this. Any explanations would be greatly appreciated.