The uncertainty on the x-axis is the difference in channel from the photo-peak to the channel in which the number of count is half that of the photo-peak count, in the range of 20 channel by average. The channel number 409 is assigned to cs-137 with the known energy level of 0.662 MeV. The calibration factor of 0.0016 MeV/Ch assigns each channel to a specific energy level.
    A large aluminium cylinder is then placed between the source and the detector. Due to the nature of scattering, only the portion of the incident photons are transmitted to the detector.  The measurement are repeated at each angle from 20 degree to 110 degree in 10 degree increments. The detector was left to gather data for 5 minutes at each interval.