Is Deuterium arc lamp a AC or DC light source?
Deuteurium lamps (I have one kicking around here somewhere) are discharge lamps, and as such the brightness is going to be dependent on the arc stability and the supply current.  

You may well be seeing variations or aliasing of variations (probably at 100Hz full wave rectified mains-frequency) ripple on the deuterium lamp supply. Aliasing is related to your microcontroller sampling frequency beating with the ripple on the lamp. 

I suggest connecting your photodetector to an oscilloscope rather than the micro ADC to observe the magnitude of any ripple directly. Any oscilloscope will be able to display 100Hz waveforms accurately. Use AC coupling if necessary, but it may be bad enough you won't have to. If you don't have access to an oscilloscope, try to get ahold of one first (it will give you lot of information) and if not, try to take samples at a known rate and compare with mains frequency.