AbslracLWe examine the effect oxygen conlamination has on the recombination centre separation, trap density and defect ionization efficiency in PDATS single cryslals. We find that, wilh no Oxygen present, recombination centres are not delecled in sample lenglhs of up lo 9.8 mm. However. for chase samples contaminated with oxygen. we find interQenlre separations of only a few hundred microns. We also show that oxygen increases the trap density and t h e defeci ionization efficiency. Finally, we demonslrale that by aclivating the oxygen presenl in lhe crystals, the recombinalion centre separalion can be further reduced.