Multimode charge transport in electron implanted PMMA

Energetic electrons directed toward an insulating target material penetrate to the interior where they progressively loose their energy through multiple atomic collisions and eventually come to rest. They therefore cause an internal charge deposition with spatial distribution largely associated with their entrance energy. An investigation of the charge deposition caused by 2 MeV incident energy electrons implanted into polymethylmethacrylate (PMMA) in air at room temperature was made by measurement of the internal charges accumulated according to the applied implantation doses, up to 500 Gy. The internal charges were measured using the electrically stimulated acoustic wave (ESAW) method and initial volume charges were 90% of the applied charge implant, and were very persistent in some cases taking months to decay. Charge transport models were compared to measured charge decay and two processes, conduction and charge drift, appear to be active simultaneously. However implantation also caused changes in the material conductivity as observed by an increase in net charge decay rates according to increased implantation levels. Because of the well-defined charge deposition process in PMMA and slow decay rates, charge accumulation can be used as a diagnostic for electron radiation exposure.