Evaluating radiation damage to scintillating plastic fibers with Monte Carlo simulations

Abstract

Current electronic portal imaging devices (EPIDs) are generally used for megavoltage imaging in radiotherapy and employ a thin Cu plate/ phosphor screen to convert x-ray energies into optical photons . In order to achieve a high spatial resolution, thin screens are used which subsequently results in low x-ray absorption and thus a low detective quantum efficiency (DQE) for megavoltage x-rays. Additionally, the high atomic number Cu/ phosphor screen materials is not ideal for dosimetric applications. To improve the imaging and dosimetry dual-functionality of EPIDs, water equivalent plastic scintillators have been proposed. Plastic scintillator fibers may however be susceptible to radiation damage caused primarily by ionizations from low energy secondary electrons. An accumulation or clustering of these ionization events, within regions corresponding to the volume of the plastic polymer chains, may lead to chain breaks. This could result in changes to the optical photon absorption properties and optical yield of the fiber, affecting the overall imaging performance of the detector. Here we used Monte Carlo radiation transport simulations for a preliminary investigation into the distribution of ionizations within a single plastic fiber. We find a large number of ionization events can accumulate along the fiber length, which over repeated exposures could lead to damage. To determine the effect of damage on the imaging performance, two fiber arrays were modeled with and without areas of damage. The damaged fiber array was found to produce approximately half the number of counts as the undamaged array.