On Monolithic Silicon Array Detectors for Small-Field Photon Beam Dosimetry

Abstract

Contemporary x-ray radiotherapy employs small radiation fields to deliver highly conformal dose distributions. Submillimeter accuracy in the measurement of the delivered dose map is a crucial requirement of detectors proposed for quality assurance applications. A 2-D monolithic silicon array detector can provide high spatial resolution by optimizing small sensitive volumes (SVs) in a large active area. They offer a stable and near energy-independent response in megavoltage photon beams, good dose linearity, and real-time read-out. The SVs are ion-implanted on a silicon wafer whose geometry and physical characteristics, such as resistivity and defects concentration, dramatically affect the detector performance. The Octa is a novel 2-D monolithic silicon array detector dedicated to small-field dosimetry. Its 512 diode SVs are arranged with a submillimeter pitch along four intersecting orthogonal linear arrays. We report on the experimental and numerical characterization (performed with Sentaurus Workbench within the Synopsys framework) of two Octa detectors manufactured, respectively, on a bulk and on an epitaxial silicon substrate. The effects of resistivity and defects concentration profiles across their large-area monolithic silicon wafers are compared and discussed in terms of the response linearity with dose, response uniformity, charge-collection efficiency, and clinical performance in the case of a small radiation field delivered with a flattening filter-free beam.