An ultra-thin Schottky diode as a transmission particle detector forbiological microbeams

Abstract

We fabricated ultrathin metal-semiconductor Schottkydiodes for use as transmission particle detectors in the biologicalmicrobeam at Columbia University's Radiological Research AcceleratorFacility (RARAF). The RARAF microbeam can deliver a precise dose of ionizingradiation in cell nuclei with sub-micron precision. To ensure an accuratedelivery of charged particles, the facility currently uses a commercialcharged-particle detector placed after the sample. We present here atransmission detector that will be placed between the particle acceleratorand the biological specimen, allowing the irradiation of samples that wouldotherwise block radiation from reaching a detector behind the sample. Fourdetectors were fabricated with co-planar gold and aluminum electrodesthermally evaporated onto etched n-type crystalline silicon substrates, withdevice thicknesses ranging from 8.5 μm - 13.5 μm. We showcoincident detections and pulse-height distributions of charged particles inboth the transmission detector and the commercial detector above it.Detections are demonstrated at a range of operating conditions, includingincoming particle type, count rate, and beam location on the detectors. The13.5 μm detector is shown to work best to detect 2.7 MeV protons(H+), and the 8.5 μm detector is shown to work best to detect 5.4 MeV alpha particles (4He++). The development of a transmissiondetector enables a range of new experiments to take place at RARAF onradiation-stopping samples such as thick tissues, targets that needimmersion microscopy, and integrated microfluidic devices for handlinglarger quantities of cells and small organisms.