Simulation of the electron field emission characteristics of a flat panel x-ray source

摘要

A distributed flat panel x ray source is designed as an alternative for medical and industrial imaging fields. The distributed x ray source corresponds to a two dimensional array of micro (93 μm) x ray cells similar in format to a field emission display. In this paper the field electron emission characteristics of a single micro x ray cell are presented. The field electron emission from a carbon-nanotube- (CNT-) based cold cathode is simulated using the particle-in-cell code oopic pro. The electron source is simulated as a triode structure, composed of an emitting cathode, extracting grid and anode. The possibility of using focusing lenses to control the trajectory of emitted electrons is also evaluated. The layer of CNT emitters is modeled as Fowler-Nordheim emitters. The field emission characteristics were analyzed for extracting voltages between 20 and 70 V and accelerating voltages between 30 and 120 kV. Under these conditions, J_(FN)-V curves, energy, and electron distributions at the anode surface were determined. Electron trajectories were determined as well. When no focusing structures were employed, electron trajectories were found to be divergent. When focusing lenses were included in the triode structure, the emitted electrons could be made to converge at the anode. In the cases where focusing structures were used, a dependency between the focal spot size and the extracting grid voltage was found. Results indicated an early feasibility of the proposed device to be employed as an electron source in the distributed flat panel x ray source.