Surface direct conversion of 511 keV gamma rays in large-area laminated multichannel-plate electron multipliers

Abstract

We have used the TOPAS simulation framework to model the direct conversion of 511 keV gamma rays to electrons in a micro-channel plate (MCP) constructed from thin laminae of a heavy-metal-loaded dielectric such as lead-glass, patterned with micro-channels (LMCP). The laminae serve as the converter of the gamma ray to a primary electron within a depth from a channel-forming surface such that the electron penetrates the channel surface (‘surface direct conversion’). The channels are coated with a secondary-emitting material to produce electron multiplication in the channels. The laminae are stacked on edge with the channels running from the top of the resulting ‘slab’ to the bottom; after assembly the slab is metalized top and bottom to form the finished LMCP.The shape of the perimeter of a lamina determines the dimensions of the slab at the lamina location in the slab, allowing non-uniform cross-sections in slab thickness, width, and length. The slab also can be non-planar, allowing curved surfaces in both lateral dimensions. The laminar construction allows incorporating structural elements in the LMCP for modular assembly in large-area arrays.The channels can be patterned on the laminae surfaces with internal shapes and structure, texture, and coatings optimized for specific applications and performance. The channels can be non-uniform across the LMCP and need not be parallel in either transverse direction.Surface direct conversion of the gamma ray to an electron eliminates the common two-step conversion of the gamma ray into an optical photon in a scintillator followed by the conversion of the photon into an electron in a photodetector. The simulations predict an efficiency for conversion of 511 keV gamma rays of ⪆ 30% for a 2.54 cm-thick lead-glass LMCP. The elimination of the photocathode allows assembly at atmospheric pressure.