Subnanosecond intensifier gating using heavy and mesh cathode underlays

Abstract

Irising time of microchannel plaie intensifiers with quartz cathode windows has been reduced to less than 100 ps. This is achieved by application of a metal underlay to reduce cathode substrata resistance. The first approach uses a 50%-transmissive Uniform nickel heavy underlay, while the second approach uses a 96%-transmissive nickel mesh.For the heavy underlay, approximately 5 nm of nickel is evaporated over the cathode side of the quartz window. For the mesh underlay, approximately 750 nm of nickel is sputtered onto the window and coated with photoresist, which is exposed through a mask of 100-micron-square spaces defined by 3.5-micron-wide lines. The photoresist is developed and washed off, exposing the nickel covering the square areas. At this point, photoresist still covers the nickel over the 3.5-micron-wide wires, protecting them when the exposed nickel covering the squares is etched away. Removal of the remaining photoresist leaves only the nickel wires, which have been reduced to 2 microns in width due to sideways etching during removal of the squares.As a prototype effort, Lawrence Livermore National Laboratory (LLNL) purchased two 1 8-mm heavy underlay tubes from Hamamatsu Photonics and formed a mesh underlay on faceplates which Hamamatsu used for construction of two additional tubes. Measurements of irising time were made on these four tubes. Irising is characterized by a bright ring, seen first at the edge as it propagates toward the center. The time lag is caused by the distributed time constant of the substrata resistance and the cathode-to-MCP capacitance. Since the capacitance is fixed by restraints of tube geometry, our goal was to reduce the distributed resistance sufficiently to achieve sub-nanosecond irising times. Testing showed no irising on one tube of each type of underlay. With these encouraging results, LLNL and Nanostructures refined the mesh application technology, and LLNL procured eight mesh tubes from ITT using meshes formed by Nanostructures. An additional 8 tubes with a 50% transmissive heavy underlay were procured from Hamamatsu. Testing of these tubes also showed no detectable irising, which leads us to conclude that tubes can be made with irising clearly faster than the time resolution of our measurement system, which we estimate to be less than 50 Ps.

Improvements in sensitivity and response times of photon imaging tubes

“…For the smaller tubes, with the best time resolution, definition is limited to no better than 1-2 million pixels. Some early 'fast shutter' image intensifier designs incorporated a photocathode built on a semi-transparent metallic under layer and for small sized (18 mm) imaging tubes, using a thick nickel underlay below the photocathode, resulted in a ∼50% transmission, and cited irising times reduced to ∼100 ps [3][4][5][6]. Thicker metallization increases response speed, but at the expense of optical transmission.…”

“…There have been a variety of approaches using patterned grid or line structures which can simultaneously increase both conductivity and transmission, without degrading the response times. Whilst these schemes offer major benefits they are still somewhat undesirable in terms of optimized image quality since fibreoptic faceplates do not have long range periodicity in the placement of the fibres, and different sections are randomly aligned relative to the deposited grid patterns [3][4][5][6][7][8]. Typically, mesh underlays allow an order of magnitude improvement in exposure time, but the procedure for making a mesh underlay involves application of photo-resist, exposure through a mask, removal of unexposed photo-resist and a final removal of exposed photo-resist and metallization steps.…”