Microchannel plate detectors

Abstract

A microchannel plate (MCP) is an array of 104-107 miniature electron multipliers oriented parallel to one another (fig. 1); typical channel diameters are in the range 10-100 μm and have length to diameter ratios (α) between 40 and 100. Channel axes are typically normal to, or biased at a small angle (~8°) to the MCP input surface. The channel matrix is usually fabricated from a lead glass, treated in such a way as to optimize the secondary emission characteristics of each channel and to render the channel walls semiconducting so as to allow charge replenishment from an external voltage source. Thus each channel can be considered to be a continuous dynode structure which acts as its own dynode resistor chain. Parallel electrical contact to each channel is provided by the deposition of a metallic coating, usually Nichrome or Inconel, on the front and rear surfaces of the MCP, which then serve as input and output electrodes, respectively. The total resistance between electrodes is on the order of 10 Ω Such microchannel plates, used singly or in a cascade, allow electron multiplication factors of 10-10 coupled with ultra-high time resolution (< 100 ps) and spatial resolution limited only by the channel dimensions and spacings; 12 μm diameter channels with 15 μm center-to-center spacings are typical. Originally developed as an amplification element for image intensification devices, MCPs have direct sensitivity to charged particles and energetic photons which has extended their usefulness to such diverse fields as X-ray) and E.U.V.) astronomy, e-beam fusion studies) and of course, nuclear science, where to date most applications have capitalized on the superior MCP time resolution characteristics). The MCP is the result of a fortuitous convergence of technologies. The continuous dynode electron multiplier was suggested by Farnsworth) in 1930. Actual implementation, however, was delayed until the 1960s when experimental work by Oschepkov et al.) from the USSR, Goodrich and Wiley) at the Bendix Research Laboratories in the USA, and Adams and Manley) at the Mullard Research Laboratories in the U.K. was described in the scientific literature. These developments relied heavily on a wealth of information on secondary electron emission) and earlier work on the technique of producing resistive surfaces in lead glasses by high temperature reduction (250-450 °C) in a hydrogen atmosphere. Finally, since most of the electrical performance characteristics of channel multipliers are not a function of channel length, l, or channel diameter, d, separately, but only a function of the ratio l/d =α, an almost arbitrary size reduction is possible. Such size reduction may be achieved by glass fiber drawing techniques which form the basis of fiber op tic device fabrication). In addition to a significant dimensional reduction resulting from these methods, a logarithmic compression of repetitive manufacturing steps is also possible, i.e., one can achieve a structure with ~10 holes requiring ~2 x 10 fiber alignment steps by a draw/multidraw technique. Prior to the application of reliable fiber drawing techniques, however, the first operational MCPs were