EFFECT OF LOCAL ELECTRIC-FIELDS ON MICROCHANNEL PLATE DETECTION OF INCIDENT 20 KEV PROTONS

We present data demonstrating the influence of an applied electric fie ld E oriented normal to the input surface of a microchannel plate (MCP ) detector on the critical operating parameters of the detector, inclu ding the quantum detection efficiency, the spatial resolution, and pul se height distribution. The MCP detector response is characterized usi ng 20 keV protons as the primary radiation. An applied electric field E<-4 V/mm, where a negative value of E corresponds to a nearby object that is biased positive relative to the input surface, results in a hi gh spatial resolution and a quantum detection efficiency that is appro ximately equal to the open area ratio of the MCP. An electric field -1 <E<5 V/mm results in low spatial resolution, in which up to 32% of the measured signal appears as a localized noise that extends several mil limeters from the point of ion impact, and a maximum quantum detection efficiency of approximately 0.87. Furthermore, a separate peak in the pulse-height distribution arises from ions striking the web of the MC P detector and has a much lower pulse magnitude than that of ions stri king channels. For E>5 V/mm, the spatial resolution increases, and the quantum detection efficiency slightly decreases from its maximum valu e with increasing E. The characteristics of each of these electric fie ld configurations are analyzed in the context of the yield and transpo rt of secondary electrons created at the web of the MCP detector, and the results can be scaled to other ions and energies according to the secondary electron yield of ions striking the web. (C) 1996 American I nstitute of Physics.