Maximizing the quantum efficiency of microchannel plate detectors: The collection of photoelectrons from the interchannel web using an electric field

Abstract

We report dependence of the extreme ultraviolet quantum efficiency (QE) of a microchannel plate (MCP) detector upon the electric field strength above its input face. Using an uncoated plate, we measured increases up to 80% as the field was raised from 0 V/μ to between 0.01 and 0.1 V/μ. Further increases in electric field resulted in a monotonic decrease in QE. Detector spatial resolution was found to degrade for these small field values but could be recovered, while maintaining most of the QE increase, by operating with fields in excess of 0.3 V/μ. Other detector parameters such as modal gain and output charge pulse-height distribution were not significantly affected by the applied electric field. We explain the QE and resolution variations in terms of photoelectrons ejected from the interchannel web and subsequently returned to the input face of the MCP by the applied electric field. We present a model and a computer simulation which quantitatively reproduce our experimental results. Applying our model to MCPs coated with photocathode materials, we conclude that the maximum QE is obtained by optimizing the combined contributions from the web area and open area of the MCP, rather than by maximizing the open area alone. In this case, most of the QE can arise from the interchannel web rather than from the directly illuminated channels.

Characterization of Efficiency Enhancement in Microchannel Plate Detectors

“…The OPoSAP has a spatial resolution of approximately 0.25mm, and no degradation has been observed with grid bias as low as 15V. A more detailed analysis of the trajectory of photoelectrons has been performed by Taylor et al [9], who considered the effect on detector resolution in a UV imaging system, and their results were similar to those given here. Mass-to charge The grid potentials given in each case are quoted with respect to the front face of the MCP, which was set to -50V.…”

“…Taylor et al [9] observed a drop off in the detection efficiency for incident photons at large grid bias, when the photoelectrons generated from the interchannel web failed to reach the nearest channel, and are returned instead to the web. However, this effect was only appreciable for larger applied fields than are applied here (up 1000V/mm), or for lower energy photoelectrons.…”

A low background-rate detector for ions in the 5 to 50 keV energy range to be used for radioisotope dating with a small cyclotron

“…36 Measurements on other microchannel plates yield collection efficiencies of about 90%, even for electron energies as low as 100 eV. 37 On scales of 10 |i to 1 mm, the collection efficiency varies less than 1% (consistent with statistical fluctuations in the measurement) over the area of the microchannel plate. 38,39 There are variations of 5% to 10% over scales larger than 5 mm.…”

Charge transfer of slow highly charged xenon ions in collisions with magnesium atoms

“…They were covered with nearly transparent grids, which had negative voltage relative to the MCP front surfaces to maximize the detection efficiency [47,48]. The MCP for the projectile detection had a delay line anode to generate a position signal.…”