UV photocathodes for space detectors

Abstract

Vacuum photodetectors have a long history in ultraviolet (UV) sensing for both astronomy and remote sensing. One of the main advantages of this technology is the ability to use solar blind photocathodes to enable high sensitivity measurements of astronomical and atmospheric sources of Far UV (FUV) and Deep UV (DUV) emission in environments with high visible light (VIS) backgrounds. The use of microchannel plates (MCP) in vacuum photodetectors also allows single photon sensitivity for extremely weak signals. However, these detectors have typically suffered from lower Quantum Efficiency (QE) than their solid-state alternatives. Recent advances in photocathode technology have resulted in significant increases in QE for several UV sensitive photocathodes. We present test results of next generation high QE photocathodes appropriate for use in a wide range of FUV and DUV astronomy and remote sensing. A newly developed opaque Cesium Iodide (CsI) photocathode deposited on microchannel plates and sealed into vacuum photodetectors with a Magnesium Fluoride (MgF2) input window demonstrates QE of < 16% @ 130 nm. An optimized transmission mode solar blind (SB) alkali-telluride photocathode demonstrates 29% peak QE and 103 to 108 suppression of NUV and visible light, a significant improvement over previous alkali-telluride photocathodes. Finally, we present data from a new high QE S20 alkali-antimonide photocathode with < 40% QE at 254 nm, suitable for instruments requiring wideband DUV through VIS coverage. Improvements in collection efficiency of vacuum photodetector MCPs from 60% to 90% will also be presented, providing a further 50% boost to detective QE.