Using a superconducting tunnel junction detector to measure the secondary electron emission efficiency for a microchannel plate detector bombarded bylarge molecular ions

An energy-sensitive superconducting tunnel junction (STJ) detector was used to measure the secondary electron emission efficiency, epsilon(e), for a m icrochannel plate (MCP) detector bombarded by large (up to 66 kDa), slow mo ving (<40 km/s) molecular ions. The method used is new and provides a more direct procedure for measuring the efficiency of secondary electron emissio n from a surface. Both detectors were exposed simultaneously to nearly iden tical ion fluxes, By exposing only a small area of the MCP detector to ions , such that the area exposed was effectively the same as the size of the ST J detector, the number of ions detected with each detector were directly co mparable. The STJ detector is 100% efficient for detecting ions in the ener gy regime investigated and therefore it can be used to measure the detectio n efficiency and secondary electron emission efficiency of the MCP, The res ults are consistent with measurements made by other groups and provide furt her characterization of the loss in sensitivity noted previously when MCP d etectors have been used to detect high-mass ions. Individual molecular ions of mass 66 kDa with 30 keV kinetic energy were measured to have about a 5% probability of producing one or more electrons when impacting the MCP, Whe n ion energy was reduced to 10 keV, the detection probability decreased to 1 %. The secondary electron yield was calculated from the secondary electro n emission efficiency and found to scale linearly with the mass of the impi nging molecular ion and to about the fourth power of ion velocity. Secondar y electrons were observed for primary ion impacts >5 km/s, regardless of ma ss, and no evidence of a velocity (detection) threshold was observed. Copyr ight (C) 2000 John Wiley & Sons, Ltd.