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Volume 78, Issue 11, November 2007
A new spectrometer is described for measuring the momentum distributions of scatteredelectrons arising from electron-atom and electron-molecule ionization experiments. It incorporates and builds on elements from a number of previous designs, namely, a source of polarized electrons and two high-efficiency electrostaticelectron energy analyzers. The analyzers each comprise a seven-element retarding-electrostatic lens system, four toroidal-sector electrodes, and a fast position-and-time-sensitive two-dimensional delay-line detector. Results are presented for the electron-impact-induced ionization of helium and the elastic scattering of electrons from argon and helium which demonstrate that high levels of momentum resolution and data-collection efficiency are achieved. Problematic aspects regarding variations in collection efficiency over the accepted momentum phase space are addressed and a methodology for their correction presented. Principles behind the present design and previous designs for electrostaticanalyzers based around electrodes of toroidal-sector geometry are discussed and a framework is provided for optimizing future devices.
- PARTICLE SOURCES, OPTICS AND ACCELERATION; PARTICLE DETECTORS
Operating a triple stack microchannel plate-phosphor assembly for single particle counting in the temperature range78(2007); http://dx.doi.org/10.1063/1.2814030View Description Hide Description
An assembly consisting of a stack of three microchannel plates (MCPs) and a phosphor screen anode has been operated over the temperature range from . We report on measurements at (using an alpha source) and with dark counts only . Without any particle source, the MCP bias current decreased by a factor of when the temperature was lowered from . Using the alpha source, and a photomultiplier tube (PMT) to monitor the phosphor screen anode, we first observed an increase in the decay time of the phosphor from when the temperature was decreased from while the decay time then decreased and reached a value of at . The pulse height distribution from the PMT was measured between 300 and and shows a spectrum typical for a MCP phosphor setup at and but is strongly degraded for intermediate temperatures. We conclude that the present MCP-phosphor detector assembly is well suited for position-sensitive particle counting operation at temperatures down to at least even for count rates beyond . This result is crucial and an important part of ongoing developments of new instrumentation for investigations of, e.g., interactions involving complex molecular ions with internal quantum state control.