AIP Digital Archive
Electrical Engineering, Measurement and Control Technology
A system has been developed to provide accurate calibration of electron detectors measuring in the energy range from approximately 10 eV to 50 keV. The principal component of the system is a large area (730 cm2), monoenergetic electron beam which is tunable in energy. This beam is produced by illuminating, in vacuum, a thin gold film deposited onto a quartz flat with ultraviolet light, such that photoelectrons are ejected from the gold surface. With the gold film floated at an adjustable negative voltage, the photoelectrons ejected from the surface are accelerated towards an electrically grounded screen which is positioned in front of the quartz flat. All high voltages in the beam system are guarded so as to eliminate any significant leakage current. The current leaving the gold surface which determines the beam intensity is measured using a specially designed picoammeter. Electron beam intensities of ∼106 electrons/cm2 s are produced by illuminating the gold photocathode with light from a 2-W mercury vapor lamp which has 90% of its output at 2537 A(ring). The resulting beam has an energy spread of 0.41 eV. Electron beam intensities of ∼107 electrons/cm2 s are produced with a 200 W medium pressure mercury arc lamp. For this case, the beam has a width in energy of 0.55 eV. Detectors to be calibrated are mounted within a set of computer-controlled rotational and translational tables. The tables allow the look direction of the detector to be oriented with respect to the beam in two orthogonal angles and two orthogonal directions. The entire calibration system operates within a set of Helmholtz coils that allow the Earth's magnetic field to be canceled. The voltage on the gold surface, the angular and spatial positioning of the detector, and the accumulation and display of the data are all controlled by a minicomputer.
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