A schematic diagram of the apparatus is shown in Fig.1 (Acrobat PDF file format). The vacuum can cylinder is size of 18 inches in diameter and 60 inches in length. A Vac-Ion pump (pumping speed 500 liter/sec), a titanium sublimation pump, and three Vac-Sorb pumps are used. These pumps provide an oil-free vacuum of three subsystems: a rotatable electron-beam source, a neutral-beam source, and fixed electron detector.

Electron Beam Source

This subsystem is comprised of an electron gun and a 127o electrostatic cylindrical energy selector. The gun has a straight 5-mil tungsten wire as a filament, a focus electrode, and an anode. This gun produces a collimated electron beam of 10-6 A and an energy half-width of approximately 1.0 eV. The electron beam enters an energy selector through a three-diaphragm lens system (called lens #1). It consist of two cylindrical grids, two plates, two focus wires, and slits. The beam is decelerated by lens #1 to make the energy half-width smaller before it enters the selector, and is accelerated to the desired energy by a second lens (lens #2), after it passes through the selector. The resultant half-width of the energy-selected electron beam is 0.06 eV. There are two electron-beam deflectors (horizontal and vertical) which are used to direct the electron beam in the precise direction desired. The eam system described produces a beam current of 10-10 - 10-9 A with a profile as shown in Fig. 2 (Acrobat PDF file format). The angular half-width is about +/- 3o at a beam energy of 5 eV.

Neutral Beam Source

The neutral beam is collimated by a fused capillary array (40 micrometer in diameter, 2 mm long, 50% open area: manufactured by Bendix Research Laboratories). The collimated neutral beam has an angular width of approximately 5o. The leak flow rate is controlled by a variable leak system, and the pressure in the flow is approximately 1 torr. The molecular-beam profile is measured by a leak probe (a hole of 0.028 inch in diameter and 0.200 inch long) which is connected to another Vac-Ion pump (pumping speed: 50 liter/sec).

Electron Detector

The detector subsystem includes a baffle, an energy analyzer, and an electron multiplier. The baffle has a 0.1 X 0.3 inches slit and is 1.5 inches long. The background counts are less than 10-6 times the main beam beyond +/- 6o from the main beam. The energy analyzer has the same dimensions as the electron-source energy selector except for the entrance and exit slits. The dimensions of both slits are 0.040 X 0.250 inches Two lens systems (called lens #3 and lens #4) are attached to the energy analyzer to achieve higher resolution of the scattered electrons. The energy resolution of the detector system is 0.04 - 0.08 eV full width at half-maximum. The electron detector (F4036, ITT product) operated at a gain of 5 X 105. The output pulses from the multiplier are fed through a pulse amplifier to a counter. The output counts are recorded on a printer and are put through a D/A converter on an XY recorder for analog representation. The incident electron beam in the horizontal plane intersects the vertically collimated neutral beam at the cylindrical center of the vacuum chamber. The incident-electron beam in the horizontal plane can be mechanically rotated from -144o to +160o with respect to the detector.
Since the full width at half-maximum of the neutral beam is well inside the acceptance angle of the detector and since the detector system has a fixed angle relative to the neutral beam, there is no angular dependence of the detecting system except for the scattering of electrons from the background gas.