Compact He-atom detector for high-resolution surface phonon measurements

A new compact detector for measuring the velocity distribution of He atoms scattered from an oriented single-crystal surface by the time-of-flight (TOF) method is described. In this system the scattered atoms are tagged by exciting them to the 23S metastable state by cross bombardment with a pulsed electron beam. After traversal of a given flight path (80.9 mm), the metastables are detected by a multichannel-plate electron multiplier, and their TOF distribution is recorded. The incident supersonic He beam is generated by a nozzle source, resulting in a very narrow energy distribution of the incident beam. The angular and energy resolutions of the detector are analyzed and its advantages and limitations are described. Finally, a demonstration of the performance of the instrument in inelastic scattering from a Cu(100) surface is presented.