A magnetic mass filter of the half-period Wien filter type, recently fully analysed by the author, is described. Formulae and graphs necessary for the design of half-period filters are given and several important aspects of the design are described in detail. The design of a molecular beam detector, incorporating a half-period filter, is discussed. In this application, the small, straight line magnet is advantageous. Many important independent requirements are fulfilled simultaneously by the relatively simple design, which utilizes the mass 'discrimination' in the ion source for good mass suppression despite the use of high transmission and low resolution. The strongly negative grid and the displaced, adjustable ribbon design solve several problems of great importance for applications of small magnets with large gaps. The detector surface stability problems often encountered in 'differential' detection of reactive scattering, especially at high intensities, are solved in a new efficient way. Finally, the requirements for absolute measurements of scattered flux density are discussed. The surface ionization efficiencies can be accurately measured by the field reversal method, which has been introduced in the design in a simple and convenient way. This appears to be the first application of this method in a beam experiment.
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