Velocity spectrometer for a neutral atomic beam

A velocity spectrometer for a neutral atomic beam has been developed to pre pare a monovelocity atomic beam. In order to disperse a relatively fast ato mic beam according to its longitudinal velocity, a magnetic quadrupole lens with a large magnetic gradient has been used. The device was made using Nd FeB permanent magnets. The magnetic gradient in the quadrupole was measured to be sufficiently large (1 T/cm) and uniform along the radial direction. The resonance fluorescence spectra of the Li atomic beam after passing thro ugh the magnetic quadrupole lens have been measured by using a single-mode tunable laser. From the peak shift of the fluorescence spectrum, the exerte d force on Li atoms by the magnetic lens was 8160 times as large as the acc eleration of gravity. Using the present spectrometer, we have measured the longitudinal velocity distribution of the thermal Li beam at 800 degreesC, which was in good agreement with the result from the fluorescence spectra. It is expected that a monovelocity Li atomic beam of 1000 m/s with a veloci ty spread narrower than 1% can be easily formed with a compact experimental arrangement using the developed velocity spectrometer. In this case, the a vailable flux given as a fraction of the incident Li flux is estimated to b e about 1%, A high-velocity resolution of 0.03% is expected at the availabl e fractional flux of about 0.01%.