Design and performance of a highly efficient mass spectrometer for molecular beams

We present a very efficient 90 degrees sector magnet mass spectrometer dete ctor, based on electron impact ionization, especially adapted for the use w ith atomic and molecular beams. In this setup, a cylindrically symmetric ma gnetic field compresses electrons emitted from a large filament into a long , narrow volume containing primarily the molecular beam. This has the advan tage of both producing as little as possible background from residual gases and increasing the interaction region between beam and ionizer. Experiment al tests using thermal He atoms show that the detector's efficiency and sen sitivity are very high, amounting to eta greater than or equal to 0.70 +/- 0.04% and S=0.22 +/- 0.03 A mbar(-1), respectively. The unconventionally lo ng ionization volume, however, may restrict the use of this mass spectromet er to experiments with no, or only moderate timing requirements. (C) 2000 A merican Institute of Physics. [S0034-6748(00)03705-9].