the pole tips of the magnet application_number US 94682686 A description the invention consists of a preselector 80 to be used between a conventional prior art ion source and a conventional prior art magnetic mass spectrometer such as a danfysik mini separator the purpose of the preselector 80 is to recycle non collimated particles back to the ionizer chamber 88 the preselector 80 is of the calutron type and includes a vacuum tank 86 between two pole tips 94 of a magnet the vacuum tank 86 is exhausted by a vacuum pump not shown within the vacuum tank 86 is a semicircular ion separation chamber 87 at one end of the ion separation chamber 87 is an ionizer chamber 88 within which is an electron gun 89 attached to the ionizer chamber 88 is an air lock 90 for charging the preselector 80 with feed material the wall 95 between the ionizer chamber 88 and the ion separation chamber 87 contains an aperture 58 adjacent the aperture 58 is an extraction electrode 92 within the main chamber are a series of baffles 96 the far wall of the ion separation chamber 87 contains an exit aperture 99 the bottom wall of the ion separation chamber 87 is a rotating wheel 98 with an axis at the center of the semicircular ion separation chamber 87 the wheel 98 is cryogenically cooled and rotates in the direction from the exit aperture 99 to the ionizer chamber 88 the top wall and side walls 95 of the ion separation chamber 87 are maintained at a temperature sufficiently high to vaporize the feed material the wheel.