Kingdon Cage as a Molecular Beam Detector

A space-charge detector has been constructed to detect beams of neutral molecules. The detector is a diode within which positive ions created by electron bombardment of the beam partially neutralize the space charge and thus produce an increase in the plate current. Beams of rare gases, hydrocarbons, alkyl iodides, nitrogen, and nitric oxide have been studied, and the detector is found to be suitable for measurements of total collision cross sections. The increase in plate current is proportional to the beam intensity raised to a power near unity. The response varies with the beam material and the electrode geometry, but is quantitative and reproducible for a given detector. The minimum detectable intensity is about 1011 molecules/cm2/sec.