A reactor for performing reactions at very low, i.e. cryogenic temperatures has a collection vessel mounted above an electron gun to collect vaporized material such as metals, metal oxides, metal salts and metal halides. The collection vessel is maintained at a very low temperature, typically between 14.degree. and 77.degree. K., by a heat pump that also serves as a support for the collection vessel. A manifold introduces a reagent into the reactor between the electron gun and the vessel to react with the vaporized material and form a product. The product is condensed on the collection vessel which may then be inverted to provide a stable support for the collected product during further experimentation.