Applications of Neutron-Activation Analysis to the Measurement of the Evaporation Rate of Solids in the Range of 10^-7 to 10^-9 g/(cm² sec)

To measure the evaporation rate of solids in vacuum, the evaporating substance was condensed on a water-cooled lead plate. The plate with the condensate was removed, irradiated by neutrons, and etched in a mixture of nitric and hydrofluoric acids, and the radioactive isotopes in the solution were identified. The possibilities of the method are illustrated by the measurement of the molybdenum metal and zirconium carbide evaporation rates. The amount of molybdenum (⁹⁹Mo) was determined by direct γ-spectrometry of the solution. In the case of zirconium, radiochemical isolation of ⁹⁵Zr from the mixture of antimony, zinc, tantalum, and hafnium present in the solution in amounts commensurable with that of zirconium was suggested. The vaporization experiment was carried out in a vacuum ∼10^-5 to 10^-6 Torr. The measured values of the molybdenum and zirconium carbide evaporation rates in the temperature range ∼1900 to 2400°C are given.