Preparation of Metallic Isotopic Targets of the Alkali and Alkaline Earth Metals by Evaporation of their Compounds

Abstract

ELECTROMAGNETICALLY enriched isotopes of the elements lithium, potassium, rubidium, calcium, strontium and barium, after the necessary chemical processing, are only available in one of the standard gravimetric forms, whereas nuclear targets for reaction studies in high-voltage accelerators are usually required as the element. It is scarcely feasible on the milligram scale to prepare the elements for evaporation; but the following simple method of making the targets is very successful. The oxide or carbonate of the isotopic element is pasted on to a boat composed of a strip of tantalum; this is directly heated in high vacuum, when oxide is reduced and the metal evaporates so that targets can be deposited using a cooled assembly. By control of the rate of heating and with suitable geometry, it is possible to achieve more than 50 per cent deposition on the backing, while the remainder of the evaporated material is recoverable. The target is then transferred to a tube which is stoppered while under vacuum and sealed off. When the target is required for use, it can be transferred to the accelerator without undue exposure to air. Alternatively, the target can be warmed in hydrogen or its isotopes to give the more stable hydrides. Deposits from a few micrograms to 1 mgm./cm.² on a variety of backings are readily made.