Coffee-ring minimization for the production of high-activity 22Na positron sources

We describe a method to deposit a uniform layer of NaCl from an aqueous solution for the purpose of fabricating high activity sources of 22 Na for experiments using positrons. For minimum attenuation of the emitted positrons, it is desirable to produce a NaCl deposit on a flat substrate without the formation of either large isolated crystals or a thick salt deposit or “coffee ring” around the edge of the evaporating drop. We have eliminated these effects using a computer-controlled syringe, the tip of which is in close proximity to the heated substrate surface, to supply the salt solution at a constant rate. In addition, dry N 2 gas at a temperature of 90 °C directed symmetrically at the surface of the droplet ensures rapid evaporation and the presence of steady advantageous fluid currents. By these means, a continuous drop size is maintained throughout the deposition of 10 mL of solution over the course of 10–12 h. The peak-to-peak variations in the resulting deposit are typically less than ±25 % of the mean density.