The ARIA project: production of depleted argon for the DarkSide experiment

The DarkSide collaboration has been working on the design of detectors operated with argon depleted from 39Ar. It has been found that the underground argon source from the Cortez CO2 wells located in Colorado is characterized by a specific content of radioactivity from 39Ar, which is at least 1000 times lower than that of atmospheric argon. However, for large detectors another depletion factor of 10–100 is needed. The ARIA project was developed within the DarkSide collaboration with the purpose of further depleting the underground argon by means of a proper technology at a reasonable cost and in a reasonable time. Isotopic separation by cryogenic distillation was found to be the proper one. Therefore, the ARIA project involves an international team that has been working on the design and installation of a cryogenic distillation plant that allows to perform the isotopic separation of argon. This is a challenging task due to the very tiny difference in the vapour pressure of the species to be separated, which requires a large number of equilibrium stages and, thus, a very tall column for the desired separation to be accomplished. This fact led the Monte Sinni mine, located in Sardinia (Italy), to be identified as the ideal place for the realization of the ARIA project. The ARIA project also finds application to other fields (e.g., medical and nuclear), where the production of stable isotopes other than those of argon is of paramount importance. Preliminary studies on the production of 15N and on the production of 13C using, respectively, nitric oxide and carbon monoxide as operating substances have been carried out by means of classical chemical engineering design methods. The aim of this work is to present the ARIA project and the studies that have been carried out about isotope separation by cryogenic distillation. This presentation was used for the Low-Radioactivity Underground Argon Workshop held at Pacific Northwest National Laboratory in Richland, Washington on March 19 - 20, 2018