  The activation of materials due to exposure to cosmic rays may become an
important background source for experiments investigating rare event phenomena.
DarkSide-20k, currently under construction at the Laboratori Nazionali del Gran
Sasso, is a direct detection experiment for galactic dark matter particles,
using a two-phase liquid-argon Time Projection Chamber (TPC) filled with 49.7
tonnes (active mass) of Underground Argon (UAr) depleted in 39Ar. Despite the
outstanding capability of discriminating gamma/beta background in argon TPCs,
this background must be considered because of induced dead time or accidental
coincidences mimicking dark-matter signals and it is relevant for low-threshold
electron-counting measurements. Here, the cosmogenic activity of relevant
long-lived radioisotopes induced in the experiment has been estimated to set
requirements and procedures during preparation of the experiment and to check
that it is not dominant over primordial radioactivity; particular attention has
been paid to the activation of the 120 t of UAr used in DarkSide-20k. Expected
exposures above ground and production rates, either measured or calculated,
have been considered in detail. From the simulated counting rates in the
detector due to cosmogenic isotopes, it is concluded that activation in copper
and stainless steel is not problematic. The activity of 39Ar induced during
extraction, purification and transport on surface is evaluated to be 2.8% of
the activity measured in UAr by DarkSide-50 experiment, which used the same
underground source, and thus considered acceptable. Other isotopes in the UAr
such as 37Ar and 3H are shown not to be relevant due to short half-life and
assumed purification methods.
