Fig. 3.

Simplified scheme of the cryostat in the dry-well of the water tank. Light blue: stainless steel walls of water tank and dry-well. Orange: copper parts (shielding and cryostat). Solid magenta, dashed black: experimental volume needed for COSINUS-1$\mathrm{\pi }$  (corresponding to a maximum total mass of $\sim$3.6 kg of NaI) and -2$\mathrm{\pi }$ (up to a total mass of $\sim$10.8 kg of NaI), respectively. Blue: temperature stages of the cryostat. Green: light-tight curtain made of e.g. Tyvek to create a passive layer at the water tank walls. This scheme is based on the favoured solution (Option 4) for the shielding concept as found by MC simulations (see Sect. 5)