The construction of the large infrastructure that LEGEND-200 requires, implies the use of major resources and time. Experience shows that such processes last several years and motivate major redesigns. The reutilization of GERDA facilities for LEGEND-200 constitutes a very economical and time efficient option. Two main features allow us to proceed with LEGEND-200:

1.  The background in GERDA is dominated by sources close to the detectors like cables, ⁴²Ar and detector surface contaminations. External sources of radiation are efficiently reduced by the water and the liquid argon (LAr) shield to ensure sufficiently low background rates for LEGEND-200¹. Muon induced events are tagged efficiently by the water Cherenkov detector and the liquid argon instrumentation.
2. The cryostat, especially the aperture of the neck, is large enough to house 12 detector strings with 200 kg of mass.

The existing GERDA infrastructure is therefore a suitable facility for LEGEND-200. The installation finished in summer 2022 and LEGEND-200 is currently taking physics data with 140 kg of ⁷⁶Ge detectors. Early 2024 the remaining detectors will be deployed to bring LEGEND-200 to its designed capacity.

To accommodate the extra strings, some re-design of the piping through the neck has been done for the detectors deployment. The detectors and liquid argon instrumentation are assembled in the clean room above the cryostat in a glove box which is flushed with nitrogen to reduce/avoid radon deposition on the detectors and protects the insulating groove between the detector p⁺ and n⁺ electrodes by an inert atmosphere. The detectors are assembled into strings and, together with the liquid argon instrumentation, placed inside the lock. When the lock is closed several pumping and purging cycles follow.

The entire assembly is lowered through the cryostat neck into the main volume. Thus the diameter of the neck is a limiting aperture. The neck of the cryostat has a nominal diameter of 800 mm while the minimum required for a maintenance hole is only 550 mm. The larger aperture is what allows the expansion of the detector mass to about 200 kg.

The cryogenic infrastructure of the cryostat includes the liquid argon and nitrogen storage tanks, a valve box which houses manual and remotely controlled valves for LN₂ / LAr and a radon trap, and piping inside the cryostat. The latter includes two copper spirals as heat exchangers for the cooling of LAr with LN₂ , a LAr filling line, a drainage pipe, temperature sensors at different positions and several devices for the fill level measurement. The available aperture for the detectors in the neck is reduced because of these pipes and sensors such that this opening becomes the limiting cross section. For LEGEND-200 the installation has been replaced to enlarge the area for detectors and LAr instrumentation.

The water tank is equipped with 66 PMTs. Easily accessible PMTs were repaired since GERDA stopped data taking and the water tank was emptied. This situation also provided the opportunity to inspect the outer wall of the cryostat for corrosion or other damage.