Infrastructure

The COSINUS experimental setup was designed keeping in mind the strict radio-purity and low-background requriements. A comprehensive Monte Carlo simulation was carried out which formed the basis for the setup, with the simulation results published in [1]. The experiment is based in Hall B of LNGS, Italy which has a rock overburden equivalent to 3,600m of water overhead.

The setup consists of four main parts:

The dilution refrigerator, located in a dry-well inside the water tank, provides the low temperatures necessary to operate the detectors at mK-temperatures.
The water tank shields ambient radioactivity.
A servicing level equipped as a clean room allows for detector mounting and includes a lifting system to move the cryostat in and out of the dry-well and external Cu shield.
A two-floor control building is located next to the tank.

To reduce the rate of cosmogenic neutron events, which is typically higher than the radiogenic background, an active muon veto system is used. The Monte Carlo simulations predict that the rate of cosmogenic neutron events is in the order of (3.5 ± 0.7) cts kg−1 yr−1 [1], which corresponds to roughly one event in 100 kg-days. The active muon veto, consisting of approximately 30 photomultiplier tubes installed in the water tank, is expected to reduce the rate of cosmogenic neutrons to the level of radiogenic neutrons, which the Monte Carlo simulation predicts to be 0.05 cts kg−1 yr−1. Ongoing radiopurity screening efforts are being conducted on all materials in the vicinity of the detectors.

The COSINUS cryogenic calorimeters require a temperature of approximately 10 mK. This is achieved using a 3He/4He dilution refrigerator. Traditionally, the pre-cooling from room temperature to 4 K has been achieved using liquid nitrogen (LN2) and liquid helium (LHe) (thus named “wet” dilution refrigirators). The successive cooling stage is realized by a closed cycle of a mixture of 3He and 4He to cool from 4 K to the base temperature of 10 mK.

For COSINUS, the newest-generation of dry dilution refrigerator is used, which provides the necessary cooling power to 4 K using a Pulse Tube cryocooler[2], thereby eliminating the need for an external supply of cryogenic liquids. The successive cooldown down to base temperature is identical to the “wet” systems.

References :

[1] Angloher, G. et al., Simulation-based design study for the passive shielding of the COSINUS dark matter experiment, Eur. Phys. J. C 82.3 (2022)

[2] K. Uhlig, W. Hehn, 3He/4He Dilution refrigerator precooled by Gifford-McMahon refrigerator, Cryogenics. 37 (5): 279, (1997)