DarkSide-Proto

The TPC of DS-Proto is a small chamber with an active mass of ~ 7 kg (5 L) of LAr and a drift length of 12 cm. It is constructed almost entirely out of acrylic employed for both the TPC walls and the upper and lower windows, separating the inner volume from the outside of the TPC. The anode and cathode are produced with a thin Clevios coating on the inner faces of the top and bottom windows, while a stainless steel grid serves to separate the active volume below it (drift volume) from the liquid-gas interface where extraction occurs. A series of parallel strips of Clevios placed on the inner side of the lateral walls (field shaping cage) is powered by a voltage divider to mitigate border effects and leakage through the grid spacing. Finally, TPB coatings are used on the inner surfaces of the TPC to shift the wavelength of argon light at 128 nm to the optical spectrum, to which acrylic is transparent and the SiPM sensors exhibit maximum efficiency.

The top acrylic window is shaped like a diving bell to contain the argon gas pocket, fed by a resistive boiler placed on the side of the diving bell. The height of liquid is instead regulated by the height of an outlet connected to a through hole on the opposite side.

The most peculiar aspect of the Proto-0 TPC is that both the diving bell and gas outlet are mechanically decoupled from the bottom half of the TPC, and are linked to two independent precision motion-control systems which allows varying the grid-anode distance and gas-pocket height. This flexibility enables a meticulous investigation of S2 properties pulse shapes across different gas pocket configurations.

DarkSide-Proto is currently hosted by the Cryogenic Laboratory for Dark Matter Research (CryoLab) at INFN Naples, which houses a complete cooling tower for argon equipped with a custom-made condensation system, recirculation and purification loops. The cryogenic setup is designed to achieve sub-mbar pressure stability to contain systematics on S2-related measurements. The CryoLab also houses the Photosensor Test Facility of DarkSide-20k, an independent system with the task of testing the full production of PDUs for the TPC of DarkSide-20k in a cryogenic environment.