The first DarkSide physics detector is to be DarkSide-50 (DS-50), currently under construction in Hall C of the Gran Sasso National Laboratory (LNGS) in Italy. DarkSide-50 will be the first of the DarkSide detectors built using low-background techniques and materials and filled with low-radioactivity underground argon (UAr). It will have an active mass of 50 kg, representing the first large-scale use of low-radioactivity argon from underground sources. In addition to having an interesting physics reach, with a projected sensitivity of 2 x 10-45cm2 for a WIMP mass of 100 GeV (90% C.L..) in a 3 year run. DarkSide-50 will allow us to fully validate important design features of the DarkSide concept, in particular those which are expected to allow the design to achieve ultra-low residual background rates.
The DarkSide-50 two-phase LAr TPC, shown in the accompanying picture, has a design based on the success of our currently-operating prototype LAr TPC called DarkSide-10 (arXiv:1204-6218). Thirty-eight 3″ Hamamatsu low-background R11065 PMTs, 19 each on the top and the bottom, will view the active UAr through fused silica windows. The windows will be coated on both sides with Indium Tin Oxide, ITO, a transparent conductor. This allows the inner window surfaces to serve as the grounded anode (top) and -60 KV cathode (bottom) of the TPC while maintaining their outer surfaces near the -1.5 KV PMT photocathode potential. A gas layer for production of the electrolumnescence signature is provided by a cylindrical rim on the fused silica anode window, which extends downward to form a “diving bell” containing the 2-cm-thick argon vapor layer (“gas pocket”) above the TPC drift volume. The cylindrical vessel containing the active region will be made of PTFE, treated to be highly reflective at visible wavelengths. The entire inner surface of the active volume will be coated with the wavelength shifter TetraPhenylButadiene (TPB) to convert the 128 nm argon scintillation into the wavelength range detectable by the PMTs. The drift field is produced by system consisting of the ITO cathode and anode planes, a field cage, and a grid that separates the drift and electron extraction regions. This construction has been fully tested with great success in DarkSide-10.
The DarkSide-50 TPC will be deployed within a borated liquid scintillator-based neutron veto (LSV), which is in turn inside a water Cherenkov muon veto. The 4 meter diameter stainless steel sphere to contain the LSV has already been fabricated, within the Borexino CTF tank which will form the muon veto. This will not only give DarkSide-50 the benefits of active background suppression, but will also give us direct experience in operating a low background experiment in these active shields, to prepare for the eventual multi-ton G2 experiment.
DarkSide-50 construction is scheduled for completion in the fall of 2012, with commissioning starting by the end of the year. With the UAr fill, the design light yield of 6 PE/keV (at zero field; a value of 9 PE/keV has been achieved in DarkSide-10), and estimated neutron veto rejection efficiency for internally-generated neutrons 99.5 per cent, the remaining background from all sources in a 0.1 ton-year exposure (3 years for the 35 kg fiducial target mass) within the expected recoil region is estimated to be less than ~0.1 event after cuts. This gives an estimated sensitivity for spin independent WIMP-nucleon scattering of 2 x 10-45cm2 for a WIMP mass of 100 GeV at the 90% C.L. The DarkSide-50 detector will thus provide valuable new scientific results and validate the basic background-free design concepts for the large DarkSide-G2 detector.