After a successful prototype (DarkSide-10 aka. DS-10) the first physics detector in the DarkSide program is DarkSide-50 (DS-50), currently situated in Hall C of the Gran Sasso National Laboratory (LNGS) in Assergi, Italy. DS-50 is the first in an eventual series of DarkSide detectors built using low-background techniques and materials and filled with low-radioactivity underground argon (UAr). It has an active mass of 50 kg Ar, representing the first large-scale use of low-radioactivity argon from underground sources, in addition to having a significant experimental reach, with a sensitivity of 2 x 10-45 cm2 for a WIMP mass of 100 GeV (90% C.L.) in a 3 year run. DarkSide-50 has allowed 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 in latter detectors.
The DS-50 two-phase LAr TPC — shown in the accompanying picture — has a design based on the success of our prototype LAr TPC, DarkSide-10 (Astroparticle Physics). Thirty-eight 3″ Hamamatsu low-background R11065 PMTs, 19 each on the top and the bottom, view the active UAr through fused silica windows. The windows are 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 is made of PTFE (aka. Teflon), treated to be highly reflective at visible wavelengths. The entire inner surface of the active volume is coated with the wavelength shifter TetraPhenylButadiene (TPB) which converts the 128 nm argon scintillation into a 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.
The DS-50 TPC is deployed within a 4-meter-diameter stainless steel sphere containing a borated liquid scintillato: this is our “neutron veto”, or “liquid scintillator veto” (LSV). The LSF is itself inside the Borexino CTF (Counting Test Facility) tank, which is filled with water and used as a Čerenkov muon veto. This “nesting doll” veto arrangement not only gives 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 DS-20k experiment.
DarkSide-50 construction was completed in late 2012, with physics data collection beginning in late 2013, using atmospheric argon (AAr). Although DS-50 was designed to achieve a light yield of 6 PE/keV (at null field) with AAr, it exceeded expectations and reached nearly 8 PE/keV. DS-50 also successfully demonstrated that a neutron veto rejection efficiency for internally-generated neutrons could be kept above 99.5%.
After an initial analysis of about 50 days of acquired AAr data, the DarkSide Collaboration published direct dark matter WIMP search results free from all sources of remaining background (less than 0.1 events after all cuts). This gives an estimated sensitivity for spin-independent WIMP-nucleon scattering of 6.1 x 10-44 cm2 for a WIMP mass of 100 GeV, at the 90% C.L.
After successful filling of the DarkSide-50 detector with UAr in April 2015, about 70 days of dark matter WIMP search data were acquired and used to assess the capabilities of the UAr. While the general characteristics (including the high light yield) of the UAr remained similar when compared to the AAr, the reduction of the 39Ar content was better than expected, with a reduction factor of about 1400.
The DarkSide Collaboration has just recently published the results of this UAr dark matter search, again showing no remaining backgrounds in the WIMP search region after all cuts. Combining these data with the AAr data, the sensitivity for spin independent WIMP-nucleon scattering is extended to 2 x 10-44 cm2 for a WIMP mass of 100 GeV, at the 90% C.L. These are the most sensitive results for a liquid-argon-based direct dark matter WIMP search to date.
The DarkSide-50 detector has already provided valuable new scientific results, and successfully validated the basic background-free design concepts for the proposed DarkSide detectors of the near-future.