The DarkSide collaboration has recently reported two significant results on the road to ton-scale liquid argon dark matter searches.
Argon is a promising target for direct dark matter searches due to the ability to distinguish heavily-ionizing WIMP-induced nuclear recoil signals from minimum-ionizing gamma- and beta-induced backgrounds by pulse shape discrimination (PSD). However, PSD performance depends exponentially on the scintillation light detection efficiency, and the overall sensitivity of argon experiments is limited by radioactive 39Ar present in commercial argon.
In the first result, the DarkSide collaboration achieved a light yield of 9.1±0.5 photoelectrons/keV in a 10 kg prototype argon time projection chamber (TPC), the highest argon light yield thus far reported by any experiment. In the second result, DarkSide collaborators measured a sample of argon extracted from underground to have less than 0.65% 39Ar activity than atmospheric argon. The figure above shows that the spectrum measured for the underground argon in consistent with the sum of known background components and essentially no 39Ar.
Extraction and purification of underground argon is currently underway. This low-radioactivity argon will be used as the target for DarkSide-50, a 50 kg TPC currently under construction at the Laboratory Nazionale del Gran Sasso (LNGS). First data-taking for DarkSide-50 should commence toward the end of 2012.
– Ben Loer