Underground Argon and URANIA Project

The Underground Argon

Imagine having a diffused source in the detection volume that decays with ~ 1 cps per kg of material mass. With a total target mass of 49.6 tonnes in DarkSide-20k (DS-20k), the effective rate from this source only is nearing 50 kHz. This is an overwhelming scenario just in terms of detector operation, let alone the rare physics goal of WIMP search. The scenario described above is true if we have Ar that is procured from the atmosphere, and the diffused source is 39Ar, which is a ꞵ-emitter with a specific activity of 1 Bq/kg and a half life of ∼ 260 years.  And the origin of this isotope is spallation of cosmic neutrons in argon, activating it mainly via 40Ar(n,2n)39Ar. The end point of this ꞵ-transition is at 565 keV. The discovery of argon present in underground (UAr) wells was of significant importance for paving the way to a detector like DS-20k. As this argon has been stored deep underground for a long time, the cosmogenic activation is avoided and also the levels of 39Ar is foreseen to be depleted.  The factor of reduction in specific activity is estimated to be > 1000 at least. With this number in mind, the contribution to the event rate from the total detector mass in DS-20k comes down to mere < 50Hz from the LAr. The advantages this provides:
  1. Significant reduction in event pile up from these events.
  2. Enhances the power of pulse shape discrimination at low energy.
  3. Due to low- pile up probability, reconstruction of event position is more efficient.
  4. Advantages in S2 only type analysis, significant for low mass searches.
Thanks to the pioneering research and successful R&D, in 2012 the first batch of UAr was extracted from a CO2 well in Cortez, Colorado. The total mass extracted was 160 kg. This was used in the DarkSide-50 detector at LNGS. The reported specific activity of 39Ar by DS-50 was 0.69±0.09 mBq/kg resulting in a depletion factor of  ∼ 1400.   
Background spectrum comparing a run in Atmospheric Argon (AAr) and Underground Argon (UAr) extracted from the URANIA site in Colorado, USA.

The URANIA Project

The total UAr requirement of DS-20k is around 90 tonnes, approx. This demand can’t be met using the small extraction plant used for DS-50. Hence, an industrial scale extraction plant is necessary. The initial analysis of the gas from CO2 wells indicated a presence of Ar at the level of ∼400 ppm only. In order to make the Ar usable, further enrichment and purification is necessary. This is where the URANIA project steps in. It is being commissioned in the same location in Cortez, Colorado, USA.  The input of The input feed stream at Urania is ∼95% CO2, plus a few percent of N2, 1% percent CH4, 430 ppm of UAr, and traces of higher hydrocarbons. The processing scheme of the UAr extraction plant is optimized for this feed composition in order to achieve a UAr output purity of ~ 99.99%. This is done by two steps reduction of CO2 concentration. In the first step there is one liquefier receiving the high-pressure CO2 with very high flow and suddenly dropping the temperature to ~ 5 °C. This partially separates the CO2 and the remaning products are sent into a stripping section. Down the line, the stream is cooled down to -50 °C and sent to the second step. Which is very similar to the first step with a addition of a pressure swing adsorption (PSA) unit. As the name suggests, it is an on-off oscillation system with short time cycle for CO2 adsorption.  This marks the end of This marks end of CO2 stripping and beginning of UAr purification unit. The final output of the first step is highly depleted in CO2 but enriched in N2 composition and Ar at the level of ~1%. This stream is fed to the purification unit consisting of 4 cryogenic distillation columns. At the end of the process the UAr reaches a purity of 99.99 %. This gas is then liquefied and filled in the skids to be transported to Aria. Throughout the extraction and purification phase, online monitoring system is put in place at each step to monitor any air infiltration into the stream.