1. Depleted Argon from Underground Sources Henning O. Back – Princeton University For the DarkSide experiment, E-1000 Augustana College – D. Alton FNAL – C. Kendziora, D. Montanari, S. Pordes Princeton U. – F. Calaprice, C. Galbiati, A. Goretti, Andrea Ianni, B. Loer, P. Mosteiro

2. Why depleted argon Naturally occurring 39 Ar is the limiting contamination in atmospheric argon – 39 Ar  39 K + e - +  ν e (Q = 535 keV) – Limits size of detectors due to pile up Atmospheric 39 Ar is produced in the upper atmosphere in 40 Ar(n, 2n) reactions Atmospheric concentration 39 Ar/ 40 Ar = 8.1×10 -16 – Corresponds to 1 Bq/kg of atmospheric argon – One ton detector Electron drift time across 1 ton detector (1m)= order 500μs (minimum time between events, equivalent to 2kHz) Atmospheric 39 Ar decay rate = 1kHz/ton 6/10/2011TIPP 20112

3. The source of depleted argon Argon from deep underground is shielded from cosmic rays CO 2 gas well in SW Colorado has 400 ppm argon Concentrate Ar, He, and N 2 through Vacuum Pressure Swing Absorption (VPSA) on Zeolite – Unwanted gases trapped on Zeolite columns under pressure – Zeolite regenerated by evacuating column – Two columns are used to have a nearly continuous flow of gas Collected gas – N 2 – 70% – He – 27.5% – Ar – 2.5% Recent plant adjustments raised argon fraction to ~4% 6/10/20113TIPP 2011 He, Ar, N 2 mixture Gas from well Absorbed gas returned to company Zeolite column Gas flows through one column under pressure. CO 2, O 2, H 2 O and CH 4 are absorbed on zeolite Simultaneously the other column is pumped on to remove the trapped gases

4. VPSA plant Production rate 0.5 – 0.8 kg/day 44 kg of depleted argon collected to date Depletion level at least a factor of 25 less than atmosphere 6/10/2011TIPP 20114

5. Argon from VPSA is born underground 6/10/2011TIPP 20115 Terrestrial 40 Ar primarily from 40 K decay 36 Ar produced in the sun and has an atmospheric natural abundance of 0.35%

6. Cryogenic Distillation Purification of gas from VPSA plant Purification by distillation based on difference in boiling points Boiling happens on the surface of column packing material Our gas has helium, which is not liquefied and just passes through system 6/10/20116TIPP 2011 Inject liquefied Ar/N 2 into column Waste gas (He and N 2 ) Product (pure Argon) Lower boiling point gas preferentially moves up column Higher boiling point liquid preferentially moves down column Packed column Temperature gradient 5.5 cm Column packing material

7. Our design 2 – 600W cryocoolers – Balanced with 700W heaters for temperature control Reboiler cooled by liquid from column – Temperature controlled with 700W heater Active PID temperature control Active mass flow control Pressure and temperature monitoring throughout Multiple input RGA measures gas at three points – Input – Waste – Product 6/10/20117TIPP 2011

8. 6/10/2011TIPP 20118

9. Control system 6/10/2011TIPP 20119 UGA

10. Commissioning Pre-safety review – Verify temperature control while under vacuum – Verify mass flow control – Measure gas composition of gas from Colorado Found concentration to be: Ar – 2.5%, He - 27.5%, and N 2 – 70% Post safety review – Temperature control with gas load Controls temperature to within 0.5K – Column cool down (Ar and N 2 ) Column can be cooled in ~5 hours – Full commissioning run (Goals) Use same gas mixture as Colorado gas (N 2 = 70%, He = 27.5%, Ar = 2.5%) Show that Distillation Column can condense all Ar, even at low concentrations Show continuous distillation is possible Show that batch distillation works 6/10/2011TIPP 201110

11. Argon condensation 5 hours after start up, we are condensing all the argon! 6/10/201111TIPP 2011 Input (Ar, N 2. He) W aste gas (He and N 2 ) P roduct

12. First continuous distillation Helium is not liquefied; it just passes through the system Normalizing to Helium allows a direct comparison of input and waste gases Input gas N 2 /He ~ 4.23 Ar/He ~ 0.22 Waste gas N 2 /He ~ 4.43 Ar/He ~ 0.02 Conclusion More argon is entering than leaving (~90% is being collected) More N 2 is leaving than entering First continuous distillation! 6/10/201112TIPP 2011

13. Batch Distillation 6/10/201113TIPP 2011 Input stream turned off Batch distillation in progress! Waste gas Distilling liquid in reboiler Only N 2 in waste

14. Batch Distillation 6/10/2011TIPP 201114 Product line shows concentration of Ar and N 2 in reboiler. Input Waste gas Product P = product line W = waste line N 2 concentration in reboiler is dropping throughout distillation run – Batch distillation works! Start – Ar:N 2 = 1:30 Stop – Ar:N 2 = 33:1 ~ Factor 1000 reduction

15. Conclusions and Future Conclusions – It is possible to condense 100% of the argon – First successful continuous distillation – Batch distillation produces very pure Argon in reboiler Future work – Increase temperature monitoring capabilities – Further study of continuous and batch distillation Goal: 1ppm N 2 – Install product gas collection system (Condenser Booster) – Increase throughput to reach 5kg/day production 6/10/2011TIPP 201115