1. Follow-up of the tests with the copper filter
CENF general meeting – 21/01/2016

2. Filter composition:
- Molecular Sieve (Type 4A mm) —> 2.46 kg
- Copper on Alumina spheres (BASF Cu-0226 S 14x28 mesh or Q-5)—> 2.96 kg (10% Cu mass)
Adsorption capacity in LAr, as estimated at FNAL: 0.5 gO2/kg
Filter prepared and regenerated by the CERN Cryolab group, flowing GAr/H2 mixture (98%/2%) through the filter at high temperature (~200°) and high flux according to FNAL prescriptions.

3. FLIC Cryo-set up - reminder
New filter with copper bed installed
BEFORE recondenser
FLIC Cryo-set up - reminder
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4. Summary of previous measurements
With the copper-based filter operated in liquid phase during filling and in gas phase during Argon recirculation (1 full volume/4-5 days), a purity higher than 10 ms (above sensitivity of our detector) was reached in a short time.
The 50 l LAr-TPC was operated for several days at max purity, then O2 was injected in steps for a total of ~16 g of O2, over the whole run.
Purity started getting worse after injection of ~1.5 g of O2 but with a very slow rate of change: still 10 us electron lifetime after injection of 16 g of O2, suggesting that the filter trapping/saturation mechanism is not completely understood.
It is also possible that other impurities are present in the detector that are not trapped by the filter. Fitting the slow component ts of the LAr scintillation light, yielded a value around 0.8 ms ( to be compared with ts > 1.2 ms prior to oxygen injection). Before the end of test the amount of N2 and H2O in the recirculating gas was measured with argon gas analyzer: 16.6 ppm and < 0.5 ppm, respectively. These are partially compatible with the measured slow light component and electron lifetime and tend to confirm the presence of additional impurities in the LAr-TPC not trapped by the molecular sieve and the copper.
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5. Overall purity plot – first run
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6. Latest tests
A new run was performed in late December, taking data for few days with the filter left as in previous test:
no tracks could be collected during this run, which seems to confirm that the Copper filter was saturated;
the measurement of the slow LAr light component was compatible with no contamination by other impurities, as it had been in the previous run, before starting Oxygen injection.
In January 2016 the filter was sent back to CERN cryolab and is now under regeneration following the prescriptions: Argon Flow at high Temperature (> 200°) to activate molecular sieve, followed by Ar(98%)+H2(~2%) flow for Copper activation. Water content is continuously recorded to get an estimate of the trapped impurities level both in Mol. Sieve and Copper.
A new run with the LAr-TPC will follow. A new bottle of Ar60+O255 (1%), with much lower content of impurities other than Oxygen, will be used for filter saturation (O255 -> H20 ≤ 0.5 ppm-mol; CO,CO2,H2 ≤ 0.1 ppm-mol).
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7. Back Up

8. Slow component LAr scintillation light, life-time as a function of Oxygen contamination
From: “Effects of Nitrogen and Oxygen contamination in liquid Argon”, doi: /j.nuclphysbps
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9. LAr scintillation light, life-time as a function of Nitrogen contamination
From: “Effects of Nitrogen and Oxygen contamination in liquid Argon”, doi: /j.nuclphysbps
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