Maintenance for DR(Pandora)

While checking the dilution unit for leaks we observed a leak rate increase from 1E-9 to 1E-6 in about 10 seconds. I did not see this leak after the last DR run, the leak was either very small or the background too high. We started opening the DR and at the same time set up N2 gas flow from the still side to the condenser side. The gas flow was left for the weekend to reduce the helium background so we can leak check. On Monday we attached the (Helium bath) replacement tube and pumped the ‘IVC’ so we can check if it is possible to detect the leak. The leak rate in the dilution unit was 1E-7 zeroed to 1E-9 and with 1 atm helium in the ‘IVC’ space we observed a leak rate started to increase after 50 seconds and reached 3E-7 in about 3 minutes. Two leaks one at the “normal” spot on the heat exchanger and one on the brass tube connecting the pancakes (see attached pdf file) were fixed.

After pumping for 3 days with only the leak detector attached to the IVC the background was 2 E-8 after zeroing it went to 0.5e-9. With the gas handling system at 0.5 atm (He gas) after 5 minutes there was no change in the leak rate.