Maintenance for DR(Pandora)
After the major maintenance done on the DR in 2019 (see 2019-01-14 entry), the room temperature leak check between the gas handling system and the IVC showed no problem. However, after pre-cooling with liquid Nitrogen a small leak (1x10-7 Torr.L/s) appeared, the DR was warmed up to about 200 K the leak disappeared, then cooled again to liquid Nitrogen temperature but the leak did not reappear (wait time 10 min). We then cooled down with Helium to below 10 K and introduced mixture into the gas handling system, both 3He and 4He were detected in the IVC (3He went from 1x10-8 to 10-5 Torr.L/s in 45 seconds). We let the DR warm back up and pressurized the dilution unit with helium gas up to 15 psi. Finally, at room temperature we were able to observe the 4He leak rate increase from 2x10-8 to 8x10-8 Torr.L/s. The DR was disassembled, and we re-soldered the same spot on the heat exchanger that failed in the past (see 2007-06-04 entry), this did not fix the leak. The next step was to re-solder the tube between the heat exchanger pancakes (failed once before, see 2012-07-10 entry) and extended the soldering of the pancake itself to the sides. This increased the size of the leak, we were able to observe with snoop two leaks one on the tube and another at the spot we stopped soldering the pancake. After few tries we were able to fix both. The re-soldered joint of the heat exchanger pancake has extended to the other side the support posts seen in the picture from 2007.
The DR was re-assembled and tested. With the dilution unit temperature below 90 K, the leak rate was 7x10-9 T.L/s and no sign of He-3 in the IVC. After cooling down we operated the DR at 20 mK with the following leak rates (2.4 10-8 (4He) and 5.5 10-9 Torr.L/s (3He)) the hydrogen contamination in the system was less than 100 ppm.
