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CCR
When using top loading cryostat (CCR), some amount of exchange gas is always needed. Without the exchange gas, you will reach base temperature of the cryostat very slowly.
With decreasing temperature, you will also decrease the pressure of exchange gas inside. From 1bar to roughly 60 mbar:
It is not possible to pump out exchange gas, when cooled down. For proper experiment, you should ALWAYS pump the exchange gas to roughly 100 mbar at room temperature. Otherwise He will produce background in you data:
We counted 10 minutes per point, blue data without pumping the He out at room temperature. After pumping the gas, background is gone.