Looking inside a fridge that cools to ultra-low temperatures

To create these ultra-low temperature environments, the cryogenics group use dilution fridges. These rely on quantum mechanical effects that occur when two isotopes of helium are mixed. By diluting a mixture of helium 4 with the much rarer isotope helium 3, we can achieve cooling due to the heat of mixing of the two isotopes. This cooling occurs when helium 3 flows from an almost pure helium 3 phase to a lower purity ‘dilute’ phase.

To gain a new insight into the real movement of helium during the cooling, and inform the development of new designs, the ISIS cryogenics team looked to one of the facility’s own instruments: IMAT. Neutrons do not interact strongly with the metal components surrounding the fridge, or the helium 4 present, but they are highly sensitive to helium 3, making them an ideal probe for this investigation.

In their study, published in Scientific Reports, the team took neutron imaging measurements of the dilution fridge under both continuous circulation and ‘single shot’ conditions. Under these different conditions, they were able to see the change in helium 3 concentration over time and at different temperatures. They found that the movement of helium during the continuous cycle operation was consistent with the accepted theory.

These experimental conditions are extremely challenging to create; even a neutron beam can make the sample too hot, as can local vibrations.

To create these ultra-low temperature environments, the cryogenics group use dilution fridges. These rely on quantum mechanical effects that occur when two isotopes of helium are mixed. By diluting a mixture of helium 4 with the much rarer isotope helium 3, we can achieve cooling due to the heat of mixing of the two isotopes. This cooling occurs when helium 3 flows from an almost pure helium 3 phase to a lower purity ‘dilute’ phase.

To gain a new insight into the real movement of helium during the cooling, and inform the development of new designs, the ISIS cryogenics team looked to one of the facility’s own instruments: IMAT. Neutrons do not interact strongly with the metal components surrounding the fridge, or the helium 4 present, but they are highly sensitive to helium 3, making them an ideal probe for this investigation.

In their study, published in Scientific Reports, the team took neutron imaging measurements of the dilution fridge under both continuous circulation and ‘single shot’ conditions. Under these different conditions, they were able to see the change in helium 3 concentration over time and at different temperatures. They found that the movement of helium during the continuous cycle operation was consistent with the accepted theory.

The video above shows the single shot experi​ment – where 3He only was extracted from the mixing chamber, without reinserting it through the dilution insert. Here you can see the difference between dilute and concentrated phase clearly, with the phase boundary moving up as the mixture ratio changes due to the single shot.

Further information

The full paper can be found at DOI: 10.1038/s41598-022-05025-0
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