Over 100 experiments each year at ISIS require ultra-low temperatures – below 1 K, or -272°C. 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 shoes the oscillations of the 3He/4He mixture condensing, precipitating into the fridge and evaporating.
They also looked at what happened when the fridge was operated incorrectly, causing it to fail. This threw up some surprises, as the effect of the failure was not what they expected.
“By altering the ratio of helium 3 to helium 4 in the dilution fridge mixture, we were able to simulate what would happen if helium 3 were to be lost… a low temperature physicist's worst nightmare! Interestingly, we found that whilst on the outside the fridge appeared to be working (albeit somewhat poorly), IMAT was able to reveal that the internal situation was far from ideal, with the boundary between the two phases seemingly absent from the mixing chamber." Dr Chris Lawson – ULT Technical Manager
Although these dilution fridges are in widespread use across the world, this insight into their operation is unique and provides a valuable resource. It also highlights the usefulness of the technique when investigating these pieces of equipment for understanding their operation and investigating failures.
IMAT has the ability to probe the inner workings of otherwise hidden processes occurring in high-tech devices. Whilst usually focussed on the samples of external users, we have shown the benefits of turning the beam on our own equipment. In the future we hope that this method will allow the development of atypical dilution fridges for space applications, diagnosis of other common dilution fridge fault modes, and be used for the training and instruction of the next generation of cryogenic technicians.
It is vital that technicians from large facilities, such as ISIS, have the freedom to run with a good idea, and drive development of the service they provide to the scientific community. The work in this publication is a fantastic example of that.
The video above shows the single shot experiment – 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.
The full paper can be found at DOI: 10.1038/s41598-022-05025-0