A Super Ultra Cold Neutron source
The cooling process starts with a 100L volume filled with liquid helium, which acts as a reserve. A small quantity of this helium is transferred to a smaller volume, called the "1-K pot", through a needle valve. The helium in this pot is cooled down to about 1K. At this temperature the 4He has already changed state and become superfluid, whereas the 3He is still liquid. Thanks to the ability of superfluid helium to "crawl" over surfaces and pass through the smallest cracks, it is possible to separate the superfluid 4He from the liquid 3He with the help of a superleak. The 4He then enters the heat exchanger, where it is further cooled down to a temperature of 0.6K or lower by a closed circuit of 3He. This is possible because the 3He requires less energy to be cooled down by pumping. The very cold 4He then fills the conversion volume.
SuperSUN is an ultracold neutron source situated at the end of the H523 beamline. This line delivers a beam of cold neutrons (including 9A wavelength neutrons), which are converted into ultacold neutrons (UCNs) in a single scattering event during which they deposit most of their energy in the helium in the conversion volume. The reverse process, where UCNs gain energy from He, is negligible at very low temperatures (0.6K or lower), which is why this temperature the maximum temperature target for SuperSUN's conversion volume. The UCNs produced are stored in this volume, which allows us to accumulate them and hence build up the UCN density in the volume. Once the density is high enough, the UCN valve opens and the UCNs are delivered to the experiment.