The purity of liquid helium revisited.

Helium has the lowest boiling point of any element, therefore it is reasonable to expect that, after purification and solid particle filtration, obtaining a completely pure liquid should be relatively straightforward, however this is not the case. In order to gain a better understanding of the origin of residual contaminants in what should be "pure" liquid helium we discuss two possible underlying mechanisms: a) those produced by the vapour pressure of the solids contaminants and, b) those dissolved in the liquid due to the solubility of the solid contaminants. We show that both mechanisms lead to a similar result: the only non-negligible residual
contaminant that is left after purification is H2 and its presence in the vapour and in the liquid is very similar (yH2=10-10). This small amount of H2 is enough to produce the blocking of fine capillary tubes used to achieve temperatures below 4.2 K in helium pumped cryostats. To solve this problem, we present a purification system based on a combination of cryocondensation and chemisorption that produces hydrogenfree clean helium, which prevents blockages in capillaries.