Alex Chuquitarqui from the group around Prof. Conrado Rillo of the University of Zaragoza is searching for hydrogen traces in liquid helium. Hydrogen vapor pressure as a function of temperature proofs that these exist. At 4.2 K the steam pressure of hydrogen is still high enough to contaminate liquid helium substantially (see fig. 1).
There are various reasons for contaminations: Traces of hydrogen can already be present in the helium source, oil degradation in pumps and compressors during recovery can lead to hydrogen formation just like the outgassing of metallic tubes and tanks and the diffusion of plastic tubes and gas balloons. This means that almost every dewar with liquid helium also contains a considerable amount of hydrogen.
Generally, dissolved hydrogen does not cause any problems. When liquid helium is pumped through capillaries, however, it cools off locally and the dissolved hydrogen freezes out as solid and acts as sealing. . A capillary is used for temperature control for example in NMR and LT-STM devices and in the PPMS and MPMS3 systems from Quantum Design. When blocked, the base temperature of the system cannot be reached any more. In this case, the system must be warmed to release the blocked capillary.
Hydrogen impurities in liquid helium are easy to detect with a special sensor developed by Prof. Rillo's group from the University of Zaragoza. It consists of a test capillary that is connected to a vacuum pump (see fig. 2).
Alex Chuquitarqui is looking for cooperation partner who are willing to test the H2-detector. If you are interested, he will send you one of the detectors for free so you can detect the hydrogen content in your liquid helium and quantify it until the capillary is blocked. This way you can avoid using hydrogen-contaminated liquid helium for delicate applications.
This project is supported by the Spanish Ministry of Science, Innovation and Universities. The project number is MAT2015-64083-R. Alex Chuquitarqui can be reached at the following e-mail address: achuquitarquiunizar.es  M. Gabal et al., Hydrogen-Free Liquid-Helium Recovery Plants: The Solution for Low-Temperature Flow Impedance Blocking, Phys. Rev. Applied 6, 024017 (2016).
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