Hydrogen purification and purity analysis

One of the main requirements of the $\mu p$ experiment is a high chemical purity of the hydrogen gas to avoid muon capture on the impurity nuclei. The contamination by N$_2$, O$_2$, and water vapour has to be below $10^{-8}$. To meet this requirement, a special gas purification apparatus and a gas purity analysis system were constructed at PNPI and installed at PSI. In this apparatus, three separate liquid N$_2$ traps with quartz wool, with special coal, and with Zeolite are used to clean the hydrogen gas with high efficiency from the water vapour, oxygen and nitrogen. The gas purity analysis module contains a gas chromatograph with a special storage column improving strongly the selectivity of the analyzed impurities. A detailed description of these systems, which were used in testing the TPC prototype, was presented in our Progress Report [3]. The level of hydrogen purity obtained with our gas purification system and the sensitivity of the gas analysis are demonstrated in Fig. 7.

Figure 7: Gas chromatographic analysis of hydrogen samples. A)-- Commercial hydrogen (class 6.0): sample $\sim$ 1L, O$_2\sim$ 0.1ppm, N$_2\sim$0.2ppm. B)-- Hydrogen after cleaning in PNPI setup: sample $\sim$ 30L, O$_2<$ 10$^{-9}$, N$_2\sim$0.02ppm.

It is seen that after cleaning the nitrogen concentration does not exceed the level of 2$\cdot10^{-8}$ that is acceptable for the $\mu p$ capture experiment. Using some additional columns for purification, it will be possible to reduce the nitrogen admixture down to $\sim 10^{-9}$. The level of water vapour concentration in the purified gas was near the required level of $\sim$ 0.04 ppm. A special device for water vapour analysis on this level of sensitivity was supplied by PNPI. Our apparatus can produce $50\div 60$ l/h of hydrogen of such purity. At present we have accumulated more than 650 l of extra pure hydrogen for future test runs.