Production of pure protium

A special requirement in the $\mu p$ experiment is a low deuterium concentration in the protium gas. It should not exceed 1 ppm. At present we can purchase the protium with $\leq$1 ppm D$_2$, and our collaboration is considering also a possibility to produce ultra-depleted protium with $\sim$ 0.3 ppm D$_2$. Usually, protium is provided as protium water that should be converted into gas phase directly in the experiment. A special setup for production of pure protium gas from the protium water was designed at PNPI and constructed and tested at PSI during 1999-2000. It containes a Whatman 75-32 hydrogen generator with a delivery capacity of 150 ml/min at a pressures up to 4 bar above atmospheric pressure. The generator utilizes the principle of electrolytic dissociation of water and subsequent diffusion through a Palladium membrane to generate ultra-pure hydrogen. For the protium gas production, we used the deuterium-depleted protium water (with deuterium admixture $\leq$1 ppm) manufactured by Ontario Power Generation Canada. To avoid additional pollution from deuterium in natural hydrogen, we prepared an electrolyte solution from sodium hydroxyde with the protium water. A system for accumulation of large amounts of protium was prepared with new tubes, valves and vessels which had no previous contacts with natural hydrogen. This system is presented in Fig. 8.

Figure 8: Setup for protium production. 1 - hydrogen generator; 2,3 - accumulation vessels; 4 - purification column; 5  - absorber-compressor; 6 - storage vessel; 7 - manometers.

The system permits to accumulate protium under 5 atm pressure in vessels 2 or 3 each of $\sim$4 l volume. After that each portion of the gas is going through the purification column 4 and is compressed in the absorber-compressor 5 operating at LN$_2$ temperature. After that the protium gas is filled into vessel 6 under $\sim$35 atm. Vessel 6 with 10 l volume permits to accumulate protium gas under $\sim$100-120 atm that is required for one filling of the TPC detector volume of 400 STP-liters. After several hours of Whatman generator training, we checked the chemical purity of the first protium sample with our chromatograph unit. The purity was: $O_2\sim$1 ppm; $N_2\sim 0.3$ ppm; $H_2O\sim 3$ ppm. After cleaning this gas in our purification system, the purity of the protium gas became better than $10^{-8}$. The sample N1 of the protium gas, taken in fall 1999 was analyzed with a special mass spectrometer in St. Petersburg. The measured concentration of HD molecules in protium was $24\pm 2$ ppm, that means that the D$_2$ admixture is on a level of $\sim 12$ ppm. The sample N2 prepared in April 2000 showed the level of D$_2$ admixture of 6$\pm$0.5 ppm. The last sample N3 prepared in December 2000 after production of 250 l protium is presently being analyzed. We expect that after sufficient training the pollution of protium by the gas system will be completely eliminated.