  The MuCap experiment is a high-precision measurement of the rate for the
basic electroweak process of muon capture, mu- + p -> n + nu . The experimental
approach is based on an active target consisting of a time projection chamber
(TPC) operating with pure hydrogen gas. The hydrogen has to be kept extremely
pure and at a stable pressure. A Circulating Hydrogen Ultrahigh Purification
System was designed at the Petersburg Nuclear Physics Institute (PNPI) to
continuously clean the hydrogen from impurities. The system is based on an
adsorption cryopump to stimulate the hydrogen flow and on a cold adsorbent for
the hydrogen cleaning. It was installed at the Paul Scherrer Institute (PSI) in
2004 and performed reliably during three experiment runs. During several months
long operating periods the system maintained the hydrogen purity in the
detector on the level of 20 ppb for moisture, which is the main contaminant,
and of better than 7 ppb and 5 ppb for nitrogen and oxygen, respectively. The
pressure inside the TPC was stabilized to within 0.024% of 10 bar at a hydrogen
flow rate of 3 standard liters per minute.
