A Circulating Hydrogen Ultra-High Purification System for the MuCap Experiment

TL;DR

This paper describes the design, implementation, and successful operation of a circulating hydrogen purification system that maintains ultra-high purity levels for the MuCap experiment's time projection chamber, enabling precise muon capture measurements.

Contribution

The paper introduces a novel circulating hydrogen purification system with cryopump and cold adsorbent, achieving stable ultra-high purity for muon capture experiments.

Findings

Hydrogen purity maintained at 20 ppb for moisture

Nitrogen and oxygen impurities kept below 7 ppb and 5 ppb

Stable pressure within 0.024% at 10 bar

Abstract

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.