A high-pressure hydrogen time projection chamber for the MuCap experiment

TL;DR

This paper details the design and performance of a high-pressure hydrogen time projection chamber used in the MuCap experiment to precisely measure muon capture rates and monitor gas purity.

Contribution

It introduces a novel high-pressure hydrogen TPC for muon capture measurement and impurity detection in the MuCap experiment.

Findings

Successful operation of the TPC at 10 bar pressure

Precise measurement of muon lifetime and capture rate

Effective impurity monitoring through nuclear recoil detection

Abstract

The MuCap experiment at the Paul Scherrer Institute performed a high-precision measurement of the rate of the basic electroweak process of nuclear muon capture by the proton, $\mu^- + p \rightarrow n + \nu_\mu$. The experimental approach was based on the use of a time projection chamber (TPC) that operated in pure hydrogen gas at a pressure of 10 bar and functioned as an active muon stopping target. The TPC detected the tracks of individual muon arrivals in three dimensions, while the trajectories of outgoing decay (Michel) electrons were measured by two surrounding wire chambers and a plastic scintillation hodoscope. The muon and electron detectors together enabled a precise measurement of the $\mu p$ atom's lifetime, from which the nuclear muon capture rate was deduced. The TPC was also used to monitor the purity of the hydrogen gas by detecting the nuclear recoils that follow muon capture by elemental impurities. This paper describes the TPC design and performance in detail.