The MuCap experiment at the Paul Scherrer Institute has measured the rate Λ{sub S} of muon capture from the singlet state of the muonic hydrogen atom to a precision of 1%. A muon beam was stopped in ...a time projection chamber filled with 10-bar, ultrapure hydrogen gas. Cylindrical wire chambers and a segmented scintillator barrel detected electrons from muon decay. Λ{sub S} is determined from the difference between the μ{sup -} disappearance rate in hydrogen and the free muon decay rate. The result is based on the analysis of 1.2×10{sup 10} μ{sup -} decays, from which we extract the capture rate Λ{sub S} =(714.9±5.4{sub stat} ±5.1{sub syst} )s{sup -1} and derive the proton’s pseudoscalar coupling g{sub P} (q{sup 2}{sub 0} =-0.88m{sup 2}{sub μ} )=8.06±0.55 .
The rate of nuclear muon capture by the proton has been measured using a new technique based on a time projection chamber operating in ultraclean, deuterium-depleted hydrogen gas, which is key to ...avoiding uncertainties from muonic molecule formation. The capture rate from the hyperfine singlet ground state of the {mu}p atom was obtained from the difference between the {mu}{sup -} disappearance rate in hydrogen and the world average for the {mu}{sup +} decay rate, yielding {lambda}{sub S}=725.0{+-}17.4 s{sup -1}, from which the induced pseudoscalar coupling of the nucleon, g{sub P}(q{sup 2}=-0.88m{sub {mu}}{sup 2})=7.3{+-}1.1, is extracted.
The project for a precision measurement of the μp-capture rate (μCAP experiment) is based on an application of a multi-wire proportional chamber (MWPC) operating in ultra-pure hydrogen at
10
bar
...pressure. A special test setup was constructed at PNPI to investigate the MWPC performance under the expected experimental conditions. The aging studies of the MWPCs were performed with intense irradiation from an α-source
(
241
Am)
and a β-source
(
90
Sr)
. After 45 days of continuous irradiation by α-particles no changes in the currents, in the signal shapes, and in the counting rates were observed. It was demonstrated that the MWPCs can operate without degradation at least up to accumulated charges of
0.1
C/cm
wire. These irradiation conditions are much more severe than in the real experiment. During the study of the MWPC we have observed an appearance of short duration signals with amplitudes an order of magnitude larger than those of normal signals from the α-particles. The number of such signals (“streamers”) strongly depend on HV. We shall continue these tests in the future with the goal of obtaining more detailed information about aging properties of MWPCs operating with high-pressure hydrogen.
.
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,
. The experimental ...approach was based on the use of a time projection chamber (TPC) that operated in pure hydrogen gas at a pressure of 10bar 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
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.
This work was carried out as part of a project aiming at a greatly improved measurement of the muon capture rate from the singlet state of the μp atom. The experiment will be performed at the intense ...muon beam of PSI using a new experimental method allowing high precision measurements of the lifetime of muons stopped in ultra-pure deuterium-depleted hydrogen (protium). The basic element of the detector is a time projection chamber operating in hydrogen gas at
10
bar
pressure. The arrival times and trajectories of the incoming muons and the outgoing decay electrons are measured with this device providing effective suppression of background. The system of chambers and electronics is designed for the large muon stop rates required for attaining high statistical accuracy. During four beam periods at PSI, data were taken. Also, various studies of the MWPC performance in hydrogen were made including ageing studies of the chambers under irradiation with stopped muons and with alpha and beta sources. It was demonstrated that the MWPCs can operate in pure hydrogen under 10 bar pressure with gas gains up to 5000, which is sufficient for the detection of relativistic electrons.
PDF
