A cylindrical drift chamber for radiative muon capture experiments at triumf

In the Standard Model, the weak interaction is purely V-A in character. However in semileptonic reactions the strong force induces additional couplings. Radiative Muon Capture (RMC), μ-Z → ν(Z-1)γ, is a process which is particularly sensitive to the induced pseudoscalar coupling constant, gp, which is still very poorly determined experimentally. Due to the extremely small branching ratio (~6 × 10-8), the elementary reaction μ-p→ νnγ has never been measured. Effort to date has concentrated on nuclear RMC where the branching ratio is much larger, but the interpretation of these results is hindered by nuclear structure uncertainties. A measurement is being carried out at TRIUMF to determine the rate of RMC on hydrogen to a precision of 8% leading to a determination of gp with an error of 10%. The detection system is based on a large volume cylindrical drift chamber, in an axial magnetic field, acting as an e+e- pair spectrometer with a solid angle of ≃2π. At a magnetic field of 2.7 kG the acceptance for 70 MeV photons is about 0.8% for a 1.0 mm thick Pb photon converter. Montecarlo calculations indicate a photon energy resolution of ≃9% FWHM at 60 MeV and ≃13% at 129 MeV. A spectrometer resolution of 12% at 129 MeV has been measured to date. The design, construction and performance of the cylindrical drift chamber are discussed.