A combined polarized target/ionization chamber for measuring the spin dependence of nuclear muon capture in laser polarized muonic ³He

We describe a device used in experiment 1231 at LAMPF and in e683 at TRIUMF to study the spin dependence of the reaction μ^- + ³He → ³H + v_μ. In order to study the spin dependence, we needed to form and polarize muonic ³He and in the same volume detect the 1.9 MeV tritons created in the reaction. Furthermore, the recoil direction of the tritons had to be determined. The apparatus served both as a polarized target and as a detector. The detector, a gridded ion chamber, was incorporated inside a 51 polarized target that was filled with 8 atm of ³He and 100 Torr of N₂ and rubidium metal. At the operating temperature of the device (205-230°C) the rubidium number density was approximately 10¹⁵ atoms/cm³. Muons that stopped in the target formed muonic helium atoms, which were then polarized by collisions with optically pumped Rb vapor. Two high-powered GaAlAs diode laser arrays were used to polarize the Rb in the 100 cm³ fiducial volume located inside the ion chamber. The ion chamber produced clean signals from the reaction tritons despite having to operate under the extreme conditions required for efficient optical pumping. The direction of the tritons was determined by analyzing the shapes of the ionization pulses. The muon polarization was measured using the decay electron asymmetry.