Measurement of the average polarization of /sup 12/B produced by polarized muons in the mu /sup -/+/sup 12/C rarr nu /sub mu /+/sup 12/B reaction

The average polarization of /sup 12/B produced by the capture of polarized muons in /sup 12/C has been measured using recoil implantation techniques. From the result the average polarization of the /sup 12/B ground state /sup 12/B(0) corresponding to the /sup 12/C rarr /sup 12/B(0) Gamow Teller reaction is deduced: /b J//sub mu /(0)=0.43+/-0.10. The sizeable deviation of this polarization from the value of 2/3, characteristic of a `bare' 0 /sup +/ rarr 1/sup +/ process, is a fair evidence for induced axial-vector interaction(s) in muon capture. The ratio of the induced pseudoscalar and the axial-vector coupling constants is deduced to be: /b g//sub P//sup mu ///b g//sub A//sup mu /=12+/-5.