MEASUREMENT OF LOW-ENERGY END OF MU+ DECAY SPECTRUM

The μ+ decay spectrum has been measured below 6.8 MeV/c using a hydrogen bubble chamber. The chamber was calibrated as an electron spectrometer using the 0.9- and the 1.4-MeV/c internal conversion lines from a Bi207 source. When the measured distribution was fitted to the Michel spectrum, the low-energy parameter η was found to be -0.13±0.20, with ρ constrained to be 34. A two-parameter fit incorporating published data from spark-chamber measurements of the upper half of the spectrum gave the values η=-0.12±0.21 and ρ=0.752±0.003. © 1969 The American Physical Society.