Measurement of the Low-Energy End of the ${\mu }^{+}$ Decay Spectrum

The ${\mu }^{+}$ 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 ${\mathrm{Bi}}^{207}$ source. When the measured distribution was fitted to the Michel spectrum, the low-energy parameter $\eta$ was found to be -0.13±0.20, with $\rho$ constrained to be $\frac{3}{4}$. A two-parameter fit incorporating published data from spark-chamber measurements of the upper half of the spectrum gave the values $\eta =-0.12\mathrm{}\pm 0.21$ and $\rho =0.752\mathrm{}\pm 0.003$.