The gamma-ray spectrum of the reaction ${\pi }^{-}+p\to n+\gamma$ has been remeasured with an improved design of the high-energy pair spectrometer. This design has taken advantage of one of the focusing properties of a 90-degree wedge-shaped magnetic field to minimize the effect on the resolving power of multiple scattering of the pair fragments in the converter. The theory of the spectrometer is developed in detail. The accuracy of the energy scale depends on magnetic field measurements and the calculation of orbits, aberrations, and resolving power. By the determination of the mesonic absorption gamma-ray energy, a precise mass value for the negative pion has been found: ${M_{\pi }}^{-}=272.7\mathrm{}\pm 0.3m_e.$

From the ${\pi }^{-}-{\pi }^0$ mass differences already obtained by Panofsky, Aamodt, and Hadley (PAH), by Chinowsky, Sachs, and Steinberger (CSS), and the ${\pi }^{-}-{\mu }^{-}$ mass difference obtained by Lederman, Tinlot, and Booth (LTB), it is possible to improve the mass values for the ${\pi }^0$ and the ${\mu }^{-}$ mesons. $\mathrm{From} {\pi }^{-}-{\pi }^0 \left(\mathrm{PAH}\right): {M_{\pi }}^0=262.2\mathrm{}\pm 2.0m_e;$ $\mathrm{from} {\pi }^{-}-{\pi }^0 \left(\mathrm{CSS}\right): {M_{\pi }}^0=263.9\mathrm{}\pm 0.7m_e;$ $\mathrm{from} {\pi }^{-}-{\mu }^{-} \left(\mathrm{LTB}\right): {M_{\mu }}^{-}=206.7\mathrm{}\pm 3.0m_e.$ If one assumes that the positive and negative pions have the same mass, the mass of the positive muon can also be deduced from the work of Birnbaum, Smith, and Barkas: ${M_{\mu }}^{+}=206.3\mathrm{}\pm 0.3 m_e.$

• Received 9 March 1954

DOI:https://doi.org/10.1103/PhysRev.96.470