Sea-Level Mesons Stopped in Thin Pb and Al Foils. II. Low-Energy (1-5 Mev) Gamma Rays

Low-energy gamma rays (1-5 Mev) associated with the stopped negative $\mu$ mesons, similar to those reported before, have been observed also in these experiments. Twenty-five meson-oriented electrons and pairs have been found in Pb associated with 23 of the 81 stopped negative $\mu$ mesons. They are "meson-oriented" in the sense that the single electron track (or apex of the pair) makes an angle less than ±20° with the imaginary line running back to the end point where the meson is stopped. None of such events has been obtained from Al. Of the 25 directed events, 4 are single electrons emitted directly from the end point of the stopped negative $\mu$ mesons, one being fairly energetic and the other three rather slow. The other 21 are single electrons and pairs observed in each case at a foil different from that where the meson stops and are meson-oriented in the above sense. The reality of this angular correlation follows from the fact that the 81 photographs of negative mesons stopped in Pb show only 6 electron tracks which make an angle of 90°±20° with the line back to the end point of the meson track ($\frac{6}{21}$ as many tracks for 6 times as much solid angle). An independent determination of the rate of the strays was made by examining a film of 575 pictures to see how many tracks point by chance toward or away from an arbitrary point in the chamber with a 20° tolerance. From both ways of counting accidentals it is concluded that, of the 21 meson oriented events, at most about $\frac{1}{5}$ can be explained as stray tracks. A crude analysis of the electron range in the Pb foils indicates that the majority of the "oriented electrons" and "oriented pairs" may be interpreted as having an energy of about 5 Mev and the rest an energy probably below 3 Mev. The multiplicity of the photons producing these electrons has also been estimated, after a correction has been made for the multiple scattering of the electrons in the first foil, where they are emitted, so as to include all the electrons other than those counted within the cone (±20°) in the forward direction of the photons. The value for the multiplicity obtained is about 4·5 per negative meson stopped. The energy and frequency of occurrence of these photons are thus consistent with that predicted in Wheeler's theory of meson transition between the Bohr orbits and with that expected from nuclear de-excitation following the $\mu$-meson capture.