PRODUCTION OF NEUTRAL MESONS IN THE CAPTURE OF $\pi^-$ MESONS IN HYDROGEN[^1]

The observed experimental[^2] spectrum of photons produced as a result of the interaction of $\pi^-$ mesons with hydrogen contains two groups of $\gamma$ rays, concentrated near 130 MeV and 70 MeV. The analysis of the experiment and of the conclusions that can be drawn from it is based on the following assumptions: a) that the $\gamma$ rays are produced by mesons, b) that these mesons come to rest, and c) that the low-energy $\gamma$ rays (70 MeV) are products of the decay of a neutral meson. In the present work these assumptions are experimentally confirmed for the group of low-energy $\gamma$ rays. The experimental arrangement is shown in Fig. 1.

Fig. 1. Experimental arrangement.

Mesons generated in the beryllium target were deflected in a magnetic field and directed into a balloon filled with hydrogen to a pressure of 1.75 atm at 80° K. The intensity of the meson beam was measured by means of two crystal counters 1 and 2, 2.5 inches in diameter, placed on the beam axis. The mesons were slowed from an initial energy of 80 MeV in absorber A and in balloon B. On both sides of the balloon, behind lead plates half a radiation unit thick, four counters (3–6) were placed.

Fig. 2. Counting rate of $\gamma$-$\gamma$ coincidences as a function of absorber thickness.

Legend: $\gamma$-$\gamma$ coincidences; differential range of the meson.

included in the coincidence circuit. Coincidences were measured with and without lead plates, for different absorber thicknesses. The counting rate without the lead plates could be neglected. This indicated that the coincidences were caused by electrons knocked out of the lead plates by $\gamma$ rays produced as a result of the decay of the neutral meson. The number of counts in the case of filling the balloons with helium was negligibly small in comparison with the number of counts for the reaction in hydrogen; this proved that the coincidences were indeed due to the reaction in hydrogen. Finally, the change in the number of counts with absorber thickness, shown in Fig. 2, shows that the $\gamma$ rays did not stop at mesons.

Thus, a certain fraction of the negative mesons is captured into the orbits of the hydrogen nucleus, with subsequent transformation into neutral mesons.

V. F.

CITED LITERATURE

1. A. Sachs and J. Steinberger, Phys. Rev. 82, 973 (1951).
2. Panofsky, Aadmon and Hadley, Phys. Rev. 81, 565 (1951).