Dependence of $\pi^+$-Meson Production on Atomic Number

The production of $\pi^+$-mesons in the scattering of protons by nuclei is of considerable interest, since the regularities revealed in this process can shed light both on the interaction of mesons with nucleons and on the structure of nuclei and nuclear forces. One of the few works in this area is the note under review,$^1$ in which the relative effective cross section for the production of $\pi^+$-mesons by protons was studied.

with an energy of 340 MeV in their scattering by nuclei of carbon, aluminum, iron, copper, silver, and lead. In this experiment, $\pi^+$ mesons with an energy of $53 \pm 4$ MeV at an angle of $0^\circ \pm 7^\circ$ to the proton beam were investigated. In order to distinguish $\pi^+$ mesons from protons, the mesons were deflected by a magnetic field by approximately $90^\circ$, after which they were registered with scintillation counters (Fig. 1).

In Fig. 2 the relative effective cross section is shown (the indicated error corresponds to statistical deviations). The results obtained—

Fig. 1.

Fig. 2.

—are in agreement with the differential cross section for the production of mesons with an energy of 20 MeV at an angle of $150^\circ \pm 15^\circ$ by protons with an energy of 240 MeV.^2 The effective cross section, referred to one particle of the nucleus, is a decreasing function of the number of nucleons in the nucleus, similar to that obtained in the photoproduction of $\pi^+$ mesons.^3 This dependence is in agreement with a theoretical prediction based on an analysis of the absorption of $\pi$ mesons in nuclear matter.^4

V. S.

References

1. D. Hamlin, M. Jakobson, J. Merritt and T. Schulz, Phys. Rev. 27, 875 (1951).
2. D. Cork, Phys. Rev. 81, 313 (1951).
3. R. F. Mozeley, Phys. Rev. 80, 493 (1950).
4. Brueckner, Serber and Watson, Phys. Rev. 84, 258 (1951).