Production of a $\pi^+$-Meson Beam at the Phasotron

When high-energy protons interact with a hydrogen target, a very intense production of $\pi^+$ mesons is observed, with a spectrum concentrated (at an angle of $0^\circ$ to the incident beam) in a narrow region of about $70$ MeV*.

This fact was used to obtain a beam of $\pi^+$ mesons at a phasotron accelerating protons to an energy of $340$ MeV**.

Protons of the indicated energy were directed onto a 5-cm polyethylene target. Owing to ionization losses, the energy of the mesons, equal (at a proton energy of $340$ MeV) to $69$ MeV, decreased after passing through the target to $54$ MeV. On the other hand, the protons that had passed through the target had an energy of $323$ MeV, which corresponds to an energy of the mesons produced by such protons, again equal to $54$ MeV. Thus, mesons were produced in a comparatively narrow energy interval. The spectrum of mesons produced in pp interaction in thin ($0.24$ cm) and thick ($5$ cm) polyethylene targets, for an initial proton energy of $340$ MeV, is shown in the figure.

Curve 1—thin target, curve 2—thick target.

The extracted proton beam ($5\cdot 10^{-10}$ A) was collimated to a diameter of $5$ cm and struck a target placed in a magnetic field. The purpose of the magnetic field was to separate the protons and $\pi^+$ mesons moving in the direction of the primary proton beam. At a field strength of $14\,300$ gauss, for mesons with an energy of $54$ MeV $\rho = 31$ cm, while for protons with an energy of $340$ MeV $\rho = 200$ cm.

To further eliminate side effects, the meson beam deflected in the magnetic field was passed through a channel in brass shielding ($5 \times 6.3$ cm), the deflection from the initial direction reaching $85^\circ$.

The production cross section of $\pi^+$ mesons in the direction of the primary beam was found to be $4\cdot 10^{-28}\ \mathrm{cm^2/steradian}$. With the proton current indicated above, approximately $5000$ mesons (with energies from $48$ to $60$ MeV) were extracted from the channel per second.

G. I.

* UFN 42, 571 (1950).
** C. Richman, M. Skinner et al., Phys. Rev. 80, 909 (1950).