Other experiments may require an external beam of mesons.[7] A meson beam is obtained in the following way (Fig. 8): A movable target such as a block of carbon is placed inside the cyclotron near the end of the outward-spiraling proton beam. When the proton beam hits this target, a shower of mesons is produced. These mesons are bent in various directions by the main magnetic field. Some of them pass through a thin metal window in the vacuum-tank wall and are focused by a magnetic lens into a beam. This meson beam then travels through a hole in the concrete shielding wall into the meson cave. The maximum intensity of this extracted meson beam depends on both the charge and energy desired. Beams of more than 100,000 mesons per second have been obtained through an aperture 4 × 4 in. in the shielding wall. CYCLOTRON EXPERIMENTS _Nuclear Physics_ About 86% of the operating time of the 184-inch synchrocyclotron is devoted to experiments in nuclear physics. Most of the experiments study the production and interaction of [pi] mesons. These particles are considered to be essential factors in the intense but short-range forces that bind the nucleus together. The three types of [pi] mesons are designated according to their electric charge as [pi]^+, [pi]^0, and [pi]^-.[8] These mesons materialize only in high-energy nuclear collisions. [Illustration: Fig. 8. Method for obtaining external meson beam.] Of great importance are those experiments that determine the probability of producing each of the three types of mesons in a nuclear collision. This type of experiment is repeated for different beam energies and target elements. Other experiments measure the energy and direction of emission of [pi] mesons from a target. [Illustration: Fig. 9. A typical experiment. Scintillation counters at A, B, C, D, and E record the passage of charged particles.]