regenerator it receives a kick. With each kick the beam
builds up its radial amplitude, until finally it enters a magnetic
channel. This channel focuses the beam and steers it outside the main
magnetic field. Once outside, the beam travels through an evacuated
tube, which is integral with the main vacuum tank. By means of a
steering magnet, the beam can be sent into either the physics cave or
the medical cave. (These experimental areas are called "caves" because
they are rooms inside the massive concrete shielding wall.)

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 x 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.]

A typical [pi]-meson experiment is represente