d in Fig. 9. The purpose
of this experiment was to detect the spin directions of protons as they
are knocked out of a liquid hydrogen target by a [pi]-meson beam. (Like
the earth, a proton spins on its axis.) An extracted proton beam from
the cyclotron enters the physics cave from the left, striking a
polyethylene target and producing [pi] mesons. A beam of these mesons is
formed by a series of two bending magnets and three focusing magnets.
This beam passes through a carbon absorber to remove unwanted particles.
The meson beam then strikes the liquid hydrogen target. A few of the
incoming mesons scatter, knocking protons out of the liquid hydrogen.
Scintillation counters at A and B record the passage of a proton, thus
defining its direction. The scattered mesons are counted by a
scintillation counter at C. A few of the protons scatter off the carbon
target and are detected by counters at E and D. From the detection of
such events, the spin directions (polarization) of the recoil protons
can be analyzed. In this way, more is learned about the fundamental
[pi]-proton interaction.

Further studies of the interactions of [pi] mesons are made in the meson
cave. Other experiments performed there are concerned with [mu] mesons.
The [mu] meson (muon) is a particle created in the decay of a [pi] meson
and is the principal constituent of cosmic rays striking the surface of
the earth. The muon is unstable, eventually undergoing a radioactive
decay into an electron. Although the muon does not experience nuclear
forces, it can interact weakly with nuclei. The behavior of the muon is
well understood, but its role as one of the elementary particles is
unknown. That is, if the muon did not exist, what effect would this have
on the structure of matter? The answer to this question, among others,
is being sought by physicists using the 184-inch cyclotron.

_Biophysics_

Experiments in biophysics are conducted in the medical cave. In these
the interest lies not in nuclear interactions but in the effect of
ionizing radiation on living tissue. High-energy beams of particles can
be used for selective destruction of specific areas of the brain. This
permits physiological mapping of the functions of the brain in
experimental animals. It further offers a therapeutic approach to the
treatment of brain tumors. One of the important investigational programs
is concerned with the relationship of the pituitary gland to the growth
rate of certain