Evening star., 1953-07-20
.Mu Meson Is 'Tumed'
In Powerful CycIotron
Shows Nucleus of Atom
Is Smaller Thdn Believed
By Thomos R. Henry

The super-elusive mu meson is being
caught and tanned.
This infiniteStimally minute funda-
mental particle of matter can serve as
a probe into the heart Of the atom,
Columbia University physicists working
under sponsorship of the once Of
Naval Research have found.
When the nucleus of an atom is split
in two-as happens when it is hit by a
coSmic ray in the upper atmosphere, it
emitS particle known as the pi meSon.
This is part of its incalculably great
binding energy, the most StupendouS
force in creation, transformed for an
instant into matter. This pi meson has
less than a billionth of a second Of
existence. It splits up into two other
particles, each about $50 times the
mass of an electron, which have an
existence of two millionths of a second.
Then they revert into maSsless energy
again. These are the mu mesons.
Produced in Big Cyclotron.
Both pi and mu mesons can be pro-
duced by bombarding atoms with pro-
tons or neutronS in superpowerful
cyclotron, such as that operated by DR..
James A. Rainwater. Columbia Univer-
sity professor of physics. and main
tanned in part by the Navy and Atomic
Energy Commission.
Now and then one of these mesons
gets into the orbit of an atom-accept-
ing the classical picture of an atom
as a group of light electrons moving
around heavy nucleus as planets move
around the sun. It becomes for an
enthusiastically brief instant another
planet-but $50 times heavier than any
of the others. 1t is able to move seven
or eight miles in its orbit during its
lifetime.
Because of it's great weight, it moves
in an orbit much closer 20 the nucleus
than any of the other atomic planets
and finally it is pulled into the nucleus
and absorbed as energy. When it does
so very powerful X-rays. running up to
six million electron volts. are emitted.
Nucleus Smaller Than Believed.
Now, from observations of what hap
pens, Dr.. Rainwater and his associate,
DR.. Val Fitch, have been able to con-
struct strikingly new picture of the
nucleus itself, the most fundamental
particle of matter in creation. For one
thing, they have determined that it is
about Is per cent smaller than has
been calculated in the past. 1t is much
smaller than the planetary electronS,
although it weighs many times as much
as all of them put together. In the
ordinary picture of the atom, electrons
can be compared to big rubber balls
filled with air whirling around a ball
of lead.
Up to now this nucleus has been pic
tured as solid hard particle, of uni
forpa density throughout, like lead
shot. The Columbia experiments with
mesons indicate, however, that this
ball had no definite boundary but
grows more and more tenuous towards
its outer edges.
Thus the comparison with the sun.
with its layers of increasing tenuous
atmosphere, is better established. The
vagueneSs and tenuousneSs of the shell
Of the nucleus can be calculated from
. the voltage of X-radiation given off
when the mu neson is pulled in.
=" Bound by Terrific Forces.
The nucleus is made up, it is gen.
orally agreed, of protons, positively
charged particles, and neutrons, parti-
cles with no electric charge, bound to-
gether by terrific forces. It's radius
has been calculated at a few tenths
of trillionths of an inch, its size vary
ing with the weight of the element
The new findings, made possible by
using the mu mesons, make it neceS-
sary, Prof. Rainwater believes, to alter
the accepted mental picture of the
nucleus. It can, of course, never be
anything but a. mental picture any,
where. The idea of the uniform hard
particle apparently must be discarded.
There are, he points out, various other
alternatives, but they are much more
disruptive to modern physical concepts.