Evening star., 1948-11-08
Stars, Men and Atoms .s4a<
Distance Factor Studied
In Creating Atom Energy
New Term in Physics-'Barn'-
Describes Measurement
By Thomas r. Henry

"
There is new term in physics-the barn..
of square centimeter.
1t has been named for the popular expires.
sion. big as a barn door"-because It is so
small .. -.
Future students will have to know all about
the barn." because 1t is O major factor in
the production Of utilizable atomic power: 1t
Is intended as a measure of the area within
which neutron can be Captured" by tie
nucleus OF an atom-and thus bring about
the splitting Of that atom with the emission
OF more neutrons and the splitting of nearby
atoms which they pass within barn" dis
tances. Much Of the major research in atomic
physics today consists in determining barns"
The number Of barns" within which neu-
tron capture will take place depends on the
speed OF the tiny non-electrihed particle the
neutron elects from the atomic nucleus When
II comes within barn dimensions it is drawn
towards the nucleus with a force estimated
at more than a billion billion times that of
gravity. 1t is moving very rapidly-at
speed OF around 10000 miles a second-it
must pass very close indeed to be drawn into
the nucleus.
Slowing Down Is Problem.
II 1t Is slowed down to about a mile n
second the probability of capture is greatly
increased.
This slowing down so that there can be a
greater barn allowance is one of the major
problems confronting those who are irving
TO devise thermal reactor-the most favored
OF the two types of atomic ovens now being
considered for the production of atomic power
The other type is the fast reactor" which
is about the same thing as an atomic bomb
slowed down by various means so that its
energy Is expended over days and hours
instead OF minute fraction of second. It
dimculties OF the order OF this measure
ment~ In barns" which give gray hairs to
nuclear engineers. These problems are COM
mon to those who are trying to devise better
atomic bombs
Among the recent calculations is that to
produce 1000 kilowats from little more than
two grams of U 235, the only naturally ocur.
ring atomic fuel. 1000 900 900 900 atomic ex.
plosions per cubic centimeter per second ara
required.
1t now Is familiar statement that everv
atomic fission releases energy equivalent to
200,00Oy0O0 electron volts This amounts to a
$500 00O0,000th of q kilowat in conventional
power terms that have any meaning. 1t has
been calculated that about u per cent of
this is a complete loss, since it is in the form
oA InfLsluie, neVer-QeCecCed particles known
as neutrinos, the existence of which is only
postulated. Approximately S3 per cent is
turned into heat in the immediate neigh-
borhood-that is, within a barn or so of the
exploding atom. The amount of heat produced
is, for all practical purposes, limited only
by the materials which can withstand it.
Fuel Source Needed
Atomic power, it generally is agreed, never
Will be OF any great practical Importance un-
less there can be an efficient method of breed
ing" fuel. There are three possible atomic fuels
Uranium 235 which is about seven n-tenths of
one percent Of the quite limited supply of uran-
ium found in nature plutonium, which is
manufactured by neutron bombardment of
Uranium 23s. the most prevalent form, and
Uranium 233, which Is manufactured from
another radioactive element, thorium. ,r.1
Scientists Of the Atomic Energy Commission
claim that 1t now is theoretically possible to
build reactor in which Uranium 23s will be
converted into plutonium by neutron bombard-
ment associated with the normal activity of
power production. I'm practice, however" the
latest report says 1T will be very difncult"
But unless this can be done, atomic power.
except for very special purposes, is only disc
tant prospect.