Nut
atoms being split.
'
According to Dr. Louis N. Rid-
fully nidden and smuggled an absolute impossibility.
Of course, the bombs couid be
smuggled into a country in ships
and exploded in the harbor, but
they could be rather easily dis-
enour, dean of the Graduate College of the University of Illinois,
“no amount of ingenuity has yet
allowed the design of an efficient
‘Small’ Bomb Found Impossible
One of the items that has been
rounding the system with a suitable neutron reflector, which reduces the loss of neutrons that
escape from the surface, the re-
causing alarm among the public
is
the
recent
talk
about
‘‘small
atomic bombs.”
It is Known that one kilogram
of the explosive material used in
duction is not very great.
The inexorable requirement for
a minimum amount of material
also puts an upper limit on the
amount of material that can he
used. This is because anything
the bomb, Uranium 235 (U-235) or
plutonium, is equivalent in energy
to 20,000 tons of TNT, a ratio of
20,000,000 to 1. So the talk about
a “small” atomic bomb is leading
to the dangerous impression that
above the critical mass would
explode spontaneously.
Let us assume, for purposes of
illustration, that the critical mass,
such an object or objects weighing
a few pounds each, could be easily
smuggled into the country and
exploded simultaneously in most,
the actual amount of which is top
secret, is ten kilograms of either
U-235 or plutonium. This would
mean that anything below this
if not all, of our important cities.
Nothing can be further from the
truth. In the handbook, the Amer-
amount
keep
ican people are given the first official assurance, without any qualifications, that “a ‘small’ effective
atomic bomb cannot be made.”
There is a sound technical reason for this. An atomic bomb is
exploded with neutrons-—atomic
bullets released when atoms of
escaping,
no
to
from
explosion
is brought together, the neutrons
would start multiplying automatically and an explosion would take
place in less than a millionth of
a second.
Henct, as explained yesterday,
to explode an atomic bomb, two
pieces, say, of one and nine kilo-
lion in less than a millionth of a
second.
Since these neutrons travel with
speeds of more than 10,000 miles a
grams, respectively, are made to
come together by a timed trigger
mechanism after the bomb is released.
second and can penetrate any sub-
Of course, the assembly could
consist of two pieces of nine kilo-
stance up to a certain thickness,
the only way to keep them from
escaping to the outside in numbers
large enough to stop the chain reaction, which would prevent the
grams each, and, theoretically, it
should be possible to
bring to-
gether several such pieces, thus
producing a much larger explosion.
explosion, is to have a quantity of
However, it must be kept in mind
that such a large assembly would
have to be brought together in
material
that the neutrons in the interior
will be unable to penetrate.
less than a millionth of a second,
The Critical Size
after which the bomb flies apart.
Hence, should two pieces join,
This quantity is known as the
critical mass, or critical size, and
while the others lag a fraction of
a millionth of a second behind,
only the two pieces that came to-
puts a definite lower limit below
explosion
that
small
a quantity totaling ten kilograms
incredible rate of two billion tril-
which no atomic
take place.
so
too
neutrons
On the other hand, as soon as
trons multiply themselves at the
(fissionable)
be
many
could ever take place.
U-235 or plutonium are split in a
chain reaction in which the neu-
explosive
would
too
can
gether first would explode while
the others would merely go off as
hot vapor, without any of their
While the critical size can be
diminished to some extent by suri0
covered
fission bomb so much as two or
three times critical size.”
Now if the atomic bomb consisted only of its explosive material, it would be very small in‘nominal atomic bomb” used on
Japan exploded with a power equal
to 20,000 tons of TNT, which is the
Matter of Timing
Not a Larger Bomb
To design a more powerful bomb
does not mean the building of a
bomb larger in size, or one con+
taining a large amount of fissionable material. What it calls for is
to improve the efficiency of the
explosion.
For example, if our hypothetical
critical mass of ten kilograms ex-
grams.
plodes with an efficiency of 10 per
cent we would have a bomb equal
in power to 20,000 tons of TNT. If
However, to bring about an efficient explosion, in other words, to
keep the bomb assembly frem fly-
ing apart at a stage when the
number of atoms split equal only
a few blockbusters, and to main-
assembly
long
for
the suspected ship before it is allowed to dock.
Even assuming that the efficiency of the explosion was no
more than 1 per cent, the critical
mass would be No more than 100
kilograms (220 pounds), while if
the efficiency was 10 per cent, the
amount
of
explosive
material
would be no more than 10 kilo-
the
measures
amounts of radioactivity, to make
possible their detection by means
of Geiger counters. This makes
necessary a thorough search of
total power of just one kilogram
{2.2 pounds) of U-235 or pluto-
tain
proper
It must be mentioned in passing
that an unexploded atomic bomb
does not give off any sizable
deed. We know that the so-called
nium.
if
searching suspicious vessels were
instituted.
the efficiency of the explosion were
improved to 20 per cent we would
have a bomb equal] to 40,000 tons.
enough
To improve efficiency would thus
mean improved auxiliary mecha-
until the amount of energy released equals 2,000 blockbusters,
nisms, such as a better tamper that
would make possible the mainte-
very intricate and heavy parts are
necessary.
nance of the bomb’s assembly for
One of these, of course, is 2
heavy shell, since a light shell
would disintegrate too quickly. An-
a few fractions of a millionth of a
second longer before it flew apart,
thus making possible the splitting
of many more atoms.
This is the
incredible thing
about the explosion of an atomic
bomb, the release of such an enor-
other is what is known as a tam-
per, which must be made of a substance of very high density. Then
there are the manyintricate de-
Vices to bring the assembly together at the instant of the ex-
mous amount of energy in such an
unbelievably short time. It takes
less than a millionth of a second
to split one gram of fissionable
Plosion.
All these auxiliary parts of the
bomb weigh many times more
than the explosive material, so
much so that it required the bomb
materia!,
equal
to
only
twenty
tons of TNT, It takes only about
a tenth of a millionth of a second
bay of a B-29 to carry it. And
the fact that the bomb needs
large amounts of material of high
longer to split a kilogram of the
fissionable
material,
equal
to
20,000 tons.
By holding it together for an-
density to Serve as a tamper and
as a reflector of neutrons, together with the other intricate
other fraction of a millionth of a
second, two kilograms, equal to
40,000 tons of TNT, would be split.
It is thus an incredible race
mechanisms to make certain that
the explosion is at the highest possible efficiency, makes a “small”
atomic bomb than can be success-
against
time,
measured
in
frac-
tions of millionths of a second.
Il