5A: 1RONNOAL«ARRES.”INTIceONAD
Thus, while the “Atomic Age", in terms of military use, teenticaliy Cega. «wi. w ine Lest of a plutonium (Pu
239
) bomb at Alamagordo,
sew »Nexico on July 16, 1945, it was officially and publicly berun
in August when,in the first use of atomic devices against civilian
populations, roughly 50 pounds of fissionable material killed over
100,000 men, women and children and injured another 100,000 more
at Hiroshima and Nagasaki in just a few minutes.
At 2:45 a.m.
local time on August 5, 1945 the B-29 Enola
Gay lumbered off the runway on the Micronesian island of Tinian,
now a part of the Mariana Islands District of the Trust Territory
of the Pacific Islands.
In the cold, metal belly of the plane
was the "Little Boy,'’ 1 uranium bomb, so called because of its
smaller size in comparison to the larger size of the "Fat Man"
plutonium bomb.
The firing mechanism of this "Little Boy" bomb
was deceptively simple.
gun barrel.
235
(U
+).
The heart of the device was a special
At one end was a "bullet" made of refined uranium
»«=At the other end was the "target" a large lump of U
estimated to be about 5 kilograms,
or 11 pounds.
235
As long as the
"bullet" and the "target'' remained apart, nothing could happen.
But when the bullet was fired into the target, the amount of
U
235
occupying the same place became critical
*
and the terribly
destructive energy of the tiny atom was unleashed.
At 08:15 on that day over Hiroshima, the Enola Gay's bomb
"Critical" means that the amount of material was large enough
to automatically cause a violent chain reaction.
This is one of the
unique qualities of radioactive material.
Such reactions cannot
take place in nature in unrefined uranium ore, but after refinement,
are possible.
44
tel .