The Greenglass Mechanism

Some atom-bomb secrets came out--or partly out--last week. During the Manhattan spy trial (see NATIONAL AFFAIRS), confessed spy David Greenglass, who worked as a machine-shop foreman at Los Alamos, described sketchily the mechanism of the A-bomb used at Nagasaki. His testimony was not transcribed. But it was not suppressed entirely. The spies on trial could not be convicted without proof that they had given real and vital secrets to the Russians.

Greenglass is no scientist (at Brooklyn Polytechnic Institute he flunked eight courses out of eight), and some of his testimony made little scientific sense. He did reveal, however, the important fact that the bomb was set off by an "implosion"; i.e., an explosion that directs much of its force inwards (see diagram).

Explosive Lenses. According to Greenglass, the plutonium in the bomb was surrounded by 36 "lenses" of some such high explosive as TNT. Such lenses are roughly analogous to the shaped charges in bazooka projectiles; when they explode, they focus much of their force in one direction. If they are arranged in a spherical shell around the plutonium, their explosion will force it toward a common center.

The general principle of an atom bomb lias been no secret. The "fissionable material" (plutonium or uranium 235) explodes spontaneously if enough of it is brought together to form a "critical mass." Smaller masses do not explode, so a bomb is made with two or more "subcritical masses." To bring about an efficient explosion, these must be slammed together as quickly as possible.

Greenglass said that the plutonium was in the form of a sphere. He may have meant a hollow sphere, but more probably he meant that the plutonium was cut into small pointed chunks that would form a sphere of more than critical mass when pushed together by the "implosion." In the unexploded bomb, the pieces were probably separated just enough to keep them from acting as a critical mass.

Neutron Source. Another interesting detail was vaguely described by Greenglass. In the center of the bomb, he said, was a beryllium sphere that provided a source of neutrons to make the plutonium explode more suddenly. He may have had in mind a mixture of beryllium and radium, the usual laboratory source of small numbers of neutrons. When bombarded by alpha-particles from radium, beryllium releases neutrons and turns into ordinary carbon. But he may have been right in saying that the central sphere was made of, pure beryllium. Plutonium itself emits alpha-particles, which might knock useful neutrons out of the beryllium.

Greenglass' bomb is not necessarily up to date, complete or accurate, but some of the information he spilled is plausible enough. His is the only description of the bomb mechanism that has reached the public since the famous Smyth Report of 1945.

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