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article number 654
article date 04-06-2017
copyright 2017 by Author else SaltOfAmerica
All Brains Together—First to Develop the Atom Bomb, Part 9: Critical Mass & Detonation
by William Laurence, Reporter, New York Times

From the 1946 book, Dawn Over Zero.

* * *

AS early as March 1939, only about a month after the announcement of the discovery of fission in this country, Dr. Fermi went to Washington to arouse the interest of our Army and Navy in the possible use of fission for military purposes. In a conference with representatives of the Navy Department, he suggested the possibility of achieving either a controllable reaction with slow neutrons, or an uncontrolled reaction, producing an explosion of a magnitude previously undreamed of, with fast neutrons.

It was recognized that the amount of U.235 that would be necessary to make a bomb would not be very large, as in pure U.235 the principal competitor for the available neutrons—namely, U.238—would not be present, and hence most of the neutrons liberated through fission would go into producing further fissions; and since these are fast neutrons, the fissions would take place at an incredibly rapid rate, producing an explosion of cataclysmic dimensions.

However, in a mass of U.235, neutrons would be lost by escape into the outer air, and if too many of them were to be lost in this way, the combustion of the U.235 atoms—that is, fission—would die out like a fire that lacks air.

Since, as explained earlier, the production of neutrons, which is a volume effect, will increase more rapidly with size than the loss by escape into the air, which is a surface effect, this loss by escape can be reduced by increasing the size of the mass to the point at which enough neutrons will be liberated and retained inside the mass to produce the explosion.

This means that no explosion can take place unless you have a minimum mass of material, a minimum known as the critical size.

It can thus be seen that an atomic explosion is very different in its mechanism from the ordinary chemical explosion of TNT and other explosives. In these, any quantity, no matter how small, can be exploded, while no quantity, no matter how large, will explode by itself if the proper precautions are taken. In atomic material, however, the explosion can occur only if the quantity reaches this critical amount.

Quantities of the material less than the critical amount are perfectly stable and safe. On the other hand, a quantity greater than this critical amount will go off automatically by itself, and no power on earth can stop it.

I can therefore reveal to you the principal secret of the atomic bomb: all you have to do to explode an atomic bomb is to unite at great rapidity two pieces of the active material, each smaller than the critical amount, but exceeding it when brought together. As Dr. Robert Williams, one of the earliest scientists to arrive at the atomic bomb center at Los Alamos put it: “Take some fissionable material in several pieces, as pure as possible, and slap them together as quickly as possible.”

This, however, is not so simple as it sounds. A the British statement points out, if an appreciable fraction of the atoms of U.235 undergo fission within a very short time, the amount of energy liberated will be so great that the mass will attain a temperature of “many million degrees and a pressure of many millions of atmospheres.”

It will consequently expand with very great rapidity. As the density of the mass decreases, the neutrons can escape more easily from it, and the chain reaction will come to end.

In order to release an appreciable fraction of the available energy, it is therefore necessary that the reaction should develop so rapidly that a substantial part of the material can react before the system has time to fly apart. The neutrons produced in the fission process are fast enough to fulfill this condition, but not if the neutrons are artificially slowed down.

How, then, can subcritical masses of U.235 be brought together rapidly enough to avoid pre-detonation? Here is atomic bomb secret number two: shoot one part as a projectile in a gun against a second part as a target.

This, again, is not too easy, of course. As Professor Smyth points out, the projectile mass, projectile speed, and gun caliber required were not far from the range of standard ordnance practice, but novel problems were introduced by the importance of achieving sudden and perfect contact between projectile and target, by the use of a tamper to reflect escaping neutrons back into the mass, and by the requirement of portability.

Gun mechanism to inject the projectile mass of U.235 in the Little Boy bomb.

In these last two phrases are revealed more vital secrets of the atomic bomb. This requires some explanation.

The critical size of a uranium-graphite pile, as well as that of an atomic bomb, may be considerably reduced by an envelope made of a substance that reflects neutrons.

In a chain-reacting pile this envelope consists of a layer of graphite. In the case of the bomb the most effective envelope is a substance having a high density.

Such a neutron-reflecting envelope is known as a tamper. In the bomb such a tamper not only reduces the critical size, thus saving precious material, but also plays an additional part: its very inertia, as Professor Smyth points out, delays the expansion of the active substance, and makes for a longer-lasting, more energetic, and more efficient explosion.

By a fortunate coincidence, materials of high density are also excellent as reflectors for neutrons. Since gold is one of those materials, it was at one time seriously contemplated putting some of the gold hoard buried at Fort Knox to work in the atomic bomb, just as the idle silver in the Treasury was put to work in one of the plants for concentrating the U.235 for the bomb.

Here, then, is the third secret of the atomic bomb: the use of an envelope surrounding the active material as a tamper, to decrease the critical size and greatly to increase the bomb’s efficiency.

Secret number four is that this tamper is made of material of high density. Gold is such a material, if anyone wants to blow it up, which may be all that it will be good for in case of a war with atomic bombs.

Secret number five is of course easily guessed at: portability. Since the atomic bomb had to be dropped from a bomber, its weight and shape had to be within the limits of what a B-29 could carry. Since the maximum load for a B-29 from the Marianas to Japan was ten tons, it is obvious that the atomic bomb could not have a total weight of more than that maximum.

How much is the critical amount? This is still one of the great secrets, but even here an approximation within rather narrow limits can be made. In the summer of 1940 a figure of one to one hundred kilograms of U.235 was commonly given as the critical size of the bomb.

Now, President Truman has stated that the bomb that wiped out Hiroshima “had more power than 20,000 tons of TNT.” According to the figure given in the Official Report, one kilogram of U.235, if all the atoms in it underwent fission, would release energy equivalent to the explosion of 20,000 short tons of TNT.

Hence, if the efficiency of the explosion over Hiroshima was one hundred per cent, the active material in that bomb weighed no more than one kilogram (2.2 pounds). If the efficiency of the nuclear explosion was only one per cent, then the amount of the material used was one hundred kilograms.

On the other hand, the Official Report states on page 63 that by the end of 1941 “it was predicted that possibly ten per cent of the total energy might be released explosively.” On the basis of that prediction (which may or may not have proved to be true), the critical amount of the Hiroshima bomb was ten kilograms.

Suppose the critical amount was found to be 100 kilograms. Anything below that amount would therefore be perfectly stable and safe; no power on earth could make it explode. Hence we divide the 100 kilograms into two parts, let’s say, of seventy and thirty kilograms respectively. We assemble these two parts in a high-speed gun in which the heavier part will serve as the target and the lighter as the projectile. When the two are far enough apart, they are, of course, harmless.

When the time comes to explode it, a mechanism is set to make the gun go off at a certain predetermined distance from the ground. When that happens, the projectile hits the target and brings the two parts together. A critical mass is formed and there is one less city in the world. As soon as the parts join, the neutrons build up so rapidly that the explosion takes place in about one tenth of a millionth of a second.

One more secret may be cleared up. The bomb was not dropped by parachute, as is commonly believed. It was dropped free. It is true that the Japanese saw something drop by parachute and thus gave rise to the common misconception.

What they saw were instruments dropped by parachutes from the accompanying plane to make blast measurements. It was through these instruments that President Truman was able to state as a fact that the bomb “had more power than 20,000 tons of TNT.”

Fat Man bomb used a rounded clamshell arrangement to bring two portions of Plutonium together to form the critical mass.

How much does the entire assembly weigh? The exact amount is being kept secret, but it may be said that it weighs several tons and, as General Farrell has stated in a public address, “it substantially fills the bomb bay of a B-29.”

The bomb is detonated in combat at such a height above the ground as to give the maximum blast effect against structures, and to disseminate the radioactive products as a cloud.

On account of the height of the explosion, as the War Department pointed out, “practically all the radioactive products are carried upward in the ascending column of hot air and dispersed harmlessly over a wide area. Even in the New Mexico test, where the height of explosion was necessarily low, only a very small fraction of the radioactivity was deposited immediately below the bomb.”

While the energy released in the fission of all the atoms in a kilogram of U.235 is equivalent to that of 20,000 tons of TNT, this energy represents the conversion of only one thousandth of the mass—namely, one gram—into energy. The conversion of one gram of matter into energy thus produces the equivalent of 20,000 tons of TNT.

Since the bomb dropped over Hiroshima had a power of about 20,000 tons, it means that the destruction of that city was caused by the conversion into energy of only one gram of matter.

This conversion took place in one tenth of a millionth of a second, or at a rate of 10,000,000 million grams, more than 10,000 tons, per second.

As the sun, which produces its radiation by the conversion of matter into energy, is one third of a million times the mass of the earth, the sun should be converting its matter at the rate of 3,000 million tons per second. Since it does this at the rate of only 4 million tons per second, the conversion of the matter in the Hiroshima bomb was at a rate 750 times faster than the rate at which energy in the sun is created out of matter.

In that one tenth of a millionth of a second vast cosmic forces are set in motion, attained, as far as our present knowledge goes, only in the super exploding stars known as super-novæ, giant stellar bodies vastly greater than our sun, which, for some unknown reason, explode in interstellar space and shine with the light of a billion suns for a relatively short time.

The temperature at the time of the atomic explosion at Hiroshima, according to testimony before the Special Committee on Atomic Energy of the United States Senate by Dr. Philip Morrison, one of our brilliant young , nuclear physicists, reached 100,000,000 degrees Fahrenheit (55,000,000 degrees centigrade) in its center.

This is about three times as great as the temperature estimated for the interior of the sun, and nearly ten thousand times the temperature of the sun’s surface. To attain an internal temperature of such magnitude would require a star with the luminosity of 400,000,000 suns, or 400 ordinary novæ combined into one.

The pressure attained during an atomic explosion is even more staggering in its dimensions, reaching hundreds of billions of atmospheres. The energy generated by an atomic bomb at the time of explosion, calculated on the basis of ten per cent efficiency, is enough to raise the entire United States wartime fleet of 9,000,000 displacement tons nearly two miles into the air.

At this point some misconception must be cleared up as to the total maximum amount of fission energy in a given mass available for the explosion, which must not be confused with the total amount of energy actually contained in that mass if all the matter in it were to be converted into energy, according to the Einstein formula for the equivalence of mass and energy.

The maximum amount of fission energy available for an explosion in any given mass of U.235 or plutonium is only one tenth of one per cent of the total energy contained in that mass. If we state, for example, that one kilogram of U.235, when exploding with one hundred per cent efficiency, would release energy equivalent to 20,000 short tons of TNT, that amount would represent only one tenth of one per cent, or one thousandth part, of the total energy contained in that kilogram. In other words, the total amount of energy present in that kilogram, on the basis of the Einstein formula (see Chapter II), if all its matter could be converted into energy, would be the equivalent of 20,000,000 short tons of TNT.

Fat Man plutonium bomb assembled.

In the process of fission, however, only one thousandth part of the matter is converted into energy. Furthermore, this one thousandth of the total energy is all that we can get or ever will get out of uranium or plutonium. It represents one hundred per cent of the total energy available for the explosion. If we state that the explosion went off with ten per cent efficiency we mean one tenth of one thousandth, not one tenth of the total energy in the entire mass.

That is the way nature has arranged it, and there is nothing man can do about it. All he can do is to take advantage of what nature has given him and use her bounty up to the limit—that is, one tenth of one per cent, which in the case of uranium and plutonium would be a full one hundred per cent of the total energy available for man’s use.

The reason for this is not hard to find. Every unit of atomic mass is the equivalent in energy of 1,000,000,000 (one billion) electron volts (an electron volt is a very small fraction of an erg, unit of work). Since an atom of U.235 is composed of 235 such units, the total mass of [the atom of U.235 has an energy equivalent of 235,000,000,000 (235 billion) electron volts.

But in the process of fission only 200,000,000 electron volts of energy are liberated; that is, only one fifth of a unit of mass of the 235 units in the atom, or 1/1175 of the atom’s mass, is converted into energy. This is roughly only 1/12 per cent, but in round figures it is generally given as 1/10 of one per cent—namely, one part per thousand.

It cannot be overemphasized that this rather small percentage is the maximum available. That is all there is; no power known to man can make the atom yield the slightest bit more. It is just as immutable a law of nature as is the law of gravitation, and we can no more change it than we can change the pull of gravity at a given point.

But this small amount is certainly a tremendous trifle. To illustrate: A bomb containing one hundred kilograms of U.235, if all its atoms were to undergo fission, would liberate the energy equivalent to 2,000,000 short tons of TNT. If only ten per cent of its atoms undergo fission, the energy liberated would still be the equivalent of 200,000 tons of TNT, or ten times more powerful than the bomb dropped on Hiroshima.

Thus the power of the bomb can be increased either by increasing its size or by improving its efficiency, or both. Since the bomb dropped on Nagasaki, three days after the Hiroshima bomb, made the first one obsolete, there can be little doubt that within the next ten years we may expect to have atomic bombs of a power equivalent to 100,000 to 250,000 tons of TNT.

Such bombs, particularly if used in quantity, will destroy not only cities but whole areas. They could cripple a nation and destroy its power of resistance in a few minutes.

Such bombs, of course, will not be carried by airplane.

Long-range rockets, using atomic energy for fuel, could travel to any part of the globe at speeds of thousands of miles per hour, at altitudes of several hundred miles. It would even be impossible under these circumstances to tell where these rockets came from. The nation attacked would be staggering in the dark in a frantic effort to find out who the enemy was.

The friendliness of a nation would be no guarantee of innocence, since the diplomacy of the atomic age would call for the outward appearance of the utmost friendliness to avert suspicion when the atomic bombs began to fall.

A nation starting such an atomic war would not necessarily do so because it was bent on aggression. Living in constant fear, day by day, hour by hour, minute by minute, and second by second, that at any moment a rain of atomic bombs might come from somewhere, the tension must sooner or later become unendurable.

It would take only a few, trigger-happy men at the pushbuttons to start pushing them frantically in the conviction that by doing so they were only beating the other fellows to the push. The nation to start it would most likely be the one whose nerves were the first to crack under the strain.

The start of an atomic war would thus probably be an expression of a primitive, animal-like fear of the unknown. Unless he takes wise measures in time, and that means now, man of the atomic age will by force of necessity revert to the animal. He will retrogress 500,000 years, all the way back to the cave he started from, except that this time he will have the means to annihilate himself.

On the other hand, since, as Professor Oppenheimer pointed out, atomic bombs of the future will be cheap, plentiful, and easy to make, small nations will have them as well as large. Hence, if atomic bombs begin to pour down from the sky some dark night, the responsibility could by no means be pinned on any one, or on several, of the small number of great industrial powers.

Since these powers would have much more to lose from an atomic bomb war, it would be more likely that an ambitious military clique that had gained control over a small nation, as is the case with Argentina, Spain, and several other small powers, might decide the time had come for them to have their fling at the dice of destiny, with everything to gain and rather little to lose.

Man collectively may become Ishmael, a “wild man”; “his hand will be against every man, and every man’s hand against him.”

Little Boy bomb fits in the bay of a B-29.
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