Franklin D. Roosevelt
Albert Einstein Old Grove Rd. Nassau Point Peconic, Long Island
August 2nd, 1939
F.D. Roosevelt, President of the United States, White House Washington, D.C.
Sir:
Some recent work by E. Fermi and L. Szilard, which has been communicated to me in manuscript, leads me to expect that the element uranium may be turned into a new and important source of energy in the immediate future. Certain aspects of the situation which has arisen seem to call for watchfulness and, if necessary, quick action on the part of the Administration. I believe therefore that it is my duty to bring to your attention the following facts and recommendations:
In the course of the last four months it has been made probable—through the work of Joliot in France as well as Fermi and Szilard in America—that it may become possible to set up a nuclear chain reaction in a large mass of uranium, by which vast amounts of power and large quantities of new radium-like elements would be generated. Now it appears almost certain that this could be achieved in the immediate future.
This new phenomenon would also lead to the construction of bombs, and it is conceivable—though much less certain—that extremely powerful bombs of a new type may thus be constructed. A single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port together with some of the surrounding territory. However, such bombs might very well prove to be too heavy for transportation by air.
Albert Einstein
The United States has only very poor ores of uranium in moderate quantities. There is some good ore in Canada and the former Czechoslovakia, while the most important source of uranium is Belgian Congo.
In view of this situation you may think it desirable to have some permanent contact maintained between the Administration and the group of physicists working on chain reactions in America. One possible way of achieving this might be for you to entrust with this task a person who has your confidence and who could perhaps serve in an inofficial capacity. His task might comprise the following:
a) to approach Government Departments, keep them informed of the further development, and put forward recommendations for Government action, giving particular attention to the problem of securing a supply of uranium ore for the United States;
b) to speed up the experimental work, which is at present being carried on within the limits of the budgets of University laboratories, by providing funds, if such funds be required, through his contacts with private persons who are willing to make contributions for this cause, and perhaps also by obtaining the co-operation of industrial laboratories which have the necessary equipment.
I understand that Germany has actually stopped the sale of uranium from the Czechoslovakian mines which she has taken over. That she should have taken such early action might perhaps be understood on the ground that the son of the German Under-Secretary of State, von Weizsäcker, is attached to the Kaiser-Wilhelm-Institut in Berlin where some of the American work on uranium is now being repeated.
Yours very truly,
(Albert Einstein)
As the quantity of uranium within which the fission chain reaction was initiated grew larger, more and more of the neutrons produced found a mark and the fission reaction would die out more and more slowly. Finally, at some particular size—the “critical size”—the fission reaction did not die at all, but maintained itself, with enough of the neutrons produced finding their mark to keep the nuclear reaction proceeding at a steady rate. At any greater size the nuclear reaction would accelerate and there would be an explosion.
It wasn’t even necessary to send neutrons into the uranium to start the process. In 1941 the Russian physicist Georgii Nikolaevich Flerov (1913- ) found that every once in a while a uranium atom would undergo fission without the introduction of a neutron. Occasionally the random quivering of a nucleus would bring about a shape that the nuclear interaction could not bring back to normal and the nucleus would then break apart. In a gram of ordinary uranium, there is a nucleus undergoing such “spontaneous fission” every 2 minutes on the average. Therefore, enough uranium need only be brought together to surpass critical size and it will explode within seconds, for the first nucleus that undergoes spontaneous fission will start the chain reaction.
First estimates made it seem that the quantity of uranium needed to reach critical size was extraordinarily great. Fully 99.3% of the metal is uranium-238, however, and, as soon as fission was discovered, Bohr pointed out that there were theoretical reasons for supposing that it was the uranium-235 isotope (making up only 0.7% of the whole) that was the one undergoing fission. Investigation proved him right. Indeed, the uranium-238 nucleus tended to absorb slow neutrons without fission, and to go on to beta-particle production that formed isotopes of neptunium and plutonium. In this way uranium-238 actually interfered with the chain reaction.
In any quantity of uranium, the more uranium-235 present and the less uranium-238, the more easily the chain reaction would proceed and the lower the critical size needed. Vast efforts were therefore made to separate the 2 isotopes and prepare uranium with a higher than normal concentration of uranium-235 (“enriched uranium”).
Of course, there was no great desire for a fearful explosion to get out of hand while the chain reaction was being studied. Before any bomb could be constructed, the mechanism of the chain reaction would have to be studied. Could a chain reaction capable of producing energy (for useful purposes as well as for bombs) be established? To test this, a quantity of uranium was gathered in the hope that a controlled chain reaction of uranium fission could be established. For that purpose, control rods of a substance that would easily absorb neutrons and slow the chain reaction were used. The metal, cadmium, served admirably for this purpose.