To James B. Conant
November 30, 1942
Dear Dr. Conant:
Your letter reached me with some delay since I returned to Berkeley only a day or so ago. I should like to answer first your P.S. You are quite right that the purities listed in Groves' compulsory memo are a little misleading. The reason for this is that Groves defined a satisfactory bomb as one that had a 50 per cent chance of exceeding a 1,000-ton TNT equivalent. The absolute requirements are figured on this basis. It is, of course, my opinion that we should be wanton to strive for such a low goal, but I believe that some good was in fact done by indicating at that time that the purity requirements are not fantastic. The desirable requirements are equally undefined since the purer the material (up to a purity of about 100-fold that given) the less must we be worried about getting the maximum speed for the firing mechanism of the detonator; and this will make for simplicity and reliability in operation. In the Washington memo all impurities were listed on the assumption that not more than five elements would reach the tabulated values. I have, in the meantime, given a much more careful account of what the actual situation is to the committee. I met with them one day in Chicago, came out with them on the train, and have spent two days with them here in Berkeley, and we have had ample opportunity to discuss the purity question and many other aspects of our problem. The information which I have given them now is con-tained in a Chicago report on the feasibility of the 49 project and is as follows: If the concentrations by weight are as given in the accompanying table, then the chance of pre-detonation is 5 per cent, if only one element is present in the listed amount. If n elements are present in the listed amount the chance of pre-detonation is 5n per cent. The chance of a pre-detonation in which the energy release is less than 10,000 tons TNT equivalent is 0.5n per cent. In this range the effects of impurities are additive, and from the actual concen-trations and figures listed one can figure out the probability of any given energy release. In any case, unless the firing mechanism fails completely the energy release will be more than sufficient to destroy the material and to make its recovery impossible. The figures given in the table are in part based on experimental values. In the case of 0 and C the figures represent highly conservative estimates based on the assumption that those isotopes which are dangerous will be as dangerous as the worst element, namely, Be.
Element Concentration by Weight
||5 x 10-6
||5 x 10-7
||2 x 10-5
||2 x 10-7
||2 x 10-5
||2 x 10-5
||5 x 10-4
(Some purity requirements on elements between P and Fe, none beyond Cu)
The only essential changes since the Washington memo are that we have sufficiently examined the experiments somewhat; and that we have studied the case of N carefully enough to be positive that there are no purity requirements on that element. The committee was of the opinion that the purity requirements as they now stand could, with a very high probability, be met. In fact, the Chicago uranium is good enough except for C and 0, and they have made no effort at all to solve this problem. If it were necessary it would be possible to work with depleted C and 0 and so considerably relax the chemical conditions on these elements. In fact, the committee was of the opinion that the major extraction processes which have to be handled automatically and the removal of traces of active material, coupled with the necessity for working in lots of less than 100 grams or of introducing suitable neutron absorbers as "safers," would present greater technical difficulties than the purification. Nevertheless, in our last discussions they seemed convinced that the helium-cooled graphite pile was a good bet.
Now to the second point, the main subject of your letter, where I feel myself on less secure ground. It is, of course, natural that the men we are after will leave a big hole. I may though, in this connection, remind you that when McMillan himself left the Radiation Laboratory for San Diego there were the same dire predictions of disastrous disruption. Nevertheless, the Radiation Laboratory has not only survived but has, as you know, flourished and expanded. In view of this and of the very large number of men of the first rank who are now working on that project, I am inclined not to take too seriously the absolute no's with which we shall be greeted. I believe that it is important to emphasize that we should in any case be willing to let these men have time enough in their old positions to try to minimize the disrup-tion of their leaving. I also agree that a fundamental clarification on this personnel problem, which can hardly be complete without Dr. Bush's participation, will be necessary. The job we have to do will not be possible without personnel substantially greater than that which we now have avail-able, and I should only be misleading you and all others concerned with the S-1 project if I were to promise to get the work done without this help.
The suggestion of Eckhardt as a substitute for Kurie is a welcome one and we shall arrange to talk with him on our next trip east. There are, however, two reasons more substantial than prejudice why the limitation to men who are known to us is sound: 1) that the technical details of this work will in large part have to do with atomic physics so that any man whose experience has been in another field will necessarily be of more limited usefulness; Kurie, for instance, would have had as one of his responsibilities the installation and servicing of the cyclotron. The second reason is that in a tight isolated group such as we are now planning, some warmth and trust in personal relations is an indispensable prerequisite, and we are, of course, able to insure this only in the case of men whom we have known in the past. You will have had from me a note on possible alternatives to Kurie. If none of our suggestions seem practicable we shall see whether Dr. Eckhardt could fill the bill.
With good wishes,
very sincerely yours,