gained only by a proportionate increase in the mass of the scarce and cxpensive } or. fe oN ent . better to usc the extra quantities to make extra bombs. | It would be much vant op fissionable matcrial within it would be very poor economy, | 1 ‘ ey re~ It so happens, however, that in atomic bombs the total anguntof/enerey Oya leased per kilogram of fissionable material (i.c., the efficiency of energy release) increases with the size of the bomb 4? This factor, weighed against \ those mentioned in the previous paragraph, indicates that there is a theoretical | . | optinun size for the bomb which has perhaps not yet been determined and which nay verywell be appreciably or even considerably larger than the Nagasaki bomb. | But it should be observed that considcrations of military cconomy are not the only factors which hold dowm the optimmm size. Onc factor, already noted, is the Steeply ascending difficulty as the number of subcritical masses increases of ‘» securing sinuitancous and perfect wmiion among them, the envelope or tamper. Another is the probiem of If the inercase of weight of ths tamper is at all pro portionate either to the increase in the amount of fissionable material used or to the amount of energy rcleased, the gross weicht of the bomb might quickly press against the technically usablc linits, In short, the fact that ean enormous increase in the power of the bomb is thcorctically conccivable docs not zican that it is Likcly to occur, cither soon or lator. It has always been theoretically possible to pour 20,000 tons of TNT together in one case and detonate it as a 45. Smyth Report, paragraph 2:18. This phonorcnon is no doubt cue to the fact that the greatcr the margin above the critical mass linits, the faster the re- action and honce the greater the proportion of matcrial which undergoes fission before the heat generated expands and disrupts the bomb, It might be noted also that even if there wore no cxpension or bursting to hait it, the reaction would cease at about the tine the fissionable matorial remaining fell below critical mass conditions, which would also tend to put a premium on having a large margin above critical nass limits. At any rate, anything like 100 per centdetonation of the explosive contents of the atomic bomb is totally out of the question. In this respect ato:ztic explosives differ markedly fron ordinary "high explosives" Like TNT or torpex, where there is no difficulty in getting a 100 per cent re- action and where the energy relcased is thereforc directly proportionate to the amount of explosive filler in the bomb. ee)