Otto Frisch and Rudolf Peierls were physicists working at the University of Birmingham. After making new calculations about the critical mass needed for an atomic explosion, they realised it amounted to pounds rather than the tons they had previously assumed. They then calculated how long it would take to separate the necessary uranium-235, and realised that a ‘super-bomb’ was a practical proposition.
Frisch was later to recall:
I have often been asked why I didn’t abandon the project there and then, saying nothing to anybody. Why start on a project which, if it was successful, would end with the production of a weapon of unparalleled violence, a weapon of mass destruction such as the world had never seen? The answer is very simple. we were at war, and the idea was reasonably obvious; very probably some German scientists had had the same idea and were working on it.
In March 1940 they submitted a joint memorandum – ‘The Frisch-Peierls Memorandum’- that succinctly set out the properties of such a bomb as well as the practical difficulties that might be encountered when actually using it. The Memorandum was taken up at the highest levels of government and started the decision making process that led to the Allied Atomic bomb.
Memorandum on the properties of a radioactive “super-bomb”
The attached detailed report concerns the possibility of constructing a “super-bomb” which utilizes the energy stored in atomic nuclei as a source of energy. The energy liberated in the explosion of such a super-bomb is about the same as that produced by the explosion of 1000 tons of dynamite. This energy is liberated in a small volume, in which it will, for an instant, produce a temperature comparable to that in the interior of the sun. The blast from such an explosion would destroy life in a wide area. The size of this area is difficult to estimate, but it will probably cover the centre of a big city.
In addition, some part of the energy set free by the bomb goes to produce radioactive substances, and these will emit very powerful and dangerous radiations. The effect of these radiations is greatest immediately after the explosion, but it decays only gradually and even for days after the explosion any person entering the affected area will be killed.
Some of this radioactivity will be carried along with the wind and will spread the contamination; several miles downwind this may kill people.
The document is remarkable not only for the proposition that the authors put forward but also for the clarity with which they consider the implications of their knowledge – whether for example the Germans have also established that such a bomb is possible:
We have no information that the same idea has also occurred to other scientists but since all the theoretical data bearing on this problem are published, it is quite conceivable that Germany is, in fact, developing this weapon. Whether this is the case is difficult to find out, since the plant for the separation of isotopes need not be of such a size as to attract attention. Information that could be helpful in this respect would be data about the exploitation of the uranium mines under German control (mainly in Czechoslovakia) and about any recent German purchases of uranium abroad. It is likely that the plant would be controlled by Dr. K. Clusius (Professor of Physical Chemistry in Munich University), the inventor of the best method for separating isotopes, and therefore information as to his whereabouts and status might also give an important clue.
At the same time it is quite possible that nobody in Germany has yet realized that the separation of the uranium isotopes would make the construction of a super-bomb possible. Hence it is of extreme importance to keep this report secret since any rumour about the connection between uranium separation and a super-bomb may set a German scientist thinking along the right lines.
They also go on to consider the strategic implications of owning or not owning such a bomb. It can be seen that the notion of deterrence through mutually assured destruction was readily apparent from the very first conception of a ‘super-bomb’.
If one works on the assumption that Germany is, or will be, in the possession of this weapon, it must be realized that no shelters are available that would be effective and that could be used on a large scale. The most effective reply would be a counter-threat with a similar bomb. Therefore it seems to us important to start production as soon and as rapidly as possible, even if it is not intended to use the bomb as a means of attack. Since the separation of the necessary amount of uranium is, in the most favourable circumstances, a matter of several months, it would obviously be too late to start production when such a bomb is known to be in the hands of Germany, and the matter seems, therefore, very urgent.
Otto Frisch was an Austrian of Jewish origin who came to England to pursue Physics in 1933 following the persecution of Jews in Vienna. Rudolf Peierls was a German of Jewish origin from Berlin. He was researching at the University of Cambridge when Hitler came to power in 1933 and decided to stay in the United Kingdom.
Stanford University has a copy of the full memorandum.