The Neutron and the Bomb by Andrew Brown

Author:Andrew Brown [Brown, Andrew]
Language: eng
Format: epub
Tags: Biographies & Memoirs
Publisher: Plunkett Lake Press
Published: 2018-08-20T22:00:00+00:00

GEORGE HOLT PHYSICS LABORATORY,

The University of Liverpool.

5th December 1939

Dear Appleton,

The examination of the uranium fission experiments and the calculation of the possibilities of an explosive chain reaction has taken much longer than I expected. The calculation is in fact quite involved, and I had to ask Pryce to look into it. In the meantime two papers have appeared, one by von Halban, Joliot and Kowarski on experiments with a large quantity of uranium oxide, and one by Peierls on the calculation of conditions for chain reactions.

The conclusions I have reached are these. It seems likely that both types of fission — the one due to thermal neutrons, the second to fast neutrons — could be developed to an explosive process under appropriate conditions. The thermal neutron fission would require a mixture of uranium and hydrogen (i.e. water); otherwise the chain reaction would be prevented by a resonance capture process which takes place. Moreover, the materials would have to be free from rare earths even in very small quantities for many of these show marked resonance effects. Given these conditions, a chain reaction will probably occur, but, as far as I can estimate from the available data, a large quantity of uranium would be necessary — perhaps more than 1 ton.

The fast neutron fission is inhibited only, as far as is known at present, by the effect of inelastic collisions. Unfortunately, we have no data whatever about inelastic collisions in uranium; the data for other heavy elements are meagre, and it would be unsafe to speculate. I think one could say that this explosion is almost certain to occur if one had enough uranium. The estimates of the amount necessary vary from about 1 ton to 30 or 40, according to the data adopted in the calculations.

There is of course a general argument against the possibility of these explosive processes — that no such explosion has occurred in pitchblende mines or in laboratories where the ore is dealt with. This argument is open to objection. First, the concentration of uranium in the crude ore is not high, and the pitchblende is said to occur in strata of only about 10 cm thickness. Secondly, the presence of impurities of rare earths, etc. etc. It does seem quite possible that the necessary conditions for explosion may never be realised either for the fast or for the thermal neutrons without the aid of man.

I am sorry I can give no definite answer to this question, for it is a very interesting one indeed. The amount of energy which might be released is of the order of the energy of the well known Siberian meteor. The difficulty is really lack of data. Very few experiments have been made which throw any light on the actual mechanism of the fission processes; they have been mainly concerned with the radioactive products of the fission — interesting and important, but apt to degenerate into a kind of botany. I think it would be desirable to get some information on the mechanism, and if I can get enough uranium oxide I will do so.

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