Story of Scientists’ Great Achievement

Manawatu Times, Volume 70, Issue 190, 13 August 1945, Page 4

Story of Scientists’ Great Achievement

(By Telegraph—Press Assn.—Copyright | Received Sunday, 8.40 p.m. LONDON, August 12. It was only alter the discovery was reported at the beginning of 1939 of the special phenomenon of the fission that tnc way was clearly seen whereby atomic or nuclear energy in matter could be released, controlled and put to use by man. This is disclosed in a report on the scientific nature of the atomic bomb issued by the Department of Scientific and industrial Research. The document is designed to be read in conjunction with American and Canadian statements and it concentrated ou the work in Britain and by British scientists in the joint project which developed the atomic bomb.

The work done in many countries was the result of full and free collaboration between scientists among whom those working in Britain played a most important part. The work confirmed the fact that more than one free neutron was produced for each fission (splitting or diversion) of the uranium nucleus. It was immediately recognised that this was a discovery of the greatest significance and that for the first time there was an experimental basis for the hope that a useful realisation of the enormous store of atomic energy in matter could be achieved. Not only did the fission reaction provide a large amount of energy, but the liberation of more than one new neutron each time the uranium nucleus underwent a fission made possible the continuation of the reaction by the development of the chain process. Once the initial step was taken such chain process would enable a reaction in a suitable mass of uranium to take place at an ever-increasing rate and involve so many atoms that there would be a sensible, indeed a possible overwhelming, liberation of energy. The process could furthermore be started by the application of only a minute direction of the energy that would be liberated.

Special reference was made to the visit of Professor Bohr to America from January to May, 1939. He was able to report directly to American physicists on the experiments carried out by Hahn, Frisch, and Meitner and their suggested interpretation of the results. One important made from this theory related to the different behaviour of various isotopes of uranium, 99.3 per cent of which consists of atoms of mass number 238. There was also 9.7 per cent of isotope of mass number 235 and .008 per cent of isotope of mass 234.

The first two were designated U 238 and U 235 respectively, which were the most important in connection with the uranium fission project. Dr. Bohr predicted in February, 1939, that the common isotope U 238 would be expected to undergo fission only when the bombarding neutrons had high energy, but that the rarer U 235 would behave differently in that it would not only show this reaction with high energy neutrons hut in addition would be particularly liable to undergo fission when the energy and therefore the velocity of the bombarding neutrons was very low. This prediction was confirmed in March, 1940, by the experiments of Miers of Minnesota, Booth Dunning and Grosse of Columbia University using a uranium sample in which U 235 content was increased over its normal value by means of the Nier mass spectrograph. In recent years the enormous effort, practically all of which had been borne by America, of utilising the energy released had been concentrated on the development of the atomic bomb. The report summarises the nature of the problems relating to the use of the fission either to produce violent explosion or liberate atomic energy under controlled conditions. When the work was organised with a new sense of urgency and importance at the outbreak of war it was generally accepted that a chain of reaction might be obtained in uranium which would yield enormous amounts of energy. This on the basis of equal weights would be millions of times greater than that produced by the combustion of coal or oil, but it was realised that if this chain reaction were to be divergent and self-sustaining certain critical conditions must be satisfied:

Firstly: The system as a whole must be of such a size that there would not be too great a possibility that neutrons produced in the fission process would escape from the system and so be unable to take a further part in the chain process.

Secondly: The system must not contain more than a limited amount of material that would absorb the neutrons, thus again removing their chance of contributing to the reaction. Thirdly: If the reaction were not to run away, it was essential to make use of neutrons of very low energy in the individual steps of the chain process. Only then would it be possible to introduce methods which would allow the rate of development of the process to be controlled.

The neutrons produced when the fission occurs have very high energies, but this is dissipated as a result of elastic collisions with the nuclei of other atoms that might be present. Frofessor Joliot and his co-workers in Paris, and Professor Farmi and other physicists in America were giving thought to the possibility of using a mixture of uranium and some suitable slowing down medium so arranged that