SCIENTIFIC DETAILS

Timaru Herald, Volume CLVIII, Issue 23277, 13 August 1945, Page 5

SCIENTIFIC DETAILS

DEVELOPMENT OF ATOMIC BOMB International Research By Telegraph—N.Z. Press Assn.—Copyright (7.30 p.m.) LONDON, August IL It was only after the discovery was reported at the beginning of 1939 of the special phenomenon of fission that the way was clearly seen by which atomic or nuclear energy in matter could be released, controlled and put to use by man. This was disclosed in a report on the scientific nature of the atomic bomb issued by the Department of Scientific and Industrial Research. The document is destined to be read in conjunction with the American and Canadian statements concentrated on work in Britain and by British scientists of 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 of uranium nucleus. It was immediately recognised that this discovery was of the greatest significance, and that for the first time there was an experimental basis for the hope that the 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 fission made possible a continuation of the reaction by the development of a 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 fraction of the energy that would be liberated. Special reference was made to the visit of Professor N. H. Bohr (Copenhagen) to America from January to May, 1939. He was able to report directly to American physicists the experiments carried out by Hahn Frisch Meitner and their suggested interpretation of the result. The overall again in energy would be eliminated. It was, therefore, only the natural activity in laboratories throughout the world resulting in these discoveries being in progress at the outbreak of war. The work was done m so many laboratories that the details of priority are not clear. One important prediction made from this theory related to the different behaviour of the various isotopes of uranium, 99.3 per cent of which consists of atoms of mass No. 238. There is also 9.7 per cent of isotope of mass No. 235 and .008 per cent of isotope of mass 234. The first two, designated U 238 and U 235 respectively, are the most important in connection with the uranium fission project. Professor 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, but in addition would be particularly liable to undergo fission when the energy, and therefore the velocity of the bombardin' neutrons, was verylow. This prediction was confirmed in March. 1940. by the experiments of Niers, of Minnesota, and Booth, Dunning- and Grosse, of Columbia University, using a uranium sample in which the U 235 content was increased over the normal value by means of the Nier mass s'ectograph. In recent years the enormous effort, practically all of which was borne by America, of utilising the energy released had been concentrated on the development of the atomic bomb. New Sense of Urgency The report summarises the nature of the problems relating to the use of the fission either to produce a 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 the chain of reaction might be 'obtained in uranium which would yield enormous amounts of energy. This on :h" ha- is. 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. (1) The system as a whole must be of such a size that there is not too great a possibility that neutrons produced in a fission process would esemv from the system and so be unable to take further part in the chain proc (2) The system must not contain more than a limited amount of material that would absorb neutrons, this again removing their chance of contributing to the reaction (3) 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 chai i process, and only then would it be possible to introduce methods which would allow of the development of the process to be controlled. The neutrons produced when the fission occurs have very hi h energies, but Ibis is dissipated ns the result of elastic collisions with nuclei of other

atoms that might be present. Professor Joliot and co-workers in Paris, Professor Farmi and other physicists of America, were giving thought to tire possibility of using a mixture of uranium and some suitable slowing down medium, so arranged that the fast neutrons produced by the fission would not have lost energy by the elastic collisons before initiating a further fission in uranium.

The most suitable materials fulfilling all conditions *for slow’ing down were heavy hydrogen, or its compounds heavy water, helium beryllium and carbon. Chadwick and Peiris inde,.' ndently in 1940 drew attention to the possibility of a fast neutron fission in the chain reaction being obtainable if pure or nearly pure, U 235 were available. A committee was established in 1940 originally under the Air Ministry, and later under the Ministry of Aircraft Production, to examine the whole problem and co-ordinate the work. Experiments in England showed the possibility of extracting U 235. Estimates were given for the cost and the time of building the plant. The committee was convinced that the atomic bomb, depending on the fission U 235, was feasible and that the effect would be comparable with some thousands of tons of T.N.T. and that sufficient quantities of material could be obtained.

New Developments Another development occurred at the time of the fall of France. Two French physicists, M. Halvan and Kowarski, sent by Joliot from Paris, arrived in England with 155 litres of heavy water obtained in Norway before the invasion. The Cavendish Laboratory provided the facilities for the experiment which provided strong evidence that by using uranium oxide or uranium metal with heavy water a divergent slow neutron fission in the chain of reaction could be realised, if the system were of sufficient size. The committee concluded that this had great potential interest for power production, but was not likely to be developed in time to use in the war.

Contact was maintained throughout with the Americans working on the fission. An organisation known as the Tube Alloys for security reasons was established and every section of the American programme was examined in detail. It was clear that the American organisation intended to make the fullest use of the enormous resources available in the universities and in industries. The programme chosen in Britain, though necessarily on a smaller scale, included the determination of the essential nuclear physical data and a theoretical investigation into the chain of reaction in the atomic bomb. The dimensions and design of the bomb and the blast effect were also investigated and the diffusion of the U 235 separation process. This included research on the process, design and construction of prototype machines, the manufacture of the materials needed, the investigation of the flow of tn? neutron divergent systems, especially with heavy water, the manufacture of uranium metal, and the manufacture of heavy water.

Manufacture of Uranium Imperial Chemical Industries was given a contract for the development of a diffusion plant and the manufacture of uranium metal was undertaken. As a result of the research on heavy water, plant designs were produced w'hich were of novel design and believed to be more simple and more efficient than any i hitherto used or suggested. After investigation remarkable work was carried on in the California University with the object of converging mass spectrograph, used for tire separation of isotopes in minute quantities, into large-scale production apparatus, but it was decided not to start corresponding research in Britain, as the most suitable physicist, Professor Oliphant, was engaged on other war work. When he was released from that work in July, 1943,fit was decided that the most efficient course in the general interest, was that Professor Oliphant and his team move to America. The British electro-magnetic programme was therefore abandoned. When Professor Oliphant returned to Britain in March, 1945, it was decided to arrange for a research to be started on some of the electrical engineering problems involved in this type of plant. With this object, research contracts have been placed with British companies.