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The Discovery of Fission Otto R The discovery of fission Otto R. FrischJohn A. Wheeler Citation: Physics Today 20, 11, 43 (1967); doi: 10.1063/1.3034021 View online: http://dx.doi.org/10.1063/1.3034021 View Table of Contents: http://physicstoday.scitation.org/toc/pto/20/11 Published by the American Institute of Physics Articles you may be interested in On the belated discovery of fission Physics Today 68, (2015); 10.1063/PT.3.2817 The Discovery of Nuclear Fission Physics Today 42, (2008); 10.1063/1.881174 The Discovery of Fission Initial formulations of nuclear fission are colored with the successes, failures and just plain bad luck of several scientists from different nations. The winning combination of good fortune and careful thought made this exciting concept a reality. by Otto R. Frisch and John A. Wheeler How It All Began by Otto R. Frisch THE NEUTRON was discovered in 1932. had little respect for theory. Once, Why, then, did it take seven years be- when one of her students suggested an fore nuclear fission was found? Fission experiment, adding that the theoreti- is obviously a striking phenomenon; it cal physicists next door thought it results in a large amount of radioactiv- hopeful, she replied, "Well, we might ity of all kinds and produces fragments try it all the same." Their disregard that have more than ten times the total of theory may have cost them the dis- Otto R. Frisch, professor of natural ionization of anything previously covery of the neutron. philosophy (physics) at Cambridge known. So why did it take so long? Cambridge is the second place wor- University, England, did research in Berlin (1927-30), Hamburg (1930- The question might be answered best thy of discussion. Ernest Rutherford, 33), London (1933-34), Copen- by reviewing the situation in Europe whose towering personality dominated hagen (1934-39) and Birmingham from an experimentalist's point of Cambridge research, had split atomic (1939-40). During the war he view. nuclei in 1919; since 1909 he had, in worked on the A-bomb at Los fact, been keenly concerned with the Alamos. He was first to observe Research in Europe energy liberated in the fission of a observation and counting of individual single uranium nuoleus. In Europe there were few laboratories nuclear particles. He first introduced in which nuclear-physics research was the scintillation method and stuck conducted, and I think the word firmly to it. His great preference was "team" had not yet been introduced for simple, unsophisticated methods, into scientific jargon. Science was and he possessed a strong distrust of still pursued by individual scientists any complicated instrumentation. who worked with only one or two stu- Even in 1932, when John Cockcroft dents and assistants. and Ernest Walton first disintegrated Paris harbored some of the most ac- nuclei by artificially-accelerated pro- tive research laboratories in Europe. tons, they used scintillations to detect It is the city in which radioactivity the process. By that time Rutherford had been discovered and where Ma- had realized that electronic methods dame Curie was working until her of particle counting must be devel- John A. Wheeler, one of the first death in 1934. She still dominated oped. The reason was that the scin- American scientists to concentrate the situation: Techniques were quite tillation method clearly had its short- on nuclear fission, worked at the similar to those used at the turn of the comings. It did not work for very low U. of Copenhagen in 1934 as a century; that is, ionization chambers or high counting rates and was not National Research Fellow with really reliable. This deficiency was Niels Bohr. Wheeler received his and electrometers. This state of af- PhD in physics at the Johns fairs is good enough for performing ac- highlighted by the results that came Hopkins University prior to his curate measurements on natural ra- from the third laboratory I want to research in Copenhagen. In 1938 dioactive elements, but it is not really mention—Vienna. he joined Princeton's physics de- partment, where he remains active. adequate for much of the work on nu- Vienna is where I began my career clear disintegration. Madame Curie and it was in those days a sort of en- PHYSICS TODAY • NOVEMBER 1967 • 43 tion method. Even at a few hundred fant terrible of nuclear physics. Sev- dent research. Hahn was working on particles a minute there were quite eral physicists were claiming that not various applications of radioactivity large corrections to be applied. only nitrogen and one or two others of for the study of chemical reactions, the light nuclei could be disintegrated structures of precipitates and similar Walther Bothe was the first to use by alpha particles but that practically subjects, whereas Lise Meitner was the coincidence method, both in an at- all of them could and did give many using radioactive materials chiefly to tempt to do something about cosmic more protons than anybody else could elucidate the processes of beta and rays and also for measuring the energy observe. I still do not know how they gamma emission and the interaction of of gamma rays by the range of the sec- found these wrong results. Apparent- gamma rays with matter. ondary electrons they produced. This ly they employed students to do the In addition, Hans Geiger was in was really the first reliable method for counting without telling them what to Germany. He had been with Ruther- measuring the energy of weak gamma expect. On the face of it, that opera- ford from 1909 onwards, in the early radiations. tion appears to be a very objective days before the nucleus was discov- Until 1932, the only source of par- method because the student would ered. Rutherford felt uncertain about ticles for doing atomic nuclear disinte- have no bias; yet the students quickly the scintillation method and asked gration was natural alpha particles: developed a bias towards high num- Geiger to develop an electric counter either polonium, which was difficult bers because they felt that they would to check on it. But as soon as Ruther- to come by (in fact one practically be given approval if they found lots of ford saw that the two gave the same had to go to Paris) or sources of one of particles. Quite likely this situation results, Rutherford returned to the the short-lived decay products of radi- caused the wrong results along with a scintillation method, which appeared um, which were very clean but were generally uncritical attitude and con- to be simpler and more reliable when short-lived and usually had lots of siderable enthusiasm over beating the used with proper precaution. Geiger gamma radiation. English at their own game. went back to Germany and perfected I still remember when I left Vienna his electric counters, and in 1928, to- The year of discovery at just about that time (after having gether with a student named But in 1932, that annus mirabilis, not escaped the duty of counting scintilla- W. Miiller, he developed an improved only the neutron was discovered but tions). My supervisor, Karl Przibram, counter that could count beta rays. two other developments took place. told me with sadness in his voice, "You Earlier counters were inadequate for In the US Ernest O. Lawrence made will tell the people in Berlin, won't this purpose, and scintillation methods the first cyclotron that showed prom- you, that we are not quite as bad as were also incapable of detecting beta ise of being useful, and in England they think?" I failed to persuade rays. However the new counters were Cockcroft and Walton built the first them. still very slow because the discharge accelerator for protons capable of pro- Germany had nuclear-physics re- between the central wire and the cy- ducing nuclear disintegrations. I search in several places. The team of lindrical envelope was quenched by a need not state that this was the begin- Otto Hahn and Lise Meitner, which large resistor of many megohms placed ning of an enormous development; had been one of the first groups to in the circuit; consequently the count- most of nuclear physics as we know it study radioactive elements, had at that ing rate was limited to numbers not would have never come about without time separated to carry out indepen- much greater than with the scintilla- at least one of those two instruments. But the interesting thing is that they played practically no role in that nar- row thread that led to the discovery of nuclear fission. I do not want to dwell on the dis- 92U+ (92U -f- n) 93EkaRe > 94EkaOs^^^^ (I) ioSek. 2,2 Min. covery of the neutron very much be- cause it was discussed in several inter- —?-> 95EkaIr esting lectures in 1962 at the History 59 Min. 66 Std! 9G JHli of Science Congress held in Ithaca, New York. The published proceed- 97EkaAu 2,5 Std. ings contained interesting contribu- tions by Norman Feather and Sir P James Chadwick, who showed that the (2) 92U + n-> (92U + n) 93EkaRe —•-• 94EkaOs ^ 40 Sek. 94 neutron was discovered in Cambridge, 16 Min. not simply by chance with everybody 95EkaIr ? else having done the groundwork, but 57Std. because a search for the neutron had been going on in Cambridge (admit- tedly with wrong ideas). The people (3) 92U+ (92U + n) 93EkaRe ? 23 Min. at Cambridge were keyed up for this discovery. They had made one obser- COMPUTATIONS, indicating chains of radioactive elements, were published in a vation that was important and that 1938 Die Naturwissenschaften article by Hahn, Meitner and Strassmann. —FIG.
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