Lawrence Berkeley National Laboratory Recent Work Title A REVISED VERSION OF A REVIEW OF NUCLEAR FISSION PART ONE - FISSION PHENOMENA AT LOW ENERGY Permalink https://escholarship.org/uc/item/33z2b4sk Author Hyde, Earl K. Publication Date 1962-04-01 eScholarship.org Powered by the California Digital Library University of California UCRL-9036-Rev. UNIVERSITY OF CALIFORNIA Lawrence Radiation Laboratory Berkeley, California Contract No. W-7405 -eng-48, A REVISED VERSION OF A REVIEW OF .-NTJCLEAR FISSION rART ONE - FISSION PHENOMENA AT LOW EERGY EarlK.Hyde April 1962 ( n TWO-WEEK LOAN COPY This is a Library Circulating Copy whIch may be borrowed for two weeks. For a personal retention copy, call Tech. Info. Division, Ext. 5545 DISCLAIMER This document was prepared as an account of work sponsored by the United States Government. 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UCRL-9036 -Rev -1- A REVIEW OF NUCLEAR FISSION PART ONE - FISSION PHENOMENA AT LOW ENERGY Earl K. Hyde April 1962 Authorts note: The original version of this report was issued in January 1960. The present version is identical to it in organization but it contains a considerable amount of recently published material particularly in the last half of the report. The author wishes to thank the many individuals who supplied comments and criticisms of the original material and who called his attention to important new data on fission phenomena. He would still be grateful for comments and suggestions but makes no promises con- cerning the preparation of a second revision. Part two of this Fission Review entitled Fission Phenomena at Moderate and High Energy was issued as report UCRL-9065 in February 1960. UCRL-9036-Rev. -2- PART ONE - FISSION PRENOMENA AT LOW ENERGY .... 11.1 HISTORICAL ACCOT OF THE DISCOY OF FISSION . • . 11.2 FISSION THEORY 11.2,1 The Liquid Drop Model of Fission . . . . 10 11.2.2 The.Unified Model and Fision Theory . . . .59 11.3 THE PROBABILITY OF FISSION 11.3.1 Empirical Relationships for Fissio,n Activation Energy ....................... 62 11.3.2 Cross Seätions for Fission with Thermal Neutrons . 66 11.3.3 Fission Cross Section as a Function of Neutron Energy in the Thermal and Resotiance.Energy Regioii. • .74 11.3.4 Fission Thresholds by the (d,p) Method ......... 82 11.3.5 Fission Cross Sections for Neutrons in the Mev Energy Range .......... 89 11.3.6 Probability of Spontaneous Fission .......... 95 11.3.7 Prob&bility of Photofission and Fission Induced by Charged Particles. (A brief cross reference to Chapter 12). ........................ 110 11. DISTRIBUTION OF MASS IN FISSION . 11,4.1 Introduction . .................. ill 11.4,2 Summary of Fission Yields in Slow Neutron Fission. 113 11.4.3 Closed Shell Effects and Fine Structure in the Mass Yield Curve ........ ................ 11.4.4 Distribution of Mass in Fission Induced by Neutrons 118 of Resonance Energy. . . . . .......... 11.+.5 Fission Product Yields in Spontaneous Fission ..... i6 11.4L6 Ternary Fission ................... 168 11.5 DISTRIBUTIONOF NUCLEAR CHARGE IN FISSION . . . . . . . . . . 178 11.6 KINETIC ENERGY OF THE FISSION FRAGMENTS. ........ 197 11.6.1 Ion-Chamber Measurements of Fragment Energy Distribution in Slow Neutron Fission of. U235 , U233 and Pu239 ................. 197 11.6.2 Ion-Chamber Measurements of Fragment Energy Distribution in Spontaneous Fission . . . . . . . . . 210 11.6.3 Time-of-flight Measurements of Fission Fragment . Velocities . , . , , . , , ......... 223 UCRL-9036-Rev. 3 - ii.6J1- Measurement of Fission Fragment Ranges ........ 2i-1 11.6.5 Calorimetric Measurement of the Energy Released in Fission .... ... ................ 26+ 11.7. PROMPT NEUTRONS EMITTED IN FISSION 11.7. 1 Measurements of V, the Average Number of Neutrons Emitted. in Fission ................... 266 11.7.2 Measurements of P (v) .................. 273 11.7.3 Measurements of P (v) as a Function of Fission Mode . 284 11.7.4 The Energy Spectrum and Angular Distribution of the Prompt Néurons from Fission ............ 301 11.7.5 Neutron Velocity and Angular. Distributions Measured in Coincidence with Fission Fragments ......... 307 11.7.6 Theoretical Calculations of Prompt-Neutron Multi- plicities . ..................... 315 11.8 DELAYED NEUTRONS IN FISSION 11.8.1 Introduction and Rsum of Early Investigations . 327 11.8.2 Recent Results on Delayed-Neutron Periods and Their Abundances ........................ 333 11.8.3 Radiocheniical Identification of Delayed Neutron Precursors ...................... 3111 11.8.4 The Shell Model Interpretation of the Delayed- Neutron Emitters ...................... 3-I-6 11.9 GAMMA RAYS IN FISSION ....................... 350 UCRL-9036-Rev. - PART ONE - FISSION PHENOMENA AT LOW ENERGY* 11.1 HISTORICAL ACCOU11T OF THE DISCOVERY OF FISSION After the neutron was discovered by CHADWICK 1 in 1932 and artificial radioactivity by I. CURIE and F. JOLIOT 2 in 1934, FERMI showed the effectiveness of paraffin-slowed neutrons in the preparation of artificial radioelements. He and his co-workers 3 at Rome exploited this technique very thoroughly by the systematic bombardment of all the eaily-available chemical elements with the neutrons emitted by a radium-beryllium source. Quite naturally this study led to the search for transuranium elements by the bombardment of uranium with slow 13,4 neutrons. FERMI and his collaborators produced a 13 minute activity by bom- bardment of uranium and succeeded in separating it from elements 82 to 92 inclusive. This led them to the logical conjecture that this activity must be element 93, particularly since it seemed to have the chemical properties at that time expected for this element (namely, properties like those of rhenium). The formation of element 93 would be expected from the capture of a neutron by uranium followed by beta decay. Continued work by the Fermi group and by other investigators, however, resulted .in the discovery of numerous additional activi- ties- -far too many to explain without postulating a very unusual pattern of isomerism. Furthermore, the radiochemical properties of many of the new "transuranium" elements differed from those to be expected of such elements. In addition to the apparent transuranium elements, four radioactivities were found which were 'reported to be - active isotopes of radium because they precipitated with barium compounds traditionally used as carriers for radium. * Published literature to the early months of 1962 was surveyed in the prepara- tion of this review. J. Chadwick, Proc. Roy. Soc. A136, 692 (1932 ). I. Curie and F. Joliot, Comptes Rendus 198, 254 (193 4). E. Amaldi, 0. D'Agostino, E. Fermi, B. Pontecorvo, F. Rasetti and E. Segr, Proc. Roy. Soc. A149, 522 (193)'; A146, 483 (193 4). E. Fermi, Nature 133, 898 (1934). UCRL-9036=Rev. -5- The investigation of those confusing products of the irradiation of uranium with neutrons occupied the period from 1935 -1939 The extent of the experimental work done on the "transuranium elements" during this period and the confusing difficulties in the way of their classification can be seen by consulting a review5 published abut oneiear before thedisoveryof:.fission. The honor of proving that the new activities were not heavy element isotopes, but isotopes of medium-weight elements produced by an entirely un- expected nuclear phenomenon fell to the German radiochemists HAHN and STRASSMANN6 '. These two chemists as well as I. CURIE and P. SAVITCH 8, who were working simultaneously in France were investigating the radiochemical properties of the new radium isotopes and finding surprising difficulty in separating them from inactive barium which had been added as a carrier element. The problem was solved by HAHN and SThASSMAHN when they added TbX(Ra22) or MsTh1 (Ra 22 ) to the mixture and carried out a partial separation of barium and radium by fractional crystallization of chloride, bromide and chromate salts. The unidentified activities isolated from neutron-bombarded uranium targets were observed to concentrate in the barium and to be separated from the ThX or MsTh1 fraction. This proved that the unknown activities must be isotopes of barium and not of radium since other elements had been eliminated in the preliminary separation. In order to clinch the identification, radiochemical experiments were performed on the daughter activities Of the strange "radium" isotopes. Previously the daughter activities had been believed to be isotopes of actinium. HAHN and STRASSMANN separated the daughter products with lanthanum carrier, then added MsTh2(Ac228) as an indicator for actinium. When a partial