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Enrico Fermi. 1901-1954Author(s): E. Bretscher and J. D. CockcroftSource: Biographical Memoirs of Fellows of the Royal Society, Vol. 1 (Nov., 1955), pp. 69-78Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/769243

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ENRICO FERMI1901-1954

THE sudden death of ENRICOFERMI t the age of 53 has filled physicists allthe world over with greatest sadness and consternation. One of the mostoutstandingand in some respectsunique scientificpersonalities,a wonderfulteacher and a marvellousrepresentativeof his native country, Italy, has leftus.Fermi was born in Rome on 29 September 1901. He was educated at theHigh School in Rome and later at the Scuola Normale Superioreof Pisawhere he obtained a Doctorate in 1922. He later studied at Gottingen withBorn and at Leiden with Ehrenfest.From 1924-26he was Lecturer n Mathe-matical Physics at Florence. In 1927 he was elected to a ProfessorshipofTheoretical Physics in Rome and in 1929 became one of the FounderMembersof the Royal Academy of Italy.Fermi'searlyworkwas mostlyconcernedwith theory,often with problemswhich arosefrom the advent of the new mechanicsof Heisenberg,Dirac andSchrodinger. One group of investigations dealt with spectroscopy: theanomalyof the intensityratio of the multipletsof the higheralkalimetals, themagnetic moments of nuclei, calculations of spectra of ions, the Ramaneffect in CO2 and in crystals, the oscillations and rotations of the NH3molecule and the hyperfinestructureseparation.Into the same period,when he was in Rome, falls his theoryof a gas whoseparticlesobey Pauli's exclusion principle. A preliminarystudy of the condi-tions under which degenerationof a gas can take place (18)* led him to thecorrectpartitionfunction at about the same time (21) as Dirac developedhistheory of an ideal gas. The method of Fermi's derivation is anything butelegant, being based on an oscillator model, but it has the advantage ofshowingup what he was doing. This clarityis one of the reasonswhy Fermi'spapers appeal to the experimentalphysicists (1). Fermi rapidly recognizedthe general usefulnessof the statistical method and applied it with con-spicuoussuccessto the calculationof variousatomic properties.This Fermi-Thomas method was later applied by F. Bloch with success to calculate thestopping power of matter for charged particles. Here again one strength ofFermi's becomes apparent: the skill and instinct with which he obtainedapproximate results where accurate calculation would be prohibitivelycomplex.Though Fermi was not a theoreticalformalist,he was greatly interestedin* Numbers in parentheses refer to the numbered entries in the bibliography.

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and contributed to the development of Dirac's radiation theory and toquantum electrodynamics at about the same time as Heisenberg and Pauliperformed their work on a similar topic. An exposition of this topic ofunsurpassable clarity was published by Fermi in the Review of ModernPhysicsof January 1932.Not unconnected with his interest in field theory was his paper on 'Anattempt at a theory of beta rays'. Here Pauli's ad hoc explanation of thecontinuous beta-ray spectrum by the neutrino is put into a theory, formallyconstructed in a manner similar to that of the emission of electro-magneticradiation. There is a certain arbitrariness in the choice of the interactionHamiltonian. Fermi apologizes for the particular choice by saying it was thesimplest. At the time, his theory was objected to, among other reasons, on thegrounds of being at variance with the experiments, particularly in so far asthe low energy spectrum of the electrons was concerned. Very characteristi-cally, Fermi pointed out that the experiments were difficult and perhapstheir precision not sufficient to be decisive to test his theory! Only shortlybefore, N. Bohr, in his Faraday Lecture (1932), had stressed the greatdilemma of physics in so far as beta decay was concerned. Though Pauli'sproposal of a neutrino was the lesser of two evils (the other meant giving upconservation of energy in the beta decay) and was a relief to all, Fermi'stheory reproduced a considerable number of experimental facts and madethe neutrino hypothesis more plausible to our minds (InternationalConferenceonPhysicsVol. 1, p. 67 (1934)). Today Fermi's theory is essentially acceptedover a wide range, though nature has turned out to be more complex thanwe thought.Both his work on quantum statistics and on the theory of the beta decayhad by this time (1934) established Fermi's international fame as a theoreticalphysicist.In 1933 Fermi's activity entered a new stage, when after Joliot-Curie'sdiscovery of the production of artificial radioactivity by a-particle bombard-ment he took up neutron research as an experimenter or a theoretician as thesituation required. By that time he had collected round him a number ofyoung Italian physicists all of whom were to obtain international repute inlater life. Fermi learnt to construct Geiger counters and was able to use theradon from the 1 gram of radium belonging to the Bureau of Public Healthto build a radon-beryllium neutron source. With this he began a systematicsearch through the elements for artificial radioactivity, starting with hydro-gen. He obtained no positive results for the elements up to and includingoxygen but found that fluorine was strongly activated. In this work he colla-borated with Amaldi, D'Agostino, Pontecorvo, Segre and Rasetti.The neutron source and the counter were kept at the end of a long corridorto prevent interference with measurements. Mrs Fermi has described howthe short-lived elements required fast running to take them from the sourceto detector and Fermi and Amaldi excelled in this. Of the 63 elementsinvestigated 37 were shown to have an easily detectable activity. The

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activation cross-section was found not to depend in any systematic way on theatomic nucleus. Two types of transmutation were discovered leading toemission of protons or a-particles and a third in which the neutron wascaptured with emission of a gamma ray. The heavy elements were usuallytransmuted by neutron capture and led to a single unstable element with asingle exponential decay of radioactivity. Uranium and thorium proved,however, to be exceptional for several radio-active elements were produced.In a letter to Natureof 16June 1934, Fermi described the existence of activitieshaving half-lives of 10 seconds, 40 seconds, 13 minutes and at least two longerperiod activities. The 13-minute activity was shown not to be isotopic withelements 82, 83, or 88-92. He therefore suggested tentatively that atransuranium nucleus with charge 93 might have been formed. It was nottill the discovery of uranium fission by Hahn and Strassman 31 years laterthat the reason for the complex radioactivities was discovered. One of theactivities of 2 -3 days half life was shown by E. McMillan in early 1940 tobe due to element 93-neptunium.During the course of the experiments in 1934, Amaldi and Pontecorvofound that the intensity of the radiocativity induced by neutrons varied withthe surroundings of the specimen. The radioactivity of silver was found to beincreased 100 times by surrounding it with paraffin. Water produped asimilar effect and it was inferred that this was due to the slowing down of theneutrons increasing the activation cross-section-in some cases, 1000 times.These neutrons were shown to make 100 collisions in paraffin before capture.This work was the precursor of the atomic pile. On the advice of Corbino ajoint patent for the production of artificial radioactivity by slow neutronswas taken out and was the subject of an award in the United States after thewar. The main results appeared in two papers (56) and (57) communicatedby Lord Rutherford on 25 July 1934 and 15 February 1935 for publicationin the Proceedingsof the Royal Society. Paper I is mostly concerned with theactivities induced by unmoderated sources; II contains a large assortment ofobservations such as efficiency of various slowing down materials, scatteringand diffusion of the slow neutrons, temperature effects, the large variationof capture cross sections for different elements, emission of gamma rays onneutron capture, separation of radioactive isotopes, and a list of all activitiesfound. A section on theoretical considerations on the properties of slowneutrons yields much of the picture of the neutron capture process, such asthe l/v law. Fermi was quite aware of the theoretical difficulties caused bythe existence of finite capture cross sections for fast neutrons, a fact whichwas only later elucidated by Bohr's postulate of the compound nucleusformation. The theory of the slowing down of neutrons, which was later tobecome so important in theoretical calculations on atomic piles, is alreadycontained in a simple form in a memorandum published by the ConsiglioNazionale delle Ricerche, Roma, 1934, under the title 'Sul Moto dei neutroninelle sostante idrogenate'. Here it is also pointed out that the large n-p lowenergy scattering is mainly due to the singlet state of the deuteron and that

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Biographical Memoirsthe neutron-capture cross section of hydrogen is due to a magnetic dipoletransition. With this one simple assumption of Fermi's many incomprehen-sible features of the deuteron system obtained a natural explanation. As thephysics of the deuteron system is of fundamental importance to nuclearphysics, just as the hydrogen atom to atomic physics, any contribution insuch a field is of particularly high merit.In the years following, i.e. 1936-7, about ten papers appeared mostly inthe RicercaScientifica n the diffusion and absorption of neutrons, work mostlycarried out in collaboration with Amaldi. The last note in this is dated July,1937.In the autumn of 1938 the anti-semitic movement in Italy, which deve-loped after the Italian-German alliance, made Fermi decide to leave Italy totake up a professorship at Columbia University. He was awarded the NobelPrize in December 1938 and the journey to Stockholm helped his move tothe United States.At Columbia Fermi joined forces with H. L. Anderson, Zinn and Szilard,to study the possibility of developing a chain reaction in uranium. About thesame time as in France it was shown that neutrons were emitted in the fissionprocess. Uranium was then surrounded with water to moderate the neutronenergy, but it was found that ordinary water absorbed too many neutrons tomake a chain reaction possible. Szilard and Fermi then decided to try a pileof graphite blocks interposed with lumps of uranium metal. When I (J.D.C.).visited Columbia in November 1940, I saw Fermi carrying out these experi-ments-at the same time as Halban and Kowarski were carrying out experi-ments on a heavy water nuclear chain reaction in Cambridge.By the spring of 1941 a small pile had been built at Columbia, but was toosmall to become divergent. At the end of 1941 the group went to Chicago andAnderson and Zinn built a larger pile in the squash court of the Universityof Chicago. Of this period Anderson has said . .. 'Fermi possessed a sure wayof starting off in the right direction, of setting aside the irrelevancies, of seizingall the essentials and proceeding to the core of the matter. The whole processof wresting from nature her secrets was for Fermi an exciting sport which heentered into with supreme confidence and great zest. No task was too menialif it sped him towards his goal. He thoroughly enjoyed the whole of the enter-prise. The piling of the graphite bricks, the running with the short livedactivated rhodium foils, and the merry clicking of the Geiger counter whicheffected the measurement. All was done with great energy and obviouspleasure, but by the end of the day, in accordance with his plan, the resultswere neatly compiled, their significance assessed, and the progress measured,so that early in the morning on the following day, the next step could begin.It was a feature of the Fermi approach never to waste time-to keep thingsas simple as possible, never to construct more elaborately or to measure withmore care than was required by the task at hand. In such matters his judge-ment was unerring. In this way, step by step, the work sped forward until inless than four short years Fermi had reached his goal. A huge pile of graphite

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EnricoFermi 73and uranium had arisen in the West stands of the University of ChicagoCampus. When, on 2 December 1942, 12 years ago, Enrico Fermi stoodbefore that silent monster he was its acknowledged master. Whatever hecommanded it obeyed. When he called for it to come alive and pour forth itsneutrons it responded with remarkable alacrity; and when at his command itquieted down again, it had become clear to all who watched that Fermi hadindeed unlocked the door to the Atomic Age.'In 1943 Fermi and his family went to Los Alamos to join Oppenheimer onthe atomic bomb project.One of us (E.B.) had the following impressions of this period:'During part of my stay at Los Alamos I was in charge of the experimentalgroup of the division for advanced development, headed by Fermi. It isnatural, therefore, that we had a good deal of contact with Fermi who tookgreat interest in what we were doing. An incident which is very characteristicof Fermi occurred when some particularly important and surprising resultswere obtained. When I told Fermi about it, he said, "Please give me theexperimental data and I will calculate the final result and if my calculationsagree with you then probably the results obtained are correct." It was verycharacteristic of Fermi that he would not accept an experimental result buttry to find out in detail how it was obtained. He was always available for anydiscussion and when he reported about the work of the division he was alwaysvery generous and fair in allotting credit where it was due.'Besides the profound influence of his research work, I believe that he hadvery considerable influence on American physics through his lecturing andteaching at the University of Chicago. He was able, practically withoutpreparation, to present any topic in nuclear and atomic physics with clarityand restriction to the essentials, which permitted nearly everybody to follow.This often happens with good lecturers, but frequently one finds that after onehas left the lecture hall most of what one thought one understood hasvanished. In Fermi's case the attraction of his exposition rested in the factthat one really understood the problem and did not, therefore, have to relyon memory. He was quite free from oratory but simply had a very penetratinginsight into a problem and could formulate his thoughts clearly. The samequality is found in most of his papers; one of them, with Marshall, practicallystarted off the whole interaction of neutrons with solids. Everything is there,just to be exploited.'During my last summer at Los Alamos, when Fermi came for a visit, Iasked him what he was doing. He said: "Well, I have one big course whereI teach children and, you know, I find it much more difficult than giving ahighbrow course." I said to Fermi: "I am really jealous of your students: Iwish someone like you would teach me physics under the same circum-stances." The next day he came to my office and said: "I am quite preparedto give you a small private course provided you find half a dozen suitablepeople to listen." I gave him a list of a dozen people: he crossed half of themoff-those he thought would not be suitable-and then started on a course of

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74 Biographical Memoirsradiation theory and quantum electrodynamics. It was a very greatexperience and extremely profitable for us. The only trouble was that Fermidid not seem to get tired and instead of one hour he lectured for two; wewere too exhausted, but it was obvious that he could easily have gone on fora further period. He just knew the topic so well, in spite of its difficulties,that it cost him no effort to reproduce it.'As a person, Fermi seemed simplicity itself. He was extraordinarilyvigorous and loved games and sport. On such occasions his ambitious naturebecame apparent. He played tennis with considerable ferocity and whenclimbing mountains acted rather as a guide. One might have called him abenevolent dictator. I remember once at the top of a mountain Fermi got upand said: "Well, it is two minutes to two, let's all leave at two o'clock"; andof course, everybody got up faithfully and obediently. This leadership andself-assurance gave Fermi the name of "The Pope" whose pronouncementswere infallible in physics. He once said: "I can calculate anything in physicswithin a factor 2 on a few sheets: to get the numerical factor in front of theformula right may well take a physicist a year to calculate, but I am notinterested in that." His leadership could go so far that it was a danger to theindependence of the person working with him. I recollect once, at a party athis house when my wife cut the bread, Fermi came along and said he had adifferent philosophy on bread-cutting and took the knife out of my wife'shand and proceeded with the job because he was convinced that his ownmethod was superior. But all this did not offend at all, but rather charmedeverybody into liking Fermi. He had very few interests outside physics andwhen he once heard me play on Teller's piano he confessed that his interestin music was restricted to simple tunes.'After the war Fermi naturally made full use of the availability of highneutron fluxes from the Argonne reactors. Again the happy ability to turn toexperiments or attack a problem theoretically bore fruit lavishly. The barestatement of titles of his papers is impressive: 'Production of low energyneutrons by filtering through graphite' (72), 'Transmission of slow neutronsthrough microcrystalline materials' (74); 'Interference phenomena of slowneutrons' (76). 'Phase of scattering of thermal neutrons by aluminium andstrontium' (75); 'Spin dependence of scattering of slow neutrons by Be, Al,Bi' (79); 'by deuterium' (82); 'On the interaction between neutrons andelectrons' (81); 'A thermal neutron velocity selector and its application to themeasurement of the cross section of boron' (78). In this large amount of workhe was essentially only supported by Leona Marshall, though in some casesother collaborators helped, such as W. J. Sturm, R. G. Sachs, H. L.Anderson. Some papers are astonishingly comprehensive; the one on inter-ference phenomena starts with the theory introducing the scattering lengths,the basic convenient datum easily extracted from experiments, Braggreflexions, filtered neutrons, diffraction by gaseous molecules, total reflexionby mirrors, concluding with a good table of scattering lengths deduced fromhis results. Only the cream of neutron optics had been skimmed off but it was

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EnricoFermi 75rich, and much in the direction of refinement could be done and was achievedat the Argonne, Oak Ridge and Brookhaven Laboratories. But Fermi's basicwork, together with his teaching of the subject at the University of Chicago,gave immense impetus to the development in this field. The effect of thisgenial atmosphere and the high standard set by Fermi can still be felt in manyplaces remote from Chicago.When the cyclotron began to operate Fermi participated in the work ofthe cyclotron group. The scattering of pions with protons and deuterons wasstudied in detail and great efforts were made to obtain the phase shifts.Fermi's theoretical work this time was concerned with a statistical, stronginteraction model for the multiple production of pions at very high particleenergies (85) suitable to evaluate the observations of the Brookhaven cosmo-tron.

In parallel with the above research, Fermi had taken a considerableinterest in W. A. Hiltner's observation of the polarization of star light and itsinterpretation by L. Davis and J. L. Greenstein as magnetic dichroism ofinterstellar material caused by weak magnetic fields of huge extension. Heshowed that wandering magnetic fields can, on the average, accelerate par-ticles to cosmic ray energies provided their initial energy is above a certainlimit. It seems at present that Fermi's basic idea is correct and that a consider-able step forward has been made in a field which has been extremely puzzling.It is worth noting that Fermi and Chandrasekhar were able to confirm bytwo independent estimates the strength of field in the spiral arms of ourgalaxy at 7 2 x 10-6 and 6 x 10-6 gauss. E. BRETSCHER

J. D. COCKCROFT

BIBLIOGRAPHYPapers(1) 1921. Electrostaticsof a uniform gravitational field. NuovoCim.22, 176-188.(2) 1921. Dynamics of a rigid system electric chargesin translatorymotion. NuovoCim.22,199-207.

(3) 1922. Electromagnetic mass and the theory of relativity; inertia of electricity. Accad.Lincei,Atti, 31, i. 184-187, 306-309.(4) 1922. X-rays. NuovoCim.24, 133-163.(5) 1922. Discrepancy between electrodynamic relativist theoriesof electromagneticmass.Phys. Z. 23, 340-344.(6) 1923. Images with rontgen rays. NuovoCim.25, 63-68.(7) 1923. Change of the lane of polarization of light in a rotating medium. Accad.Lincei,Atti, 32, 115-118.(8) 1923. A mechanical normal system is in general quasi-ergodic.Phys. Z. 24, 261-265.(9) 1923. Adiabatic invariants of mechanical systems. NuovoCim.25, 171-175.(10) 1923. Electromagnetic mass. NuovoCim.25, 159-170.(11) 1923. Applicability of Ehrenfest'sprinciple. NuovoCim.25, 271-285.6

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76 BiographicalMemoirs(12) 1923. The photoelectric effect. NuovoCim.26, 97-104.(13) 1923. Entropy in a monatomic gas. Accad.Lincei,Atti, 32, ii, 395-398.(14) 1923. Probability of quantic states. Accad.Lincei,Atti, 32, ii, 493-495.(15) 1924. Optical resonance reflection and diffusion. Accad.Lincei,Atti, 33, i. 90-93.(16) 1924. The probability of the quantum states. Z. Phys.26, 54-56.(17) 1924. The thermic equilibrium of ionisation. NuovoCim. 1, 153-158.(18) 1924. Quantising systems containing identical elements. NuovoCim. 1, 145-152.(19) 1924. Shock between atoms and electrically-charged particles. Z. Phys.29, 315-327.(20) 1926. Quantisation of the monatomic perfect gas. Accad.Lincei,Atti, 3, 145-149.(21) 1926. Quantisation of the ideal monoatomic gas. Z. Phys.36, 902-912.(22) 1926. Radiation in intense magnetic fields. Accad.Lincei,Atti, 3, 478-483.(23) 1926. (With E. PERSICO.)Adiabatic invariance and kinetic energy in undulatorymechanics. Accad.Lincei,Atti, 4, 452-457.(24) 1926. Probability formula. NuovoCim.3, 313-318.(25) 1926. Wave mechanics of collision. Z. Phys.,40, 399-402.(26) 1927. (With F. RASETTI.)Measurement of k/h. Z. Phys.43, 379-383.(27) 1927. Mechanism of emission according to undulatory mechanics. Accad.Lincei,Atti,5, 795-800.(28) 1927. Application of statisticalgas methods to electronics system. Accad.Lincei,Atti, 6,602-607.(29) 1928. Statisticalmethod of investigating electrons in atoms. Z. Phys.,48, 73-79.(30) 1928. Statistical deduction of atomic properties.Accad.Lincei,Atti, 7, 342-346.(31) 1928. Statistical calculation of the Rydberg corrections.Z. Phys.,49, 550-554.(32) 1928. The atom. Accad.Lincei,Atti, 7, 726-730.(33) 1929. Quantistic electrodynamics.Accad.Lincei,Atti, 9, 881-886.(34) 1929. Motion of a body of variable mass. Accad.Lincei,Atti,9, 984-986.(35) 1929. Quantum theory of interferencefringes. Accad.Lincei,Atti, 10, 72-77.(36) 1929. Complex 4d terms of the helium molecule. Accad.Lincei,Atti, 10, 515-517.(37) 1930. Doublet components of the alkali metals. Z. Phys.,59, 680-686.(38) 1930. Magnetic moments of atomic nuclei. Z. Phys., 60, 320-333.(39) 1930. Interpretation of the principle of causality in quantistic mechanics. Accad.Lincei, Atti, 11, 980-985.(40) 1930. Calculation of the spectra of ions. Accad.d'Italia, Mem. 1, (Fis.) 2, (10 pp.);Nuovo Cim. 8, 7-14, 1931.(41) 1930. Magnetic moments of atomic nuclei. Accad. d'Italia, Mem. 1 (Fis.), 1 (12 pp.).(42) 1930. Quantistic electrodynamics. Accad. Lincei, Atti, 12, 431-435.(43) 1931. Electrodynamic masses in quantistic electrodynamics. Nuovo Cim. 8, 121-132.(44) 1931. Theory of radiation. Ann. Inst. H. Poincare, 1, 53-74.(45) 1931. Raman effect in CO2. Z. Phys., 71, 250-259.(46) 1931. (With F. RASETTI.) Raman effect in rock salt. Z. Phys., 71, 689-695.(47) 1932. Quantum theory of radiation. Rev. Mod. Phys., 4, 87-132.(48) 1932. (With H. BETHE.) Interaction of two electrons. Z. Phys., 77, 296-306.(49) 1932. Raman effect in molecules and crystals. Accad. d'Italia, Mem. 3, 3 (Fisica), (22

pp.).(50) 1932. Oscillation and rotation of the ammonia molecule. Accad. Lincei, Atti, 16, 179-185.(51) 1933. (With B. Rossi.) Action of the earth's magnetic field on penetrating radiation.Accad. Lincei, Atti, 17, 346-350.(52) 1933. (With E. SEGRE.) Theory of hyperfine structure. Z. Phys., 82, 729-749.(53) 1934. Theory of /-rays. Nuovo Cim. 11, 1-19. Z. Phys., 88, 61-771.(54) 1934. Displacement by pressure of the high lines of the spectral series. Nuovo Cim. 11,157-166.(55) 1934. Possible production of elements of atomic number higher than 92. Nature, Lond.133, 898-899.

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EnricoFermi 77(56) 1934. (With E. AMALDI,O. D'AGOSTINO,F. RASETTI& E. SEGRE.) Artificial radio-activity produced by neutron bombardment. Nuovo Cim. 11, 429-460; Proc.Roy.Soc.A, 146, 483-500.(57) 1935. (With E. AMALDI,O. D'AGOSTINO,B. PONTECORVO,. RASETTI& E. SEGRE.)Artificial radioactivity produced by neutron bombardment. Part II. Proc.Roy.Soc.A, 149, 522-558.(58) 1934. Artificial radioactivity by neutron bombardment. InternationalConferencen

Physics, London, Vol. I. Nuclear Physics. pp. 75-77, Physical Society.(59) 1934. (With H. A. BETHE, W. M. ELSASSER,H. O. W. RICHARDSON K. SITTE.)Natural f-decay. International Conferenceon Physics, London, Vol. I. Nuclear

Physics, pp. 66-71, Physical Society. Discussion.(60) 1935. (With F. RASETTI.)Slow neutrons. Nuovo Cim. 12, 201-210.(61) 1936. (With E. AMALDI.)Absorption of slow neutrons. Ric. Sci. 1, 56-59.(62) 1936. (With E. AMALDI.)Mean free path of slow neutrons in paraffin wax. Ric. Sci. 1,223-225.(63) 1936. (With E. AMALDI.) Groups of slow neutrons. Ric. Sci. 1, 310-313.(64) 1936. (With E. AMALDI.)Diffusion of slow neutrons. Ric. Sci. 1, 393-395.(65) 1936. (With E. AMALDI.)Absorption and diffusion of slow neutrons. Ric. Sci. 1, 454-503.(66) 1936. Motion of neutrons in hydrogenous substances. Ric. Sci. 2, 13-52.(67) 1936. (With E. AMALDI.)Absorption and diffusion of slow neutrons. Phys. Rev. 50, 899-928.(68) 1937. (With E. AMALDI& F. RASETTI.)Artificial generator of neutrons. Ric. Sci. 2,40-43.(69) 1939. (With H. L. ANDERSON& L. SZILARD.)Neutron production and absorption inU. Phys. Rev. 56, 284-286.(70) 1940. Ionization loss of energy in gases and in condensed materials. Phys. Rev. 57,485-493.(71) 1941. (With H. L. ANDERSON& A. V. GROSSE.)Branching ratio in the fission of U(235).Phys.Rev.59, 52-56.(72) 1946. (With H. L. ANDERSON& L. MARSHALL.)Production of low energy neutrons byfiltering through graphite. Phys. Rev. 70, 815-817.(73) 1947. (With E. TELLER & V. WEISSKOPF.)The decay of negative mesons in matter.Phys.Rev.71, 314-315.(74) 1947. (With W. J. STURM& R. G. SACHS.)The transmission of slow neutrons throughmicrocrystalline materials. Phys. Rev. 71, 589-594.(75) 1947. (With L. MARSHALL.)Phase of scattering of thermal neutrons by aluminumand strontium. Phys. Rev. 71, 915.(76) 1947. (With L. M{ARSHALL.) Interference phenomena of slow neutrons. Phys. Rev. 71,666-677.(77) 1947. (With H. L. ANDERSON,A. WATTENBERG,G. L. WEIL & W. ZINN.) Method formeasuring neutron-absorption cross sections by the effect on the reactivity of a

chain-reacting pile. Phys. Rev. 72, 16-23.(78) 1947. (With J. MARSHALL& L. MARSHALL.)A thermal neutron velocity selector andits application to the measurement of the cross section of boron. Phys. Rev. 72,193-196.(79) 1947. (With L. MARSHALL.)Spin dependence of scattering of slow neutrons by Be, Aland Bi. Phys. Rev. 72, 408-410.(80) 1947. (With E. TELLER.) The capture of negative mesotrons in matter. Phys. Rev. 72,

399-408.(81) 1947. (With L. MARSHALL.) On the interaction between neutrons and electrons.Phys.Rev.72, 1139-1146.(82) 1949. (With L. MARSHALL.)Spin dependence of slow neutron scattering by deuterons.Phys. Rev. 75, 578.

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78 BiographicalMemoirs(83) 1949. On the origin of the cosmic radiation. Phys.Rev.75, 1169.(84) 1949. (With C. N. YANG.)Are mesons elementary particles? Phys.Rev.76, 1739.(85) 1950. High energy nuclear events. Progr.Theor.Phys.,5, 570.(86) 1951. Angular distribution of the pions produced in high energy nuclear events.Phys.Rev.81, 683.(87) 1952. The nucleus. Phys. Today,5, 6-9.(88) 1952. (With H. L. ANDERSON,E. A. LONG,R. MARTIN& D. E. NAGLE.) Total crosssections of negative pions in hydrogen. Phys.Rev.85, 934-935.(89) 1952. (With H. L. ANDERSON, . LUNDBY,D. E. NAGLE& G. B. YODH.) Ordinary andexchange scattering of negative pions by hydrogen. Phys.Rev.85, 935-936.(90) 1952. (With H. L. ANDERSON,E. A. LONG & D. E. NAGLE.) Total cross sections ofpositive pions in hydrogen. Phys.Rev.85, 936.(91) 1952. (With H. L. ANDERSON, . E. NAGLE& G. B. YODH.)Deuterium total cross sectionfor positive and negative pions. Phys.Rev.86, 413.(92) 1952. (With H. L. ANDERSON,D. E. NAGLE& G. B. YODH.) Angular distribution of

pions scattered by hydrogen. Phys.Rev.86, 793.(93) 1952. (With H. L. ANDERSON.)Scattering and capture of pions by hydrogen. Phys.Rev.86, 794.(94) 1953. (With H. L. ANDERSON,R. MARTIN & D. E. NAGLE.) Angular distribution ofpions scattered by hydrogen. Phys.Rev.91, 155-168.(95) 1953. Nuclear polarization in pion-proton scattering. Phys.Rev.91, 947.(96) 1953. (With M. GLICKSMAN,. MARTIN& D. E. NAGLE.) Scattering of negative pionsby hydrogen. Phys.Rev.92, 161-163.(97) 1953. Multiple production of pions in nucleon-nucleoncollisions at cosmotronenergies.Phys.Rev.92, 452-453.(98) 1954. (With N. METROPOLIS& E. FELIXALEI.) Phase shift analysis of scattering of

negative pions by hydrogen. Phys.Rev.95, 1581.(99) 1954. Polarization of high energy protons scattered by nuclei. Nuovo Cim. (9) 11,407-411.Books

1928. Introduzionella Fisica Atomica.Bologna; N. Zanichelli.1934. Molecole cristalli.Bologna; N. Zanichelli.1937. Thermodynamics.ew York; Prentice-Hall, Inc.1951. Elementaryparticles. New Haven; Yale University Press.