XIII. Properties of Atomic Nuclei

XIII. PROPERTIES OF ATOMIC NUCLEI ATOMKERNERNES EGENSKABER Nordiska (19. skandinaviska) naturforskarmotet i Helsingfors den 11-15 augusti 1936, Helsinki-Helsingfors 1936, pp. 73-81

Address to the Nordic Scientists’ Meeting in Helsinki/Helsingfors given on 12 August 1936 TEXT AND TRANSLATION

See Introduction, sect. 3, ref. 67

P A R T I: P A P E R S A N D MANUSCRIPTS RELATING T O N U C L E A R PHYSICS

This lecture was also published in Fysisk Tidsskrift 34 (1936) 186-194. Apart from correcting a misprint (“the sulphur isotope ;is’’ - conference proceedings, p. 76), Bohr only introduced minor linguistic improvements.

Pohjoismainen (19. skandinaavinen) luonnontutkijain kokous Helsingissa 1936. Nordiska (19. skandinaviska) naturforskarmotet i Helsingfors 1936. Eripaiiios. - SZrtryck.

ATOMKERNERNES EGENSKABER

AV

NIELS BOHR

HELSINKI - HELSINGFORS 1936

This Page Intentionally Left Blank

Professor WIELS BOHR,Kabenhavn:

Atomkernernes Egenskaber. Foredraget indlededes rned en kort Orntale af den Udvikling i Fysiken, der har f m t tii Kendskabet ti1 Atornernes Byggestene, og som tog sin Begyndelse ved Elektronernes Opdagelse ornkring Aarhundredeskiftet og fandt en forelubig ilfslutning ved Lord RUTHERFORDS Opdagelse i 1911 af, a t ethvert Atom indeholder en positivt ladet Kerne af overordentlig srnaa Dirnensioner, hvori Starstedelen af Atomets Masse er koncentreret, og hvorom de rneget lettere negativt ladede Elektroner grupperer sig. Dette simple Billede af Atoniet gjorde det muligt a t skelne skarpt inellem de Egenskaber hos Stofferne, der skyldes Atornkernens indre Bygning, og de der har deres Oprindelse i det ydre Elektronsystems Struktur. hfedens vi ved de szdvanlige fysiske og kemiske Egenskaber ved Stofferne liar a t g0re med Wndringer i det ydre Elektronsystern, skyldes de radioaktive Fznornener hos visse Grundstoffer Processer i Sedenstaaende er en Satnnieniattiing af 1n:iholdet 31 Foredraget, der LIev holdt i nere fri Form rned Benytttlse af e t stort Antal Lysbilleder.

74

selve Atomkernen. Den omtrent samtidige Opdagelse af Isotopernes Eksistens understregede yderligere Forskellen mellem disse t o Grupper af Egenskaber, idet man fandt Grundstoffer, der med iervrigt identiske fysiske og kemiske Egenskaber havde forskellige Atomvagte og ofte tilmed forskellige radioaktive Egenskaber. Det omtaltes dernast, hvorledes den ejendommelige Modsatning mellem Atomernes Stabilitet og sadvanlige mekaniske Modellers Egenskaber har opnaaet en Forklaring gennem Opdagelsen af det PLAxCKske T'irkningskvantum. Medens de T'irkninger, der indgaar i Beskrivelsen af Modeller i sadvanlig Maalestok, er saa store, a t man ganske kan se bort fra Virkningskvantets Eksistens, galder dette ikke mere for Atomerne, hvorfor vi her finder helt nye Lovmaessigheder. Saaledes har det vist sig, a t enhver .endring i e t Atoms Tilstand kan beskrives som en individuel Proces, hvorved Atomet fsres fra en af de saakaldte stationaere Tilstande ti1 en anden af disse, og navnlig har det vzret muligt i vid Udstrakning a t gsre Rede for de optiske Spektres og Rsntgenspektrenes Lovmassigheder ved a t antage, a t Udsendelsen af enhver af Linierne i disse Spektre skyldes en saadan Overgangsproces, hvorved e t Lyskvantum udsendes. De ferlgende Aars gradvise matematiske Trdformning af disse Forestillinger fgrtes ti1 en forelsbig Afslutning gennem Skabelsen af en rationel Kvantemekanik, der fremtrader som en konsekvent Almindeliggerrelse af den klassiske Mekanik. Den Erkendelse, a t enhver med Virkningskvantets Eksistens forenelig Maaleproces medforer en ukontrollerbar Vekselvirkning mellem Maaleobjekt og Maaleinstrument, har endvidere bragt en dybtgaaende Revision af hele Iagttagelsesproblemet med sig, der har f s r t ti1 en fuldstzndig Opklaring af de tilsyneladende Paradoxer, som indeholdes i Kvantemekanikkens principielt statistiske Beskrivelsesmaade. Trods alle disse nye T r a k bevarer imidlertid Problemet om Atombygningen en overordentlig Simpelhed, der ferrst og fremmest skyldes Elektronsystemets aabne Struktur, soin betinger, a t de enkelte Elektroners Bindinger i ferrste Tilnarmelse kan beskrives uafhangigt af hverandre ved H j a l p af en Systematik, der i alle Enkeltheder gOr Rede for Lovmassighederne i Grundstoffernes periodiske System. Ved Problemet om Atonikernernes Opbygning og Egenskaber stilles man derimod paa Grund af den t a t t e Sammenpakning af Partikler i Kernerne overfor en helt ny Situation, hvor man maa vente a t msde vasentlig andre Lovmassigheder end de, der g d d e r for Elektronbindingen i Atomet. Gennem de senere Aars store eksperimentelle Opdagelser indenfor Kernefysiken er imidlertid fremskaffet e t righoldigt Materiale, der allerede paa nuvzrende Tidspunkt aabner Muligheder for en sammenhangende Beskrivelse af Atomkernernes Egenskaber.

75

Grundlaget for hele denne Udvikling skabtes ved det berermte, af RUTHERi 1919 foretagne fsrste Kernespr~engningsforserg,hvor det lykkedes ved Beskydning af Kvalstofatomer med a-Partikler a t udslynge Protoner. Processen kan skrives: FORD

14X 7

+ 4 He-+'JO +

H,

hvor Tallene foroven og forneden angiver hendoldsvis Atonivzgten og Kerneladningen. Dette banebrydende Arbejde efterfulgtes snart af en he1 R a k k e Forserg over Kerneomdannelser, hvor det naste afgerrende Fremskridt bestod i, a t man ti1 Beskydningen af Stofferne ikke som hidtil anvendte de naturligt forekommende a-Straaler, men derimod kunstigt accellererede Protoner. Saaledes lykkedes det COCKROFTog WALTOX i I932 ved Bombardement af Lithium a t sernderdele dette i t o a-Partikler efter Skemaet:

Denne Proces var szrlig interessant, fordi de Protoner, der anvendtes ved Beskydningen, havde en saa ringe Energi, at de ikke efter klassisk-fysiske Forestillinger vilde viere i Stand ti1 a t overvinde den elektrostatiske Frasterdning, som indtil ineget smaa Afstande virker mellem Kernerne. At en saaden Reaktion dog efter Kvanteteorien har en vis Sandsynlighed, var imidlertid allerede tidligere paavist af GAMOW i Forbindelse med hans smukke kvanteteoretiske Forklaring af Lovene for Udsendelse af a-Partikler fra radioaktive Stoffer, hvor det netop drejer sig om en lignende Gennemgang af Partikler gennem Omraader, hvor de iferlge den klassiske Fysik ikke har Mulighed for a t kornnie. Endelig udmarker den omhandlede Proces sig derved, a t man her i alle Enkeltheder kunde gerre Rede for den ved Reaktionen frigjorte kinetiske Energi (ca. 16 Mill. Volt) ved Hjaelp af EINSTEINS Relation for Wquivalens mellem Masse og Energi, idet alle de i Processen optmdende Partiklers Masser var meget nsjagtigt kendt fra ASTOSS massespektroskopiske Maalinger. Vort Kendskab ti1 Atomkernerne er endvidere i overordentlig Grad blevet beriget gennem CHADWICKSOpdagelse i I932 af den saakaldte Neutron, en neural Partikel nied meget n a r samme Masse son1 Protonen, og som fsrst blev iagttaget ved Beskydning af Beryllium med a-Partikler. Reaktionen kan her skrives:

Be

+- 4 He-+':

C $-

Denne Xeutron kunde, som man snart fandt, opstaa ved mange forskellige Kerneprocesser, hvorfor det var naturligt, som navnlig fremhzvet af HEISES-

BERG, a t betragte den som en fundamental Byggesten i alle Kerner. Herefter skulde enhver Atomkerne kun indeholde Protoner og Neutroner, hvis samlede Antal svarer ti1 Atomvzgten, medens Protonantallet er lig med Kerneladningen. Efter denne Opfattelse, hvorved man undgaar de Vanskeligheder, som det efter Kvanteteorien medferrer a t antage Eksistensen af Elektroner i selve Kernen, er de ved b-Straaleomdannelse udsendte Elektroner a t betragte som skabte ved selve Omdannelsesprocessen i lignende Forstand som Lyskvanterne skabes ved Overgange mellem et Atomsystems stationme Tilstande. E n helt ny Epoke indenfor Kernefysiken indlededes allerede Aaret efter ved Wgteparret CURIE-JOLIOTS Opdagelse af, a t visse af de ved Beskydning med a-Partikler dannede nye Grundstofisotoper var ,&Straale-radioaktive, idet de med en vis Periode omdannedes under Udsendelse af enten sedvanlige negative eller i visse Tilfdde positive Elektroner. Disse saakaldte Positroner er a t betragte som en ny Elementarpartikel, hvis Eksistens var forudsagt af DIRACS relativistiske Elektronteori, og som kort forinden var opdaget af ANDERSON og BLACKETT ved Undersergelser over de ved kosmiske Straaler frembragte Sekundzereffekter. Det ferrste Eksempel paa F'rembringelsen af denne saakaldte kunstige Radioaktivitet var den ferlgende Reaktion:

:z

hvorved der under Neutronudsendelsen dannes en radioaktiv Fosforisotop P, der atter med en Halveringstid pan. 3 Minutter omdannes ti1 en Siliciumisotop

30Si under samtidig Udsendelse af en Positron. 14

Navnlig efter FERMISPaavisning af Neutronernes store Evne ti1 ved Sainmensterd med Atomkerner a t frembringe Onidannelser af disse har vi i de allersidste Aar laert et overordentlig stort Antal nye radioaktive Isotoper a t kende. Denne Evne hidrerrer derfra, a t Neutroner paa Grund af deres manglende Ladning ikke kan ionisere og ferlgelig ikke p a samnie Maade soni a-Straaler niiste Energi ved a t passere Stof, nien kun ved Saniniensterd nied Atonikerner, i hvilke de endvidere kan trznge ind chindret af det Kernen omgivende elektriske Felt. Som Eksenipel paa en Kerneonidarinelse nied Neutroner kan anfares: 32 16

S

+ Ln-t

::P +

H,

hvor den dannede Fosforisotop er radioaktiv og omdannes ti1 Svovlisotopen

93 S under Udsendelse af negative Elektroner med en Halveringstid paa ca.

14 Dage. Netop denne usacdvanlig lange Halveringstid har muliggjort en

77

k e k k e vigtige Underswgelser over Posforonisztningen ved kemiske og biologiske Processer ved Hj z l p af den af HEVESY udviklede radioaktive Indikatornietode, der netop ved Opdagelsen af den kunstige Radioaktivitet har faaet et saa overordentlig udvidet og betydningsfuldt Xnvendelsesomraade. Medens alle de hidtil nzttvnte Kernereaktioner er ledsaget af Udsendelsen af niaterielle Partikler, har man genneni FERMIS og h a m Medarhejderes Underbogelser tillige l w t en smlig Cruppe Xeutronreaktioner a t kende, hvor den indfaldende Neutron sinipeltheri indfanges af Atomkernen under 'I'dsendelse af den overskydende Xnergi i Form af elektroniagiietisk Straaling (y-Straalei). Ht t? piik IIksempel er Processen:

hx-or den dannede radioaktive nye Jodisotop har en Halveringstid paa zG Minutter. Ti1 Processer af denne Art knytter der sig den saxlige Interesse, a t de giver 0 s e t nyt Indblik i Nekanisnien ved Kernereaktionerne. Fra en nzrniere Underssgelse af de radioaktive Stoffers y-Straalespektre kan nian nenilig slutte, a t den Tid, en anslaaet Xtonikerne bruger ti1 a t udsende elektromagnetisk Straaling, er nieget lang i Forhold ti1 det 'I'idsruni, en Neutron vilde hrunge on; sinipelthen a t passere igenneni en Xtomkerne. Det betyder, a t hvis der skal v z r e en rimelig Sandsynlighed for Indfangning af Neutronen, niaa dens Samnienstod nied den oprindelige Kerne fare ti1 Dannelsen af e t Melleniprodukt, der fsrst kan ssnderdeles efter forholdsvis lang Tids Forlsb. Dette hanger netop saninien rned den fsr n a v n t e store Tatthed af Partikler i Atomkernen, der niedforer, a t den indfaldende Neutrons Energi ojeblikkelig fordeles mellem samtlige Kernepartikler, saaledes a t ingen af disse faar Energi nok ti1 straks a t forlade Kernen. E n eventuel senere Bortgang af en af Partiklerne vil derfor fordre en tilfzldig Koncentration af Energien paa vedkommende Partikel, hvad der for en tung Kerne paa Grund af det store Antal Partikler i Almindelighed vil k m v e saa lang en Tid, a t der forinden er en betydelig Sandsynlighed for en Straalingsudsendelse. Dette Forhold blev i Foredraget illustreret ved et Lysbillede, der er gengivet i Fig. I , og son1 viser en cirkelforniet Fordybning i en Plade, hvori der befinder sig e t Antal Billardkugler. Hvis en Kugle udefra strirdes ind i Fordybningen, og der ingen andre Kugler befandt sig i denne, vilde Kuglen passere op over Randen paa den modsatte Side og gaa videre med sin oprindelige Hastighed. Paa Grund af de ovrige Kuglers Nmvzrelse vil iniidlertid den ankommende Kugle hurtigt dele sin Xnergi med de svrige Kugler, der forudsat gnidningslss Bevzgelse, under hyppige indbyrdes Saninienstod vil

hevage sig frem og tilbage i Ford! bningen, indtil tilfzldigvis en af deni, tler er i Kxrheden af Kanden,. son1 Folge af Samnienstodene modtager en tilstrzkkelig Energi ti1 a t slippe ud. Fir der selv en ringe Gnidning ti1 Stede nielleni Kuglerne og Pladen eller bIulighed for, a t Kuglens kinetiske Energi ved Saniniestod omsattes i T-arnie, er der derimcd en betydelig Sandsynlighed for, a t ingen af Kuglerne nogensinde slipper Lid af Fordybnirigen, ganske svarende ti1 en Indfangning af en Seutron i en Kerne a n d t r Udsendelse af Straaling . Den oiwordentiige Lethed, hx-ornierl Energi fordeler sig iiielleni Atonikernens enkelte Dele, hetinger en gennenigribende E'orskel nielleni selve

Fig.

I.

Atonikernernes Egenskaber og de Egenskaber hos Xtomet, der vzsentlig a f h m g e r af det ydre Elektronsystem, Dette afspejler sig fmst og fremmest i den meget forskellige Fordeling af Energixzrdirrne for de mulige Tilstande for Kernen og for Blektronsystemet. Medens i det sidste T i l f d d e enhver Xndring af ,4toniets Energi i Slmindelighed kan henfmes ti1 en Z n d r i n g af Rindingsforholdene for en enkelt Elektron, er Kernens Energivzrclier bestemte ved de niulige kollektil-e Bevzgelsesformer for alle Kernens Dele. I Fig. z er der givet en skeniatisk Oversigt over Fordelingen af de saakaldte Energiniveauer for en Kerne nied en Atonivagt onitrent soni Jodets. De laveste Niveauer svarer her ti1 de Tilstande, der er bestenimende for de radioaktive Stoffers y-Straalespektre og har en gennenisnitlig Afstand paa nogle Hundrede Tusinde Elektron Volt. Med voksende Anslagsenergi rykker h'iveauerne meget hurtigt tzettere samnien og lader sig ikke lzngere rent adskille, naar vi komnier op ti1 saadanne Energier, der svarer ti1 Mellenitil-

79

standen for Indfangningen af hurtige Neutroner. Anslagsenergien for disse Tilstande f'indes ved ti1 Keutroneris kinetiske Energi (nogle 3Iillioner T'olt) a t l q g e dens Bitidirigsenergi i Kerrien (ca. 9 Millioner T'olt), der hidrorer fra de s t a r k e Tiltrakningskrafter iiielleni Kernedelene i smaa ;lfstande. Paa E'iguren er den ointrentlige Beliggenhed af de paagzldende Niveauer angivet ved det overste Forstorrelsesglas, der illustrerer, a t Xiveauerne i dette Omraade ligger saa tat, a t de nappe kan skelnes fra hverandre, se!v oili Maalestokken for I:iguren var valgt 100,000 Gange stnrre. De uskarpc Linier, sorii man ser igenneni Classet, skal endvidere illustrere, a t Niveauerne i dette Oiiiraade ikke eiigang kan ventes a t v z r e skarpt adskilte paa Grund af den Fig. 2 . endelige Levetid af de paagaldende Tiistande, der vzsentlig betinges af hluligheder, for en Xeutrons Undslipning fra Kernen. For lavere Anslagsenergier vil Niveauerne v z r e skarpere, idet Tilstandens Levetid alene begrmses af Sandsynligheden for Straalingsprocesser. Dette er illtistreret ved det andet Forstmrelsesglas, der er anbragt saaledes, a t det dzkker over et Energiomraade, der svarer ti1 en Forening af en hvilende Neutron ined den oprindelige Kerne. Deiine Energivzrdi er angivet ved den punkterede Linie i Forstmrelsesglassets Felt, niedens de fuldt optrukne Linier skal antyde Beliggenheden af nogle nzrliggende Kerneniveauer.

80

Et overbevisende Vidnesbyrd om en saadan tzt Fordeling af skarpe Energiniveauer i dette Ornraade har man faaet igennem Undersragelser over Kerners Indfangning af Neutroner med Hastigheder svarende ti1 Brrakdele af en Elektron Volt. Saadanne langsornme Neutroner frembringes, soni FEKMI fmrst har paavist, naar szdvanlige hurtige Neutroner passerer genneni tykke Lag af Paraffin eller andre brintholdige Stoffer. Sorn F d g e af Sarnrnenstmdene ined de disse indeholdende Protoner vil nemlig Neutronerne efterhaanden fordele deres kinetiske Energi nied Protonerne, indtil Neutronerne befinder sig i alniindelig Varrneligevzgt med det Stof, hvorigennern de passerer. I Modsatning ti1 den store Lighed, sorn hurtige Neutroners Reaktioner nied Kerner med ikke altfor forskellige Atornvzgte udviser, har det vist sig, a t Virkningen af Samrnensterd niellem Neutroner med Ternperaturhastigheder og Atomkerner varierer paa den tilsyneladende mest lunefulde Maade, naar man gaar fra e t Grundstof ti1 e t andet. Medens de fleste Stoffer ikke viser nogen specifik Virkning overfor langsomnie Keutroner, har andre Stoffer en overordentlig Evne ti1 a t reagere rned disse. Vi har her a t gerre med et typisk kvantemekanisk Resonansfznomen, der niaa forventes a t optrade, naar Sumnien af den indfaldende Neutrons kinetiske Energi og den oprindelige Kernes Energi i Kormaltilstanden tilfzldigvis falder sanimen med en anslaaet Energitilstand af den Kerne, der vilde dannes ved Keutronens Indfangning. Disse Fznomener laerer os derfor direkte om Energiniveauernes Fordeling og deres Skarphed i det paagaldende Ornraade, og en nzrmere Undersergelse har vist, a t Niveauernes gennernsnitlige Afstand her er omtrent 10 Volt, niedens deres Rredde kun belerber sig ti1 en Brerkdel af en Volt, saaledes sorn det er antydet paa Figuren igenneni Billedet i det paagzldende Forstmrelsesglas. Selv om mange Kernereaktioner paa Grund af den staerke elektriske Frasterdning melleni ladede Partikler ofte frembyder mere indviklede Forhold end Sammenstpd mellem Neutroner og Kerner, har det vist sig a t viere e t for alle Kernereaktioner f d l e s T r a k , a t deres Forlab kan beskrives som foregaaende i t o adskilte Stadier, af hvilke det fmste er en forelrabig Sammensrnelten af de t o sammenst~dendePartikler ti1 et mere eller mindre stabilt Mellemprodukt, medens det sidste bestaar i dette hlelleniprodukts senere Smderdeling under Udsendelse af materielle Partikler eller i en Straalingsudsendelse, hvorved dets endelige Stabilitet sikres. Hvilket Resultat en Kernereaktion giver, er derfor bestemt saavel af Mellemproduktets mulige Energitilstande som af den relative Sandsynlighed for de forskellige Sranderdelings- og Straalingsprocesser, son1 disse Tilstande kan give Anledning til. Ti1 Trods for Kerneproblernernes sterrre Komplikation i rnekanisk Henseende

81

samnienlignet med de szdvanlige Atomproblemer, betyder netop denne Mulighed for en Opdeling af Kernereaktionerne i vel adskilte Stadier en for tlisse ejendominelig Simplifikation, der i h0j Grad letter Oversigten over det baa hastigt voksende experimentelle Materiale vedrmende Atoinkernernes Egenskaher .

Hclsingfors 1936 Finska Litteratursiillskapets T r \ ckeri Ab.

P A R T I: PAPERS A N D MANUSCRIPTS RELATING TO NUCLEAR PHYSICS

TRANSLATION Professor Niels Bohr; Copenhagen: Properties of Atomic Nuclei' The lecture was introduced by a brief review of the development in physics which led to the knowledge of the constituents of the atom, and which started with the discovery of the electron at the turn of the century, and found its temporary completion in Lord Rutherford's discovery in 191 1 that every atom contains a positive nucleus of extremely small dimensions, in which most of the atomic mass is concentrated, and around which the much lighter negatively charged electrons are distributed. This simple picture of the atom made it possible to distinguish clearly between those properties of materials which are due to the internal structure of the atomic nucleus, and those which have their origin in the structure of the outer system of electrons. Whereas the usual physical and chemical properties of materials are concerned with changes in the outer electron system, radioactive phenomena in certain elements are due to processes in the nucleus itself. The discovery of the existence of isotopes at about the same time further emphasised the difference between these two sets of properties, since there were elements with otherwise identical physical and chemical properties which had different atomic weights and often even different radioactive properties. It was then discussed how the peculiar contradiction between the stability of the atom and the behaviour of the usual mechanical models found its explanation in the discovery of Planck's quantum of action. Whereas the actions occurring in the description of models on the everyday scale are so large that one can disregard the existence of the quantum of action, this is no longer true for atoms, and we therefore find here quite new regularities. Thus it turned out that any change in the state of an atom can be described as an individual process, in which the atom passes from one of the so-called stationary states to another, and in particular it has been possible to account to a large extent for the regularities of optical and X-ray spectra by the assumption that the emission of any line in these spectra is due to such a transition, with the emission of a light quantum. In the following years the gradual mathematical formulation of these ideas was brought to a temporary completion by the creation of a rational quantum mechanics, which forms a consistent generalisation of classical mechanics. The realisation that any measurement which is consistent with the existence of the quantum of

'

This is a summary o f the contents of the lecture which was given more informally, with the aid of numerous slides.

P A R T 1: P A P E R S A N D M A N U S C R I P T S R E L A T I N G T O N U C L E A R P H Y S I C S

action is accompanied by an uncontrollable interaction between the measured object and the measuring apparatus, has also brought a fundamental revision of the entire problem of observations, which has led to a complete clarification of the apparent paradoxes contained in the quantum mechanical description, which is fundamentally statistical. However, in spite of all these new features the problem of atomic structure preserves an extraordinary simplicity, which is, above all, due to the open structure of the electron system, which leads to the result that the binding of each individual electron can, to a first approximation, be described independently of the others, with the help of a classification which accounts in all details for the regularities in the periodic system of elements. In the problem of the structure and properties of the nucleus, on the other hand, one faces an entirely new situation, because of the tight packing of particles in the nuclei, and one must expect to meet here essentially different regularities from those valid for the binding of electrons in the atom. In recent years great experimental discoveries in nuclear physics have however provided a wealth of material, which already at this time opens up the possibility of a consistent description of the properties of atomic nuclei. The foundation for this whole development was laid by Rutherford's famous first nuclear disintegration experiment in 1919, where he succeeded in expelling protons by bombarding nitrogen atoms with a-particles. The reaction can be written in the form:

where the upper and lower indices represent the atomic weight and the nuclear charge, respectively. This pioneering work was soon followed by a whole series of experiments on nuclear transmutations, and the next decisive step consisted in using for the bombardment of matter artificially accelerated protons, instead of the naturally occurring a-rays used previously. Thus Cockcrof t and Walton succeeded in 1932 in splitting lithium into two a-particles by proton bombardment according to the scheme: :Li

+

:H

+

$He

+

;He.

This reaction was particularly interesting because the protons used in the bombardment had such a low energy that, by the ideas of classical physics, they would not have been able to overcome the electrostatic repulsion which acts between nuclei down to very small distances. However, in quantum theory such a reaction has a finite probability, as was already shown earlier by Gamow in con-

P A R T I: PAPERS A N D MANUSCRIPTS RELATING TO NUCLEAR PHYSICS

nection with his beautiful quantum theoretical explanation of the emission of aparticles from radioactive substances, where one is concerned just with a similar transition of particles through regions which they could not reach according to classical physics. Finally, the process which we have mentioned was remarkable because one could here in full detail account for the kinetic energy released in the reaction (about 16 MeV) with the help of Einstein’s relation for the equivalence of mass and energy, since the masses of all particles participating in the reaction were very accurately known from Aston’s mass spectroscopic measurements. Our knowledge of the atomic nucleus was further enriched to an extraordinary degree by Chadwick’s discovery in 1932 of the so-called neutron, a neutral particle with very nearly the same mass as the proton, which was first observed in the bombardment of beryllium with a-particles. The reaction can here be written as: :Be

+

:He

+

‘62C

+ 6n

This neutron could, as was soon found, occur in many different nuclear reactions, and it was therefore natural to consider it as a basic constituent of all nuclei, as stressed especially by Heisenberg. Accordingly each nucleus should contain only protons and neutrons, with their total number representing the atomic weight, while the number of protons gives the nuclear charge. According to this view, with which one eliminates the difficulties arising in quantum theory from the assumption of the presence of electrons in the nucleus itself, the electrons emitted in &ray transformations have to be regarded as created in the transformation itself, just as light quanta are created in the transitions between stationary atomic states. An entirely new epoch in nuclear physics was introduced already a year later by the discovery by the Curie-Joliots of the fact that certain new isotopes created by a-particle bombardment were /3-radioactive, and transformed with a certain lifetime with the emission of ordinary negative electrons or, in some cases, positive ones. These so-called positrons have to be regarded as new elementary particles, whose existence was predicted by Dirac’s relativistic electron theory, and which had been discovered a short time previously by Anderson and BIackett in the study of the secondary effects caused by cosmic rays. The first example of the production of this so-called artificial radioactivity was the following reaction:

where the neutron emission leaves a radioactive isotope of phosphorus, ?!P,

PART I: PAPERS A N D MANUSCRlPTS RELATING TO NUCLEAR PHYSICS

which in turn transforms with a half-life of 3 minutes into a silicon isotope, %i, together with the emission of a positron. In particular after Fermi demonstrated the great power of neutrons to cause transmutations in their collisions with nuclei we have learned in the last few years of an extraordinarily large number of new radioactive isotopes. This power results from the fact that neutrons do not ionise because they have no charge, and therefore lose energy not in passing through matter, as d o cx-particles, but only in collisions with nuclei, into which they can penetrate without being prevented by the nuclear electric field. As an example of a nuclear transmutation with neutrons we can quote: 32 16s + 6n ?:P + !H, -+

where the resultant phosphorus isotope is radioactive and transforms into the sulphur isotope with the emission of a negative electron and with a half-life of about 14 days. Precisely this unusually long half-life has made possible a number of important investigations about the transfer of phosphorus in chemical and biological processes, using Hevesy's radioactive indicator method which the discovery of artificial radioactivity has provided with an extraordinarily expanded and meaningful field of application. Whereas all nuclear reactions so far mentioned involve the emission of material particles, the investigations of Fermi and his collaborators have also acquainted us with a particular group of neutron reactions in which the incident neutron is simply captured by the nucleus, with the emission of the excess energy in the form of electromagnetic radiation (y-rays). A typical example is the reaction : 127 531

+

6n

-+

':$I

+

y,

where the resultant new radioactive iodine isotope has a half-life of 26 minutes. Processes of this type are of particular interest, as they give us a new insight into the mechanism of nuclear reactions. Indeed one can conclude from a closer study of the y-ray spectra of radioactive substances that the time needed by an excited nucleus for emitting electromagnetic radiation is very long compared to the time a neutron would take to pass simply through an atomic nucleus. This means that, in order to have a reasonable probability for capture of the neutron, its collision with the original nucleus must lead to the formation of an intermediate product, which can disintegrate only after a relatively long time. This is connected with the above-mentioned great density of particles in the nucleus, as a result of which the energy of the incident neutron is immediately shared with all other nuclear particles, so that none of them have enough energy to escape from the nucleus at once. A possible later escape of one of the particles therefore requires an ac-

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PART I: PAPERS A N D MANUSCRIPTS RELATING TO NUCLEAR PHYSICS

cidental concentration of energy on the particle concerned, and for a heavy nucleus this will, because of the large number of particles, generally require so long a time that meanwhile there is an appreciable probability for the emission of radiation. This situation was illustrated in the lecture by a slide, which is reproduced as fig. 1, and which shows a circular well in a board containing a number of billiard balls. If a ball was projected from outside into the well with no other balls in it, the ball would pass over the opposite rim and continue with its original speed. Because of the presence of the other balls the incident ball will rapidly share its energy with the others, which, assuming the motion frictionless, will move back and forth in the well with frequent mutual collisions, until by accident one of them near the edge acquires by the collisions enough energy to escape. If there is even a little friction between the balls and the board, or the possibility that the kinetic energy of the ball will be transformed into heat in the collisions, there will however be a strong probability that none of the balls will ever be able to get out of the well, in complete analogy to the capture of a neutron with the emission of radiation.

Fig. 1.

The extraordinary facility with which the energy gets shared between the individual particles in the nucleus implies a fundamental difference between the properties of the nucleus and the properties of the atom, which depend essentially on the outer electron system. This manifests itself above all in the very different distribution of energy values for the possible states of the nucleus and for the electron system. Whereas in the latter case any change in the energy of the atom can in general be attributed to a change in the quantum numbers of a single electron, the energy levels of the nucleus are determined by the possible forms

P A R T I : PAPERS A N D MANUSCRIPTS RELATING TO N U C L E A R PHYSICS

of collective motion of all its particles. Fig. 2 gives a schematic indication of the distribution of the so-called energy levels for a nucleus with an atomic weight similar to iodine. The lowest levels here correspond to the states which determine the y-spectra of radioactive substances, and have an average spacing of a few hundred thousand electron volts. With increasing excitation energy the levels rapidly accumulate more closely and can no longer be distinguished clearly when we come to those energies which correspond t o the intermediate state for the capture of a fast neutron. To find the excitation energy of these states we have to add t o the kinetic energy of the neutron (a few MeV) its binding energy in the nucleus (about 9 MeV), which results from the strong forces of attraction between nuclear particles at small distances. The figure shows the approximate position of the levels in question by the upper magnifying glass, which indicates that the levels in this region lie so close that they can hardly be separated from each other even on a scale increased 100 000 times. The diffuse lines seen through the glass are intended to illustrate further that the Fig. 2. levels in this region cannot even be expected to be sharply separated because of the finite lifetime of the relevant states, which is essentially due to the possibility of neutron escape from the nucleus. For lower excitation energy the levels become sharper, since the lifetime of the state is limited only by the probability for radiative processes. This is illustrated by the second magnifying glass, which is placed so as to cover the energy region corresponding to the addition of a neutron at rest to the original nucleus. This energy value is indicated by the

PART I: PAPERS AND MANUSCRIPTS RELATING TO NUCLEAR PHYSICS

broken line in the field of the magnifying glass, whereas the full lines are meant to indicate the position of some nearby levels. Convincing evidence of such a dense distribution of sharp energy levels in this region has been obtained from the studies of the capture by nuclei of neutrons at speeds corresponding to a fraction of an electron volt. Such slow neutrons are produced, as was first shown by Fermi, when the usual fast neutrons pass through thick layers of paraffin or other substances containing hydrogen. This is because in the collisions with the protons contained in these, the neutrons will share their kinetic energy with the protons until the neutrons are in thermal equilibrium with the material through which they are passing. In contrast to the great similarity between the reactions of fast neutrons with nuclei of not too different atomic weight, it has turned out that the effect of collisions of neutrons at thermal velocities with nuclei varies in an apparently most capricious manner from one element to another. While most substances show no specific effect for slow neutrons, other substances have an extraordinary power for reacting with them. We are here concerned with a typical quantum-mechanical resonance phenomenon, which must be expected to occur when the sum of the kinetic energy of the incident neutron and the energy of the ground state of the original nucleus happens to coincide with an excited state of the nucleus formed by the capture of the neutron. These phenomena therefore provide direct information about the distribution of energy levels and their sharpness in the region in question, and more detailed research has shown that the average spacing between the levels is about 10 volts, whereas their width amounts to only a fraction of a volt as is indicated in the figure in the view through the appropriate magnifying glass. Although many nuclear reactions often present a more complicated behaviour than the collisions between neutrons and nuclei because of the strong electric repulsion between charged particles, it has been found as a common feature of all nuclear reactions that their course can be described as proceeding in two separate stages of which the first is a temporary fusion of the two colliding particles into a more or less stable intermediate product, whereas the second consists of a later disintegration of the intermediate product with the emission of material particles or in the emission of radiation, ensuring its final stability. The result of the nuclear reaction is therefore determined both by the possible states of the intermediate product, and by the relative probabilities of the various disintegration and radiative processes to which these states can give rise. In spite of the great complexity of the mechanical picture for nuclei, compared to the relevant atomic problems, just the possibility of dividing the nuclear reactions into well separated stages represents for these a peculiar simplification, which greatly facilitates the survey of the so rapidly growing experimental material concerning the properties of atomic nuclei.