from alchemy to atoms · the top portion of the ﬁgure shows the periodic table in 1941. each...

To this day, plutonium—the famous isotope plutonium-239 in particular—continues to play

a crucial role in nuclear weapons andpeaceful nuclear energy. Having atomicnumber Z = 94, plutonium is the nextelement after neptunium in the periodictable and is two elements up from ura-nium. Yet until 1940, no elementsbeyond uranium were known to exist.They were not found because alltransuranic elements are radioactive,with lifetimes that are short comparedwith geologic times. While large quan-tities may have existed very early inEarth’s history, all natural accumula-tions have long since disappeared.1

The understanding of how to createplutonium and other transuranic ele-

ments was the result of several break-throughs in nuclear physics in the1930s, including the discovery of the neutron in 1932 and of artificial radioactivity in 1934. Those remarkabledevelopments encouraged nuclear sci-entists, the modern-day alchemists, topursue the lofty goal of reintroducingthese elements to the Earth. But withthe discovery of fission in 1938 camethe potential to liberate huge amountsof nuclear energy. Once it was realizedthat plutonium-239 might undergo fis-sion by slow neutrons, the eruditedesire to simply create it was quicklysuperseded by another: to create enoughmaterial to build a weapon, one sopowerful that it would change the affairs of man.

Chain Reactions

The year 1932 is considered the beginning of modern nuclear physics.In that annus mirabilis, the neutron wasdiscovered by James Chadwick, thepositron was identified by Carl Ander-son, and the particle accelerator of JohnCockroft and Ernest Walton was firstused to perform artificial disintegrationsof the atomic nucleus.

Before the discovery of the neutron,

62

1Minute amounts of transuranic elements arecontinuously produced on Earth. Through aprocess described in this article, uranium in uranium ore will absorb neutrons from natural radioactivity and get transmuted into neptuniumand plutonium. Also, transuranic elementsformed in violent cosmic explosions may spreadinto the cosmos and fall to Earth.

From Alchemy to Atoms The making of plutonium

André Michaudon

Los Alamos ScienceNumber 26 2000

From Alchemy to Atoms

several groups had shown that bom-barding boron and beryllium withα-particles resulted in the emission of apenetrating radiation—one that couldknock out protons from absorbers con-taining hydrogen. Through numerousexperiments, Chadwick was able todemonstrate that this radiation was actually a neutral particle with a massnearly identical to that of the proton.

Dubbed a neutron, such a particlehad already been envisaged by ErnestRutherford as early as 1920, but as aproton-electron combination. Togetherwith Royds, Rutherford had alreadyproved (in 1908) that α-particles werehelium nuclei. By 1911, Rutherfordshowed that essentially the entire massof an atom was contained in a tiny,positively charged nucleus. After thediscovery of the neutron as an un-charged elementary particle, thenucleus was understood to consist ofneutrons and protons. The number ofprotons Z determines the electriccharge, or elemental identity, of thenucleus, whereas the mass number Adetermines the isotopic identity (seeFigure 1).

Chadwick’s discovery also openedthe door to neutron-induced nucleartransmutations. The alchemists’ dreamof transmuting the elements had alreadybecome a reality as early as 1919, whenRutherford demonstrated that bombard-ing nuclei with α-particles resulted inreaction products that were differentfrom the target nuclei. For example, nitrogen-14 was transformed into oxy-gen-17 by the reaction

(1)

The α-particle (helium-4 nucleus) isabsorbed by the nitrogen to create afleeting, unstable nuclear state, whichrapidly decays to oxygen-17 by emittinga proton. This reaction and the othersknown at the time were induced withpositively charged projectiles—namely,protons or α-particles—whose energyhad to be sufficient to overcome the

Coulomb repulsion from the target nucleus. But neutrons have no electriccharge and, consequently, no Coulombbarrier to overcome. Therefore, they canpenetrate matter and be absorbed by nuclei more easily than charged particles.

The first to realize that the neutronwas also the key to releasing the energyembedded in the nucleus was Leo Szi-

lard, a Hungarian physicist. On Septem-ber 12, 1933, Szilard envisionedneutron-mediated chain reactions. Aneutron could induce a nuclear reactionthat would emit two or more neutronsand thus act as a neutron multiplier.This nuclear reaction would also liber-ate energy. Because they are uncharged,emitted neutrons would not lose energy

14N + 4He 17O + p .7 2 8

Number 26 2000 Los Alamos Science 63

Proton

Hydrogen Deuterium Helium-4

UHeHH

Uranium-238

NeutronHAtomicnumber, Z

Element symbol

Massnumber, A

Element Key

11

11

21

42

23892

Figure 1. Elements and NucleiThe top portion of the figure shows the periodic table in 1941. Each column contains

elements with similar chemical properties. Each row contains elements arranged in order

of increasing atomic number Z, the number of protons in the atomic nucleus. In 1941,

no elements beyond uranium had been identified, and thorium,

protactinium, and uranium were thought to be transition metals.

James Chadwick discovered the neutron, a neutral particle with

nearly the same mass as the positively charged proton. His dis-

covery led to a concise model of the nucleus. Each has Z protons,

N neutrons, and a mass number A = Z + N. Hydrogen, the first

element, has the simplest nucleus (a lone proton). Deuterium is

an isotope of hydrogen. (Isotopes of the same element have the

same number of protons but different numbers of neutrons.)

The nucleus of the next element, helium, is also known as an

α-particle. Nuclei increase in size roughly as A1/3. Thus,

uranium-238 (with 238 nucleons—protons plus neutrons) is a huge nucleus, and it is dis-

torted like a football. Uranium has two common isotopes, uranium-235 and uranium-238.

**

*

3

58 59 63626160 64 65 66 67 68 69 7170

1

4

11 12

19 20 21

37 38 39

55 57

87 88 89

22

40

72

23

73

41

24

42

74

25

43

75

91 92 93 94

26 27 28 29 30

44 45 46 47

76 77 78 79

48

80

5 6 7 8

2

9 10

13 14 15 16 17 18

3635343331 32

545352515049

86858483828156

90

Li

H

Na

K

Rb

Cs

Fr

Be

Mg

Ca

Sr

Ba

Ra

He

Ne

Ar

Kr

Xe

Rn

F

Cl

Br

I

At

O

S

Se

Te

Po

N

P

As

Sb

Bi

C

Si

Ge

Sn

Pb

B

Al

Ga

In

Tl

Sc

Y

La

Ac

Ti

Zr

Hf

V

Nb

Ta

Cr

Mo

W

Mn

Tc

Re

Fe

Ru

Os

Zn

Cd

Hg

Co

Rh

Ir Au

Ag

CuNi

Pd

Pt

Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Th Pa U X X

Rh

*Lanthanides

Periodic Table of the Elements—1941


James Chadwick

to electrons in matter and would there-fore race through a material until theycollide with other nuclei. These neu-trons could then induce similarreactions, again followed by neutronmultiplication. The same process wouldcontinue with more and more reactions.If two neutrons were emitted, one reac-tion would be followed by two, whichwould be followed by four, then eight,sixteen, thirty-two, sixty-four, and soon. The number of reactions would increase geometrically, as would thetotal energy released. A self-sustainingchain reaction would be established, resulting in an enormous release of

nuclear energy (see Figure 2). If theneutrons were fast, each generation ofthe chain reaction would occur in ashort time, and there would be manygenerations before the energy liberatedby the process blew the material apart.A massive explosion would occur witha force millions of times greater thananything man had previously unleashed.

Harnessing nuclear energy wastherefore inextricably linked to creatinga nuclear bomb, and Szilard acceptedthis woeful connection as he beganlooking for elements that could act asneutron multipliers when bombarded byneutrons. He thought that beryllium, for

example, might emit two neutrons whenit absorbed one. Szilard also realizedthat neutrons had to induce neutron-multiplying reactions before diffusingout of the material. Along those lines,Szilard introduced the concept of criti-cal mass (of still unknown elements), or the minimum amount of materialneeded to sustain a chain reaction. Although Rutherford had stated that energy could not possibly be releasedfrom atomic nuclei, by 1934 Szilardhad filed a patent on this subject.

Nuclear Transmutations

Before Szilard began thinking aboutchain reactions, nuclear transmutationshad been achieved but were initiallythought to produce stable nuclei likethe oxygen-17 of Reaction (1). In fur-ther studies, positrons were observedafter light elements such as boron, alu-minum, or magnesium had beenbombarded with α-particles. Positrons(e+) are the antimatter counterparts ofelectrons. They are positively chargedand have a mass equal to that of theelectron. Frederic Joliot and Irène Curieobserved that positron emission contin-ued after α-ray irradiation had beenstopped. Furthermore, the number ofemitted positrons decreased exponen-tially with time. The positron signalwas therefore similar to what would beexpected from the decay of a radioac-tive element.

Using a chemical precipitationmethod, Joliot and Curie separated thesource of this persistent positron emis-sion from an irradiated aluminum targetand showed that the source was an unstable isotope of phosphorus thatsubsequently decayed into silicon-30 bypositron emission:

(2)

(3)

Joliot and Curie had thus discoveredartificial radioactivity (1934). Whereas

30P 30Si + e+ + ν .15 14

27Al + 4He 30P + n , and13 2 15


64 Los Alamos ScienceNumber 26 2000

(a) Absorption of only one neutron per nuclear reaction leads to sustainable nuclear power.

Neutronabsorbed

Neutrons emitted,energy released

(b) Absorption of more than one neutronper nuclear reaction can lead to an exponentiallyincreasing number of nuclear reactionsand a powerful explosion.

Figure 2. A Nuclear Chain Reaction Leo Szilard realized that a self-sustaining chain reaction could occur if absorption of

one neutron causes a nucleus to emit several others. Each reac-

tion releases energy, and the amount of energy released per unit

time depends on the rate at which emitted neutrons are reab-

sorbed, inducing more reactions. (a) If one neutron is reabsorbed

per reaction on average, the chain proceeds in a linear fashion.

This is the concept of a nuclear reactor, wherein the number of

neutrons is purposely limited to keep the chain reaction under

control. (b) If more than one neutron is reabsorbed, on average,

the number of nuclear reactions increases geometrically. Without

controls, the chain can grow so quickly and release so much

energy that a massive explosion occurs.

Leo Szilard

natural radioactivity had been observedin many of the heaviest elements (fromthallium to uranium), artificially induced nuclear transmutations couldnow create new nuclei across the peri-odic table. Furthermore, Joliot-Curiehad rigorously proved their result bychemically isolating their product. In this way, they established a prece-dent for identifying transmuted nuclei.

Spurred on by these remarkableevents, several groups pursued usingneutrons, rather than charged particles,to induce nuclear transmutations. The idea was given its greatest impetusin Rome, under Fermi’s leadership. Beginning in early 1934, Fermi and hiscollaborators carried out a systematicstudy of nearly every element in the periodic table. Stable elementswould be bombarded with neutrons,and Fermi would measure the activity,that is, the intensity of the radiation induced in the irradiated sample.

A major step forward occurred whenFermi and colleagues accidentally discovered that the activity increaseddramatically when the incident neutronswere slowed down. Neutrons lose

energy when they scatter from hydro-gen or other light elements, and the insertion of paraffin (which containslots of hydrogen atoms) between a neu-tron source and the irradiated samplewas sufficient to slow the neutronsdown. Even neutrons that had sloweddown to room temperature—so-calledthermal neutrons with energies of onlya fraction of an electron volt (eV)—would lead to a high activity.

The enhanced activity, an indicationthat many unstable nuclei were beingcreated, was the result of neutron-radiative capture. A slow neutron canbe absorbed by a target nucleus to forma relatively long-lived intermediate stateknown as the compound nucleus. (Thecompound nucleus model was proposedby Niels Bohr in 1936.) The bindingenergy of the absorbed neutron is con-verted into excitation energy of thecompound nucleus, which quickly de-cays by γ-ray emission to its groundstate (or sometimes to an isomericstate). The newly created nucleus—anisotope one mass unit higher than thetarget nucleus—can be unstable, inwhich case it decays after a characteris-

tic half-life by emitting either α-parti-cles (α-decay) or β-particles (β-decay).

When neutron absorption is followedby β-decay, a neutron (or a proton) inthe unstable nucleus transforms into aproton (neutron), creating a β-particleand a type of neutrino. If a neutrontransforms, the β-particle is an electronand the neutrino an electron antineutrino,both of which flee the nucleus. Thenewly created proton remains in the nucleus, so that a new element—oneatomic number higher but with the samemass number—is created. Neutron bom-bardment could therefore be used toproduce transuranic elements. As seen in Figure 3, neutron irradiation of uranium-238 would create uranium-239,which was expected to β-decay to ele-ment X-239 with atomic number Z = 93.

Confident that he would produce the first transuranic element, Fermitried his neutron source on uranium.But while he observed β-activity tocome from the sample, direct confirma-tion of a new element escaped him. Thechemistry of transuranic elements wasnot known at that time, and separationof β-emitters from irradiated samples



Figure 3. Creating Transuranic ElementsEnrico Fermi discovered that irradiating elements with low-energy (slow) neutrons greatly

increased the probability of neutron capture. Often, the result was the creation of an unstable nucleus

that would radioactively decay. If uranium-238 captured a slow neutron, it would become the com-

pound nucleus uranium-239* (an excited state of uranium-239), which would cool down to the ground

state by emitting γ-rays. It was expected that uranium-239 would then undergo β-decay, wherein a

neutron in uranium-239 decays into a proton, an electron, and an electron antineutrino, thus creating

the first transuranic element, X-239, with atomic number Z = 93.

Neutron-radiative capture (10–16 s) Beta decay (23 min)

n + U U* U + γ23892

23992

23992 U X + e + ν239

92239

93

e

ν

X23993

U239

92U*239

92U

23892

Slow neutronis absorbed.

Compound nucleus formsin an excited state.

Compound nucleus cools down by radiating γ-rays.

Unstable uranium nucleus in its ground state β-decays to element 93.

Enrico Fermi

proved difficult. Using chemical separa-tion techniques, Fermi could only provethat the activity did not come from uranium, lead, or any element betweenthem in the periodic table. Fermi postu-lated that an unexplained β-activity witha half-life of 13 minutes came from atransuranic element, but in the absenceof chemical proof would not go beyondthis suggestion.

More troublesome was the fact thatthe radiations emitted by the neutron-activated samples were remarkablycomplex. Besides the 13-minute activitywere several others that could not beidentified nor simply interpreted interms of transuranic radioactive decays.

In September 1934, Ida Noddack sug-gested that the complex radiations mightbe coming from nuclear products lighterthan lead. Because lead has 10 protonsless than uranium, Noddack was sug-gesting that absorption of a singlelow-energy neutron resulted in a sub-stantial breakup of the uranium nucleus.This was unimaginable. At the time, all known nuclear reactions resulted inonly minor changes of the nucleus. Nod-dack’s suggestion was largely ignoredand Fermi himself, after some calcula-tions, patently dismissed the idea.

Noddack was correct however, andironically, Fermi could have seen evi-dence for such a nuclear breakup withinhis own laboratory in early 1935. Atthat time, Fermi’s group made anotherattempt to detect transuranic elementsfrom irradiated uranium samples, usinga recently built ionization chamber.They postulated that, if neutron absorp-tion followed by β−decay producedtransuranic nuclei and if those nucleihad a short half-life forα−decay, theywould emit energetic α-particles (according to the Geiger-Nuttal law).To reduce low-energy background fromnatural α-radioactivity, the Italian sci-entists placed a thin aluminum foilbetween the uranium sample and thedetector. While stopping low-energy α-particles of natural radioactivity, thisfoil would be partially transparent tothe higher-energy α-particles that mightbe emitted from short-lived transuranic



Slow neutrons are absorbed.

92U

Prompt neutron

Prompt neutron

Fission

Heavyfragment

Lightfragment

Neutron capture

56Ba

36Kr

Figure 4. Fission(a) Hahn and Strassmann chemically isolated radioactive barium from a neutron-

irradiated sample of pure uranium. They realized that the uranium nucleus had split

into two nuclei, one of which was barium. Fission could explain the confusing activity

of neutron-irradiated uranium samples that was observed by groups in Rome, Paris,

and Berlin. The fission fragments are born with a significant amount of excitation

energy that is dissipated through prompt-neutron and γ-ray emission. Because

de-excited fission products are far from the β-stability valley, they subsequently

β-decay, and then always emit γ-rays and sometimes delayed neutrons. Also, fission

does not produce a unique pair of primary fission fragments before prompt-neutron

emission or a unique pair of fission products after prompt-neutron emission. (b) The

β-decay chains of fission products are illustrated here for barium-141 ( Z = 56) and kryp-

ton-92 ( Z = 36). They were obtained after prompt emission of 3 neutrons from the

primary fission fragments produced in uranium-235 neutron-induced fission. The γ-ray

emission that follows β-decay is not shown. The β-decays from krypton-92 and rubidi-

um-92 (Z = 37) are also followed by neutron emission (the delayed neutrons) from

rubidium-92 and strontium-92, respectively. Later, experiments proved that both prompt

β− β− β−Ba La Ce Pr14156

14157

14158

14159

β− β− β−Rb Sr Y Zr9137

9138

9139

9140

β− β− β−Rb Sr Y Zr9237 β−Kr92

369238

9239

9240

18.2 min 3.9 h

58 s 9.5 h 58.5 d Stable

4.5 s1.84 s

Delayed-neutronemission (0.03%)

(b) Fission-Product Decay Chains

Delayed-neutronemission (0.012%)

2.71 h 3.54 h Stable

32.5 d Stable

Fritz Strassmann Otto Hahn and Lise Meitner Otto Frisch

(a)



elements. The experiment proved unsuccessful—no α-particles were detected—but fission products were undoubtedly produced and would havebeen detected if the aluminum foil hadnot been there to stop them!

The multitude of β-activities fromneutron-irradiated uranium remained amystery for quite a while. Indeed, by1935, two other groups had observedsimilar startling results: Irene Curie andPavel Savitch in Paris, and Lise Meit-ner, Otto Hahn, and Fritz Strassmann inBerlin. A 23-minute activity seemed tooriginate from uranium-239 and wouldimply the creation of X-239, but neitherthe Paris nor the Berlin scientists couldexplain their results convincingly.

All these studies and speculationstook a radically different turn with thediscovery of fission by Hahn and Strass-mann in December 1938. (Lise Meitner,who was Jewish, had to flee the Naziregime. She left Berlin in July.) Usingradiochemical methods, the team clearlydemonstrated that a pure uranium sam-ple contained radioactive barium (Z =56) after having been irradiated by neu-trons. The barium could only come fromthe splitting, or fission, of the uraniumnucleus. Fission could explain the con-fusing spectrum of radiation that wasobserved after neutron-induced activa-tion of uranium—the radiation came inpart from the great variety of fissionfragments and their descendants (seeFigure 4). Also, Szilard’s vision of nu-clear energy released from nuclear chainreactions suddenly switched from dreamto reality. The prompt emission of neu-trons from the hot fission fragmentswould provide the neutron multiplicationmechanism needed for a chain reaction.

Fission

Although nuclear fission had escapedtheoretical prediction, it was immediatelyexplained in terms of existing nuclearmodels. Like neutron-radiative capture,neutron-induced fission was interpretedas one mode of decay of the compoundnucleus. The details could easily

EfbEne

rgy

Increasing nuclear deformation

Increasing nuclear deformation

LDM fission barrier

LDM

−5

0

0

5

10

Ene

rgy

(MeV

)

and delayed neutrons were emitted during fission. (c) Lise Meitner and Otto Frisch

used the liquid drop model (LDM) to explain fission. The nucleus is considered to be a

positively charged drop of fluid acted upon by two opposing forces. The attractive

strong force holds the nucleons together and results in a surface tension that shapes

the drop into a compact sphere. The electric (Coulomb) force tries to break the drop

apart. As the size of the nucleus increases, the surface tension grows less rapidly than

the long-range Coulomb force because the strong force has very limited range. The net

result is that larger nuclei become increasingly unstable with respect to shape distor-

tions. It requires additional energy to further distort the nucleus, seen by the fission

barrier in the potential-energy diagram. When uranium absorbs a neutron, the excita-

tion energy sets the nucleus oscillating, and the extra energy allows it to deform so

much that it tops the fission barrier ( E > Efb). The nucleus breaks in two. The fission

fragments carry off about 170 MeV of kinetic energy, more than a factor of 20 greater

than the energy released by α- or β-decay, and tens of millions of times greater than

that released by breaking chemical bonds. (d) A modern calculation of the fission

barrier and nuclear deformation in plutonium-240 is shown here. The double hump

(solid line) arises from considering the detailed quantum mechanics of the nucleus.

(Calculation courtesy of Peter Möller, Los Alamos National Laboratory.)

(c)

(d)

be described within the context of the liquid-drop model (LDM), originally putforward by George Gamow in 1928 andthen given a much more serious develop-ment by Bohr in 1937. The LDMignores the discrete nature of the protonsand neutrons. Instead, it models the nucleus as an electrically charged liquiddrop that can deform, vibrate, or split. It can also merge with another drop as infusion reactions. The model provided astraightforward prescription for calculat-ing basic nuclear properties, such as themass and size of nuclei.

Meitner had remained in close com-

munication with the Berlin group, andwithin a week of the discovery of fis-sion, she and her nephew Otto Frischused the LDM to describe the process.In the heuristic picture presented in Fig-ure 4, the drop-like compound nucleusdeforms so much upon absorbing a neu-tron that it elongates into a dumbbellshape that finally scissions into twodroplets, also known as fission frag-ments. (Frisch was the one who coinedthe term “fission” after a discussion withthe American biologist William Arnoldabout the splitting of bacterial cells.)The electrically charged droplets repel

each other, and they instantly fly apart torapidly reach about 170 million electronvolts (MeV) of total kinetic energy afterfull acceleration. By mid-January of1939, Frisch had observed these ener-getic fission fragments with an ionizationchamber, and subsequently observedneutron-induced fission in thorium-232.

Experimental studies quickly focusedon whether neutron emission accompa-nied fission. R. B. Roberts, R. C. Meyer,and P. Wang in the United States firstshowed that “delayed” neutrons wereemitted following the β-decay of fissionproducts. But other studies soon revealed



236U*

239U*

Efb

Pot

entia

l ene

rgy

(MeV

)

Deformation

Fissionbarrier

Figure 5. Fission Cross Sections(a) By February 1939, Niels Bohr realized that the two

uranium isotopes behave differently with respect to fis-

sion. Both isotopes have an even number of protons

(Z = 92). Uranium-235 has an odd

number of neutrons ( N = 143),

whereas uranium-238 has an

even number (N = 146). When

uranium-238 absorbs a neutron, it forms the even-odd nucleus uranium-239 and gains about 4.8 MeV of bind-

ing energy, which goes into nuclear excitation. This amount of energy is not enough to top the fission barrier

of about 6 MeV, so uranium-238 needs to absorb a neutron with kinetic energy greater than about 1 MeV in

order to fission. But the strong force that binds the nucleus together “likes” to pair up protons and neutrons;

hence, when uranium-235 absorbs a neutron to form the even-even nucleus uranium-236, it gains about

6.5 MeV of binding energy. That amount is more than enough to allow the excited nucleus to fission regard-

less of the incident-neutron energy. Uranium-235 is fissile. (b) The top graph shows the different fission

cross-sections for uranium isotopes as a function of neutron energy. At thermal energies of about 0.025 eV,

the uranium-235 cross section is about 500 barns. In the bottom graph, the energy spectrum of prompt neutrons emitted in the sl ow-

neutron-induced fission of uranium-235 is quite wide, with an average energy of about 2 MeV. About half of these neutrons cannot

induce fission in uranium-238 (hatched area). The fraction that cannot induce fission is even higher in nuclear devices because t he

neutrons lose energy through nuclear interaction with device components.

��

��

��

��

��

1.5

1.0

0.5

0

0.4

0.3

0.2

0.1

02 31

Neutron energy En (MeV)

Neu

tron

-indu

ced

fissi

on c

ross

sec

tion

N (

En)

235U

238U

238U fission threshold

Niels Bohr

(a) (b)

that, as the initial fission fragments form,they immediately release “prompt” neu-trons. Fermi, Szilard and collaborators atColumbia University, and Joliot and hiscollaborators at Paris demonstrated thatmore than one prompt neutron was emit-ted during fission. Joliot andcollaborators first reported this findingfor uranium, with the precise averagenumber of 3.5 ± 0.7 prompt neutrons.The modern figure is now an average of2.4 prompt neutrons per fission.

Therefore, the prospect of nuclearenergy liberated from fission chain reactions came within reach, but as understood by groups in the UnitedStates, Western Europe, the SovietUnion, and later Japan, the prospects ofachieving an explosive chain reactionusing natural uranium were dim. Short-ly after the discovery of fission inthorium, Bohr had analyzed the avail-able data and concluded that onlyuranium-235, not uranium-238, fis-sioned when bombarded with slowneutrons. Neutrons of any energy couldinduce the lighter isotope to fission (itwas said to be a fissile material),whereas neutrons had to have energieson the order of 1 MeV or more to induce uranium-238 to fission.

The difference stems from the factthat uranium-235 has an odd number ofneutrons and uranium-238 has an evennumber (both isotopes have an evennumber of protons). Thus, when uranium-235 absorbs a neutron, it pairswith the odd neutron, creating additionalexcitation energy (or pairing energy)and pushing the resulting uranium-236compound nucleus above the fissionbarrier (see Figure 5). Uranium-238 hasno unpaired neutron to match up withthe absorbed neutron, and thus no pair-ing energy and no fission results untilthe incident neutron brings in the need-ed energy. Bohr and John Wheeler usedthe LDM to strengthen the argument intheir 1939 landmark paper on fission. InMarch 1940, by comparing slow-neutron-induced fission in separateduranium isotopes, Alfred O. Nier et al.finally provided experimental proof thaturanium-235 was fissile.

In a nuclear device, however, fastneutrons would have to induce enoughfission reactions and release enough energy before the material is blown off.In this respect, the role of the two ura-nium isotopes strongly depends on theenergy spectrum of fission neutrons. Itwas later shown that fission neutronshave a wide energy spectrum with anaverage energy of about 2 MeV. Giventhis spectrum, only half of these fissionneutrons can induce fission in uranium-238 but this fraction would be evenlower in a nuclear device, where theneutron energy is degraded through nuclear interactions with the compo-nents of the device (see Figure 5). Evenwith nuclear data available at that time,uranium-238 appeared unsuitable fornuclear detonations. Uranium-235 ismuch more suitable, but its isotopicabundance in natural uranium is only0.7 percent. Thus, a bomb made of nat-ural uranium would be very inefficient.In May 1940, French physicist FrancisPerrin estimated the critical mass ofnatural uranium to be on the order oftens of tons. Shortly thereafter, more-precise calculations by Rudolf Peierlsreduced this number to some tons. Butsimilar calculations for pure uranium-235 made by Frisch and Peierls led to acritical mass of a few pounds for thisisotope. This was well within the realmof a bomb.

Obtaining pure uranium-235, however,was a severe problem. Because thechemical properties of isotopes areidentical, chemical separation of urani-um-235 from uranium-238 was notpossible. The fissile isotope could be obtained in large amounts only througha separation process that exploited theslight 1.2 percent difference in mass between uranium-235 and uranium-238.The most promising method was agaseous diffusion method,which had previously been developed by the Ger-man physical chemist Klaus Clusius. But the equipment and facilities neededto achieve the separation would have tobe constructed on a gigantic scale. Frischestimated that approximately 100,000tubes would be necessary. This number

was, however, drastically reduced to5,000 tubes in subsequent developments.Still, with the European continent at warand the possibility that German scientistsmight be developing a bomb, plans weremade to separate uranium isotopes andbuild a uranium-235 bomb.

Almost as these plans were beingdrawn up, Bohr’s analysis of fissionablenuclei had another implication. Thetransuranic element 239 with Z = 94protons and 145 neutrons, could also befissile. In May 1940, Turner outlinedthis possibility in a letter to Szilard,who urged secrecy, for he realized thatif element-239 with Z = 94 were creat-ed, subsequent separation of thistransuranic element from uranium couldbe done through chemistry, thus obviat-ing the difficult and expensive need toseparate isotopes.

Seven months later, Fermi and Segrècame to the same conclusion. They alsounderstood that the neutron flux within anuclear reactor—a device that allows fora self-sustaining, controlled chain reac-tion—would be extremely high. Fermihad believed that a controlled chain re-action could be achieved in naturaluranium if slow neutrons were used. Herealized that neutron-radiative capture bythe uranium-238 reactor fuel would thenproduce large quantities of element-239.

Studies on chain reactions were soonconcentrated at Columbia. The use ofgraphite as a neutron moderator provedfeasible when measurements of neutronattenuation in a large graphite assemblyproved that the absorption of neutronsby carbon was lower than anticipated.Also, measurements of the multiplica-tion factor in a lattice structure ofgraphite and uranium gave hope that amultiplication factor greater than 1could be achieved. Therefore, a nuclearreactor could possibly be made withsuch a lattice.

The Discovery of Neptunium

The discovery of fission in the wan-ing days of 1938 had cleared the wayfor more-precise studies on transuranic



elements. Taking into account the radioactivity of fission products, the German team confirmed that a previously observed β-activity with a 23-minute half-life actually came fromuranium-239. While they failed to detectthe first transuranic, their work helpedan American research effort to succeed.

At Berkeley, Edwin McMillan beganusing cyclotron-produced neutrons to irradiate a thin foil of uranium. The energetic fission fragments would flyout of the foil. McMillan discoveredthat the foil retained some activity. Inaddition to the known 23-minute activity, indicative of uranium-239, another 2.3-day activity remained. Although this last activity could nothave originated from fission fragments,the source behaved chemically like therare earths, which are abundant in mostfission fragments. This puzzling incon-sistency led McMillan and PhilipAbelson to repeat the experiment. Thesecond time, they found that the 2.3-day activity came from an elementwith chemical properties different from

those of any other element but close tothose of uranium.

Moreover, McMillan and Abelsondemonstrated in May 1940 that the 2.3-day activity grew from uranium-239. They carefully purifiedthe activated uranium sample and thenobtained precipitates at 23-minute inter-vals, using equal amounts of cerium asa carrier. The activity of the precipitatewas measured a day later to allow anyuranium-239 to totally decay awaywhile preserving most of the 2.3-dayactivity. The intensity of the latter activity showed a 23-minute exponen-tial decay as a function of the time theprecipitate was taken from the activateduranium sample. This demonstrated thaturanium-239 decays with a 23-minutehalf-life to a new radioactive transuranicelement that has a 2.3-day half-life.

McMillan and Abelson had thus dis-covered the first transuranic element withmass number 239 and atomic number Z = 93. They called it neptunium—afterthe planet Neptune, which was next inline beyond Uranus in the solar system.

Because the chemical properties of neptunium-239 were different from thoseof rare earths but close to those of urani-um, McMillan and Abelson suggestedfor the first time that neptunium mightbelong to a second “rare earth” group ofsimilar elements starting with uranium.

Their suggestion was prophetic. GlennSeaborg later proved that this second“rare earth” group actually starts after actinium (Z = 89). Today, the group isreferred to as the actinides. (See the box“The Actinide Concept” on page 368.)

The Discovery of Plutonium

The next step was the formation andidentification of the transuranic elementwith Z = 94, soon to be dubbed “pluto-nium” even before its discovery. (Thenew element was named after Pluto,after the next planet beyond Neptune inthe solar system.) As a follow-up oftheir studies on neptunium-239, McMillan and Abelson had already attempted to identify plutonium-239 as

Figure 6. Discovery of Plutonium at Berkeley

After failing to detect plutonium-239 from the neptunium-239 sample produced in

neutron-irradiated uranium, the Berkeley team irradiated uranium with deuterons.

This deuteron-induced reaction produced two neptunium isotopes—neptunium-239

with a β-decay half-life of 2.3 days and neptunium-238 with a β-decay half-life of

2.1 days. The data in (a) show the α-activity in the neptunium sample from

deuteron-irradiated uranium. It was observed with a new high-sensitivity ionization

chamber. Although neptunium-238 contributed only 5% of the total β-activity,

the product of that decay (plutonium-238) produced more than 98% of the observed

α-activity. The α-activity in (b) originated from plutonium-239 that had grown in a neptunium sample from neutron-irradiated uranium.

(Early experiments by McMillan and Abelson were not sensitive enough to detect this small plutonium-239 α-activity.)



300

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365 α c/min / mCi β

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Glenn Seaborg Art Wahl

(a) (b)

the product of the 2.3-day decay of nep-tunium-239. Plutonium was expected todecay by emission of α- rather than β-particles, and so McMillan and Abel-son looked for radiation from α-decay(or spontaneous fission2) from a purifiedsample of neptunium-239 obtained fromneutron-irradiated uranium. They failedto detect any, and the negative result ledthem to postulate that the half-life forthese two processes in plutonium-239was greater than a million years.

McMillan left Berkeley shortly there-after to help develop radar for the wareffort while Kennedy, Seaborg, Segrè,and graduate student Art Wahl contin-ued the pursuit of plutonium. A majorstep forward occurred when the groupused the 60-inch cyclotron at Berkeleyto irradiate uranium with deuterons withthe hope that another plutonium isotope,shorter-lived than plutonium-239, couldbe formed, whose α-decay could bemore easily detected. Studies of γ-raysand β-rays emitted from these deuteron-irradiated uranium samples revealed thepresence of two neptunium isotopes—neptunium-239 and another thataccounted for about 5 percent of the β-ray activity. This other isotope (lateridentified as neptunium-238) had a half-life of 2.1 days, close to that of nep-tunium-239. These neptunium samplesalso showed the existence of an α-emitter whose chemical propertieswere found to be different from those ofneptunium, uranium, and all other knownelements. The only possibility was thatthis α-emitter was the second transuranicelement—plutonium—which was createdby the β-decay of neptunium.

The growth of plutonium from the neptunium samples was later stud-ied in the spring of 1941 from theradiation emitted by these samples withthe help of a newly developed ioniza-tion chamber that could be placedbehind the poles of the magnet. Withthis arrangement, β−rays were deflectedaway from the ionization chamber but

not the α−rays, which could then bemore easily detected. The growth stud-ies showed that the α-decay detectedfrom neptunium chemically separatedfrom deuteron-irradiated uranium sam-ples came mostly from plutonium-238,which was produced by the β-decay ofthe neptunium-238. (see Figure 6). Although this decay accounted only for5 percent of the total β-decay in thesample, the grown-in plutonium-238 hasa relatively short half-life (88 years) andtherefore a relatively high α-activity.

The same sensitive technique wasused with neptunium samples chemicallyseparated from neutron-irradiated uranium, and the α-ray activity from plutonium-239 was finally detected. Pre-vious experiments with neutron-irradiateduranium were not sensitive enough to detect the relatively small α-activity ofplutonium-239, which is due to its rela-tively long half-life (24,000 years).

Making Bulk Plutonium

Small but ponderable quantities of plutonium-239 (0.5 microgram), producedusing cyclotrons, soon became availableand were used to demonstrate that fissioninduced by thermal neutrons was largerby a factor of 1.28 than for uranium-235.A larger plutonium-239 sample (3.5 mi-crograms) was used in July 1941 todemonstrate that its fission cross sectionwith fast neutrons was also larger thanthat for uranium-235. Although uncertain-ties still existed about the number ofprompt fission neutrons emitted, it already appeared that plutonium-239 wasa strong competitor to uranium-235 fornuclear explosives.

Then, a remarkable effort followedfor making enough plutonium to build a bomb. At the beginning of 1942, all work on plutonium was supervisedby the Metallurgical Laboratory at theUniversity of Chicago and was placedunder the leadership of Arthur Compton.Three missions were given to this labo-ratory: (1) to find a system using naturaluranium in which a controlled chain reaction could take place, (2) to find a

chemical separation technique for pluto-nium formed in the irradiated uraniumfuel, and (3) to obtain theoretical and experimental nuclear data relevant to anexplosive chain reaction with uranium or plutonium.

The first objective, which had already been started by Fermi and Szi-lard at Columbia, gained momentumwhen the work shifted to the Universityof Chicago. In an abandoned squashcourt, Fermi assembled what he knewwould be a self-sustaining nuclear reac-tor. Twenty-five feet high and 20 feetwide, the reactor was built from349,263 kilograms of pure carbon cutas large graphite bricks. Blind holeswere drilled into about a quarter of thebricks, and about 36,500 kilograms ofuranium oxide and 5600 kilograms of pure uranium metal were pressedinto thousands of fuel elements anddropped into the holes. An active bricklayer contained an array of evenlyspaced uranium plugs. These weresandwiched between two dead layersof solid graphite. As the two types oflayers were alternated and stacked, afull 3-dimensional lattice of uraniumplugs was formed (see photo on arti-cle’s opening page).

Collisions with the carbon atoms inthe graphite would slow the neutrons tothermal energies, whereupon theywould diffuse until they encountered auranium-235 atom. The chain reactionwas held in check by cadmium controlrods that would absorb neutrons with-out fissioning. As the number of layersincreased, the reactor came increasinglyclose to critical mass. On December 2,1942, with the final layers in place andthe control rods removed, the reactorwent critical and sustained the world’sfirst manmade nuclear chain reaction.

Several steps were taken to meetthe second objective—the bulk-scalechemical separation of plutonium fromreactor fuel. By the end of 1942, about 500 micrograms of mostly pluto-nium-239 had been obtained from largequantities of neutron-irradiated uranium.The irradiation had been carried out atthe cyclotrons at Berkeley and Wash-



2Spontaneous fission, wherein the nucleus under-goes fission without an initiating neutron, wasdiscovered in uranium by George Flerov and K. A. Petrjak of the Soviet Union in June, 1940.

ington University. Chemical studieswith these first milligram samplesshowed that it would be possible to sep-arate plutonium from other elements in the reactor fuel.

Subsequent developments on a largerscale included the construction of a pilot nuclear plant at Clinton, Tennessee.This plant was to produce plutonium inquantities larger than the fraction of amilligram obtained from cyclotrons inorder to demonstrate the possibility ofplutonium separation on an industrialscale and study the biological effects of radiation. The Clinton reactor startedoperation on November 4, 1943, by producing 500 kilowatts, and it reached1800 kilowatts in May 1944. Plutoniumwas separated from slugs of irradiateduranium fuel by remote control and behind thick shields. The first slug wastreated for plutonium separation on December 20, 1943, and by March1944, several grams of plutonium wereavailable. These larger quantities of plu-tonium were also essential in obtainingnuclear data on plutonium.

The last step toward the productionand separation of plutonium-239 was the construction of three nuclear piles onthe Hanford site, whose purchase wasapproved by General Groves almost immediately after the creation of MetLab. These reactors were designed tosupply enough plutonium-239 for nucleardevices. The first pile started operation inSeptember 1944, and the three piles werein operation by the summer of 1945.

Met Lab’s third objective—obtainingprecise nuclear data on plutonium-239—was met by experiments carried out atBerkeley and at the new, secret LosAlamos site, where J. Robert Oppen-heimer had been appointed director.Although early data already indicated thatplutonium would be fissile, more preciseand sometimes new data were needed todesign a weapon. Most of these data wereobtained by measurements of fissioncross sections and fission neutrons. Thenumber of prompt neutrons emitted perfission of plutonium-239 proved to behigher than for uranium-235, which enhanced the potential for using plutoni-

um-239 as a weapons material. Although these plutonium-239 data

were encouraging, other data seemed toindicate that plutonium-239 might not be used in the weapon that was being designed at Los Alamos. The plutoniumsamples obtained from the Clinton pilotplant were emitting an unexpectedly highneutron intensity from spontaneous fission, which had not been seen in thesamples from the Berkeley cyclotron.

The high neutron flux and long irradia-tion times in the Clinton reactor allowedplutonium-239 itself to capture neutronsand become plutonium-240, which has ahigh spontaneous fission rate. The spon-taneous fission rate in plutonium-239 ishindered by a spin effect that makes the fission barrier higher than for plutoni-um-240 (see Figure 7). The plutoniumfrom the Hanford piles would have anisotopic composition similar to that of the



1020

235U239Pu

240Pu

249Cf

255Fm

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100

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Hal

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e T

gf (

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40 42

Figure 7. Spontaneous Fission Half-LivesExperimental spontaneous-fission half-lives Tgf for isotopes of transuranic elements

are plotted as a function of the fissility parameter Z2/A (Z and A are the atomic and

mass number, respectively, for each isotope). The solid lines are drawn through

the half-lives of isotopes having even numbers of protons ( Z) and neutrons ( N).

These even-even isotopes have spin 0 and positive parity. The solid square is for plu-

tonium-240. The spontaneous fission half-lives of isotopes having even Z but odd N

(such as uranium-235, plutonium-239, californium-249, and fermium-255), indicated by

filled circles, are systematically higher than those for the even-even isotopes of the

same element. These even-odd isotopes have a spin different from zero and a parity

that can be either positive or negative. Their spontaneous-fission half-lives are longer

because the fission barrier is higher to accommodate their spin and parity. Similarly,

the α-decay half-life of even-odd isotopes is higher than for neighboring even-even

isotopes (see the discussion of plutonium-238 and plutonium-239 in the text) because

the α-particles have to penetrate a higher potential barrier—another manifestation of

a spin effect—in these even-odd nuclei.

plutonium produced in the Clinton pile.At the time of this discovery,

the favored weapon design was the“gun-type” design, wherein a subcriticalamount of fissile material would be liter-ally fired—like a bullet from a gun—into another subcritical mass. The rapidassembly of a critical mass from thesetwo fissile pieces would result in a run-away chain reaction and a nuclearexplosion. But the ever-present emissionof neutrons from spontaneous fissionwould cause premature fissioning of the plutonium before a critical mass wasachieved. A plutonium gun-type weaponwould thus fizzle.

The use of plutonium in a nucleardetonation, therefore, required the for-mation of the critical mass with a fastermethod. This new method was foundwith the implosion of a plutonium shell, whose efficiency was laterdemonstrated at the Alamogordo test. �

Further Reading

Amaldi, E. 1984. Physics Reports111: 1.

Bohr, N., and Wheeler, J. A. 1939. Phys. Rev. 56: 426.

Chamberlain, O., G. W. Farwell, and E. Segrè, 1944. “94240 and Its Spontaneous Fission” (September). Los Alamos Scientific Laboratory report LAMS-131.

Curie, I., and F. Joliot.1934a. Comptes Rendus198: 254.

———.1934b. Comptes Rendus198: 559.

Fermi, E. 1934a. Nature. 133: 757.

———1934b. Nature. 133: 898.

Flerov, G. N, and K. A. Petrjak 1940. Phys. Rev. 58: 89.

Frisch, O. R. 1939. Nature 143: 276.

Frish, O. 1979. What Little I Remember.New York: Cambridge University Press.

Goldschmidt, B. 1982. La Recherche131(13): 366.

Hahn, O., and F. Strassmann. 1939. Naturwissenschaften27:11.

Holloway, D. 1994. Stalin and the Bomb—the Soviet Union and Atomic Energy 1939–1956. New Haven: Yale University Press.

Hughes, J. 2000. Phys. World. July issue:43.

Joliot, F., and I. Curie. 1934. Nature133: 201.

Kennedy, J. W., G. T. Seaborg, E. Segrè, and A. C. Wahl. 1946. Phys. Rev. 70: 555.

Lanouette, W., and B. Silard. 1994. Genius in the Shadows, A Biography of Leo Szilard. Chicago: The University of Chicago Press.

McMillan, E., and P. H. Abelson. 1940. Phys. Rev. 57: 1185.

Meitner, L., and O. R. Frisch. 1939. Nature143: 239.

Nier, A. O., E. T. Booth, J. R. Dunning, and A. V. Grosse. 1940. Phys. Rev. 57: 546.

Noddack, I. 1934. Angew. Chemie47: 653.

Rhodes. R. 1987. The Making of the AtomicBomb. New York: Simon & Schuster.

Roberts, R., R. Meyer, and P. Wang. 1939. Phys. Rev. 55: 510.

Seaborg, G. T., and E. Segrè. 1947. Nature4052: 863.

Kathren, R. L. J. B. Gough, and G. T. Benefiel, eds. 1994. The Plutonium Story: The Journals of Professor Glenn T. Seaborg.Columbus, OH: Battelle Press.

Seaborg, G. T., E. M. McMillan, J. W. Kennedy,and A. C. Wahl. 1946. Phys. Rev. 69: 367.

Segré, E. 1970. Enrico Fermi: Physicist. Chicago:The University of Chicago Press.

Segré, R. 1993. A Mind Always in Motion: The Autobiography of Emilio Segrè. Los Angeles: The University of California Press.

Sime, R. L. 1998. Scientific American. January issue: 80.

Szilard, L., and W. H. Zinn. 1939. Phys. Rev. 55: 799.

Turner, L. A. 1940. Rev. Mod. Phys. 12: (1): 1.

von Halban, H., F. Joliot, and L. Kowarski. 1939.Nature 143: 680.

Wahl, A. Discovery of Plutonium. 1990. In Proceedings of the Robert A. Welch Foundation Conference on Chemical ResearchXXXIV, Fifty Years with Transuranium Elements, October 22–23, Houston, Texas.

Wheeler, J. A. 1989. Ann. Rev. Nucl. Part. Sci.39: xiii.



André Michaudon obtained two engineering degrees from the French Grandes Ecoles. He thenserved as Lieutenant in the French Navy. After ashort appointment as a microwave research engi-

neer in a privatecompany, he had a longcareer at the Commissari-at à l’Energie Atomique.He served as a groupleader at the Centre d’Etudes Nucléaires at Saclay, where he pioneered neutron time-of-flight work with anelectron linear accelerator

used as a pulsed-neutron source. Results of thesestudies, which focused on fission, formed thebackbone of a Ph.D. thesis submitted at the Sor-bonne. Intermediate resonances in theneptunium-237 subthreshold fission cross sectionwere discovered in the course of these studies.Work at Saclay was interrupted by a sabbaticalyear of studies in theoretical nuclear physics atthe Massachusetts Institute of Technology underthe supervision of H. Feshbach. André thenmoved to Bruyeres le Chatel, as Head of the Nuclear Physics Division, and later to Limeil asDeputy Head of the Department of GeneralPhysics. Later, André was appointed as Frenchco-Director of the Institut Laue Langevin, inGrenoble. In this position, he was instrumental inobtaining the decision to build the European Synchrotron Research Facility at Grenoble. André Michaudon was a Professor of NuclearPhysics at the Institut des Sciences et Techniques Nucléaires. In addition, he was a member andchairman of many national and international scientific committees and served on the ExecutiveCouncil of the European Science Foundation. He has been associated with the Los Alamos National Laboratory first as a consultant and thena staff member. André Michaudon is a Fellow of the American Physical Society and of the American Nuclear Society.

from alchemy to atoms · the top portion of the ﬁgure shows the periodic table in 1941. each...

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