From Alchemy to Atoms
From Alchemy to Atoms The making of plutonium
o this day, plutonium—the ments was the result of several break- Chain Reactions famous isotope plutonium-239 throughs in nuclear physics in the Tin particular—continues to play 1930s, including the discovery of The year 1932 is considered the a crucial role in nuclear weapons and the neutron in 1932 and of artiﬁcial beginning of modern nuclear physics. peaceful nuclear energy. Having atomic radioactivity in 1934. Those remarkable In that annus mirabilis, the neutron was number Z = 94, plutonium is the next developments encouraged nuclear sci- discovered by James Chadwick, the element after neptunium in the periodic entists, the modern-day alchemists, to positron was identiﬁed by Carl Ander- table and is two elements up from ura- pursue the lofty goal of reintroducing son, and the particle accelerator of John nium. Yet until 1940, no elements these elements to the Earth. But with Cockroft and Ernest Walton was ﬁrst beyond uranium were known to exist. the discovery of ﬁssion in 1938 came used to perform artiﬁcial disintegrations They were not found because all the potential to liberate huge amounts of the atomic nucleus. transuranic elements are radioactive, of nuclear energy. Once it was realized Before the discovery of the neutron, with lifetimes that are short compared that plutonium-239 might undergo ﬁs- with geologic times. While large quan- sion by slow neutrons, the erudite 1Minute amounts of transuranic elements are continuously produced on Earth. Through a tities may have existed very early in desire to simply create it was quickly process described in this article, uranium in Earth’s history, all natural accumula- superseded by another: to create enough uranium ore will absorb neutrons from natural tions have long since disappeared.1 material to build a weapon, one so radioactivity and get transmuted into neptunium and plutonium. Also, transuranic elements The understanding of how to create powerful that it would change the formed in violent cosmic explosions may spread plutonium and other transuranic ele- affairs of man. into the cosmos and fall to Earth.
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several groups had shown that bom- Periodic Table of the Elements—1941 barding boron and beryllium with 1 2 α H He -particles resulted in the emission of a 3 4 567 8 910 penetrating radiation—one that could Li Be B C N O F Ne 11 12 13 14 15 16 17 18 knock out protons from absorbers con- Na Mg Al Si P S Cl Ar 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 taining hydrogen. Through numerous K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 experiments, Chadwick was able to Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe 5556 57 * 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 demonstrate that this radiation was Cs Ba La Hf Ta W Re Os IrPt Au Hg Tl Pb Bi Po At Rn actually a neutral particle with a mass 87 88 89 ** 90 91 92 93 94 Fr Ra Ac Th Pa U X X nearly identical to that of the proton.
Dubbed a neutron, such a particle 58 596061 62 63 64 65 66 67 68 69 70 71 *Lanthanides had already been envisaged by Ernest Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Rutherford as early as 1920, but as a Proton Uranium-238 proton-electron combination. Together Element Key with Royds, Rutherford had already Mass proved (in 1908) that α-particles were number, A 1 Element Neutron helium nuclei. By 1911, Rutherford symbol Atomic 1H showed that essentially the entire mass number, Z of an atom was contained in a tiny, positively charged nucleus. After the discovery of the neutron as an un- charged elementary particle, the Hydrogen Deuterium Helium-4 nucleus was understood to consist of neutrons and protons. The number of protons Z determines the electric charge, or elemental identity, of the 1 2 4 238 1H 1H 2He 92U nucleus, whereas the mass number A determines the isotopic identity (see Figure 1. Elements and Nuclei Figure 1). The top portion of the ﬁgure shows the periodic table in 1941. Each column contains Chadwick’s discovery also opened elements with similar chemical properties. Each row contains elements arranged in order the door to neutron-induced nuclear of increasing atomic number Z, the number of protons in the atomic nucleus. In 1941, transmutations. The alchemists’ dream no elements beyond uranium had been identiﬁed, and thorium, of transmuting the elements had already protactinium, and uranium were thought to be transition metals. become a reality as early as 1919, when James Chadwick discovered the neutron, a neutral particle with Rutherford demonstrated that bombard- nearly the same mass as the positively charged proton. His dis- ing nuclei with α-particles resulted in covery led to a concise model of the nucleus. Each has Z protons, reaction products that were different N neutrons, and a mass number A = Z + N. Hydrogen, the ﬁrst from the target nuclei. For example, element, has the simplest nucleus (a lone proton). Deuterium is nitrogen-14 was transformed into oxy- an isotope of hydrogen. (Isotopes of the same element have the gen-17 by the reaction same number of protons but different numbers of neutrons.) The nucleus of the next element, helium, is also known as an James Chadwick α-particle. Nuclei increase in size roughly as A1/3. Thus, 14 4 17 (1) uranium-238 (with 238 nucleons—protons plus neutrons) is a huge nucleus, and it is dis- 7N + 2He 8O + p . torted like a football. Uranium has two common isotopes, uranium-235 and uranium-238.
The α-particle (helium-4 nucleus) is Coulomb repulsion from the target lard, a Hungarian physicist. On Septem- absorbed by the nitrogen to create a nucleus. But neutrons have no electric ber 12, 1933, Szilard envisioned ﬂeeting, unstable nuclear state, which charge and, consequently, no Coulomb neutron-mediated chain reactions. A rapidly decays to oxygen-17 by emitting barrier to overcome. Therefore, they can neutron could induce a nuclear reaction a proton. This reaction and the others penetrate matter and be absorbed by that would emit two or more neutrons known at the time were induced with nuclei more easily than charged particles. and thus act as a neutron multiplier. positively charged projectiles—namely, The ﬁrst to realize that the neutron This nuclear reaction would also liber- protons or α-particles—whose energy was also the key to releasing the energy ate energy. Because they are uncharged, had to be sufﬁcient to overcome the embedded in the nucleus was Leo Szi- emitted neutrons would not lose energy
Number 26 2000 Los Alamos Science 63 From Alchemy to Atoms
(a) Absorption of only one neutron per nuclear reaction leads to sustainable nuclear power. example, might emit two neutrons when it absorbed one. Szilard also realized that neutrons had to induce neutron- multiplying reactions before diffusing out of the material. Along those lines, Szilard introduced the concept of criti- Neutron Neutrons emitted, cal mass (of still unknown elements), absorbed energy released or the minimum amount of material needed to sustain a chain reaction. (b) Absorption of more than one neutron per nuclear reaction can lead to an exponentially Although Rutherford had stated that increasing number of nuclear reactions energy could not possibly be released and a powerful explosion. from atomic nuclei, by 1934 Szilard had ﬁled a patent on this subject.
Before Szilard began thinking about chain reactions, nuclear transmutations had been achieved but were initially thought to produce stable nuclei like the oxygen-17 of Reaction (1). In fur- ther studies, positrons were observed after light elements such as boron, alu- Figure 2. A Nuclear Chain Reaction minum, or magnesium had been Leo Szilard realized that a self-sustaining chain reaction could occur if absorption of bombarded with α-particles. Positrons one neutron causes a nucleus to emit several others. Each reac- (e+) are the antimatter counterparts of tion releases energy, and the amount of energy released per unit electrons. They are positively charged time depends on the rate at which emitted neutrons are reab- and have a mass equal to that of the sorbed, inducing more reactions. (a) If one neutron is reabsorbed electron. Frederic Joliot and Irène Curie per reaction on average, the chain proceeds in a linear fashion. observed that positron emission contin- This is the concept of a nuclear reactor, wherein the number of ued after α-ray irradiation had been neutrons is purposely limited to keep the chain reaction under stopped. Furthermore, the number of control. (b) If more than one neutron is reabsorbed, on average, emitted positrons decreased exponen- the number of nuclear reactions increases geometrically. Without Leo Szilard tially with time. The positron signal controls, the chain can grow so quickly and release so much was therefore similar to what would be energy that a massive explosion occurs. expected from the decay of a radioac- tive element. to electrons in matter and would there- nuclear energy (see Figure 2). If the Using a chemical precipitation fore race through a material until they neutrons were fast, each generation of method, Joliot and Curie separated the collide with other nuclei. These neu- the chain reaction would occur in a source of this persistent positron emis- trons could then induce similar short time, and there would be many sion from an irradiated aluminum target reactions, again followed by neutron generations before the energy liberated and showed that the source was an multiplication. The same process would by the process blew the material apart. unstable isotope of phosphorus that continue with more and more reactions. A massive explosion would occur with subsequently decayed into silicon-30 by If two neutrons were emitted, one reac- a force millions of times greater than positron emission: tion would be followed by two, which anything man had previously unleashed. 27 4 30 would be followed by four, then eight, Harnessing nuclear energy was 13 Al + 2He 15P + n , and (2) sixteen, thirty-two, sixty-four, and so therefore inextricably linked to creating on. The number of reactions would a nuclear bomb, and Szilard accepted 30 30 + ν increase geometrically, as would the this woeful connection as he began 15P 14Si + e + . (3) total energy released. A self-sustaining looking for elements that could act as chain reaction would be established, neutron multipliers when bombarded by Joliot and Curie had thus discovered resulting in an enormous release of neutrons. He thought that beryllium, for artiﬁcial radioactivity (1934). Whereas
64 Los Alamos Science Number 26 2000 From Alchemy to Atoms
Neutron-radiative capture (10–16 s) Beta decay (23 min)
238 239 239 γ 239 239 X ν n + 92 U 92 U* 92 U + 92 U 93 + e + e
238 239 239 239 92U U 92U* 92 93X
Figure 3. Creating Transuranic Elements Enrico 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. Enrico Fermi natural radioactivity had been observed energy when they scatter from hydro- tic half-life by emitting either α-parti- in many of the heaviest elements (from gen or other light elements, and the cles (α-decay) or β-particles (β-decay). thallium to uranium), artiﬁcially insertion of parafﬁn (which contains When neutron absorption is followed induced nuclear transmutations could lots of hydrogen atoms) between a neu- by β-decay, a neutron (or a proton) in now create new nuclei across the peri- tron source and the irradiated sample the unstable nucleus transforms into a odic table. Furthermore, Joliot-Curie was sufﬁcient to slow the neutrons proton (neutron), creating a β-particle had rigorously proved their result by down. Even neutrons that had slowed and a type of neutrino. If a neutron chemically isolating their product. down to room temperature—so-called transforms, the β-particle is an electron In this way, they established a prece- thermal neutrons with energies of only and the neutrino an electron antineutrino, dent for identifying transmuted nuclei. a fraction of an electron volt (eV)— both of which ﬂee the nucleus. The Spurred on by these remarkable would lead to a high activity. newly created proton remains in the events, several groups pursued using The enhanced activity, an indication nucleus, so that a new element—one neutrons, rather than charged particles, that many unstable nuclei were being atomic number higher but with the same to induce nuclear transmutations. created, was the result of neutron- mass number—is created. Neutron bom- The idea was given its greatest impetus radiative capture. A slow neutron can bardment could therefore be used to in Rome, under Fermi’s leadership. be absorbed by a target nucleus to form produce transuranic elements. As seen Beginning in early 1934, Fermi and his a relatively long-lived intermediate state in Figure 3, neutron irradiation of collaborators carried out a systematic known as the compound nucleus. (The uranium-238 would create uranium-239, study of nearly every element in compound nucleus model was proposed which was expected to β-decay to ele- the periodic table. Stable elements by Niels Bohr in 1936.) The binding ment X-239 with atomic number Z = 93. would be bombarded with neutrons, energy of the absorbed neutron is con- Conﬁdent that he would produce and Fermi would measure the activity, verted into excitation energy of the the ﬁrst transuranic element, Fermi that is, the intensity of the radiation compound nucleus, which quickly de- tried his neutron source on uranium. induced in the irradiated sample. cays by γ-ray emission to its ground But while he observed β-activity to A major step forward occurred when state (or sometimes to an isomeric come from the sample, direct conﬁrma- Fermi and colleagues accidentally state). The newly created nucleus—an tion of a new element escaped him. The discovered that the activity increased isotope one mass unit higher than the chemistry of transuranic elements was dramatically when the incident neutrons target nucleus—can be unstable, in not known at that time, and separation were slowed down. Neutrons lose which case it decays after a characteris- of β-emitters from irradiated samples
Number 26 2000 Los Alamos Science 65 From Alchemy to Atoms
proved difﬁcult. Using chemical separa- tion techniques, Fermi could only prove that the activity did not come from uranium, lead, or any element between them in the periodic table. Fermi postu- lated that an unexplained β-activity with a half-life of 13 minutes came from a transuranic element, but in the absence of chemical proof would not go beyond Fritz Strassmann Otto Hahn and Lise Meitner Otto Frisch this suggestion. More troublesome was the fact that (a) Neutron capture Fission the radiations emitted by the neutron- Prompt neutron activated samples were remarkably Ba Heavy complex. Besides the 13-minute activity 56 92U fragment were several others that could not be identiﬁed nor simply interpreted in Light Kr terms of transuranic radioactive decays. 36 fragment In September 1934, Ida Noddack sug- Slow neutrons are absorbed. Prompt neutron gested that the complex radiations might be coming from nuclear products lighter than lead. Because lead has 10 protons (b) Fission-Product Decay Chains less than uranium, Noddack was sug- gesting that absorption of a single 18.2 min 3.9 h 32.5 d Stable low-energy neutron resulted in a sub- 141 141 141 141 Ba β− La β− Ce β− Pr stantial breakup of the uranium nucleus. 56 57 58 59 This was unimaginable. At the time, all known nuclear reactions resulted in s 4.5 s1.84 2.71 h 3.54 h Stable only minor changes of the nucleus. Nod- dack’s suggestion was largely ignored 92 − 92 − 92 − 92 − 92 36Kr β 37Rb β 38 Sr β 39 Y β 40 Zr and Fermi himself, after some calcula- tions, patently dismissed the idea. Delayed-neutron Delayed-neutron Noddack was correct however, and emission (0.03%) emission (0.012%) ironically, Fermi could have seen evi- 91 91 91 91 Rb β− Sr β− Y β− Zr dence for such a nuclear breakup within 37 38 39 40 his own laboratory in early 1935. At 58 s 9.5 h 58.5 d Stable that time, Fermi’s group made another attempt to detect transuranic elements Figure 4. Fission from irradiated uranium samples, using (a) Hahn and Strassmann chemically isolated radioactive barium from a neutron- a recently built ionization chamber. irradiated sample of pure uranium. They realized that the uranium nucleus had split They postulated that, if neutron absorp- into two nuclei, one of which was barium. Fission could explain the confusing activity tion followed by β−decay produced of neutron-irradiated uranium samples that was observed by groups in Rome, Paris, transuranic nuclei and if those nuclei and Berlin. The ﬁssion fragments are born with a signiﬁcant amount of excitation had a short half-life for α−decay, they energy that is dissipated through prompt-neutron and γ-ray emission. Because would emit energetic α-particles de-excited ﬁssion products are far from the β-stability valley, they subsequently (according to the Geiger-Nuttal law). β-decay, and then always emit γ-rays and sometimes delayed neutrons. Also, ﬁssion To reduce low-energy background from does not produce a unique pair of primary ﬁssion fragments before prompt-neutron natural α-radioactivity, the Italian sci- emission or a unique pair of ﬁssion products after prompt-neutron emission. (b) The entists placed a thin aluminum foil β-decay chains of ﬁssion products are illustrated here for barium-141 (Z = 56) and kryp- between the uranium sample and the ton-92 (Z = 36). They were obtained after prompt emission of 3 neutrons from the detector. While stopping low-energy primary ﬁssion fragments produced in uranium-235 neutron-induced ﬁssion. The γ-ray α-particles of natural radioactivity, this emission that follows β-decay is not shown. The β-decays from krypton-92 and rubidi- foil would be partially transparent to um-92 (Z = 37) are also followed by neutron emission (the delayed neutrons) from the higher-energy α-particles that might rubidium-92 and strontium-92, respectively. Later, experiments proved that both prompt be emitted from short-lived transuranic
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elements. The experiment proved (c) unsuccessful—no α-particles were detected—but ﬁssion products were undoubtedly produced and would have been detected if the aluminum foil had not been there to stop them! The multitude of β-activities from neutron-irradiated uranium remained a mystery for quite a while. Indeed, by Efb Energy 1935, two other groups had observed LDM fission barrier similar startling results: Irene Curie and Pavel Savitch in Paris, and Lise Meit- ner, Otto Hahn, and Fritz Strassmann in Berlin. A 23-minute activity seemed to originate from uranium-239 and would 0 Increasing nuclear deformation imply the creation of X-239, but neither the Paris nor the Berlin scientists could explain their results convincingly. (d) All these studies and speculations took a radically different turn with the 10 discovery of ﬁssion by Hahn and Strass- mann in December 1938. (Lise Meitner, who was Jewish, had to ﬂee the Nazi regime. She left Berlin in July.) Using 5 radiochemical methods, the team clearly demonstrated that a pure uranium sam- ple contained radioactive barium (Z = 56) after having been irradiated by neu-
Energy (MeV) trons. The barium could only come from 0 LDM the splitting, or ﬁssion, of the uranium nucleus. Fission could explain the con- fusing spectrum of radiation that was observed after neutron-induced activa- −5 tion of uranium—the radiation came in Increasing nuclear deformation part from the great variety of ﬁssion fragments and their descendants (see and delayed neutrons were emitted during ﬁssion. (c) Lise Meitner and Otto Frisch Figure 4). Also, Szilard’s vision of nu- used the liquid drop model (LDM) to explain ﬁssion. The nucleus is considered to be a clear energy released from nuclear chain positively charged drop of ﬂuid acted upon by two opposing forces. The attractive reactions suddenly switched from dream strong force holds the nucleons together and results in a surface tension that shapes to reality. The prompt emission of neu- the drop into a compact sphere. The electric (Coulomb) force tries to break the drop trons from the hot ﬁssion fragments apart. As the size of the nucleus increases, the surface tension grows less rapidly than would provide the neutron multiplication the long-range Coulomb force because the strong force has very limited range. The net mechanism needed for a chain reaction. 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- Fission 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 Although nuclear ﬁssion had escaped fragments carry off about 170 MeV of kinetic energy, more than a factor of 20 greater theoretical prediction, it was immediately than the energy released by α- or β-decay, and tens of millions of times greater than explained in terms of existing nuclear that released by breaking chemical bonds. (d) A modern calculation of the ﬁssion models. Like neutron-radiative capture, barrier and nuclear deformation in plutonium-240 is shown here. The double hump neutron-induced ﬁssion was interpreted (solid line) arises from considering the detailed quantum mechanics of the nucleus. as one mode of decay of the compound (Calculation courtesy of Peter Möller, Los Alamos National Laboratory.) nucleus. The details could easily
Number 26 2000 Los Alamos Science 67 From Alchemy to Atoms
(a) (b) 1.5 235U
238 E U fb 0.5 239U*
Potential energy (MeV) Fission