From Alchemy to Atoms From Alchemy to Atoms The making of plutonium André Michaudon 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to simply create it was quickly process described in this article, uranium in Earth’s history, all natural accumula- superseded by another: to create enough uranium ore will absorb neutrons from natural tions have long since disappeared.1 material to build a weapon, one so radioactivity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. 62 Los Alamos Science Number 26 2000 From Alchemy to Atoms 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. Nuclear Transmutations 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.