Fusion in the Universe: Where Your Jewellery Comes From

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Fusion in the Universe: where your jewellery comes from

Does alchemy sound too good to be true? Paola Rebusco, Henri Boffin and Douglas Pierce-Price, from ESO in Garching, Germany, describe how creating gold – and other heavy metals – is possible, though sadly not in the laboratory.

ow are heavy elements formed? and diagram on page 53). Nature HThe last episode of the ‘Fusion cherishes stable configurations and in the Universe’ saga (Boffin & Pierce- therefore the fusion process described Price, 2007) ended with the produc- in our last article, which brings us tion of iron, but the nucleosynthesis from hydrogen up to heavier, more adventure – in which atomic nuclei stable nuclei, will not continue are created – does not stop there. Let beyond iron-56. So, where do heavier us refresh our memory. In the first elements such as lead, silver, gold few minutes after the Big Bang, the and uranium come from? There is no temperature of the newborn Universe magic: the Universe provides other cooled down (to a few billion fascinating ways to produce all the degrees!) to form hydrogen and heavy elements. In the high tempera- helium. Stars spend most of their ture and pressure of a star, fusion is neutrons (the s- and r-processes) and life burning hydrogen into helium. as spontaneous as rolling down a one with the capture of protons (the Only when temperature and pressure hill (a process that releases energy). p-process). become high enough do they start to However, these new mechanisms fuse helium atoms, forming new ele- are more laborious, like climbing a Neutron capture ments. Lighter elements are the bricks hill (a process that needs energy). One route to create elements heav- that successively fuse together to pro- Furthermore the next stages of ier than iron-56 starts when extra neu- duce heavier elements, up to iron-56. nucleosynthesis are quite hectic, as trons collide and fuse with an existing Iron-56 has the most stable nucleus they involve captures and explosions. nucleus. In this way we get neutron- because it has the maximum nuclear Three types of capture are involved, richer, heavier nuclei, but with the binding energy (see box on page 54 two dealing with the capture of same number of protons, or the same

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Image courtesy of Mafalda Martins, ESO Binding energy plot: the graph shows the nuclear binding energy per nucleon (i.e. per proton or neutron), expressed in MeV (1MeV=1.6x10-13J). For increasing atomic number the binding energy increases (downwards in this plot), until it reaches its maximum for iron-56. The nucleosynthesis from hydrogen to iron-56 is ener- getically favourable and occurs through consecutive fusion reactions. If you want to climb the rest of the periodic table, then new mechanisms, such as the s- process, r-process and p-process, are needed. Note that one can go in the opposite direction (from heavy to light nuclei) through nuclear fission

Image courtesy of Mafalda Martins, ESO Examples of the s-process (top) and r- process (bottom). Each position on the grid represents a different possible nucleus, with the number of neutrons varying horizontally, and number of protons varying vertically. Thus, each horizontal row represents isotopes of a single element. In the paths shown, a step to the right corresponds to a neutron being acquired by the nucleus. A diagonal step up and to the left corresponds to a beta- decay in which a neutron turns into a proton, releasing an electron and an anti- neutrino.

Image courtesy of Mafalda Martins, ESO Notice that the horizontal track in the s- process is shorter than in the r-process (in the s-process fewer neutrons are captured); as a consequence the movement in the vertical direction is also shorter (there are fewer neutrons that can be converted into protons)

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atomic number. These nuclei are just whether the initial neutron capture is Where in the Universe can we find heavier isotopes of the original ele- slow or rapid relative to the beta the right conditions for the s-process ment, so we have not yet achieved decay. The two cases, referred to to occur? It turns out that it can occur our aim of creating a heavier, different respectively as the s-process and r- during the late stages of the life of element. process, produce different elements Sun-like stars. We already know (see, However, the process has not yet and occur in different circumstances for example, Boffin & Pierce-Price finished. These new isotopes may be in the Universe. 2007) that if the initial mass of a star stable or unstable, depending on their is comparable to that of the Sun, then number of protons and neutrons. If Slow neutron capture: at the end of the star’s life, it runs out the neutron capture produces an the s-process of fuel and cools to become a white unstable isotope, then it can undergo Each neutron capture in the s-process dwarf. Before it cools down, free neu- a spontaneous radioactive decay. One converts a nucleus to an isotope of the trons are produced (mainly from the such decay is ‘beta decay’, in which same element with one more neutron. decay of carbon and neon): they are an electron and an anti-neutrino are Eventually, these single increases in plentiful enough to produce heavy emitted, so that one of the nucleus’ neutron number lead to an unstable elements via slow neutron capture. neutrons is converted into a proton. isotope. Because the neutron capture is In this way, elements such as barium, The net result of this conversion is a relatively slow in the s-process, the copper, osmium, strontium and tech- nucleus with one more proton and unstable nucleus beta-decays before netium are produced. one fewer neutrons. Since the number any more neutrons can be captured. of protons has changed, this has In other words, as soon as the first Rapid neutron capture: indeed produced a new, different ele- unstable configuration is reached, a the r-process ment. beta decay turns the nucleus into one If, instead, the neutrons are pro- In this process of neutron capture with one more proton and one fewer duced at a very high rate, then the followed by beta decay, it is important neutrons, see diagram on page 53. unstable nuclei that are formed have

The mystery of the vanished mass

The nuclear binding energy is the amount of energy u and mn = 1.00866 u, respectively. The measured

needed to break a nucleus apart into protons and neu- mass of a nucleus of helium-4 is mHe = 4.00150 u,

trons. It is also the energy that two particles release while the sum of the mass of its components is 2mP +

when they merge. Let’s imagine you have a proton 2mn = 4.03188. The difference gives the mass 4.03188 and a neutron and that they have the same mass (a u – 4.00150 u = 0.03038 u, which corresponds to a very good approximation). Push them together until total binding energy of approximately 28.3 MeV (the they merge and they will form a deuterium nucleus. binding energy per nucleon is 28.3/(2 + 2) = 7.07 What is its mass? If the proton has mass 1 and the neu- MeV). tron has mass 1, you would expect 2, wouldn’t you? If you repeat the same steps for iron-56 (which con- Not so: the mass of a deuterium nucleus is lower than sists of 26 protons and 30 neutrons), the total binding the sum of the two – some mass has vanished! The energy is much greater: about 492.2 MeV, or 8.79 2 solution lies in the famous Einstein equation, E = mc . MeV per nucleon. This extreme stability places iron- When two particles merge, they release the nuclear 56 at the lowest point of the curve in the binding ener- binding energy EB, but since energy and mass are gy plot, and fusion to heavier elements would be an equivalent, this means that the correspondent mass, ‘uphill’ process, requiring the input of energy. This is 2 mB= EB/c , is lost. why, although helium-4 nuclei can be readily fused Let’s consider first helium-4 and then iron-56. In into heavier elements, more extreme processes atomic mass units (u = 1.66 x 10-27 kg = 931.5 MeV/c2) (described in this article) are required to obtain ele-

BACKGROUND the mass of a proton and a neutron are mP = 1.00728 ments heavier than iron-56.

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enough time to swallow many neu- The onion-like structure in the final stage of a massive star: the outermost envelope is trons that subsequently decay in cas- composed of hydrogen and helium, and progressively heavier nuclei (up to iron) are cade into protons (see diagram): this layered, due to successive fusion reactions is how the elements with the highest atomic number are synthesised in nature. Let us discover where the r-process Hydrogen Burning takes place in the Universe. As was Helium Burning also discussed in the previous article, Oxygen Burning when the mass of a star is greater Carbon Burning than about eight solar masses, the Silicon Burning temperature and pressure at its centre Iron Core become high enough to trigger the fusion of carbon and oxygen and, ulti- mately, to form a core of iron. In this final stage, a star’s interior is very like an onion (see right): the outermost envelope is composed of hydrogen and helium, with the inner layers consisting of progressively heavier nuclei, due to successive fusion reactions.

Iron is too stable to start to burn, Image courtesy of Mafalda Martins, ESO hence it accumulates and the iron core continues to increase. There is, sion (see below). This phenomenon is capture many neutrons before it beta- however, a mass limit (called the called a supernova explosion, specifi- decays. Gold, europium, lanthanum, Chandrasekhar limit) above which cally a Type II supernova polonium, thorium and uranium are the iron core can no longer grow, as (SN II). some of the elements produced its gravity becomes too high for it to It is in the collapsing iron core of through the r-process. support itself. At this point a cata- SN II that the r-process occurs. strophic collapse (with the outer lay- During collapse, electrons and pro- Proton capture ers of the core reaching velocities up tons merge to produce neutrons and Another process by which heavier to 250 million km/h) shrinks the core, neutrinos. The flux (the number per nuclei are produced is via proton cap- until the infalling matter bounces unit time and unit area) of neutrons is ture (p-process). However, a large back and all the energy is transferred so high (of the order of 1022 neutrons nucleus containing many protons has to the outer layers, in a titanic explo- per cm2/s) that a nucleus has time to a high positive charge, which repels Image courtesy of Mafalda Martins, ESO Implosion Supernova Explosion Remnant

The different phases for Type II supernovae: core contraction, explosion, and supernova remnant

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Margaret Burbidge and the B2HF team

The mechanisms behind the production of the heavier her popular books on astronomy: “I saw my fascination elements (the s- and r-processes) were first pointed out with the stars, born at age 4”, she writes in her witty in a long theoretical paper published in 1957: autobiography (Burbidge, 1994), “linked with my other ‘Synthesis of the elements in stars’ (Burbidge et al., delight, large numbers.” Her life has been full of scien- 1957). This revolutionary and still up-to-date paper is tific discoveries and political fights; it was not always 2 signed B HF – not a strange chemical compound but easy to be a female scientist but she never gave up. “If the initials of the surnames of the scientists who wrote you meet with a blockage, find a way around it,” she it: Margaret Burbidge, Geoffrey Burbidge, William suggests. The rest of the group is no less notable: Fred Fowler and Fred Hoyle. Hoyle and Margaret’s husband, Geoffrey Burbidge, are The British astronomer Margaret Burbidge was born in most famous for their iconoclastic theories opposing 1919 and is still active in research, as professor emeri- the Big Bang theory, while William Fowler shared the tus of physics at the University of California, San Diego, 1983 Nobel Prize in Physics for his theoretical and USA. When she was a teenager, her grandfather gave experimental studies on nucleosynthesis. BACKGROUND

additional approaching protons. This Conclusion References repulsion (the Coulomb barrier) is We have seen how, although Boffin H, Pierce-Price D (2007) Fusion very high, and ensures that proton nuclear fusion in stars produces ele- in the Universe: we are all stardust. capture is a much rarer event than ments only up to iron-56, heavier ele- Science in School 4: 61-63. neutron capture. To be absorbed by ments are produced by a variety of www.scienceinschool.org/2007/ the nucleus, a free proton must be processes. These nucleosynthesis issue4/fusion/ very energetic, so this process only processes, involving the capture of takes place at very high temperatures. neutrons or protons, and radioactive Burbidge EM, Burbidge GR, Fowler So where can we find high enough decays, happen in exotic situations in WA, Hoyle F (1957) Synthesis of the temperatures for proton capture? the Universe. Slow neutron capture elements in stars. Reviews of Modern Again, we look to the stars. Although can occur late in the lives of Sun-like Physics 29: 547-650 our own Solar System has only one stars, before they end their days as star – the Sun – a large number of white dwarfs. Proton capture is a Burbidge EM (1994) Watcher of the stars are actually in systems with at result of a white dwarf or neutron skies. Annual Review of Astronomy least two stars. When two stars are star cannibalising gas from an unfor- and Astrophysics 32: 1-36 orbiting each other, they form a tunate companion star. And rapid ‘binary system’. If the stars are close neutron capture takes place during Resources enough, it is possible for one star with the catastrophic stellar collapse which To discover when and where the a strong gravitational pull to ‘steal’ occurs just before the dramatic explo- latest supernovae have detonated, gas from its companion star. This can sion of a Type II supernova. By chang- see the Supernovae website, where happen, for example, when a massive, ing one element into another, these scientists and amateurs hunt and compact white dwarf or neutron star fascinating natural processes achieve register new supernova explosions: pulls hydrogen-rich gas down onto its what mediaeval alchemists could not www.supernovae.net surface from its partner. This material – the transformation of base metals provides a flow of free protons, hot into (among other elements) gold. and energetic enough to overcome the Nevertheless, we cannot blame the Coulomb barrier and fuse with other alchemists. Their laboratories may nuclei. Lanthanum, ruthenium and have been well equipped, but they samarium are typical elements pro- lacked a key piece of apparatus: a duced in the p-process. supernova explosion.

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