SIS_5_41-65_x1 15.05.2007 16:58 Uhr Seite 52 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 met- als – 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 52 Science in School Issue 5 : Summer 2007 www.scienceinschool.org SIS_5_41-65_x1 15.05.2007 16:58 Uhr Seite 53 Science topics 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 nucleosyn- thesis 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 nucle- us, with the number of neutrons varying horizontally, and number of protons varying vertically. Thus, each horizontal row represents isotopes of a single ele- ment. 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 cap- tured); as a consequence the movement in the vertical direction is also shorter (there are fewer neutrons that can be converted into protons) Science in School Issue 5 : Summer 2007 53 www.scienceinschool.org SIS_5_41-65_x1 15.05.2007 16:58 Uhr Seite 54 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. 54 Science in School Issue 5 : Summer 2007 www.scienceinschool.org SIS_5_41-65_x1 15.05.2007 16:58 Uhr Seite 55 Science topics 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.