The term NUCLEOSYNTHESIS means the creation of new elements in nuclear reactions. Before we can consider how the present-day mix of elements may have come into being, we need to look at the various processes that could have contributed. It used to be sup¬posed that the Universe has always had the composition we observe today, but since we now know that nuclear reactions are occurring inside stars, we must consider the possibility of a composition that has changed significantly, because stars are modifying the elemental mix.

Stars on the main sequence derive their energy from converting hydrogen to helium: hydrogen nuclei essentially fuse together in a series of reactions. The consumption of a nuclear fuel in this way is called burning, though it is not combustion in the normal sense. The two main processes are called the proton-proton (p-p) chain and the carbon-nitrogen (CN) cycle. Energy is released in these reactions, which only occur in the first place because the tempera¬ture (about 107K) becomes sufficiently hot in the core of a star to give the particles the necessary speeds. The p-p chain can occur in material that is initially pure hydrogen. If there is already some carbon or nitrogen present, then the CN cycle can operate. When all the hydrogen that is hot enough has been converted into helium, no further reactions occur in the stellar core until the temperature exceeds 108K, at which stage helium burning can commence. Helium nuclei combine to form the isotopes of carbon and oxygen, 12C and 160. These reactions and successive changes in the internal processes of a star cause structural changes that move the star away from the main sequence.

Another increase in temperature to about 109K takes place when all the helium has been consumed, and this allows carbon and oxygen burning reactions to occur with the production of heavier elements such as neon, sodium, magnesium and silicon. Up until this point, only elements with a mass number that is an exact multiple of four have been created (i.e. 12C, 16O, 20Ne, 24Mg, 28Si, 32S, etc., up to 56Fe) in the more massive stars that are able to reach the requisite central pressures. At a temperature of 3.5xl09K silicon burning will take place in the later evolution of 20 M0 stars for example. Two silicon nuclei (28Si) combine to form nickel-56 (56Ni), which emits two positrons to make cobalt (56Co) and ultimately iron (56Fe). In this way all the iron we find in our everyday surroundings is thought to have been cooked up in a stellar furnace at a temperature of 3.5 x 109K.

Now we must examine the origin of elements with mass numbers that are not multiples of four. Any further temperature rise is sufficient to cause protons and a-particles (helium nuclei) to be removed again from some of the nuclei that have been built up and thus trigger a whole host of new reactions. They can then re-combine until, gradually, all the elements with mass numbers up to the iron peak (56Ni, 56Co, 56Fe etc.) are created with the con¬sequential release of energy. By the time the temperature has soared to 4 x 109K, nearly all the nuclei will have become iron-peak elements. However, there the build-up ends because any further fusion demands the input of energy, and it is therefore necessary to look for other means to create the elements heavier than iron.

A popular explanation for the origin of many of the heavy elements is that they have built up gradually by the NEUTRON CAPTURE PROCESS. Relatively plentiful iron-peak nuclei act as ‘seeds’ that pick up neutrons one at a time. After the capture of a neutron, a nucleus may be unstable and, if so, the emission of an electron ($-decay) follows. When this happens, the number of protons in the nucleus increases by one, and thus a new element is created. If there is a large number of neutrons about, several may be captured by one seed nucleus before it has time to decay via emission of an electron. This rapid capture of several neutrons has been termed the R-PROCESS.. If the flux of neutrons is small, so that any unstable nucleus has time to decay before the next neutron comes along, the situation is described as the slow, or S-PROCESS. Some examples will make this clear. If we add neutrons one at a time to 56Fe we make 57Fe, 58Fe, and 59Fe. Now 57Fe and 58Fe are stable, but 59Fe is not. In the r-process (many neutrons around) 60Fe, and 61Fe, form successively, but 61Fe decays to 61Co in around 6 minutes. In s-process synthesis (not many neutrons) the 59Fe decays to 59Co before it can pick up an unattached neutron. A continuation of our argument would show that the two mixtures of elements that result from r-process and s-process syntheses are not identical. The r-process forms nuclei that are neutron-rich relative to the s-process. This leads us to the interesting possibility of determining which route led to the formation of the heavy elements in the Solar System.

There is remarkable evidence that the material of the Solar System has been subjected to the s-process. Nuclei that easily capture neutrons are quickly destroyed when there are neutrons about, so it would be expected that the greater an element’s ability to capture neutrons, the less there will be of that element. The quantity that is a measure of a nucleus’ ability to catch neutrons is called its capture cross-section (a). The s-process predicts that the product of the abundance, N, and a is roughly constant over certain ranges of atomic mass. The Solar System abundances fit the s-process prediction rather well, where the solid line is the theoretical prediction that fits the observations best. Unfortunately. the r-process cannot be tested in the same way. The prediction is a father random distribution of (? N) with atomic mass that cannot be reliably connected with the observed abundances.
Some heavy isotopes cannot be manufactured by either of the two neutron-capture processes notably theme isotopes that are relatively rich in protons. If Is thought that they may be produced by proton capture (P-PROCESS)The source of neutrons and protons to allow these processes to occur is uncertain, but it is thought that they may be produced in large numbers in the later stages of stellar evolution, or it may be that these all happened under special circumstances early in the life of the Galaxy

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