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==Modern nuclear physics== {{Main|Liquid-drop model|Nuclear shell model|Nuclear structure}} A heavy nucleus can contain hundreds of [[nucleon]]s. This means that with some approximation it can be treated as a [[Newtonian mechanics|classical system]], rather than a [[quantum mechanics|quantum-mechanical]] one. In the resulting [[liquid-drop model]],<ref>J.M.Blatt and V.F.Weisskopf, Theoretical Nuclear Physics, Springer, 1979, VII.5</ref> the nucleus has an energy that arises partly from [[surface tension]] and partly from electrical repulsion of the protons. The liquid-drop model is able to reproduce many features of nuclei, including the general trend of [[binding energy]] with respect to mass number, as well as the phenomenon of [[nuclear fission]]. Superimposed on this classical picture, however, are quantum-mechanical effects, which can be described using the [[nuclear shell model]], developed in large part by [[Maria Goeppert Mayer]]<ref>{{Cite journal |doi = 10.1103/PhysRev.75.1969|title = On Closed Shells in Nuclei. II|journal = Physical Review|volume = 75|issue = 12|pages = 1969–1970|year = 1949|last1 = Mayer|first1 = Maria Goeppert|bibcode = 1949PhRv...75.1969M}}</ref> and [[J. Hans D. Jensen]].<ref>{{Cite journal |doi = 10.1103/PhysRev.75.1766.2|title = On the "Magic Numbers" in Nuclear Structure|journal = Physical Review|volume = 75|issue = 11|pages = 1766|year = 1949|last1 = Haxel|first1 = Otto|last2 = Jensen|first2 = J. Hans D|last3 = Suess|first3 = Hans E|bibcode = 1949PhRv...75R1766H}}</ref> Nuclei with certain "[[Magic number (physics)|magic]]" numbers of neutrons and protons are particularly stable, because their [[Nuclear shell model|shells]] are filled. Other more complicated models for the nucleus have also been proposed, such as the [[interacting boson model]], in which pairs of neutrons and protons interact as [[boson]]s. [[ab initio methods (nuclear physics)|Ab initio methods]] try to solve the nuclear many-body problem from the ground up, starting from the nucleons and their interactions.<ref>{{cite journal|last1=Stephenson|first1=C.|last2=et.|first2=al.|title=Topological properties of a self-assembled electrical network via ab initio calculation|journal=Scientific Reports |volume=7|issue=1|pages=932|date=2017|doi=10.1038/s41598-017-01007-9|pmid=28428625|pmc=5430567|bibcode=2017NatSR...7..932B}}</ref> Much of current research in nuclear physics relates to the study of nuclei under extreme conditions such as high [[Spin (physics)|spin]] and excitation energy. Nuclei may also have extreme shapes (similar to that of [[Rugby ball]]s or even [[pear]]s) or extreme neutron-to-proton ratios. Experimenters can create such nuclei using artificially induced fusion or nucleon transfer reactions, employing ion beams from an [[particle accelerator|accelerator]]. Beams with even higher energies can be used to create nuclei at very high temperatures, and there are signs that these experiments have produced a [[phase transition]] from normal nuclear matter to a new state, the [[quark–gluon plasma]], in which the [[quark]]s mingle with one another, rather than being segregated in triplets as they are in neutrons and protons. ===Nuclear decay=== {{Main|Radioactivity|Valley of stability}} Eighty elements have at least one [[stable isotope]] which is never observed to decay, amounting to a total of about 251 stable nuclides. However, thousands of [[isotope]]s have been characterized as unstable. These "radioisotopes" decay over time scales ranging from fractions of a second to trillions of years. Plotted on a chart as a function of atomic and neutron numbers, the binding energy of the nuclides forms what is known as the [[valley of stability]]. Stable nuclides lie along the bottom of this energy valley, while increasingly unstable nuclides lie up the valley walls, that is, have weaker binding energy. The most stable nuclei fall within certain ranges or balances of composition of neutrons and protons: too few or too many neutrons (in relation to the number of protons) will cause it to decay. For example, in [[beta decay]], a [[nitrogen]]-16 atom (7 protons, 9 neutrons) is converted to an [[oxygen]]-16 atom (8 protons, 8 neutrons)<ref>Not a typical example as it results in a "doubly magic" nucleus</ref> within a few seconds of being created. In this decay a neutron in the nitrogen nucleus is converted by the [[weak interaction]] into a proton, an electron and an [[antineutrino]]. The element is transmuted to another element, with a different number of protons. In [[alpha decay]], which typically occurs in the heaviest nuclei, the radioactive element decays by emitting a helium nucleus (2 protons and 2 neutrons), giving another element, plus [[helium-4]]. In many cases this process continues through [[decay chain|several steps]] of this kind, including other types of decays (usually beta decay) until a stable element is formed. In [[gamma decay]], a nucleus decays from an excited state into a lower energy state, by emitting a [[gamma ray]]. The element is not changed to another element in the process (no [[nuclear transmutation]] is involved). Other more exotic decays are possible (see the first main article). For example, in [[internal conversion]] decay, the energy from an excited nucleus may eject one of the inner orbital electrons from the atom, in a process which produces high speed electrons but is not beta decay and (unlike beta decay) does not transmute one element to another. ===Nuclear fusion=== In [[nuclear fusion]], two low-mass nuclei come into very close contact with each other so that the strong force fuses them. It requires a large amount of energy for the strong or [[nuclear force]]s to overcome the electrical repulsion between the nuclei in order to fuse them; therefore nuclear fusion can only take place at very high temperatures or high pressures. When nuclei fuse, a very large amount of energy is released and the combined nucleus assumes a lower energy level. The binding energy per nucleon increases with mass number up to [[nickel]]-62. [[Star]]s like the Sun are powered by the fusion of four protons into a helium nucleus, two [[positron]]s, and two [[neutrinos]]. The uncontrolled fusion of hydrogen into helium is known as thermonuclear runaway. A frontier in current research at various institutions, for example the [[Joint European Torus]] (JET) and [[ITER]], is the development of an economically viable method of using energy from a controlled fusion reaction. Nuclear fusion is the origin of the energy (including in the form of light and other electromagnetic radiation) produced by the core of all stars including our own Sun. ===Nuclear fission=== [[Nuclear fission]] is the reverse process to fusion. For nuclei heavier than nickel-62 the binding energy per nucleon decreases with the mass number. It is therefore possible for energy to be released if a heavy nucleus breaks apart into two lighter ones. The process of [[alpha decay]] is in essence a special type of spontaneous [[nuclear fission]]. It is a highly asymmetrical fission because the four particles which make up the alpha particle are especially tightly bound to each other, making production of this nucleus in fission particularly likely. From several of the heaviest nuclei whose fission produces free neutrons, and which also easily absorb neutrons to initiate fission, a self-igniting type of neutron-initiated fission can be obtained, in a [[chain reaction]]. Chain reactions were known in chemistry before physics, and in fact many familiar processes like fires and chemical explosions are chemical chain reactions. The fission or [[Nuclear chain reaction|"nuclear" chain-reaction]], using fission-produced neutrons, is the source of energy for [[nuclear power]] plants and fission-type nuclear bombs, such as those detonated in [[Hiroshima]] and [[Nagasaki, Nagasaki|Nagasaki]], Japan, at the end of [[World War II]]. Heavy nuclei such as [[uranium]] and [[thorium]] may also undergo [[spontaneous fission]], but they are much more likely to undergo decay by alpha decay. For a neutron-initiated chain reaction to occur, there must be a [[critical mass]] of the relevant isotope present in a certain space under certain conditions. The conditions for the smallest critical mass require the conservation of the emitted neutrons and also their slowing or [[neutron moderator|moderation]] so that there is a greater [[Neutron cross section|cross-section]] or probability of them initiating another fission. In two regions of [[Oklo]], Gabon, Africa, [[natural nuclear fission reactor]]s were active over 1.5 billion years ago.<ref>{{cite journal |last=Meshik |first=A. P. |date=November 2005 |title=The Workings of an Ancient Nuclear Reactor |journal=Scientific American |volume=293 |issue=5 |pages=82–91 |url=http://www.sciam.com/article.cfm?id=ancient-nuclear-reactor |access-date=2014-01-04 |doi=10.1038/scientificamerican1105-82 |pmid=16318030 |bibcode=2005SciAm.293e..82M |archive-date=2009-02-27 |archive-url=https://web.archive.org/web/20090227134554/http://www.sciam.com/article.cfm?id=ancient-nuclear-reactor |url-status=live |url-access=subscription }}</ref> Measurements of natural neutrino emission have demonstrated that around half of the heat emanating from the Earth's core results from radioactive decay. However, it is not known if any of this results from fission chain reactions.<ref>{{cite web |last1=Biello |first1=David |title=Nuclear Fission Confirmed as Source of More than Half of Earth's Heat |url=https://blogs.scientificamerican.com/observations/nuclear-fission-confirmed-as-source-of-more-than-half-of-earths-heat/ |website=Scientific American |access-date=25 January 2023 |date=July 18, 2011 |archive-date=25 January 2023 |archive-url=https://web.archive.org/web/20230125171628/https://blogs.scientificamerican.com/observations/nuclear-fission-confirmed-as-source-of-more-than-half-of-earths-heat/ |url-status=live }}</ref> === Production of "heavy" elements === {{main|nucleosynthesis}} According to the theory, as the Universe cooled after the [[Big Bang]] it eventually became possible for common subatomic particles as we know them (neutrons, protons and electrons) to exist. The most common particles created in the Big Bang which are still easily observable to us today were protons and electrons (in equal numbers). The protons would eventually form hydrogen atoms. Almost all the neutrons created in the Big Bang were absorbed into [[helium-4]] in the first three minutes after the Big Bang, and this helium accounts for most of the helium in the universe today (see [[Big Bang nucleosynthesis]]). Some relatively small quantities of elements beyond helium (lithium, beryllium, and perhaps some boron) were created in the Big Bang, as the protons and neutrons collided with each other, but all of the "heavier elements" (carbon, element number 6, and elements of greater [[atomic number]]) that we see today, were created inside stars during a series of fusion stages, such as the [[proton–proton chain]], the [[CNO cycle]] and the [[triple-alpha process]]. Progressively heavier elements are created during the [[stellar evolution|evolution]] of a star. Energy is only released in fusion processes involving smaller atoms than iron because the binding energy per [[nucleon]] peaks around iron (56 nucleons). Since the creation of heavier nuclei by fusion requires energy, nature resorts to the process of neutron capture. Neutrons (due to their lack of charge) are readily absorbed by a nucleus. The heavy elements are created by either a ''slow'' neutron capture process (the so-called [[s-process|''s''-process]]) or the ''rapid'', or [[r-process|''r''-process]]. The ''s'' process occurs in thermally pulsing stars (called AGB, or asymptotic giant branch stars) and takes hundreds to thousands of years to reach the heaviest elements of lead and bismuth. The ''r''-process is thought to occur in [[supernova explosions]], which provide the necessary conditions of high temperature, high neutron flux and ejected matter. These stellar conditions make the successive neutron captures very fast, involving very neutron-rich species which then beta-decay to heavier elements, especially at the so-called waiting points that correspond to more stable nuclides with closed neutron shells (magic numbers).
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