Nuclear physics

Nuclear physics
Nucleus Nucleons p n Nuclear matter Nuclear force Nuclear structure Nuclear reaction
Models of the nucleus Liquid drop Nuclear shell model Interacting boson model Ab initio
Nuclides' classification Isotopes– equalZ Isobars– equalA Isotones– equalN Isodiaphers– equalN−Z Isomers– equal all the above Mirror nuclei–Z↔N Stable Magic Even/odd Halo Borromean
Nuclear stability Binding energy p–n ratio Drip line Island of stability Valley of stability Stable nuclide
Radioactive decay Alpha α Beta β 2β 0v β+ K/L capture Isomeric Gamma γ Internal conversion Spontaneous fission Cluster decay Neutron emission Proton emission Decay energy Decay chain Decay product Radiogenic nuclide
Nuclear fission Spontaneous Products pair breaking Photofission
Capturing processes electron 2× neutron s r proton p rp
High-energy processes Spallation by cosmic ray Photodisintegration
Nucleosynthesisand nuclear astrophysics Nuclear fusion Processes: Stellar Big Bang Supernova Nuclides: Primordial Cosmogenic Artificial
High-energy nuclear physics Quark–gluon plasma RHIC LHC
Scientists Alvarez Becquerel Bethe A. Bohr N. Bohr Chadwick Cockcroft Ir. Curie Fr. Curie Pi. Curie Skłodowska-Curie Davisson Fermi Hahn Jensen Lawrence Mayer Meitner Oliphant Oppenheimer Proca Purcell Rabi Rutherford Soddy Strassmann Świątecki Szilárd Teller Thomson Walton Wigner
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Nuclear physicsis the field ofphysicsthat studiesatomic nucleiand their constituents and interactions, in addition to the study of other forms ofnuclear matter.

Nuclear physics should not be confused withatomic physics, which studies theatomas a whole, including itselectrons.

Discoveries in nuclear physics have led toapplicationsin many fields. This includesnuclear power,nuclear weapons,nuclear medicineandmagnetic resonance imaging, industrial and agricultural isotopes,ion implantationinmaterials engineering, andradiocarbon datingingeologyandarchaeology. Such applications are studied in the field ofnuclear engineering.

Particle physicsevolved out of nuclear physics and the two fields are typically taught in close association.Nuclear astrophysics, the application of nuclear physics toastrophysics, is crucial in explaining the inner workings ofstarsand theorigin of the chemical elements.

The history of nuclear physics as a discipline distinct fromatomic physics, starts with the discovery ofradioactivitybyHenri Becquerelin 1896,^[1]made while investigatingphosphorescenceinuraniumsalts.^[2]The discovery of theelectronbyJ. J. Thomson^[3]a year later was an indication that the atom had internal structure. At the beginning of the 20th century the accepted model of the atom was J. J. Thomson's"plum pudding" modelin which the atom was a positively charged ball with smaller negatively charged electrons embedded inside it.

In the years that followed, radioactivity was extensively investigated, notably byMarie Curie, a Polish physicist whose maiden name was Sklodowska,Pierre Curie,Ernest Rutherfordand others. By the turn of the century, physicists had also discovered three types ofradiationemanating from atoms, which they namedalpha,beta, andgammaradiation. Experiments byOtto Hahnin 1911 and byJames Chadwickin 1914 discovered that the beta decayspectrumwas continuous rather than discrete. That is,electronswere ejected from the atom with a continuous range of energies, rather than the discrete amounts of energy that were observed in gamma and alpha decays. This was a problem for nuclear physics at the time, because it seemed to indicate thatenergy was not conservedin these decays.

The 1903Nobel Prizein Physics was awarded jointly to Becquerel, for his discovery and to Marie and Pierre Curie for their subsequent research into radioactivity. Rutherford was awarded the Nobel Prize in Chemistry in 1908 for his "investigations into the disintegration of the elements and the chemistry of radioactive substances".

In 1905,Albert Einsteinformulated the idea ofmass–energy equivalence. While the work on radioactivity byBecquerelandMarie Curiepredates this, an explanation of the source of the energy of radioactivity would have to wait for the discovery that the nucleus itself was composed of smaller constituents, thenucleons.

In 1906,Ernest Rutherfordpublished "Retardation of the α Particle from Radium in passing through matter."^[4]Hans Geigerexpanded on this work in a communication to theRoyal Society^[5]with experiments he and Rutherford had done, passing alpha particles through air, aluminum foil and gold leaf. More work was published in 1909 by Geiger andErnest Marsden,^[6]andfurther greatly expanded work was published in 1910 by Geiger.^[7]In 1911–1912 Rutherford went before the Royal Society to explain the experiments and propound the new theory of the atomic nucleus as we now understand it.

Published in 1909,^[8]with the eventual classical analysis by Rutherford published May 1911,^{[9][10][11][12]}the key preemptive experiment was performed during 1909,^{[9][13][14][15]}at theUniversity of Manchester. Ernest Rutherford's assistant, Professor^[15]Johannes^[14]"Hans" Geiger, and an undergraduate, Marsden,^[15]performed anexperiment in which Geiger and Marsdenunder Rutherford's supervision fired alpha particles (helium 4 nuclei^[16]) at a thin film ofgoldfoil. Theplum pudding modelhad predicted that the alpha particles should come out of the foil with their trajectories being at most slightly bent. But Rutherford instructed his team to look for something that shocked him to observe: a few particles were scattered through large angles, even completely backwards in some cases. He likened it to firing abulletat tissue paper and having it bounce off. The discovery, with Rutherford's analysis of the data in 1911, led to the Rutherford model of the atom, in which the atom had a very small, very densenucleuscontaining most of its mass, and consisting of heavy positively charged particles with embedded electrons in order to balance out the charge (since the neutron was unknown). As an example, in this model (which is not the modern one) nitrogen-14 consisted of a nucleus with 14 protons and 7 electrons (21 total particles) and the nucleus was surrounded by 7 more orbiting electrons.

Around 1920,Arthur Eddingtonanticipated the discovery and mechanism ofnuclear fusionprocesses instars, in his paperThe Internal Constitution of the Stars.^[17][18]At that time, the source of stellar energy was a complete mystery; Eddington correctly speculated that the source wasfusionof hydrogen into helium, liberating enormous energy according to Einstein's equationE = mc². This was a particularly remarkable development since at that time fusion and thermonuclear energy, and even that stars are largely composed ofhydrogen(seemetallicity), had not yet been discovered.

The Rutherford model worked quite well until studies ofnuclear spinwere carried out byFranco Rasettiat theCalifornia Institute of Technologyin 1929. By 1925 it was known that protons and electrons each had a spin of±+1⁄2. In the Rutherford model of nitrogen-14, 20 of the total 21 nuclear particles should have paired up to cancel each other's spin, and the final odd particle should have left the nucleus with a net spin of1⁄2. Rasetti discovered, however, that nitrogen-14 had a spin of 1.

In 1932 Chadwick realized that radiation that had been observed byWalther Bothe,Herbert Becker,IrèneandFrédéric Joliot-Curiewas actually due to a neutral particle of about the same mass as the proton, that he called theneutron(following a suggestion from Rutherford about the need for such a particle).^[19]In the same yearDmitri Ivanenkosuggested that there were no electrons in the nucleus — only protons and neutrons — and that neutrons were spin1⁄2particles, which explained the mass not due to protons. The neutron spin immediately solved the problem of the spin of nitrogen-14, as the one unpaired proton and one unpaired neutron in this model each contributed a spin of1⁄2in the same direction, giving a final total spin of 1.

With the discovery of the neutron, scientists could at last calculate what fraction ofbinding energyeach nucleus had, by comparing the nuclear mass with that of the protons and neutrons which composed it. Differences between nuclear masses were calculated in this way. When nuclear reactions were measured, these were found to agree with Einstein's calculation of the equivalence of mass and energy to within 1% as of 1934.

Alexandru Procawas the first to develop and report the massive vectorbosonfield equationsand a theory of themesonicfield ofnuclear forces. Proca's equations were known toWolfgang Pauli^[20]who mentioned the equations in his Nobel address, and they were also known to Yukawa, Wentzel, Taketani, Sakata, Kemmer, Heitler, and Fröhlich who appreciated the content of Proca's equations for developing a theory of the atomic nuclei in Nuclear Physics.^{[21][22][23][24][25]}

In 1935Hideki Yukawa^[26]proposed the first significant theory of thestrong forceto explain how the nucleus holds together. In theYukawa interactionavirtual particle, later called ameson, mediated a force between all nucleons, including protons and neutrons. This force explained why nuclei did not disintegrate under the influence of proton repulsion, and it also gave an explanation of why the attractivestrong forcehad a more limited range than the electromagnetic repulsion between protons. Later, the discovery of thepi mesonshowed it to have the properties of Yukawa's particle.

With Yukawa's papers, the modern model of the atom was complete. The center of the atom contains a tight ball of neutrons and protons, which is held together by the strong nuclear force, unless it is too large. Unstable nuclei may undergo alpha decay, in which they emit an energetic helium nucleus, or beta decay, in which they eject an electron (orpositron). After one of these decays the resultant nucleus may be left in an excited state, and in this case it decays to its ground state by emitting high-energy photons (gamma decay).

The study of the strong and weak nuclear forces (the latter explained byEnrico FermiviaFermi's interactionin 1934) led physicists to collide nuclei and electrons at ever higher energies. This research became the science ofparticle physics, the crown jewel of which is thestandard model of particle physics, which describes the strong, weak, andelectromagnetic forces.

A heavy nucleus can contain hundreds ofnucleons. This means that with some approximation it can be treated as aclassical system, rather than aquantum-mechanicalone. In the resultingliquid-drop model,^[27]the nucleus has an energy that arises partly fromsurface tensionand partly from electrical repulsion of the protons. The liquid-drop model is able to reproduce many features of nuclei, including the general trend ofbinding energywith respect to mass number, as well as the phenomenon ofnuclear fission.

Superimposed on this classical picture, however, are quantum-mechanical effects, which can be described using thenuclear shell model, developed in large part byMaria Goeppert Mayer^[28]andJ. Hans D. Jensen.^[29]Nuclei with certain "magic" numbers of neutrons and protons are particularly stable, because theirshellsare filled.

Other more complicated models for the nucleus have also been proposed, such as theinteracting boson model, in which pairs of neutrons and protons interact asbosons.

Ab initio methodstry to solve the nuclear many-body problem from the ground up, starting from the nucleons and their interactions.^[30]

Much of current research in nuclear physics relates to the study of nuclei under extreme conditions such as highspinand excitation energy. Nuclei may also have extreme shapes (similar to that ofRugby ballsor evenpears) or extreme neutron-to-proton ratios. Experimenters can create such nuclei using artificially induced fusion or nucleon transfer reactions, employing ion beams from anaccelerator. Beams with even higher energies can be used to create nuclei at very high temperatures, and there are signs that these experiments have produced aphase transitionfrom normal nuclear matter to a new state, thequark–gluon plasma, in which thequarksmingle with one another, rather than being segregated in triplets as they are in neutrons and protons.

Eighty elements have at least onestable isotopewhich is never observed to decay, amounting to a total of about 251 stable nuclides. However, thousands ofisotopeshave 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 thevalley 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, inbeta decay, anitrogen-16 atom (7 protons, 9 neutrons) is converted to anoxygen-16 atom (8 protons, 8 neutrons)^[31]within a few seconds of being created. In this decay a neutron in the nitrogen nucleus is converted by theweak interactioninto a proton, an electron and anantineutrino. The element is transmuted to another element, with a different number of protons.

Inalpha 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, plushelium-4. In many cases this process continues throughseveral stepsof this kind, including other types of decays (usually beta decay) until a stable element is formed.

Ingamma decay, a nucleus decays from an excited state into a lower energy state, by emitting agamma ray. The element is not changed to another element in the process (nonuclear transmutationis involved).

Other more exotic decays are possible (see the first main article). For example, ininternal conversiondecay, 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.

Innuclear 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 ornuclear forcesto 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 tonickel-62.Starslike the Sun are powered by the fusion of four protons into a helium nucleus, twopositrons, and twoneutrinos. The uncontrolled fusion of hydrogen into helium is known as thermonuclear runaway. A frontier in current research at various institutions, for example theJoint European Torus(JET) andITER, 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 fissionis 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 ofalpha decayis in essence a special type of spontaneousnuclear 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 achain 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, using fission-produced neutrons, is the source of energy fornuclear powerplants and fission-type nuclear bombs, such as those detonated inHiroshimaandNagasaki, Japan, at the end ofWorld War II. Heavy nuclei such asuraniumandthoriummay also undergospontaneous fission, but they are much more likely to undergo decay by alpha decay.

For a neutron-initiated chain reaction to occur, there must be acritical massof 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 ormoderationso that there is a greatercross-sectionor probability of them initiating another fission. In two regions ofOklo, Gabon, Africa,natural nuclear fission reactorswere active over 1.5 billion years ago.^[32]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.^[33]

According to the theory, as the Universe cooled after theBig Bangit 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 intohelium-4in the first three minutes after the Big Bang, and this helium accounts for most of the helium in the universe today (seeBig 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 greateratomic number) that we see today, were created inside stars during a series of fusion stages, such as theproton–proton chain, theCNO cycleand thetriple-alpha process. Progressively heavier elements are created during theevolutionof a star.

Energy is only released in fusion processes involving smaller atoms than iron because the binding energy pernucleonpeaks 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 aslowneutron capture process (the so-calleds-process) or therapid, orr-process. Thesprocess 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. Ther-process is thought to occur insupernova 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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• Ernest Rutherford's biography at the American Institute of PhysicsArchived2016-07-30 at theWayback Machine
• American Physical Society Division of Nuclear PhysicsArchived2017-09-20 at theWayback Machine
• American Nuclear SocietyArchived2008-12-02 at theWayback Machine
• Annotated bibliography on nuclear physics from the Alsos Digital Library for Nuclear Issues
• Nuclear science wikiArchived2013-10-21 at theWayback Machine
• Nuclear Data Services – IAEAArchived2021-03-18 at theWayback Machine
• Nuclear PhysicsArchived2017-12-23 at theWayback Machine, BBC Radio 4 discussion with Jim Al-Khalili, John Gribbin and Catherine Sutton (In Our Time, Jan. 10, 2002)