Nuclear physics is the field of physics that studies atomic nuclei and
their constituents and interactions. Other forms of nuclear matter are
Nuclear physics should not be confused with atomic
physics, which studies the atom as a whole, including its electrons.
Discoveries in nuclear physics have led to applications in many
fields. This includes nuclear power, nuclear weapons, nuclear medicine
and magnetic resonance imaging, industrial and agricultural isotopes,
ion implantation in materials engineering, and radiocarbon dating in
geology and archaeology. Such applications are studied in the field of
Particle physics evolved out of nuclear physics and the two fields are
typically taught in close association. Nuclear astrophysics, the
application of nuclear physics to astrophysics, is crucial in
explaining the inner workings of stars and the origin of the chemical
1.1 Rutherford's team discovers the nucleus
James Chadwick discovers the neutron
1.3 Proca's equations of the massive vector boson field
1.4 Yukawa's meson postulated to bind nuclei
2 Modern nuclear physics
2.1 Nuclear decay
2.2 Nuclear fusion
2.3 Nuclear fission
2.4 Production of "heavy" elements (atomic number greater than five)
3 See also
6 External links
Since 1920s cloud chambers played an important role of particle
detectors and eventually lead to the discovery of positron, muon and
The history of nuclear physics as a discipline distinct from atomic
physics starts with the discovery of radioactivity by Henri Becquerel
in 1896, while investigating phosphorescence in uranium salts.
The discovery of the electron by J. J. Thomson 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" model in 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 by Marie and
Pierre Curie as well as by Ernest
Rutherford and his collaborators. By the turn of the century
physicists had also discovered three types of radiation emanating from
atoms, which they named alpha, beta, and gamma radiation. Experiments
Otto Hahn in 1911 and by
James Chadwick in 1914 discovered that the
beta decay spectrum was continuous rather than discrete. That is,
electrons were 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 that energy was not
conserved in these decays.
Nobel Prize in
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".
Albert Einstein formulated the idea of mass–energy
equivalence. While the work on radioactivity by Becquerel and Marie
Curie predates 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, the nucleons.
Rutherford's team discovers the nucleus
Ernest Rutherford published "Retardation of the α Particle
from Radium in passing through matter."
Hans Geiger expanded on
this work in a communication to the Royal Society 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
and Ernest Marsden, and further greatly expanded work was published
in 1910 by Geiger. 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.
The key experiment behind this announcement was performed in 1910 at
the University of Manchester: Ernest Rutherford's team performed a
remarkable experiment in which Geiger and Marsden under Rutherford's
supervision fired alpha particles (helium nuclei) at a thin film of
gold foil. The plum pudding model had 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 a bullet at 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 dense nucleus containing 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
Arthur Eddington anticipated the discovery and mechanism
of nuclear fusion processes in stars, in his paper The Internal
Constitution of the Stars. At that time, the source of stellar
energy was a complete mystery; Eddington correctly speculated that the
source was fusion of hydrogen into helium, liberating enormous energy
according to Einstein's equation E = mc2. This was a particularly
remarkable development since at that time fusion and thermonuclear
energy, and even that stars are largely composed of hydrogen (see
metallicity), had not yet been discovered.
The Rutherford model worked quite well until studies of nuclear spin
were carried out by
Franco Rasetti at the California Institute of
Technology in 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 of 1⁄2. Rasetti discovered, however, that
nitrogen-14 had a spin of 1.
James Chadwick discovers the neutron
Main article: Discovery of the neutron
In 1932 Chadwick realized that radiation that had been observed by
Walther Bothe, Herbert Becker, Irène and
Frédéric Joliot-Curie was
actually due to a neutral particle of about the same mass as the
proton, that he called the neutron (following a suggestion from
Rutherford about the need for such a particle). In the same year
Dmitri Ivanenko suggested that there were no electrons in the nucleus
— only protons and neutrons — and that neutrons were spin 1⁄2
particles 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 of 1⁄2 in the same direction, giving a final
total spin of 1.
With the discovery of the neutron, scientists could at last calculate
what fraction of binding energy each 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.
Proca's equations of the massive vector boson field
Alexandru Proca was the first to develop and report the massive vector
boson field equations and a theory of the mesonic field of nuclear
forces. Proca's equations were known to Wolfgang Pauli 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.
Yukawa's meson postulated to bind nuclei
In 1935 Hideki Yukawa proposed the first significant theory of the
strong force to explain how the nucleus holds together. In the Yukawa
interaction a virtual particle, later called a meson, 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
attractive strong force had a more limited range than the
electromagnetic repulsion between protons. Later, the discovery of the
pi meson showed 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 (or positron). 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
Enrico Fermi via
Fermi's interaction in 1934) led physicists to
collide nuclei and electrons at ever higher energies. This research
became the science of particle physics, the crown jewel of which is
the standard model of particle physics which describes the strong,
weak, and electromagnetic forces.
Modern nuclear physics
Main articles: Liquid-drop model, Nuclear shell model, and Nuclear
A heavy nucleus can contain hundreds of nucleons. This means that with
some approximation it can be treated as a classical system, rather
than a quantum-mechanical one. In the resulting liquid-drop model,
the nucleus has an energy which 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 and
J. Hans D. Jensen. Nuclei with certain numbers of neutrons and
protons (the magic numbers 2, 8, 20, 28, 50, 82, 126, ...) are
particularly stable, because their 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 bosons, analogously to Cooper pairs of electrons.
Ab initio methods try to solve the nuclear many-body problem from the
ground up, starting from the nucleons and their interactions.
Much of current research in nuclear physics relates to the study of
nuclei under extreme conditions such as high spin and excitation
energy. Nuclei may also have extreme shapes (similar to that of Rugby
balls or even pears) or extreme neutron-to-proton ratios.
Experimenters can create such nuclei using artificially induced fusion
or nucleon transfer reactions, employing ion beams from an
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 quarks
mingle with one another, rather than being segregated in triplets as
they are in neutrons and protons.
Radioactivity and Valley of stability
Eighty elements have at least one stable isotope which is never
observed to decay, amounting to a total of about 254 stable isotopes.
However, thousands of isotopes 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) 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 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.
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 forces 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.
Stars like the Sun are powered by the fusion of four protons into a
helium nucleus, two positrons, 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
Joint European Torus (JET) and ITER, is the development of an
economically viable method of using energy from a controlled fusion
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 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
From certain 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, 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, 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 moderation so that there is a greater cross-section or
probability of them initiating another fission. In two regions of
Oklo, Gabon, Africa, natural nuclear fission reactors were active over
1.5 billion years ago. 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.
Production of "heavy" elements (atomic number greater than five)
Main article: 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
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 evolution of a star.
Since the binding energy per nucleon peaks around iron (56 nucleons),
energy is only released in fusion processes involving smaller atoms
than that. 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) or the rapid, or 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).
Nuclear technology portal
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