Nuclear physics is the field of
physics that studies the building blocks and
interactions of
atomic nuclei.The most
commonly known applications of nuclear physics are
nuclear power and
nuclear weapons, but the research has
provided wider applications, including those in medicine (
nuclear medicine,
magnetic resonance imaging),
materials engineering
(
ion implantation) and
archaeology (
radiocarbon dating).
The field of
particle physics
evolved out of nuclear physics and, for this reason, has been
included under the same term in earlier times.
History
The discovery of the
electron by
J. J. Thomson was the first indication that the atom
had internal structure. At the turn of the 20th century the
accepted model of the atom was J. J. Thomson's
"plum pudding" model in which the atom
was a large positively charged ball with small negatively charged
electrons embedded inside of it. By the turn of the century
physicists had also discovered three types of
radiation coming from atoms, which they named
alpha,
beta,
and
gamma radiation. Experiments in 1911
by
Lise Meitner and
Otto Hahn, 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 range of energies,
rather than the discrete amounts of energies that were observed in
gamma and alpha decays. This was a problem for nuclear physics at
the time, because it indicated that
energy was not conserved in these
decays.
In 1905,
Albert Einstein formulated
the idea of
mass–energy
equivalence. While the work on radioactivity by
Becquerel, Pierre 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
In 1907
Ernest Rutherford
published "Radiation of the α Particle from Radium in passing
through Matter".
Geiger expanded on this
work in a communication to the Royal Society with experiments he
and Rutherford had done passing α particles through air, aluminum
foil and gold leaf. More work was published in 1909 by
Geiger and
Marsden
and further greatly expanded work was published in 1910 by Geiger,
In 1911-2 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 happened in 1909 as
Ernest Rutherford's team performed a remarkable
experiment in which
Hans Geiger and
Ernest
Marsden under his supervision fired alpha particles (helium
nuclei) at a thin film of
gold foil. The
plum pudding model predicted that
the alpha particles should come out of the foil with their
trajectories being at most slightly bent. Rutherford had the idea
to instruct his team to look for something that shocked him to
actually 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, beginning with Rutherford's analysis of the data in
1911, eventually led to the Rutherford model of the atom, in which
the atom has 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 electrons.
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 had a spin of 1/2, and 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 has a spin of 1.
James Chadwick discovers the neutron
In 1932 Chadwick realized that radiation that had been observed by
Walther Bothe,
Herbert L. 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 about the need for such a particle, by
Rutherford). In the same year
Dmitri
Ivanenko suggested that neutrons were in fact spin 1/2
particles and that the nucleus contained neutrons to explain the
mass not due to protons, and that there were no electrons in the
nucleus-- only protons and neutrons. 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 contribute a
spin of 1/2 in the same direction, for a final total spin of
1.
With the discovery of the neutron, scientists at last could
calculate what fraction of
binding
energy each nucleus had, from comparing the nuclear mass with
that of the protons and neutrons which composed it. Differences
between nuclear masses calculated in this way, and when nuclear
reactions were measured, were found to agree with Einstein's
calculation of the equivalence of mass and energy to high accuracy
(within 1% as of in 1934).
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 by
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 unifies the strong, weak, and electromagnetic
forces.
Modern nuclear physics
A heavy nucleus can contain hundreds of
nucleons which 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.
Nuclei with certain numbers of neutrons and protons (the
magic numbers 2, 8, 20, 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.
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 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.
Modern topics in nuclear physics
Spontaneous changes from one nuclide to another: nuclear
decay
There are 80 elements which have at least one stable
isotope (defined as isotopes never observed to
decay), and in total there are about 256 such
stable isotopes. However, there are thousands
more well-characterized isotopes which are unstable. These
radioisotopes may be unstable and decay in all timescales ranging
from fractions of a second to weeks, years, or many billions of
years.
For example, if a nucleus has too few or too many neutrons it may
be unstable, and will decay after some period of time. For example,
in a process called
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 turned into a proton and an
electron and
antineutrino, by the
weak nuclear force. The element
is transmuted to another element in the process, because while it
previously had seven protons (which makes it nitrogen) it now has
eight (which makes it oxygen).
In
alpha decay 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, until a stable element is formed.
In
gamma decay, a nucleus decays from an
excited state into a lower state by emitting a
gamma ray. It is then stable. The element is not
changed in the process.
Other more exotic decays are possible (see the main article). For
example, in
internal conversion
decay, the energy from an excited nucleus may be used to 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
When two low mass nuclei come into very close contact with each
other it is possible for the strong force to
fuse the two together. It takes a great deal
of energy to push the nuclei close enough together for the strong
or
nuclear forces to have an effect,
so the process of nuclear fusion can only take place at very high
temperatures or high densities. Once the nuclei are close enough
together the strong force overcomes their electromagnetic repulsion
and squishes them into a new nucleus. A very large amount of energy
is released when light nuclei fuse together because the binding
energy per nucleon increases with mass number up until
nickel-62.
Stars like our 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.
Research to find an
economically viable method of using energy from a
controlled fusion reaction is currently being undertaken
by various research establishments (see JET
and ITER
).
Nuclear fission
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. This splitting of atoms is known as nuclear fission.
The process of
alpha decay may be
thought of as a special type of spontaneous
nuclear fission. This process produces 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.
For certain of the heaviest nuclei which produce neutrons on
fission, and which also easily absorb neutrons to initiate fission,
a self-igniting type of neutron-initiated fission can be obtained,
in a so-called
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 the two that the United States
used against Hiroshima and
Nagasaki at the end of World War II. Heavy nuclei such as
uranium and
thorium
may 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 element present
in a certain space under certain conditions (these conditions slow
and conserve neutrons for the reactions).
There is one known
example of a natural nuclear fission reactor
, which was active in two regions of Oklo
, Gabon,
Africa, 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
According to the theory, as the Universe cooled after the
big bang it eventually became possible for
particles as we know them to exist. The most common particles
created in the big bang which are still easily observable to us
today were protons (
hydrogen) and electrons
(in equal numbers). Some heavier elements were created as the
protons collided with each other, but most of the heavy elements we
see today were created inside of 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, energy is only released in
fusion processes occurring below this point. Since the creation of
heavier nuclei by fusion costs 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 by 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
because the conditions of high temperature, high neutron flux and
ejected matter are present. 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). The
r
process duration is typically in the range of a few seconds.
See also
References
- Philosophical Magazine (12, p
134-46)
- Proc. Roy. Soc. July 17, 1908
- Proc. Roy. Soc. A82 p 495-500
- Proc. Roy. Soc. Feb. 1, 1910
- Nuclear Physics by Irving Kaplan 2Nd edition, 1962
Addison-Wesley
- General Chemistry by Linus Pauling 1970 Dover Pub. ISBN
0-486-65622-5
- Introductory Nuclear Physics by Kenneth S. Krane Pub.
Wiley
- Models of the Atomic Nucleus by N. Cook, Springer Verlag
(2006), ISBN 3540285695
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