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In physics, the neutron is a subatomic particle with no net electric charge
and a mass of 940 MeV (very slightly more than a proton). The nucleus of
most atoms (all except the most common isotope of Hydrogen, which consists
of a single proton only) consists of protons and neutrons. Outside the
nucleus, neutrons are unstable and have a half-life of about 15 minutes,
decaying by emitting an electron and antineutrino to become a proton. The
same decay method (beta decay) occurs in some nuclei. Particles inside the
nucleus are typically resonances between neutrons and protons, which
transform into one another by the emission and absorption of pions. A
neutron is classified as a baryon, and consists of two down quarks and
one up quark.
The characteristic of neutrons which most differentiates them from other
common subatomic particles is the fact that they are uncharged. This
property of neutrons delayed their discovery, makes them very penetrating,
makes it impossible to observe them directly, and makes them very important
as agents in nuclear change.
Although atoms in their normal state are also uncharged, they are ten
thousand times larger than a neutron and consist of a complex system of
negatively charged electrons widely spaced around a positively charged
nucleus. Charged particles (such as protons, electrons, or alpha particles)
and electromagnetic radiations (such as gamma rays) lose energy in passing
through matter. They exert electric forces which ionize atoms of the
material through which they pass. The energy taken up in ionization equals
the energy lost by the charged particle, which slows down, or by the gamma
ray, which is absorbed. The neutron, however, is unaffected by such forces;
it is affected only by the very short-range strong nuclear force which comes
into play when the neutron comes very close indeed to an atomic nucleus.
Consequently a free neutron goes on its way unchecked until it makes a
"head-on" collision with an atomic nucleus. Since nuclei have a very small
cross section, such collisions occur but rarely and the neutron travels a
long way before colliding.
In the case of a collision of the elastic type, the ordinary laws of
momentum apply as they do in the elastic collision of billiard balls. If the
nucleus that is struck is heavy, it acquires relatively little speed, but if
it is a proton, which is approximately equal in mass to the neutron, it is
projected forward with a large fraction of the original speed of the
neutron, which is itself correspondingly slowed. Secondary projectiles
resulting from these collisions may be detected, for they are charged and
The uncharged nature of the neutron makes it not only difficult to detect
but difficult to control. Charged particles can be accelerated, decelerated,
or deflected by electric or magnetic fields which have no effect on
neutrons. Furthermore, free neutrons can be obtained only from nuclear
disintegrations; there is no natural supply. The only means we have of
controlling free neutrons is to put nuclei in their way so that they will be
slowed and deflected or absorbed by collisions. These effects are of great
practical importance in nuclear reactors and nuclear weapons.
In 1930 W. Bothe and H. Becker in Germany found that if the very energetic
natural alpha particles from polonium fell on certain of the light elements,
specifically beryllium, boron, or lithium, an unusually penetrating
radiation was produced. At first this radiation was thought to be gamma
radiation although it was more penetrating than any gamma rays known, and
the details of experimental results were very difficult to interpret on this
basis. The next important contribution was reported in 1932 by Irene Curie
and F. Joliot in Paris. They showed that if this unknown radiation fell on
paraffin or any other hydrogen-containing compound it ejected protons of
very high energy. This was not in itself inconsistent with the assumed gamma
ray nature of the new radiation, but detailed quantitative analysis of the
data became increasingly difficult to reconcile with such an hypothesis.
Finally (later in 1932) the physicist James Chadwick in England performed a
series of experiments showing that the gamma ray hypothesis was untenable.
He suggested that in fact the new radiation consisted of uncharged particles
of approximately the mass of the proton, and he performed a series of
experiments verifying his suggestion. Such uncharged particles are now
The existence of stable clusters of four neutrons, or tetraneutrons, has
been hypothesised by a team led by Francisco-Miguel Marqus at the CNRS
Laboratory for Nuclear Physics based on observations of the disintegration
of beryllium-14 nuclei. This is particularly interesting, because current
theory suggests that these clusters should not be stable, and therefore not