In physics, the neutron is a subatomic particle with no net electric charge and a mass of 939.6 MeV/''Speed_of_light''² ( kg, slightly more than a proton). Its spin (physics) is ½. Its antiparticle is called the antineutron.
The atomic 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 mean lifetime of 886 seconds (about 15 minutes), decaying by emitting an electron and neutrino to become a proton. Neutrons in this unstable form are known as Free neutron. 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 neutron's antimatter equivalent is the antineutron.
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 atomic nucleus. Charged particles (such as protons, electrons, or alpha particles) and electromagnetic radiations (such as gamma ray) 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 and weak nuclear force nuclear forces 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 collision 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 produce ion.
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 electricity or magnetic fields which have about no effect on neutrons (there is a small effect of a magnetic field on the free neutron because of its magnetic moment). Furthermore, free neutrons (neutron radiation) 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. The capture of free neutrons often results in neutron activation, inducing radioactivity. Free neutron beams are obtained from neutron source by neutron transport.
One use of neutron emitters is the detection of light nuclei, particularly the hydrogen found in water molecules. When a fast neutron collides with a light nucleus, it loses a large fraction of its energy. By measuring the rate at which slow neutrons return to the probe after reflecting off of hydrogen nuclei, a neutron probe may determine the water content in soil.
==Discovery==
In 1930Walther Bothe and H. Becker in Germany found that if the very energetic alpha particles emitted 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 Irène Joliot-Curie and Frédéric 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 were eventually called ''neutrons'', apparently from the Latin root for ''neutral'' and the Greek language ending ''-on'' (by imitation of ''electron'' and ''proton'').
== Current developments ==
The existence of stable clusters of four neutrons, or tetraneutrons, has been hypothesised by a team led by Francisco-Miguel Marques 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 do not exist.
==Antineutron==
The antineutron is the antiparticle of the neutron. It was discovered by Bruce Cork in the year 1956, a year after the antiproton was discovered.
CPT-symmetry puts strong constraints on the relative properties of particles and antiparticles and, therefore, is open to stringent tests. The masses of the neutron and antineutron are equal to one part in (9±5)×10-5.
==See also==
*particle physics
*subatomic particle
*list of particles
*quark model
*chemistry
*neutron star
*neutron transport
*neutron radiation
*thermal neutron
*fast neutron
*pulse neutron generatorNucleonHadronQuantum chromodynamicsNuclear physics
Neutron
I'm not sure the description of particles in a nucleus as
resonances between protons and neutrons by exchange of pions
is up to date. I remember it was one of the first :strong interaction models that appear, but that was before :quarks
and :QCD. Is that description still valid in a quark context?
Yep. QCD provides a little more detail to the picture, of course, but you still get triples of up-downs which correspond to proton-neutrons, and since these are colorless and gluons very sticky you tend to get quark-antiquark pairs connecting them, and these are pions.
Here is the thing, My name is Jessi and I am in 8th grade we just started learning about Atoms,Protons,Neutrons,Elcrtrons,exc, and i am just haveing some trouble understanding it all. It is so comfusing
== Importance in chemistry ==
I did a Google search and I got a page saying that neutrons are not important in chemistry. Please complete this sentence:
Although neutrons are not important in chemistry, they are important in... User:66.245.84.123 00:53, 28 Sep 2004 (UTC) (No, my IP address is not a part of this sentence.)
:Ummm... It's not actually true, although many people who should know better do say that it is. The number of neutrons in its nucleus doesn't make much difference to the chemical behavior of an atom, but it does make some. If you drank enough heavy water to make it replace somewhere between a third and two-thirds of your body water (nobody has tried it), your hair would fall out and any fast-growing cancers would go into remission, owing to the slowing down of some very critical chemical processes in your body. And much chemical research uses radioactive tracers. And depending what you mean by ''important in chemistry'', there's the little detail that the only stable nucleus without any neutrons is hydrogen! So without neutrons, chemistry would be a little on the boring side, if indeed there were any chemists to do it. User:Andrewa 01:40, 26 Oct 2004 (UTC)
::It is mainly important for hydrogen where the speed of the reaction changes by about a factor of 6 under most circumstances where hydrogen is important. Otherwise, it isn't so important... none the less most living things do show significant preference for certain isotopesUser:Pdbailey 04:09, 23 Nov 2004 (UTC)
== how many neutrons are in helium? ==
Depends on the isotope. Helium-3 and helium-4 have one and two neutrons respectively and are the only stable isotopes of helium. Helium-6 and helium-8 (4 and 6 neutrons) are beta decayers with half-lives under a second. -- User:Xerxes314 20:35, 2005 Jan 22 (UTC)
== Half-life of free neutrons ==
I've noticed that there seems to be a very wide range of values given for the half-life of free neutrons. A quick Google search showed up values ranging from around ten minutes to as much as 17 minutes. Even Wikipedea's two entries on the neutron and the free neutron currently give values of 886s and "about ten minutes" respectively.
I have found a reference which gives a measurement with a range of error:
http://prola.aps.org/abstract/PRD/v5/i7/p1628_1
The value they give is 10.61 ± 0.16 min (about 636.6 +/- 9.6 seconds).
I've not edited any entries to reflect this as I have not found another reference confirming this value and the error range. Perhaps someone will be able to confirm the figures.
I assume that the wide range in published values is a reflection of how tricky it is to make the measurement. It would be interesting to hear if this is the case.
:Indeed these conflicting Half-life can certainly lead one into dubious waters. This simply comes down to the instrumentation used by laboratories independent to each other. One lab might use a Geiger counter while another might use a Scintillation counter with less dead time. I understand the consensus among many nuclear physicist and particle physicist physicists is that the half-life lies somewhere between 10.2 and 10.6 minutes. — ''oo64eva (AJ)'' (User: oo64eva | User_talk:Oo64eva | Special:Contributions/Oo64eva) @ 14:53, Apr 15, 2005 (UTC)
