1. What is inside the nucleus?

By 1910 the atom was thought to consist of a massive nucleus orbited by electrons, but measurements of atomic mass indicated that all nuclei must contain integer numbers of some other particle. What were these particles inside the nucleus?

One of these particles was the proton. The proton was discovered during investigations of positive rays, and can be produced by ionising hydrogen. Hydrogen is the lightest type of atom, consisting of a single proton and a single electron. Ionisation separates the electron from the atom, so only the proton remains.

If more massive nuclei contained only protons their charge would be much higher than measurements suggested. With the exception of hydrogen all atoms have a higher mass number than charge number. Rutherford thought that the nucleus consisted of protons and 'neutral doublets' formed from closely bound protons and electrons. This could explain both the mass and the charge that had been measured for different nuclei.

2. The elusive neutron

Rutherford described his 'neutral doublet', or neutron, in 1920. The particle would be uncharged but with a mass only slightly greater than the proton. Because it was uncharged there would be no electrical repulsion of the neutron as it passed through matter, so it would be much more penetrating than the proton. This would make the neutron difficult to detect.
The discovery of the neutron was made by James Chadwick, who spent more than a decade searching. Chadwick had accompanied Rutherford in his move from Manchester to Cambridge. He later became the Assistant Director of Research in the Cavendish, and was responsible for keeping Rutherford informed of any new developments in physics. Chadwick and Rutherford often discussed neutrons, and suggested 'silly' experiments to discover them, but the inspiration for Chadwick's discovery came from Europe, not Rutherford.

3. Beryllium radiation

In 1930 the German physicists Bothe and Becker bombarded the light metal beryllium with alpha particles, and noticed that a very penetrating radiation was emitted. This radiation was non-ionising, and they assumed it was gamma rays.
In 1932 Irène and Frédéric Joliot-Curie investigated this radiation in France. They let the radiation hit a block of paraffin wax, and found it caused the wax to emit protons. They measured the speeds of these protons and found that the gamma rays would have to be incredibly energetic to knock them from the wax.
Chadwick reported the Joliot-Curie's experiment to Rutherford, who did not believe that gamma rays could account for the protons from the wax. He and Chadwick were convinced that the beryllium was emitting neutrons. Neutrons have nearly the same mass as protons, so should knock protons from a wax block fairly easily.

4. Chadwick's discovery

Chadwick worked day and night to prove the neutron theory, studying the beryllium radiation with an ionisation counter and a cloud chamber. He found that the wax could be replaced with other light substances, even beryllium, and that protons were still produced.
Within a month Chadwick had conclusive proof of the existence of the neutron. He published his findings in the journal, Nature, on February 27, 1932.

5. Neutrons from beryllium

The alpha-particles from the radioactive source hit the beryllium nuclei and transformed them into carbon nuclei, leaving one free neutron. When this neutron hit the hydrogen nuclei in the wax it could knock a proton free, in the same way that a white snooker ball can transfer all its energy to a red snooker ball.

Rutherford gave the best description of a neutron as a highly penetrating neutral particle with a mass similar to the proton. We now know it is not a combination of an electron and a proton. Quantum mechanics restricts an electron from getting that close to the proton, and measurements of nuclear 'spin' provide experimental proof that the nucleus does not contain electrons.

6. The decaying particle

Chadwick knew the neutron wasn't formed from an electron and a proton, and explained in his Nobel lecture that it seemed 'useless to discuss whether the neutron and proton are elementary particles or not'. He knew that a more powerful investigation of the neutron was necessary to decide if it was made up of anything else. We now believe that the neutron and the proton are made of even tinier particles called quarks.
To further confuse matters, free neutrons are not stable. If a neutron is outside the nucleus for several minutes it will transform into a proton, an electron, and an extremely light particle called a neutrino. The decay occurs because one of the quarks inside the neutron has transformed into a different quark, producing an additional positive charge in the particle.

7. The nuclear bomb

Neutrons are very penetrating because they are uncharged. This makes them very useful to nuclear physicists, as they can be fired into the nucleus without being repelled like the proton. A neutron can even be made to stop inside a nucleus, transforming elements into more massive types.
This understanding of the neutron allowed scientists to develop nuclear power, and nuclear weapons during the Second World War. Chadwick helped in the theory behind the first nuclear bombs, and used a particle accelerator in Liverpool to show that it is possible to construct them with only a few kilograms of uranium.

Neutron

From Wikipedia, the free encyclopedia

This article is about the subatomic particle. For other uses, see Neutron (disambiguation).

Neutron
The quark structure of the neutron. (The color assignment of individual quarks is not important, only that all three colors are present.)
Classification	Baryon
Composition	1 up quark, 2 down quarks
Statistics	Fermionic
Interactions	Gravity, Weak, Strong,Electromagnetic
Symbol	n, n0 , N0
Antiparticle	Antineutron
Theorized	Ernest Rutherford[1][2] (1920)
Discovered	James Chadwick[1] (1932)
Mass	1.674927351(74)×10−27 kg[3] 939.565378(21) MeV/c2[3] 1.00866491600(43) u[3]
Mean lifetime	881.5(15) s (free)
Electric charge	0 e 0 C
Electric dipole moment	<2.9×10−26 e·cm
Electric polarizability	1.16(15)×10−3 fm3
Magnetic moment	−0.96623647(23)×10−26 J·T−1[3] −1.04187563(25)×10−3 μB[3] −1.91304272(45) μN[3]
Magnetic polarizability	3.7(20)×10−4 fm3
Spin	1⁄2
Isospin	1⁄2
Parity	+1
Condensed	I(JP) = 1⁄2(1⁄2+)

The neutron is a subatomic hadron particle which has the symbol n or n0
, no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen,nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of protons in a nucleus is the atomic number and defines the type of element the atom forms. Neutrons are necessary within an atomic nucleus as they bind with protons via the strong force; protons are unable to bind with each other due to their mutual electromagnetic repulsion being stronger than the attraction of the strong force. The number of neutrons is the neutron number and determines the isotope of an element. For example, the abundant carbon-12 isotope has 6 protons and 6 neutrons, while the very rare radioactive carbon-14 isotope has 6 protons and 8 neutrons.

While bound neutrons in stable nuclei are stable, free neutrons are unstable; they undergobeta decay with a mean lifetime of just under 15 minutes (881.5±1.5 s).^[4] Free neutrons are produced in nuclear fission and fusion. Dedicated neutron sources like research reactors andspallation sources produce free neutrons for use in irradiation and in neutron scatteringexperiments. Even though it is not a chemical element, the free neutron is sometimes included in tables of nuclides.^[5] It is then considered to have an atomic number of zero and amass number of one, and is sometimes referred to as neutronium.^{[citation needed]}

The neutron has been the key to nuclear power production. After the neutron was discovered in 1932, it was realized in 1933 that it might mediate a nuclear chain reaction. In the 1930s, neutrons were used to produce many different types of nuclear transmutations. Whennuclear fission was discovered in 1938, it was soon realized that this might be the mechanism to produce the neutrons for the chain reaction, if the process also produced neutrons, and this was proven in 1939, making the path to nuclear power production evident. These events and findings led directly to the first man-made nuclear chain reaction which was self-sustaining (Chicago Pile-1, 1942) and to the first nuclear weapons (1945).

Contents   [hide]  ·         1 Discovery ·         2 Intrinsic properties o    2.1 Stability and beta decay o    2.2 Electric dipole moment o    2.3 Magnetic moment o    2.4 Anti-neutron o    2.5 Structure and geometry of charge distribution within the neutron ·         3 Neutron compounds o    3.1 Dineutrons and tetraneutrons o    3.2 Neutronium and neutron stars ·         4 Detection o    4.1 Neutron detection by neutron capture o    4.2 Neutron detection by elastic scattering ·         5 Uses ·         6 Sources ·         7 Protection ·         8 Production ·         9 Neutron temperature o    9.1 Thermal neutrons o    9.2 Cold neutrons o    9.3 Ultracold neutrons o    9.4 Fission energy neutrons o    9.5 Fusion neutrons o    9.6 Intermediate-energy neutrons o    9.7 High-energy neutrons ·         10 See also o    10.1 Neutron sources o    10.2 Processes involving neutrons ·         11 References ·         12 Further reading

[edit]Discovery

In 1920, Ernest Rutherford conceptualized the possible existence of the neutron.^[2] In particular, Rutherford considered that the disparity found between the atomic number of an atom and its atomic mass could be explained by the existence of a neutrally charged particle within the atomic nucleus. He considered the neutron to be a neutral double consisting of an electron orbiting a proton.

In 1930 Viktor Ambartsumian and Dmitri Ivanenko in USSR found that, contrary to the prevailing opinion of the time, the nucleus cannot consist of protons and electrons. They proved that some neutral particles must be present besides the protons.^[6]

In 1931, Walther Bothe and Herbert Becker in Germany found that if the very energetic alpha particles emitted from polonium fell on certain 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 a hypothesis.

In 1932, James Chadwick performed a series of experiments at the University of Cambridge, showing that the gamma ray hypothesis was untenable. He suggested that the new radiation consisted of uncharged particles of approximately the mass of the proton, and he performed a series of experiments verifying his suggestion.^[7] These uncharged particles were called neutrons, apparently from the Latin root for neutraland the Greek ending -on (by imitation of electron and proton).

The discovery of the neutron explained a puzzle involving the spin of the nitrogen-14 nucleus, which had been experimentally measured to be 1 ħ. It was known that atomic nuclei usually had about half as many positive charges than if they were composed completely of protons, and in existing models this was often explained by proposing that nuclei also contained some "nuclear electrons" to neutralize the excess charge. Thus, nitrogen-14 would be composed of 14 protons and 7 electrons to give it a charge of +7 but a mass of 14 atomic mass units. However, it was also known that both protons and electrons carried an intrinsic spin of ¹⁄₂ ħ, and there was no way to arrange an odd number (21) of spins ±¹⁄₂ ħ to give a spin of 1 ħ. Instead, when nitrogen-14 was proposed to consist of 3 pairs of protons and neutrons, with an additional unpaired neutron and proton each contributing a spin of ¹⁄₂ ħ in the same direction for a total spin of 1 ħ, the model became viable. Soon, nuclear neutrons were used to naturally explain spin differences in many different nuclides in the same way, and the neutron as a basic structural unit of atomic nuclei was accepted.

[edit]Intrinsic properties

[edit]Stability and beta decay

The Feynman diagram for beta decay of a neutron into a proton, electron, andelectron antineutrino via an intermediate heavy W boson

Under the Standard Model of particle physics, because the neutron consists of three quarks, the only possible decay mode without a change of baryon number is for one of the quarks to change flavour via the weak interaction. The neutron consists of two down quarks with charge −¹⁄₃ e and one up quarkwith charge +²⁄₃ e, and the decay of one of the down quarks into a lighter up quark can be achieved by the emission of a W boson. By this means the neutron decays into a proton (which contains one down and two up quarks), an electron, and an electron antineutrino.

Outside the nucleus, free neutrons are unstable and have a mean lifetime of 881.5±1.5 s (about 14 minutes, 42 seconds); therefore the half-life for this process (which differs from the mean lifetime by a factor of ln(2) = 0.693) is 611.0±1.0 s (about 10 minutes, 11 seconds).^[4] Free neutrons decay by emission of an electron and an electron antineutrino to become a proton, a process known as beta decay:^[8]

n0
 → p+
 + e−
 + ν
e

Neutrons in unstable nuclei can also decay in this manner. However, inside a nucleus, protons can also transform into a neutron via inverse beta decay. This transformation occurs by emission of anantielectron (also called positron) and an electron neutrino:

p+
 → n0
 + e+
 + ν
e

The transformation of a proton to a neutron inside of a nucleus is also possible through electron capture:

p+
 + e−
 → n0
 + ν
e

Positron capture by neutrons in nuclei that contain an excess of neutrons is also possible, but is hindered because positrons are repelled by the nucleus, and quickly annihilate when they encounter electrons.

When bound inside of a nucleus, the instability of a single neutron to beta decay is balanced against the instability that would be acquired by the nucleus as a whole if an additional proton were to participate in repulsive interactions with the other protons that are already present in the nucleus^{[clarification needed]}. As such, although free neutrons are unstable, bound neutrons are not necessarily so. The same reasoning explains why protons, which are stable in empty space, may transform into neutrons when bound inside of a nucleus.

[edit]Electric dipole moment

Main article: Neutron electric dipole moment

The Standard Model of particle physics predicts a tiny separation of positive and negative charge within the neutron leading to a permanentelectric dipole moment.^[9] The predicted value is, however, well below the current sensitivity of experiments. From several unsolved puzzles in particle physics, it is clear that the Standard Model is not the final and full description of all particles and their interactions. New theories going beyond the Standard Model generally lead to much larger predictions for the electric dipole moment of the neutron. Currently, there are at least four experiments trying to measure for the first time a finite neutron electric dipole moment, including:

§  Cryogenic neutron EDM experiment being set up at the Institut Laue-Langevin^[10]

§  nEDM experiment under construction at the new UCN source at the Paul Scherrer Institute^[11]

§  nEDM experiment being envisaged at the Spallation Neutron Source^[12]

§  nEDM experiment being built at the Institut Laue-Langevin^[13]

[edit]Magnetic moment

Main article: neutron magnetic moment

Even though the neutron is a neutral particle, the magnetic moment of a neutron is not zero because it is a composite particle containing three charged quarks.

[edit]Anti-neutron

Main article: 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, so studying antineutrons yields provide stringent tests on CPT-symmetry. The fractional difference in the masses of the neutron and antineutron is 9±6×10⁻⁵. Since the difference is only about two standard deviations away from zero, this does not give any convincing evidence of CPT-violation.^[4]

[edit]Structure and geometry of charge distribution within the neutron

An article published in 2007 featuring a model-independent analysis concluded that the neutron has a negatively charged exterior, a positively charged middle, and a negative core.^[14] In a simplified classical view, the negative "skin" of the neutron assists it to be attracted to the protons with which it interacts in the nucleus. However, the main attraction between neutrons and protons is via the nuclear force, which does not involve charge.

[edit]Neutron compounds

[edit]Dineutrons and tetraneutrons

Main articles: Dineutron and Tetraneutron

The existence of stable clusters of 4 neutrons, or tetraneutrons, has been hypothesised by a team led by Francisco-Miguel Marqués 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.

The dineutron is another hypothetical particle.

[edit]Neutronium and neutron stars

Main articles: Neutronium and Neutron star

At extremely high pressures and temperatures, nucleons and electrons are believed to collapse into bulk neutronic matter, called neutronium. This is presumed to happen in neutron stars.

The extreme pressure inside a neutron star may deform the neutrons into a cubic symmetry, allowing tighter packing of neutrons.^[15]

[edit]Detection

Main article: Neutron detection

The common means of detecting a charged particle by looking for a track of ionization (such as in a cloud chamber) does not work for neutrons directly. Neutrons that elastically scatter off atoms can create an ionization track that is detectable, but the experiments are not as simple to carry out; other means for detecting neutrons, consisting of allowing them to interact with atomic nuclei, are more commonly used. The commonly used methods to detect neutrons can therefore be categorized according to the nuclear processes relied upon, mainly neutron capture or elastic scattering. A good discussion on neutron detection is found in chapter 14 of the book Radiation Detection and Measurement by Glenn F. Knoll (John Wiley & Sons, 1979).

[edit]Neutron detection by neutron capture

A common method for detecting neutrons involves converting the energy released from neutron capture reactions into electrical signals. Certain nuclides have a high neutron capture cross section, which is the probability of absorbing a neutron. Upon neutron capture, the compound nucleus emits more easily detectable radiation, for example an alpha particle, which is then detected. The nuclides 3
He, 6
Li, 10
B, 233
U, 235
U, 237
Np and 239
Pu are useful for this purpose.

[edit]Neutron detection by elastic scattering

Neutrons can elastically scatter off nuclei, causing the struck nucleus to recoil. Kinematically, a neutron can transfer more energy to light nuclei such as hydrogen or helium than to heavier nuclei. Detectors relying on elastic scattering are called fast neutron detectors. Recoiling nuclei can ionize and excite further atoms through collisions. Charge and/or scintillation light produced in this way can be collected to produce a detected signal. A major challenge in fast neutron detection is discerning such signals from erroneous signals produced by gamma radiation in the same detector.

Fast neutron detectors have the advantage of not requiring a moderator, and therefore being capable of measuring the neutron's energy, time of arrival, and in certain cases direction of incidence.

[edit]Uses

Science with Neutrons

Foundations

§  Neutron temperature §  Flux · Radiation · Transport §  Cross section · Absorption · Activation

Neutron scattering

§  Neutron diffraction §  Small-angle neutron scattering §  GISANS §  Reflectometry §  Inelastic neutron scattering §  Triple-axis spectrometer §  Time-of-flight spectrometer §  Backscattering spectrometer §  Spin-echo spectrometer

Other applications

§  Radiography and tomography §  Activation analysis ·Prompt gamma activation analysis §  Fundamental research with neutrons:Ultracold neutrons · Interferometry

Infrastructure

§  Neutron sources: Research reactor ·Spallation · Neutron moderator §  Neutron optics: Guide · Supermirror ·Polarizer §  Detection

Neutron facilities

§  America: HFIR · LANSCE · NIST CNR -SNS §  Australia: OPAL §  Asia: J-PARC §  Europe: BER II · FRM II · ILL · ISIS · JINR ·LLB · PINS · SINQ §  Africa: Necsa §  Historic: IPNS · HFBR §  Planned: ESS §  User Affiliations: ENSA

§  v §  t §  e

The neutron plays an important role in many nuclear reactions. For example, neutron capture often results in neutron activation, inducing radioactivity. In particular, knowledge of neutrons and their behavior has been important in the development of nuclear reactors and nuclear weapons. The fissioning of elements like uranium-235 and plutonium-239 is caused by their absorption of neutrons.

Cold, thermal and hot neutron radiation is commonly employed in neutron scattering facilities, where the radiation is used in a similar way one uses X-rays for the analysis of condensed matter. Neutrons are complementary to the latter in terms of atomic contrasts by different scattering cross sections; sensitivity to magnetism; energy range for inelastic neutron spectroscopy; and deep penetration into matter.

The development of "neutron lenses" based on total internal reflection within hollow glass capillary tubes or by reflection from dimpled aluminum plates has driven ongoing research into neutron microscopy and neutron/gamma ray tomography.^[16][17][18]

A major use of neutrons is to excite delayed and prompt gamma rays from elements in materials. This forms the basis of neutron activation analysis (NAA) and prompt gamma neutron activation analysis (PGNAA). NAA is most often used to analyze small samples of materials in a nuclear reactor whilst PGNAA is most often used to analyze subterranean rocks around bore holes and industrial bulk materials on conveyor belts.

Another use of neutron emitters is the detection of light nuclei, particularly the hydrogen found inwater 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.

[edit]Sources

Main article: Neutron source

Main article: Research reactor

Because free neutrons are unstable, they can be obtained only from nuclear disintegrations, nuclear reactions, and high-energy reactions (such as in cosmic radiation showers or accelerator collisions). Free neutron beams are obtained from neutron sources by neutron transport. For access to intense neutron sources, researchers must go to a specialist neutron facility that operates a research reactor or a spallation source.

The neutron's lack of total electric charge makes it difficult to steer or accelerate them. Charged particles can be accelerated, decelerated, or deflected by electric or magnetic fields. These methods have little effect on neutrons beyond a small effect of an inhomogeneous magnetic field because of the neutron's magnetic moment. Neutrons can be controlled by methods that includemoderation, reflection and velocity selection.

[edit]Protection

Exposure to free neutrons can be hazardous, since the interaction of neutrons with molecules in the body can cause disruption to moleculesand atoms, and can also cause reactions which give rise to other forms of radiation (such as protons). The normal precautions of radiation protection apply: avoid exposure, stay as far from the source as possible, and keep exposure time to a minimum. Some particular thought must be given to how to protect from neutron exposure, however. For other types of radiation, e.g. alpha particles, beta particles, or gamma rays, material of a high atomic number and with high density make for good shielding; frequently lead is used. However, this approach will not work with neutrons, since the absorption of neutrons does not increase straightforwardly with atomic number, as it does with alpha, beta, and gamma radiation. Instead one needs to look at the particular interactions neutrons have with matter (see the section on detection above). For example, hydrogen-rich materials are often used to shield against neutrons, since ordinary hydrogen both scatters and slows neutrons. This often means that simple concrete blocks or even paraffin-loaded plastic blocks afford better protection from neutrons than do far more dense materials. After slowing, neutrons may then be absorbed with an isotope which has high affinity for slow neutrons without causing secondary capture-radiation, such as lithium-6.

Hydrogen-rich ordinary water affects neutron absorption in nuclear fission reactors: usually neutrons are so strongly absorbed by normal water that fuel-enrichment with fissionable isotope is required. The deuterium in heavy water has a very much lower absorption affinity for neutrons than does protium (normal light hydrogen). Deuterium is therefore used in CANDU-type reactors, in order to slow (moderate) neutron velocity, to increase the probability of nuclear fission compared to neutron capture.

[edit]Production

Institut Laue–Langevin (ILL) in Grenoble,France – one of the most important neutron research facilities worldwide

Various nuclides become more stable by expelling neutrons as a decay mode; this is known asneutron emission, and happens commonly during spontaneous fission.

Cosmic radiation interacting with the Earth's atmosphere continuously generates neutrons that can be detected at the surface. Even stronger neutron radiation is produced at the surface of Mars where the atmosphere is thick enough to generate neutrons from cosmic ray spallation, but not thick enough to provide significant protection from the neutrons produced. These neutrons not only produce a Martian surface neutron radiation hazard from direct downward-going neutron radiation, but also a significant hazard from reflection of neutrons from the Martian surface, which will produce reflected neutron radiation penetrating upward into a Martian craft or habitat from the floor.^[19]

Nuclear fission reactors naturally produce free neutrons; their role is to sustain the energy-producing chain reaction. The intense neutron radiation can also be used to produce various radioisotopes through the process of neutron activation, which is a type of neutron capture.

Experimental nuclear fusion reactors produce free neutrons as a waste product. However, it is these neutrons that possess most of the energy, and converting that energy to a useful form has proved a difficult engineering challenge. Fusion reactors which generate neutrons are likely to create around twice the amount of radioactive waste of a fission reactor, but the waste is composed of neutron-activated lighter isotopes, which have relatively short (50–100 years) decay periods as compared to typical half lives of 10,000 years for fission waste, which is long primarily due to the long half life of alpha-emitting transuranic actinides.^[20]

[edit]Neutron temperature

Main article: Neutron temperature

[edit]Thermal neutrons

A thermal neutron is a free neutron that is Boltzmann distributed with kT = 0.0253 eV (4.0×10⁻²¹ J) at room temperature. This gives characteristic (not average, or median) speed of 2.2 km/s. The name 'thermal' comes from their energy being that of the room temperature gas or material they are permeating. (see kinetic theory for energies and speeds of molecules). After a number of collisions (often in the range of 10–20) with nuclei, neutrons arrive at this energy level, provided that they are not absorbed.

In many substances, thermal neutrons have a much larger effective cross-section than faster neutrons, and can therefore be absorbed more easily by any atomic nuclei that they collide with, creating a heavier — and often unstable — isotope of the chemical element as a result.

Most fission reactors use a neutron moderator to slow down, or thermalize the neutrons that are emitted by nuclear fission so that they are more easily captured, causing further fission. Others, called fast breeder reactors, use fission energy neutrons directly.

[edit]Cold neutrons

Cold neutrons are thermal neutrons that have been equilibrated in a very cold substance such as liquid deuterium. Such a cold source is placed in the moderator of a research reactor or spallation source. Cold neutrons are particularly valuable for neutron scattering experiments.

[edit]Ultracold neutrons

Ultracold neutrons are produced by inelastically scattering cold neutrons in substances with a temperature of a few kelvins, such as soliddeuterium or superfluid helium. An alternative production method is the mechanical deceleration of cold neutrons.

[edit]Fission energy neutrons

A fast neutron is a free neutron with a kinetic energy level close to 2 MeV (3.2×10⁻¹³ J), hence a speed of ~20,000 km/s (~ 6% of the speed of light). They are named fission energy or fast neutrons to distinguish them from lower-energy thermal neutrons, and high-energy neutrons produced in cosmic showers or accelerators. Fast neutrons are produced by nuclear processes such as nuclear fission.

Fast neutrons can be made into thermal neutrons via a process called moderation. This is done with a neutron moderator. In reactors, typically heavy water, light water, or graphite are used to moderate neutrons.

[edit]Fusion neutrons

The fusion reaction rate increases rapidly with temperature until it maximizes and then gradually drops off. The DT rate peaks at a lower temperature (about 70 keV, or 800 million kelvins) and at a higher value than other reactions commonly considered for fusion energy.

For more details on this topic, see Nuclear fusion#Criteria and candidates for terrestrial reactions.

D-T (deuterium-tritium) fusion is the fusion reaction that produces the most energetic neutrons, with 14.1 MeV of kinetic energy and traveling at 17% of the speed of light. D-T fusion is also the easiest fusion reaction to ignite, reaching near-peak rates even when the deuterium and tritium nuclei have only a thousandth as much kinetic energy as the 14.1 MeV that will be produced.

14.1 MeV neutrons have about 10 times as much energy as fission neutrons, and are very effective at fissioning even non-fissile heavy nuclei, and these high-energy fissions produce more neutrons on average than fissions by lower-energy neutrons. This makes D-T fusion neutron sources such as proposed tokamak power reactors useful for transmutation of transuranic waste. 14.1 MeV neutrons can also produce neutrons by knocking them loose from nuclei.

On the other hand, these very high energy neutrons are less likely to simply be captured without causing fission or spallation. For these reasons, nuclear weapon design extensively utilizes D-T fusion 14.1 MeV neutrons to cause more fission. Fusion neutrons are able to cause fission in ordinarily non-fissile materials, such asdepleted uranium (uranium-238), and these materials have been used in the jackets of thermonuclear weapons. Fusion neutrons also can cause fission in substances that are unsuitable or difficult to make into primary fission bombs, such as reactor grade plutonium. This physical fact thus causes ordinary non-weapons grade materials to become of concern in certain nuclear proliferation discussions and treaties.

Other fusion reactions produce much less energetic neutrons. D-D fusion produces a 2.45 MeV neutron and helium-3 half of the time, and produces tritium and a proton but no neutron the other half of the time. D-³He fusion produces no neutron.

[edit]Intermediate-energy neutrons

Transmutation flow in LWR which is a thermal-spectrum reactor

A fission energy neutron that has slowed down but not yet reached thermal energies is called an epithermal neutron.

Cross sections for both capture and fission reactions often have multiple resonancepeaks at specific energies in the epithermal energy range. These are of less significance in a fast neutron reactor where most neutrons are absorbed before slowing down to this range, or in a well-moderated thermal reactor where epithermal neutrons mostly interact with moderator nuclei, not with either fissile or fertileactinide nuclides. However, in a partially moderated reactor with more interactions of epithermal neutrons with heavy metal nuclei, there are greater possibilities fortransient changes in reactivity which might make reactor control more difficult.

Ratios of capture reactions to fission reactions are also worse (more captures without fission) in most nuclear fuels such as plutonium-239, making epithermal-spectrum reactors using these fuels less desirable, as captures not only waste the one neutron captured but also usually result in a nuclide which is not fissile with thermal or epithermal neutrons, though still fissionable with fast neutrons. The exception is uranium-233 of the thorium cycle which has good capture-fission ratios at all neutron energies.

[edit]High-energy neutrons

These neutrons have more energy than fission energy neutrons and are generated as secondary particles by particle accelerators or in the atmosphere from cosmic rays. They can have energies as high as tens of joules per neutron. These neutrons are extremely efficient ationization and far more likely to cause cell death than X-rays or protons.^[21][22]

[edit]See also

Wikimedia Commons has media related to: Neutron

§  Ionizing radiation

§  Isotope

§  List of particles

§  Neutron capture nucleosynthesis

§  Neutron magnetic moment

§  Neutron radiation and the Sievert radiation scale

§  Nuclear reaction

§  R-process

§  S-process

§  Thermal reactor

[edit]Neutron sources

§  Neutron generator

§  Neutron sources

[edit]Processes involving neutrons

§  Neutron bomb

§  Neutron diffraction

§  Neutron flux

§  Neutron transport

Discovery of the Neutron

It is remarkable that the neutron was not discovered until 1932 when James Chadwick used scattering data to calculate the mass of this neutral particle. Since the time of Rutherford it had been known that the atomic mass number A of nuclei is a bit more than twice the atomic number Z for most atoms and that essentially all the mass of the atom is concentrated in the relatively tiny nucleus. As of about 1930 it was presumed that the fundamental particles were protons and electrons, but that required that somehow a number of electrons were bound in the nucleus to partially cancel the charge of A protons. But by this time it was known from the uncertainty principle and from "particle-in-a-box" type confinement calculations that there just wasn't enough energy available to contain electrons in the nucleus.
A rough scale of the energy required for the confinement of a particle to a given dimension can be obtained by setting the DeBroglie wavelength of the particle equal to that dimension. For example, if we presume that the dimension of a hydrogen atom is about 0.2 nm, then the corresponding confinement energy is about 38 eV, the correct order of magnitude for atomic electrons. But to confine an electron to a nuclear dimension of about 5 fermis requires an energy of about 250 MeV. The maximum available confinement energy from the electrical attraction to the nucleus is given by

Electrostatic potential energy

So it is clear that there are no electrons in the nucleus.
An experimental breakthrough came in 1930 with the observation by Bothe and Becker that bombardment of beryllium with alpha particles from a radioactive source produced neutral radiation which was penetrating but non-ionizing. They presumed it was gamma rays, but Curie and Joliot showed that when you bombarded a paraffin target with this radiation, it ejected protons with energy about 5.3 MeV. This proved to be inconsistent with gamma rays, as can be shown from momentum and energy analysis:

This analysis follows that for a headon elastic collision where a small particle strikes a much more massive one. Again, the necessary energy for the gamma ray explanation was much greater than any energy observed to be available from the nucleus, so the neutral radiation must be some kind of neutral particle.
The 5.3 MeV energy of the ejected protons could be easily explained if the neutral particle had a mass comparable to that of the proton. For headon collisions, this would require only 5.3 MeV from the neutral particle, a value in the range of observed nuclear particle emissions.

Chadwick was able to prove that the neutral particle could not be a photon by bombarding targets other than hydrogen, including nitrogen, oxygen, helium and argon. Not only were these inconsistent with photon emission on energy grounds, the cross-section for the interactions was orders of magnitude greater than that for Compton scattering by photons.

The task which remained for Chadwick was that of determining the mass of the neutral particle. He chose to bombard boron with alpha particles and analyze the interaction of the neutral particles with nitrogen. These particlular targets were chosen partly because the masses of boron and nitrogen were well known. Conservation of energy applied to the combined interactions gives the following expressions:

Solving for the mass energy of the neutron gives

The remaining unknown on the right hand side of the equation is the speed of the neutron. Assuming that the neutron mass was close to that of the proton, Chadwick bombarded hydrogen atoms with his produced neutrons to learn the speed of the protons after the collisions. Then setting the neutron speed equal to those proton speeds, he used the above energy expression to get a neutron mass of 938 +/- 1.8 MeV. With a consistent set of experiments, Chadwick had obtained the first value for the neutron mass which compared well with the presently accepted value of 939.57 MeV.

In 1920, Ernest Rutherford postulated that there were neutral, massive particles in the nucleus of atoms. This conclusion arose from the disparity between an element's atomic number (protons = electrons) and its atomic mass (usually in excess of the mass of the known protons present). James Chadwick was assigned the task of tracking down evidence of Rutherford's tightly bound "proton-electron pair" or neutron. In 1930 it was discovered that Beryllium, when bombarded by alpha particles, emitted a very energetic stream of radiation. This stream was originally thought to be gamma radiation.  However, further investigations into the properties of the radiation revealed contradictory results. Like gamma rays, these rays were extremely penetrating and since they were not deflected upon passing through a magnetic field, neutral. However, unlike gamma rays, these rays did not discharge charged electroscopes (the photoelectric effect). Irene Curie and her husband discovered that when a beam of this radiation hit a substance rich in protons, for example paraffin, protons were knocked loose which could be easily detected by a Geiger counter. In 1932, Chadwick proposed that this particle was Rutherford's neutron. In 1935, he was awarded the Nobel Prize for his discovery. Using kinematics, Chadwick was able to determine the velocity of the protons. Then through conservation of momentum techniques, he was able to determine that the mass of the neutral radiation was almost exactly the same as that of a proton. This is Chadwick's equation: With Chadwick's announcement, Heisenberg then proposed the proton-neutron model for the nucleus. Rutherford was incorrect in his "proton-electron" pair - there were no "free electrons" in the nucleus. However, once free of the nucleus, evidence was mounting that these neutrons were unstable. By 1932, the products of beta decay had been thoroughly examined. To account for a broad spectrum of electron energies from a typical beta emitter, discussions were taking place in which leading physicists were considering abandoning the concepts of conservation of momentum and conservation of energy in radioactive decays. To bring empirical evidence back into alignment with these fundamental basic principles, Wolfgang Pauli proposed in 1930 the existence of an invisible particle that would carry off the missing energy and momentum. He called this particle the neutrino, or little neutral one. It wasn't until 1955 that Cowan and Reines, working with discharging radiation from the Savannah River Nuclear Power Plant with its abundant supply of antineutrinos released through the decay of free neutrons, discovered concrete experimental data to support the existence of neutrinos. Forty years later, in 1995, Frederick Reines was awarded theNobel Prize for his pioneering work.