As soon as possible It them
Neutron, neutral subatomic particle that is a constituent of every atomic nucleus except ordinary hydrogen. It has no electric charge and a rest mass equal to 1.67493 × 10−27 kg—marginally greater than that of the proton but nearly 1,839 times greater than that of the electron. Neutrons and protons, commonly called nucleons, are bound together in the dense inner core of an atom, the nucleus, where they account for 99.9 percent of the atom's mass. Developments in high-energy particle physics in the 20th century revealed that neither the neutron nor the proton is a true elementary particle; rather, they are composites of extremely small elementary particles called quarks. The nucleus is bound together by the residual effect of the strong force, a fundamental interaction that governs the behaviour of the quarks that make up the individual protons and neutrons.
Neutron
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James Chadwick
Bertram N. Brockhouse
Luis Alvarez
Walter M. Elsasser
Robert Hofstadter
Felix Bloch
Walther Bothe
Clifford G. Shull
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The neutron was discovered in 1932 by the English physicist James Chadwick. Within a few years after this discovery, many investigators throughout the world were studying the properties and interactions of the particle. It was found that various elements, when bombarded by neutrons, undergo fission—a type of nuclear reaction that occurs when the nucleus of a heavy element is split into two nearly equal smaller fragments. During this reaction each fissioned nucleus gives off additional free neutrons, as well as those bound to the fission fragments. In 1942 a group of American researchers, under the leadership of the physicist Enrico Fermi, demonstrated that enough free neutrons are produced during the fission process to sustain a chain reaction. This development led to the construction of the atomic bomb. Subsequent technological breakthroughs resulted in the large-scale production of electric power from nuclear energy. The absorption of neutrons by nuclei exposed to the high neutron intensities available in nuclear reactors has also made it possible to produce large quantities of radioactive isotopes useful for a wide variety of purposes. Furthermore, the neutron has become an important tool in pure research. Knowledge of its properties and structure is essential to an understanding of the structure of matter in general. Nuclear reactions induced by neutrons are valuable sources of information about the atomic nucleus and the force that binds it together.

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subatomic particle
>neutrons. But these basic atomic components are by no means the only known subatomic particles. Protons...
A free neutron—one that is not incorporated into a nucleus—is subject to radioactive decay of a type called beta decay. It breaks down into a proton, an electron, and an antineutrino (the antimatter counterpart of the neutrino, a particle with no charge and little or no mass); the half-life for this decay process is 614 seconds. Because it readily disintegrates in this manner, the neutron does not exist in nature in its free state, except among other highly energetic particles in cosmic rays. Since free neutrons are electrically neutral, they pass unhindered through the electrical fields within atoms and so constitute a penetrating form of radiation, interacting with matter almost exclusively through relatively rare collisions with atomic nuclei.
Neutrons and protons are classified as hadrons, subatomic particles that are subject to the strong force. Hadrons, in turn, have been shown to possess internal structure in the form of quarks, fractionally charged subatomic particles that are thought to be among the fundamental components of matter. Like the proton and other baryon particles, the neutron consists of three quarks; in fact, the neutron possesses a magnetic dipole moment—i.e., it behaves like a minute magnet in ways that suggest that it is an entity of moving electric charges.
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Hadron, any member of a class of subatomic particles that are built from quarks and thus react through the agency of the strong force. The hadrons embrace mesons, baryons (e.g., protons, neutrons, and sigma particles), and their many resonances. All observed subatomic particles are hadrons except for the gauge bosons of the fundamental interactions and the leptons. Except for protons and neutrons that are bound in atomic nuclei, all hadrons have short lives and are produced in the high-energy collisions of subatomic particles. The other three basic forces of nature also affect hadron behaviour: all hadrons are subject to gravitation; charged hadrons obey electromagnetic laws; and some hadrons break up by way of the weak force (as in radioactive decay), while others decay via the strong and the electromagnetic forces.
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What Is an Atom?
By Tim Sharp
First Published 1 year ago
Reference Article: Facts about atoms, the building blocks of matter.
Atoms consist of a nucleus made of protons and neutrons orbited by electrons. (Image credit: Shutterstock)
Atoms are the basic units of matter and the defining structure of elements. The term "atom" comes from the Greek word for indivisible, because it was once thought that atoms were the smallest things in the universe and could not be divided. We now know that atoms are made up of three particles: protons, neutrons and electrons — which are composed of even smaller particles, such as quarks.
Atoms were created after the Big Bang 13.7 billion years ago. As the hot, dense new universe cooled, conditions became suitable for quarks and electrons to form. Quarks came together to form protons and neutrons, and these particles combined into nuclei. This all took place within the first few minutes of the universe's existence, according to CERN.
It took 380,000 years for the universe to cool enough to slow down the electrons so that the nuclei could capture them to form the first atoms. The earliest atoms were primarily hydrogen and helium, which are still the most abundant elements in the universe, according to Jefferson Lab. Gravity eventually caused clouds of gas to coalesce and form stars, and heavier atoms were (and still are) created within the stars and sent throughout the universe when the star exploded (supernova).

Atomic particles
Protons and neutrons are heavier than electrons and reside in the nucleus at the center of the atom. Electrons are extremely lightweight and exist in a cloud orbiting the nucleus. The electron cloud has a radius 10,000 times greater than the nucleus, according to the Los Alamos National Laboratory.
Protons and neutrons have approximately the same mass. However, one proton is about 1,835 times more massive than an electron. Atoms always have an equal number of protons and electrons, and the number of protons and neutrons is usually the same as well. Adding a proton to an atom makes a new element, while adding a neutron makes an isotope, or heavier version, of that atom.
Nucleus
The nucleus was discovered in 1911 by Ernest Rutherford, a physicist from New Zealand. In 1920, Rutherford proposed the name proton for the positively charged particles of the atom. He also theorized that there was a neutral particle within the nucleus, which James Chadwick, a British physicist and student of Rutherford's, was able to confirm in 1932.
Virtually all the mass of an atom resides in its nucleus, according to Chemistry LibreTexts. The protons and neutrons that make up the nucleus are approximately the same mass (the proton is slightly less) and have the same angular momentum, or spin.
The nucleus is held together by the strong force, one of the four basic forces in nature. This force between the protons and neutrons overcomes the repulsive electrical force that would otherwise push the protons apart, according to the rules of electricity. Some atomic nuclei are unstable because the binding force varies for different atoms based on the size of the nucleus. These atoms will then decay into other elements, such as carbon-14 decaying into nitrogen-14.
Here's a simple drawing of the structure of an atom. (Image credit: Shutterstock)
Protons
Protons are positively charged particles found within atomic nuclei. Rutherford discovered them in experiments with cathode-ray tubes that were conducted between 1911 and 1919. Protons are about 99.86% as massive as neutrons.
The number of protons in an atom is unique to each element. For example, carbon atoms have six protons, hydrogen atoms have one and oxygen atoms have eight. The number of protons in an atom is referred to as the atomic number of that element. The number of protons also determines the chemical behavior of the element. Elements are arranged in the Periodic Table of the Elements in order of increasing atomic number.
Three quarks make up each proton — two "up" quarks (each with a two-thirds positive charge) and one "down" quark (with a one-third negative charge) — and they are held together by other subatomic particles called gluons, which are massless.
Electrons
Electrons are tiny compared to protons and neutrons, over 1,800 times smaller than either a proton or a neutron. Electrons are about 0.054% as massive as neutrons, according to Jefferson Lab.
Joseph John (J.J.) Thomson, a British physicist, discovered the electron in 1897, according to the Science History Institute. Originally known as "corpuscles," electrons have a negative charge and are electrically attracted to the positively charged protons. Electrons surround the atomic nucleus in pathways called orbitals, an idea that was put forth by Erwin Schrödinger, an Austrian physicist, in the 1920s. Today, this model is known as the quantum model or the electron cloud model. The inner orbitals surrounding the atom are spherical but the outer orbitals are much more complicated.
An atom's electron configuration refers to the locations of the electrons in a typical atom. Using the electron configuration and principles of physics, chemists can predict an atom's properties, such as stability, boiling point and conductivity, according to the Los Alamos National Laboratory.
Neutrons
The neutron's existence was theorized by Rutherford in 1920 and discovered by Chadwick in 1932, according to the American Physical Society. Neutrons were found during experiments when atoms were shot at a thin sheet of beryllium. Subatomic particles with no charge were released – the neutron.
Neutrons are uncharged particles found within all atomic nuclei (except for hydrogen). A neutron's mass is slightly larger than that of a proton. Like protons, neutrons are also made of quarks — one "up" quark (with a positive 2/3 charge) and two "down" quarks (each with a negative one-third charge).
History of the atom
The theory of the atom dates at least as far back as 440 B.C. to Democritus, a Greek scientist and philosopher. Democritus most likely built his theory of atoms upon the work of past philosophers, according to Andrew G. Van Melsen, author of "From Atomos to Atom: The History of the Concept Atom" (Duquesne University Press, 1952).
Democritus' explanation of the atom begins with a stone. A stone cut in half gives two halves of the same stone. If the stone were to be continuously cut, at some point there would exist a piece of the stone small enough that it could no longer be cut. The term "atom" comes from the Greek word for indivisible, which Democritus concluded must be the point at which a being (any form of matter) cannot be divided any more.
His explanation included the ideas that atoms exist separately from each other, that there are an infinite amount of atoms, that atoms are able to move, that they can combine together to create matter but do not merge to become a new atom, and that they cannot be divided, according to Universe Today. However, because most philosophers at the time — especially the very influential Aristotle — believed that all matter was created from earth, air, fire and water, Democritus' atomic theory was put aside.
John Dalton, a British chemist, built upon Democritus' ideas in 1803 when he put forth his own atomic theory, according to the chemistry department at Purdue University. Dalton's theory included several ideas from Democritus, such as atoms are indivisible and indestructible and that different atoms form together to create all matter. Dalton's additions to the theory included the following ideas: That all atoms of a certain element were identical, that atoms of one element will have different weights and properties than atoms of another element, that atoms cannot be created or destroyed and that matter is formed by atoms combining in simple whole numbers.
Thomson, the British physicist who discovered the electron in 1897, proved that atoms can be divided, according to the Chemical Heritage Foundation. He was able to determine the existence of electrons by studying the properties of electric discharge in cathode-ray tubes. According to Thomson's 1897 paper, the rays were deflected within the tube, which proved that there was something that was negatively charged within the vacuum tube. In 1899, Thomson published a description of his version of the atom, commonly known as the "plum pudding model." An excerpt of this paper is found on the Chem Team site. Thomson's model of the atom included a large number of electrons suspended in something that produced a positive charge giving the atom an overall neutral charge. His model resembled plum pudding, a popular British dessert that had raisins suspended in a round cake-like ball.
The next scientist to further modify and advance the atomic model was Rutherford, who studied under Thomson, according to the chemistry department at Purdue University. In 1911, Rutherford published his version of the atom, which included a positively charged nucleus orbited by electrons. This model arose when Rutherford and his assistants fired alpha particles at thin sheets of gold. An alpha particle is made up of two protons and two neutrons, all held together by the same strong nuclear force that binds the nucleus, according to the Jefferson Lab.
The scientists noticed that a small percentage of the alpha particles were scattered at very large angles to the original direction of motion while the majority passed right through hardly disturbed. Rutherford was able to approximate the size of the nucleus of the gold atom, finding it to be at least 10,000 times smaller than the size of the entire atom with much of the atom being empty space. Rutherford's model of the atom is still the basic model that is used today.
Several other scientists furthered the atomic model, including Niels Bohr (built upon Rutherford's model to include properties of electrons based on the hydrogen spectrum), Erwin Schrödinger (developed the quantum model of the atom), Werner Heisenberg (stated that one cannot know both the position and velocity of an electron simultaneously), and Murray Gell-Mann and George Zweig (independently developed the theory that protons and neutrons were composed of quarks).
Additional resources:
Read more about the early universe, from CERN.
Learn more about the history of atomic chemistry in this video from Khan Academy.
Check out this simple slide show about atoms from the Jefferson Lab.
This article was updated on Sept. 10, 2019, by Live Science contributor Traci Pedersen.
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Neutrons
Neutrons play a definitive role in understanding the material world. They can show where atoms are and what atoms do.
By scattering neutrons off materials, scientists can visualise the positions and motions of atoms and make discoveries that have the potential to affect almost every aspect of our lives. Results from neutron experiments can help us to develop new materials for every-day uses.
Neutrons are used to study the dynamics of chemical reactions at interfaces for chemical and biochemical engineering, food sciences, drug synthesis and molecular biology.
Neutrons can probe deep into solid objects such as turbine blades, gas pipelines and welds to give a unique microscopic insight into the strains and stresses that affect the operational lifetimes of these crucial engineering components.
Neutron studies of nano-particles, low-dimensional systems and magnetism impact upon next generation computer and IT technology, data storage, sensors and superconducting materials.
Neutrons can be used for studying geological samples, new materials for energy production and storage, chemicals which affect the environment, and polymers and plastics. They can be used to study materials for health – from new materials for hip implants to gels that can help babies with clef palates. They have a very wide variety of uses!
Almost all of the major changes in our society, the dramatic revolutions in transport and manufacturing, the growth of computing and the internet and the steady increase in average life span, have their origin in understanding and exploiting the physics and chemistry of materials.
The goal of modern materials science is to understand the properties of matter on the atomic scale, and to use this knowledge to optimise the properties or develop new materials.
In neutron scattering experiments, materials are exposed to intense beams of neutrons inside specialised instruments at large research centres. The images that are made are used to reveal the molecular structure inside the material which can be directly linked to the physical and chemical properties experienced in the everyday world.
The UK is home to Europe's largest neutron scattering community and operates a world-leading short-pulse spallation source, ISIS at the STFC Rutherford Appleton Laboratory (RAL) and manages the UK subscription to the world-leading reactor source, Institut Laue-Langevin (ILL) in Grenoble. We have invested heavily in these facilities for the UK neutron scattering community through the development of a second target station at ISIS and through our support for the ILL Millennium Programme at ILL. The UK is also making a significant contribution to the construction of the European Spallation Source (ESS) currently being built in Lund, Sweden.
From this knowledge emerges fascinating new science or an understanding of current problems in industry.
STFC ensures that research using neutron scattering continues to make a valuable contribution to society through its on-going funding and development of the ISIS Neutron and Muon Source in the UK and the Institut Laue-Langevin in France.
More...
Key facts
Neutrons have unique advantages as a probe of atomic-level properties:
The process of neutron scattering is non-destructive, so that delicate or valuable samples can be studied
Neutrons are penetrating, so that they can look deep inside engineering samples to study, for example, welds
Neutrons with energies in the range of atomic motions have wavelengths of the order of the distances between atoms – making them very good at studying both where atoms are and how they are moving
Neutrons are good at seeing light atoms, such as hydrogen, in the presence of heavier ones
Neutrons are good at distinguishing neighbouring elements in the periodic table
Different isotopes of the same element scatter neutrons differently. For example, extra information can be gained by swapping hydrogen atoms with their deuterium isotopes in part of a sample
Neutrons have a magnetic moment, meaning that they can be used to study the magnetic properties of materials
Neutrons provide complementary information to other techniques such as x-ray scattering, so many researchers use neutron facilities such as ISIS and ILL alongside x-ray sources such as Diamond or the ESRF.
STFC manages access for UK scientists and researchers to both facilities. It wholly owns ISIS and has a one third stake in the ILL facility, with the remaining two thirds being owned by France and Germany and additional support being provided from 10 other European member countries.
More on key facts
Science
Neutrons are neutral sub-atomic particles with no electrical charge. Because of this, these unassuming particles are non-destructive and can penetrate into matter much deeper than charged particles such as electrons. In addition, because they have a property called spin, neutrons can be used to probe magnetism on an atomic scale.
There are two main methods of producing neutrons for materials research. One is by splitting uranium atoms in a nuclear fission reactor. The other, called spallation, involves firing high-energy protons into a metal target, such as mercury or tungsten, to induce a nuclear reaction that produces neutron beams.
ILL is the most intense reactor neutron source in the world. ISIS is the most productive spallation neutron source in the world.
Neutron sources play a crucial role in research across the scientific spectrum, from nuclear and elementary particle physics, chemistry and materials science to engineering and life sciences.
Neutron techniques complement synchrotron X-ray techniques for studying materials. Through ILL and ISIS, and the synchrotron facilities ESRF and Diamond, STFC is helping to keep the UK at the forefront of ground-breaking research worldwide.
Resources
The original neutron scattering brochure is available
See the 80 year history of neutrons in a timeline
The Institute of Physics factsheet about neutron scattering
ILLand ISIS websites
You can find out more about the impact ISIS is having on everyday life
How STFC funded science is changing the world
Links
Neutron scattering can be performed at neutron sources such as ISIS in the UK and the ILL in Grenoble, France – please see these websites for more detailed information and how to get time at one of these neutron facilities.
ISIS
ILL
Neutron impact brochure
Contacts
Jennifer Scratcher
Programme Manager, Light Sources and Neutrons Division
Tel: +44 (0)1793 418 038
ILL website
ISIS website
For media enquiries please telephone: +44 (0)1235 445 627
Case studies
Neutron case studies
History
Nobel Laureate Louis Néel was one of ILL's founding fathers in 1967. His work on magnetism underpins, for example, the magnetic recording technology used in modern computers.
Experiments at ILL proved the theoretical work of another Nobel Laureate, Pierre-Gilles de Gennes; this basic physics research was crucial for improving the industrial production of many plastics and fibres.
Nobel Laureate Norman Ramsey's work helped develop the technology behind the STFC-Sussex CryoEDM experiment at ILL. This will test some of most fundamental theories of our universe such as the Standard Model of particle physics and the reasons for an imbalance between matter and antimatter.
Nobel Laureate Sir Harry Kroto used ISIS in 1991 to determine the crystal structure and bonding of C60 carbon 'buckyballs'. His early research into this new form of carbon has since produced an entirely new field of chemistry and industrial applications.
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Related terms:
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RADIATION — EFFECTS AND USES
JERRY B. MARION, in Physics in the Modern World (Second Edition), 1981
Neutrons
Neutrons are not emitted in ordinary radioactive decay events, and because they are electrically neutral particles, neutrons cannot be accelerated in machines as can electrons and nuclei. But neutrons can be produced in nuclear reactions initiated by high-energy particles in accelerator beams. A variety of target materials yield neutrons when bombarded by high-speed particles. For example, the bombardment of lithium by protons produces neutrons according to the reaction,
7Li+ 1H→ 7Be+n
The absence of electric charge makes the neutron an interesting and important particle. When a neutron strikes a piece of matter, it does not interact with the atomic electrons (this happens only with charged particles); instead, neutrons interact with the nuclei. These neutron-nucleus interactions can result in the transfer of energy from the neutron to the nucleus (see the discussion of neutron moderators in Section 20-7), or in a neutron-induced disintegration. The capture of a neutron by a nucleus often results in the formation of a radioactive isotope. (In the case of a heavy nucleus, the result can be fission.)
In traveling through a piece of matter, a neutron does not produce any ionization. When the neutron strikes a nucleus, the nucleus recoils as a result of the collision. As the nucleus moves through the surrounding atoms, some of the atomic electrons are stripped away. Thus, the collision produces ionization along the path of the recoiling nucleus. In a material that contains a large fraction of hydrogen (for example, biological tissue), neutrons interact primarily with the nuclear protons of the hydrogen atoms. The knocked-on protons are the particles that produce almost all of the ionization in such materials.
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Interaction of Neutrons With Matter
Ilya Obodovskiy, in Radiation, 2019
7.1 Properties of Neutrons
Neutrons are uncharged particles, and therefore they do not participate in the electromagnetic interaction and do not produce ionization of the atoms. The interaction of a neutron magnetic moment with matter is very weak and unlikely.
All the main processes of interaction are caused by nuclear forces, as a result of various manifestations of which energetic charged particles appear in the substance. These are charged particles produced by neutrons that transmit their energy to matter, mainly due to ionization.
Unlike charged particles, which practically continuously lose energy in small portions, neutrons experience rare collisions with atoms, in which they can lose either all or a large part of their energy, which is caused by the short-range nature of nuclear forces.
The physical nature of the interaction of neutrons with atoms is fundamentally different from that of gamma quanta, but, formally, they are identical. Both gamma quanta and neutrons are penetrating radiations, whose fluxes are attenuated exponentially. For both types of radiations, it is possible to use the similar parameters—absorption and scattering coefficients.
Let us note that a free neutron is an unstable particle, it experiences a beta decay with a half-life of 614 s. But all the processes of neutrons passing through matter usually end up with the capture of a neutron by some nucleus in the time much shorter than a second. Therefore, analyzing all processes of neutron interaction with matter, the neutron instability can be ignored.
Because neutrons do not have an electric charge, they freely penetrate through the electron shells of atoms and are not repelled by the Coulomb field of the nucleus. Therefore, neutrons are an excellent tool with which you can study the nucleus, solids, biological structures, and create new elements that are absent in the surrounding world and are useful for medicine, industry, agriculture, and science.
About neutron sources see Section 17.10.
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Neutron Radiation
Michael F. L'Annunziata, in Radioactivity (Second Edition), 2016
10.4.5 Nuclear Fission
The reaction of neutron-induced fission occurs when a neutron interacts with a fissile or fissionable nucleus and the nucleus becomes unstable, taking on the characteristics of an oscillating liquid droplet, which then fragments into two nuclides (fission fragments). At the same time, there is the release of neutrons (2.4 neutrons on the average for neutron-induced 235U fission) and a relatively high amount of energy (∼194 MeV for neutron-induced 235U fission). Neutron-induced fission of natural 235U and man-made 233U and 239Pu is optimal at thermal incident neutron energies; whereas neutron-induced fission in 232Th, 238U, and 237Np requires neutron energies of at least 1 MeV. A more detailed treatment of nuclear fission was provided previously in this chapter.
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Structure of Melt and Liquid Alloys
Jean-Pierre Gaspard, in Handbook of Crystal Growth (Second Edition), 2015
9.5.2 Neutron Sources
Neutrons are the most abundant elementary particles on earth in mass, but the production of free neutrons is not easy. Unlike X-rays, neutrons are produced almost only in large-scale facilities, national or international. There are two main production tools: fission reactors and spallation sources. Once produced, free neutrons have a lifetime of about a quarter of an hour (886 s), enough to perform scattering experiments. There are many neutron sources for research worldwide. A list is given in http://www.ncnr.nist.gov/nsources.html. The sources of neutrons are fully incoherent and their brilliance is orders of magnitude smaller than the synchrotron sources. At the Laue-Langevin Institute (ILL), the brightest steady-state source in the world, the unperturbed flux is 1.5·1015 n s–1 cm–2.
The relation between energy E, wave vector k and wavelength λ is
(9.43)ÅÅE(meV)=2.073k2(Å−2)=81.82λ2(Å2)
The neutrons are massive particles and their de Broglie associated wavelength depends on their velocity, hence on their temperature. The average wavelength, in the Maxwellian distribution, follows the relation
(9.44)Åλ¯(Å)=0.28E(eV)=30.8T(K)
where λ¯ is the average wavelength of the Maxwellian spectrum. Usually, neutrons are thermalized either in heavy water at room temperature (thermal neutrons) or in liquid hydrogen or deuterium at 20 K (cold neutrons) or heated on a block of graphite at 2000 K (hot neutrons). Table (9.11) shows the different wavelengths.
For the study of liquids and amorphous materials, the k-range should extend up to about 10 Å-1, i.e., an energy of the order of 200 meV. Because neutrons penetrate deeply into matter, it is rather straightforward to use them even in complex and bulky sample environments. This is the case of high-temperature liquids that require a sample holder, a furnace, and possibly a vacuum vessel.
9.5.2.1 Fission Reactors
Neutrons are produced by fission of 235U. The excited nucleus decays in a cascade of fission products, producing an average of 2.5 neutrons of about 2 MeV per 235U nucleus. Using a moderator (e.g., D2O), the fast neutrons are slowed down to meV energies in order to sustain the nuclear chain reaction and to get neutrons with suitable wavelengths. The production of neutrons is at a constant rate. A schematic of neutron production in a fission reactor, slowed by D2O, is shown in Figure 9.18.

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FIGURE 9.18. Neutrons are produced in the core of a steady-state reactor by the fission of 235U. The fast neutrons are slowed down by a moderator (here D2O).
9.5.2.2 Spallation Sources
For different reasons (brilliance, safety), the new trend is to produce neutrons by spallation. High-energy protons generated in a linear accelerator hit a target of heavy metal: mercury, lead, uranium, or tungsten. The excited nucleus emits a wealth of particles, among others, 20 high-energy neutrons that are moderated. Unlike the reactors, the spallation source produces pulsed neutrons, because the protons are generated in bunches. Thanks to their time structure, the neutron energies are simply measured by a time of flight method. Spallation sources are operated in the United Kingdom (ISIS), the United States (SNS), Japan (J-SNS), and Switzerland (SINQ). Currently, in Lund, Sweden, a European spallation source (ESS) is being built, which will be around 30 times brighter than today's leading facilities. The ESS investment cost is estimated at approximately €1900 million (2013), with €140 million annual operations cost.
All these facilities are well documented, and a virtual tour of their operations can be found at http://www.ill.eu/about/movies/presentation-movie/.
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Neutron Scattering - Magnetic and Quantum Phenomena
Janos Major, ... Ferenc Mezei, in Experimental Methods in the Physical Sciences, 2015
1.1 Introduction
Neutrons are magnetic particles, they possess a magnetic moment, coupled to its spin s = 1/2. The value of this magnetic moment is μ = 1.913 Bohr magneton and its direction is opposite to that of the spin. This property makes neutron radiation particularly well suited for the study of magnetism in condensed matter. Indeed, the specific interaction of the magnetic moment of the neutrons with the microscopic magnetic fields created by magnetic atoms offers unique opportunities to probe magnetism on the microscale by neutron scattering, often with a sensitivity not equaled by any other microscopic probe. In addition, manipulating and observing the direction of the neutron magnetic moment in spin-polarized neutron beams is a very powerful tool to single out in neutron scattering experiments what is related to the magnetic behavior of the sample in the scattering signal, which is a mixture of contributions of different origins.
Going a step further, there also is another side to the story. The magnetic moment of each neutron can also be used to keep track of other relevant parameters of a neutron propagating in a beam, notably the value and direction of its velocity. In doing this, the neutron magnetic moment is used as a measuring device attached individually to each neutron, which can deliver information on the neutrons individually. Such methods are called "spin labeling" and they can be advantageously used to observe fine changes in the neutron parameters in a scattering process. For this reason, they can offer valuable opportunities for exploring matter by any neutron scattering process independently of whether it is related to the magnetic properties of the sample or not at all.
Thus when we talk about neutron scattering and magnetism, on the one hand, we need to consider exploration of magnetism in a broad variety of materials, and on the other hand, the opportunities the magnetism of the neutron offers to study nonmagnetic and/or magnetic phenomena in another broad variety of materials. While most of the rest of this volume is devoted to the first of these two large subject cases, this chapter focuses on the overview of the second one.
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Lattice Vibrations-2
Joginder Singh Galsin, in Solid State Physics, 2019
7.6.1.1 Time-of-Flight Method
Neutrons are produced in a nuclear reactor with an average energy of 2 MeV. They are slowed down by passing through a material (moderator) to acquire thermal energy (≈ 0.025 eV for T = 300 K). The thermal neutrons are then Bragg reflected from a single large crystal, for example of Al or Pb, to produce a monochromatic beam of neutrons. In the time-of-flight method a monochromatic beam of neutrons is allowed to fall on the specimen crystal. The beam after reflection through an angle ϕ is collected by a counter and the time of flight is measured. In this method neutrons scattered at two or more than two angles can be analyzed. The main disadvantage of this method is that the phonon wave vector q is chosen randomly.
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The Atomic Nucleus
Michael F. L'Annunziata, in Radioactivity (Second Edition), 2016
20.8.2 Neutron Radioactivity
The term "neutron radioactivity" should not be confused with the emission of neutrons from heavy nuclei following nuclear fission reactions, or neutron emission as the result of the decay of nuclei by spontaneous fission. Somewhat analogous to proton radioactivity, described previously, neutron radioactivity refers to the direct emission of neutrons from light nuclei that possess an excess number of neutrons—that is, a number of neutrons beyond their neutron drip line.
20.8.2.1 Beta-Delayed Neutron Emission
Neutron radioactivity most commonly occurs as a beta-delayed process; that is, neutron emission from the atomic nucleus occurs following beta (β−) decay. Beta-delayed neutron emission by fission products is an important source of neutrons that contributes to the total number of neutrons responsible for the continuity of nuclear fission chain reactions in nuclear power reactors.
Beta-delayed neutron emission involves the emission of one or more neutrons from a neutron-rich nucleus when the neutrons exist in neutron-unbound states in a daughter nucleus at an elevated energy following beta decay (Birch et al., 2014). This decay process is abbreviated as β−n, β−2n, or β−3n, for the emission of one, two, or three neutrons following beta decay. The emission of four neutrons from a beta-decay daughter nucleus at an excited state is possible, but not yet confirmed. The half-lives of nuclei that exhibit neutron radioactivity are considerably shorter than those that undergo proton radioactivity, because a neutron, due to its lack of charge, does not encounter any coulomb barrier to hinder its emission from a nucleus. When half-lives are very short (<10−12 s), the process may be termed neutron emission rather than neutron radioactivity, as described by Thoennessen et al. (2013).
An example of beta-delayed neutron emission, namely β−n and β−2n, in the decay of 94Br is illustrated in Fig. 20.23. The element bromine has an atomic number Z = 35, whereby the isotope 94Br with a neutron number of 59 (N = 94 − 35 = 59) is very rich in neutrons. The beta decay of 94Br leaves neutrons of the daughter nuclei 94Kr at elevated unbound energy states, permitting their emission from the 94Kr nucleus to form 93Kr or 92Kr, respectively. Either one or two neutrons are emitted simultaneously from the 94Kr nucleus, and the neutron-emission decay sequences may be written as follows:

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Figure 20.23. Beta-delayed neutron emission (β−n and β−2n) from neutron-rich 94Br.
Adapted from Rykaczewski (2008), Holifield Radioactive Ion Beam Facility for the US Department of Energy: https://www.phy.ornl.gov/hribf/app/decay/neutrons.shtml.
[20.54]B3594r→t1/2=70msβ−K3694r+β−+ν¯→nK3693r+n
[20.55]B3594r→t1/2=70msβ−K3694r+β−+ν¯→2nK3692r+2n
Among the light nuclei (Z ≤ 28), there are numerous isotopes for which the beta-delayed neutron emission probabilities have been measured. By the year 2014 a total of 203 nuclei had been identified as potential precursors to beta-delayed neutron emission; among these, the probabilities of β−n emission have been measured experimentally for 109 nuclei, whereas probabilities of β−2n emission have been measured experimentally for only 19, and the probabilities of β−3n emission measured in four nuclei (Birch et al., 2014). The beta-delayed emission of four neutrons (ie, β−4n), is yet to be measured. The nuclides that exhibit neutron radioactivity are found in the neutron-rich region of the Chart of the Nuclides, which is a Z versus N plot of the nuclides. These are illustrated in Fig. 20.24 for nuclides in the region of Z ≤ 28. The radionuclides that decay by beta-delayed 1n emission (β−n) are marked in red. Nuclides decaying by β−2n and β−3n are fewer in number, as mentioned previously. Also, as noted by Birch et al. (2014), all nuclides that decay by β−2n also decay by β−1n precursors; and all β−3n precursors are also β−2n and β−1n. Nuclides that exhibit negatron emission and neutron radioactivity reside in the neutron-rich side of the Chart of the Nuclides, ie, the region to the right of the black stable line of nuclides; whereas nuclides that decay by positron emission and exhibit proton radioactivity are found in the region to the left of the black line. Nuclides that decay by proton emission are not identified in Fig. 20.24; however, they reside among the yellow boxes of proton-rich nuclides to the left of the black boxes of stable nuclides.

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Figure 20.24. Chart of nuclides for Z ≤ 28 nuclei. The coloring of the boxes indicates the measurement status of single-neutron decay emission probability, P1n. Small white boxes indicate the existence of an isomer for that isotope.
Adapted from the NUCLEUS display software of the Atomic Mass Data Center: http://amdc.in2p3.fr/web/nubdisp_en.html.
20.8.2.2 Direct Neutron Emission
The direct emission of neutrons from neutron-rich nuclei can occur; and in such cases the half-life is generally much shorter than proton emission, because neutrons will not encounter any coulomb barrier to their escape from the atomic nucleus. An example is the dineutron decay of neutron-rich 16Be measured by Spyrou et al. (2012), which decays with a 200 ns half-life according to the following decay sequence:
[20.56]B416e→t1/2=200nsB414e+2n
The emission of only one neutron or the sequential emission of two neutrons from 16Be is not possible energetically, because the ground state of 15Be is at a significantly higher energy than the parent 16Be nuclide (Spyrou et al., 2012).
20.8.2.3 Detection and Measurement of Neutron Radioactivity
The detection and measurement of neutron radioactivity require instrumental techniques that are different to the gas ionization detection and measurement techniques described previously for proton radioactivity. The neutral charge on the neutron will not permit its detection by gas ionization. However, certain inorganic and organic plastic scintillators are sensitive to neutron interactions such as proton recoil following elastic scattering in organic and plastic scintillators. A review of methods for the detection and measurement of neutrons is provided by the writer in an earlier book (L'Annunziata, 2012c).
A method proposed by Caesar et al. (2013) for the detection and measurement of neutron radioactivity and the very short half-lives involved is described in detail by Thoennessen et al. (2013). A diagram of the instrumental arrangement is illustrated in Fig. 20.25, with the decay scheme of 16B as an example. The neutron-rich nucleus of 16B emits a neutron with a half-life of <190 ps according to the decay sequence
[20.57]B516→t1/2<190psB515+n
The very short half-life requires the measurement of the neutron emissions from the onset of the production of the 16B. A method of half-life measurement illustrated in Fig. 20.25 is the measurement of the neutron distribution along position-sensitive scintillation detectors, known as the decay in a magnetic field method. As described by Thoennessen et al. (2013), the 16B is produced by accelerating a beam of 17C against a target material that knocks out a proton from the 17C, producing 16B. The reaction 17C(-p)16B occurs at the location identified by a red dot in Fig. 20.25. The decay sequence 16B → 15B + n at various times in flight is illustrated by three blue dots along the flight path of the 16B, bent by a deflecting magnetic field with a bend radius r and a deflecting angle θ after a possible drift distance d. The deflecting magnetic field will bend the path of the charged particles away from the paths of the emitted neutrons, permitting the detection and counting of 15B. The distribution of the neutrons measured along the distance x of the position-sensitive scintillation detectors (see Fig. 20.25) will provide a measure of the half-life of the nuclear decay. Decay times as short as 10 ps will show a concentration of neutrons detected near the zero-degree line (little deflection) at a distance l from the target. A longer half-life of the order of 500 ps to 1 ns will exhibit a distribution of neutrons further away from the zero-degree line along the distance x. Longer decay times of up to 100 μs will exhibit a spread further away from the zero-degree line.

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Figure 20.25. Schematics of the decay in a magnetic field technique. The incoming 17C beam produces 16B in the target (red dot), which enters a magnetic field (bend radius r and deflection angle θ) after a drift distance d. The 16B can then decay at different positions along the flight path (blue dots) into 15B and a neutron. The 15B fragments and neutrons are detected and identified in a set of charged-particle detectors and position-sensitive scintillation detectors (at a distance l of 1500 cm from the target), respectively.
Thoennessen, M., et al., 2013. Novel techniques to search for neutron radioactivity. Nuc. Instrum. Methods Phys. Res. Sect. A 729, 207–211, reprinted with permission from Elsevier © 2013.
For additional reading on neutron radioactivity the reader may peruse papers by Spyrou et al. (2012), Thoennessen et al. (2013), and Birch et al. (2014).
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Neutron-Spin-Echo Spectroscopy and Magnetism
C. Pappas, ... F. Mezei, in Neutron Scattering from Magnetic Materials, 2006
Abstract
Neutron-spin-echo (NSE) spectroscopy uses the precession of neutron spins in a magnetic field to measure the energy transfer at the sample and decouples the energy resolution from beam characteristics like monochromatisation and collimation. The resolution can therefore be improved substantially, up to the 10-5 range, while keeping the high intensity advantage of a beam, which is only 10–20% monochromatic. The typical time range covered by NSE, i.e., 10-12 s to about 2×10-7 s, spans from typical microscopic times to the mesoscopic time scale. For this reason NSE is in particular used for the study of a large variety of slow motion phenomena (critical slowing down, relaxation effects, disordered dynamics, soft matter), which occur on a mesoscopic time scale between microscopic collision times and macroscopic dynamics. NSE applications in the field of magnetism benefit from the unique combination of high energy resolution with polarisation analysis, which allows for a direct and unambiguous separation of the (often) weak magnetic scattering from all other structural contributions.
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Fusion Power Plants
Garry McCracken, Peter Stott, in Fusion (Second Edition), 2013
13.3 Radiation Damage and Shielding
Fusion neutrons interact with the atoms of the walls, blanket, and other structures surrounding the plasma. They undergo nuclear reactions and scatter from atoms, transferring energy and momentum to them. Neutrons have a number of deleterious effects. First, they damage the structural materials. Second, they cause the structure to become radioactive, which requires the material to be carefully recycled or disposed of as waste at the end of the power plant's life. However, the activity of materials in fusion power plants is confined to the structural materials, since the waste product of the fusion reaction is helium.
Radiation damage processes (Box 13.2) have been studied in considerable detail in fission reactors, providing a good basis for assessing the problems in fusion power plants. The fusion neutron spectrum consists of a primary component of 14 MeV neutrons from the DT reaction together with a spectrum of lower-energy neutrons resulting from scattering. The damage caused by fusion neutrons is expected to be more severe than that from fission neutrons because the spectrum of fusion neutrons extends to higher energies. The higher-energy neutrons cause reactions that deposit helium in the solid lattice, and this has a marked effect on the behavior of materials under irradiation. To fully characterize the materials for a fusion power plant requires the construction of a powerful test facility with neutrons in the appropriate energy range.
Box 13.2
Radiation Damage
When an energetic neutron collides with an atom of the first wall or blanket structure, it can knock the atom out of its normal position in the lattice. The displaced atom may come to rest at an interstitial position in the lattice, leaving a vacancy at its original lattice site (Figure 13.2). In fact, the displaced atom may have enough energy to displace other atoms before coming to rest, and so the damage usually occurs in cascades. The damage is quantified in terms of the average number of displacements per atom (dpa) experienced during the working life of the material. In places like the first wall, where the neutron flux is highest, the damage rate is expected to reach the order of hundreds of dpa. At these levels of damage, the strength of the material will be reduced significantly, and some of the wall components will have to be renewed several times during the lifetime of the power plant. In principle, the neutron damage can be reduced by reducing the neutron flux—but this requires a larger structure for the same output power and increases the capital costs. Thus, there has to be a careful optimization of the size of the plant to strike a balance between the capital and maintenance costs.
A neutron can also undergo a nuclear reaction with a lattice atom, leaving a transmuted atom (or atoms) in place of the original. Usually the new atom is radioactive, and this is the main source of radioactivity in a fusion power plant. The neutron can also eject a proton or an alpha particle from the target nucleus, and these reactions are referred to as (n,p) and (n,α) reactions. A typical (n,p) reaction is
F56e+n→p+M56n
The (n,α) reactions are particularly important for the 14 MeV neutrons from the DT fusion reaction. The protons and alpha particles produced in these reactions pick up electrons and form hydrogen and helium atoms in the lattice. Individual atoms tend to coalesce, forming bubbles of gas in the lattice, and can be deleterious to the structural strength. Further damage to the lattice can be produced by energetic recoil of the reaction product.
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Radioactivity and Our Well-Being
Michael F. L'Annunziata, in Radioactivity (Second Edition), 2016
1.1.5.d Boron Neutron Capture Therapy
Neutron capture therapy (NCT) involves the treatment of malignant tumors that cannot be treated effectively by other methods due to the particular invasive character of the tumor or its inoperable characteristics. The method encompasses two stages. A pharmaceutical or drug containing in its molecular structure an element with a high cross-section for the capture of thermal or slow neutrons, such as boron-10, is administered. The drug should ideally concentrate in the tumor cells—that is, it should have a specificity to settle after administration in tumor cells rather than healthy tissue. The patient and tumor are irradiated with epithermal neutrons, which have an energy in the range of 0.4–100 eV. While passing through tissue and before reaching the tumor, the epithermal neutrons lose energy via collisions, mainly with protons in the bodily tissue, and thus are slowed down to the thermal energy range of <0.4 eV. When the thermal neutrons reach the tumor cells, they are captured mainly by the boron-10 previously administered to the patient in a tumor-specific drug. The capture of thermal neutrons by the boron-10 isotopes distributed throughout the tumor cells results in the production of boron-11 isotopes, which in turn undergo fission into high-energy alpha particles (helium nuclei) and high-energy nuclei of lithium-7, as illustrated in Fig. 1.19. The range of travel of these ionizing particles is very short, ∼5–9 μm, which is the size of a single cell. Both particles possess high energy, a positive charge, and relatively high mass, which gives them great power to deposit much energy via ionization (ie, high linear energy transfer) over a short distance (a few μm); this causes much ionization damage within the cancerous cell and delivers a lethal blow. When boron-10 is used in this way to kill malignant tumor cells selectively, the method is known as boron neutron capture therapy (BNCT).

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Figure 1.19. Schematic of the boron neutron capture fission reaction. BNCT is based on the neutron capture reaction 10B(n,α)7Li, where a 10B atom captures a low-energy thermal neutron (Eth < 0.4 eV) and spontaneously decays to produce the linear recoiling particles 4He (α particle) and 7Li. In tissues these particles have short penetration ranges, approximately the width of a single cell (5 μm for 7Li and 9 μm for 4He). The average linear energy transfer is high (7Li, 162 keV/μm; 4He, 196 keV/μm), which results in densely ionizing radiation restricted to the track of each particle.
From Chandra, S., Lorey II, D.R., 2007. SIMS ion microscopy imaging of boronophenylalanine (BPA) and 13C15N-labeled phenylalanine in human glioblastoma cells: relevance of subcellular scale observations to BPA-mediated boron neutron capture therapy of cancer. Int. J. Mass Spec. 260, 90–101, reprinted with permission from Elsevier © 2007.
BNCT has a potential advantage over other therapeutic methods previously described, because it is both biologically and physically specific: it is biologically specific when sufficient boron-loaded drug can be administered to concentrate in tumor cells, so it is biologically focused at the cellular level, as illustrated in Fig. 1.20; and it is physically specific because the neutron capture fission reaction will occur only with boron-10 isotopes and not with other isotopes of the elements C, H, O, N, or P common in human tissue. As noted by Professor Akira Matsumura of the Faculty of Medicine, University of Tsukuba, Japan and then president of the International Society for Neutron Capture Therapy, in his announcement of the 15th International Congress on Neutron Capture Therapy in September 2012:

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Figure 1.20. Therapeutic mechanism of BNCT. The boron-10 residing within a tumor cell captures a thermal neutron and undergoes fission, with the release of high-energy 4He and 7Li nuclei which deal a lethal blow of ionization within the cell.
Figure from Evangelisti, L., et al., 2013. Boron neutron capture therapy and 18F-labelled borophenylalanine positron emission tomography: a critical and clinical overview of the literature. Appl. Radiat. Isot. 74, 91–101, reprinted with permission from Elsevier © 2013.
Boron Neutron Capture Therapy is attracting increasing interest and demand as the "next-generation charged particle therapy" after proton and carbon ion therapy since it is the only radiation therapy that can focus on the cellular level.
http://square.umin.as.jp/ICNCT15/general/.
Much research and clinical trials are now ongoing around the world to establish BNCT "as an accepted and mainstream modality for several cancers" (Kreiner et al., 2011), including head and neck cancers. Research is under way to determine the most efficient boron-loaded drugs that will settle in sufficient concentration in tumor cells, and the most efficient means of delivery of epithermal neutrons via research reactors or accelerator-created neutrons in the hospital setting. This was highlighted in the announcement of the 16th International Congress on Neutron Capture Therapy, held in Helsinki, Finland in June 2014, which included the statement by Dr. Leena Kankaanranta, oncologist and president of the International Society of Neutron Capture Therapy, noting that key developments to be discussed are "the development of hospital-deployable accelerator-based neutron sources and next-generation boron targeting agents for BNCT, as well as highlights of the latest biological, biophysical and clinical research" (http://icnct16.org/welcome-message/).
Another isotope receiving attention in NCT is gadolinium-157, which has a thermal neutron capture cross-section much higher than that of boron-10; however, research into the application of gadolinium-157 in NCT is far behind developments made with boron-10. An excellent review of NCT is provided by Issa et al. (2013). The IAEA reports that 20 countries now have BNCT facilities, although the technique is still in clinical trial stages (http://www.naweb.iaea.org/napc/physics/research_reactors/database).
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Neutrons
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RADIATION — EFFECTS AND USES
JERRY B. MARION, in Physics in the Modern World (Second Edition), 1981
Neutrons
Neutrons are not emitted in ordinary radioactive decay events, and because they are electrically neutral particles, neutrons cannot be accelerated in machines as can electrons and nuclei. But neutrons can be produced in nuclear reactions initiated by high-energy particles in accelerator beams. A variety of target materials yield neutrons when bombarded by high-speed particles. For example, the bombardment of lithium by protons produces neutrons according to the reaction,
7Li+ 1H→ 7Be+n
The absence of electric charge makes the neutron an interesting and important particle. When a neutron strikes a piece of matter, it does not interact with the atomic electrons (this happens only with charged particles); instead, neutrons interact with the nuclei. These neutron-nucleus interactions can result in the transfer of energy from the neutron to the nucleus (see the discussion of neutron moderators in Section 20-7), or in a neutron-induced disintegration. The capture of a neutron by a nucleus often results in the formation of a radioactive isotope. (In the case of a heavy nucleus, the result can be fission.)
In traveling through a piece of matter, a neutron does not produce any ionization. When the neutron strikes a nucleus, the nucleus recoils as a result of the collision. As the nucleus moves through the surrounding atoms, some of the atomic electrons are stripped away. Thus, the collision produces ionization along the path of the recoiling nucleus. In a material that contains a large fraction of hydrogen (for example, biological tissue), neutrons interact primarily with the nuclear protons of the hydrogen atoms. The knocked-on protons are the particles that produce almost all of the ionization in such materials.
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Interaction of Neutrons With Matter
Ilya Obodovskiy, in Radiation, 2019
7.1 Properties of Neutrons
Neutrons are uncharged particles, and therefore they do not participate in the electromagnetic interaction and do not produce ionization of the atoms. The interaction of a neutron magnetic moment with matter is very weak and unlikely.
All the main processes of interaction are caused by nuclear forces, as a result of various manifestations of which energetic charged particles appear in the substance. These are charged particles produced by neutrons that transmit their energy to matter, mainly due to ionization.
Unlike charged particles, which practically continuously lose energy in small portions, neutrons experience rare collisions with atoms, in which they can lose either all or a large part of their energy, which is caused by the short-range nature of nuclear forces.
The physical nature of the interaction of neutrons with atoms is fundamentally different from that of gamma quanta, but, formally, they are identical. Both gamma quanta and neutrons are penetrating radiations, whose fluxes are attenuated exponentially. For both types of radiations, it is possible to use the similar parameters—absorption and scattering coefficients.
Let us note that a free neutron is an unstable particle, it experiences a beta decay with a half-life of 614 s. But all the processes of neutrons passing through matter usually end up with the capture of a neutron by some nucleus in the time much shorter than a second. Therefore, analyzing all processes of neutron interaction with matter, the neutron instability can be ignored.
Because neutrons do not have an electric charge, they freely penetrate through the electron shells of atoms and are not repelled by the Coulomb field of the nucleus. Therefore, neutrons are an excellent tool with which you can study the nucleus, solids, biological structures, and create new elements that are absent in the surrounding world and are useful for medicine, industry, agriculture, and science.
About neutron sources see Section 17.10.
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Neutron Radiation
Michael F. L'Annunziata, in Radioactivity (Second Edition), 2016
10.4.5 Nuclear Fission
The reaction of neutron-induced fission occurs when a neutron interacts with a fissile or fissionable nucleus and the nucleus becomes unstable, taking on the characteristics of an oscillating liquid droplet, which then fragments into two nuclides (fission fragments). At the same time, there is the release of neutrons (2.4 neutrons on the average for neutron-induced 235U fission) and a relatively high amount of energy (∼194 MeV for neutron-induced 235U fission). Neutron-induced fission of natural 235U and man-made 233U and 239Pu is optimal at thermal incident neutron energies; whereas neutron-induced fission in 232Th, 238U, and 237Np requires neutron energies of at least 1 MeV. A more detailed treatment of nuclear fission was provided previously in this chapter.
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Structure of Melt and Liquid Alloys
Jean-Pierre Gaspard, in Handbook of Crystal Growth (Second Edition), 2015
9.5.2 Neutron Sources
Neutrons are the most abundant elementary particles on earth in mass, but the production of free neutrons is not easy. Unlike X-rays, neutrons are produced almost only in large-scale facilities, national or international. There are two main production tools: fission reactors and spallation sources. Once produced, free neutrons have a lifetime of about a quarter of an hour (886 s), enough to perform scattering experiments. There are many neutron sources for research worldwide. A list is given in http://www.ncnr.nist.gov/nsources.html. The sources of neutrons are fully incoherent and their brilliance is orders of magnitude smaller than the synchrotron sources. At the Laue-Langevin Institute (ILL), the brightest steady-state source in the world, the unperturbed flux is 1.5·1015 n s–1 cm–2.
The relation between energy E, wave vector k and wavelength λ is
(9.43)ÅÅE(meV)=2.073k2(Å−2)=81.82λ2(Å2)
The neutrons are massive particles and their de Broglie associated wavelength depends on their velocity, hence on their temperature. The average wavelength, in the Maxwellian distribution, follows the relation
(9.44)Åλ¯(Å)=0.28E(eV)=30.8T(K)
where λ¯ is the average wavelength of the Maxwellian spectrum. Usually, neutrons are thermalized either in heavy water at room temperature (thermal neutrons) or in liquid hydrogen or deuterium at 20 K (cold neutrons) or heated on a block of graphite at 2000 K (hot neutrons). Table (9.11) shows the different wavelengths.
For the study of liquids and amorphous materials, the k-range should extend up to about 10 Å-1, i.e., an energy of the order of 200 meV. Because neutrons penetrate deeply into matter, it is rather straightforward to use them even in complex and bulky sample environments. This is the case of high-temperature liquids that require a sample holder, a furnace, and possibly a vacuum vessel.
9.5.2.1 Fission Reactors
Neutrons are produced by fission of 235U. The excited nucleus decays in a cascade of fission products, producing an average of 2.5 neutrons of about 2 MeV per 235U nucleus. Using a moderator (e.g., D2O), the fast neutrons are slowed down to meV energies in order to sustain the nuclear chain reaction and to get neutrons with suitable wavelengths. The production of neutrons is at a constant rate. A schematic of neutron production in a fission reactor, slowed by D2O, is shown in Figure 9.18.

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FIGURE 9.18. Neutrons are produced in the core of a steady-state reactor by the fission of 235U. The fast neutrons are slowed down by a moderator (here D2O).
9.5.2.2 Spallation Sources
For different reasons (brilliance, safety), the new trend is to produce neutrons by spallation. High-energy protons generated in a linear accelerator hit a target of heavy metal: mercury, lead, uranium, or tungsten. The excited nucleus emits a wealth of particles, among others, 20 high-energy neutrons that are moderated. Unlike the reactors, the spallation source produces pulsed neutrons, because the protons are generated in bunches. Thanks to their time structure, the neutron energies are simply measured by a time of flight method. Spallation sources are operated in the United Kingdom (ISIS), the United States (SNS), Japan (J-SNS), and Switzerland (SINQ). Currently, in Lund, Sweden, a European spallation source (ESS) is being built, which will be around 30 times brighter than today's leading facilities. The ESS investment cost is estimated at approximately €1900 million (2013), with €140 million annual operations cost.
All these facilities are well documented, and a virtual tour of their operations can be found at http://www.ill.eu/about/movies/presentation-movie/.
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Neutron Scattering - Magnetic and Quantum Phenomena
Janos Major, ... Ferenc Mezei, in Experimental Methods in the Physical Sciences, 2015
1.1 Introduction
Neutrons are magnetic particles, they possess a magnetic moment, coupled to its spin s = 1/2. The value of this magnetic moment is μ = 1.913 Bohr magneton and its direction is opposite to that of the spin. This property makes neutron radiation particularly well suited for the study of magnetism in condensed matter. Indeed, the specific interaction of the magnetic moment of the neutrons with the microscopic magnetic fields created by magnetic atoms offers unique opportunities to probe magnetism on the microscale by neutron scattering, often with a sensitivity not equaled by any other microscopic probe. In addition, manipulating and observing the direction of the neutron magnetic moment in spin-polarized neutron beams is a very powerful tool to single out in neutron scattering experiments what is related to the magnetic behavior of the sample in the scattering signal, which is a mixture of contributions of different origins.
Going a step further, there also is another side to the story. The magnetic moment of each neutron can also be used to keep track of other relevant parameters of a neutron propagating in a beam, notably the value and direction of its velocity. In doing this, the neutron magnetic moment is used as a measuring device attached individually to each neutron, which can deliver information on the neutrons individually. Such methods are called "spin labeling" and they can be advantageously used to observe fine changes in the neutron parameters in a scattering process. For this reason, they can offer valuable opportunities for exploring matter by any neutron scattering process independently of whether it is related to the magnetic properties of the sample or not at all.
Thus when we talk about neutron scattering and magnetism, on the one hand, we need to consider exploration of magnetism in a broad variety of materials, and on the other hand, the opportunities the magnetism of the neutron offers to study nonmagnetic and/or magnetic phenomena in another broad variety of materials. While most of the rest of this volume is devoted to the first of these two large subject cases, this chapter focuses on the overview of the second one.
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Lattice Vibrations-2
Joginder Singh Galsin, in Solid State Physics, 2019
7.6.1.1 Time-of-Flight Method
Neutrons are produced in a nuclear reactor with an average energy of 2 MeV. They are slowed down by passing through a material (moderator) to acquire thermal energy (≈ 0.025 eV for T = 300 K). The thermal neutrons are then Bragg reflected from a single large crystal, for example of Al or Pb, to produce a monochromatic beam of neutrons. In the time-of-flight method a monochromatic beam of neutrons is allowed to fall on the specimen crystal. The beam after reflection through an angle ϕ is collected by a counter and the time of flight is measured. In this method neutrons scattered at two or more than two angles can be analyzed. The main disadvantage of this method is that the phonon wave vector q is chosen randomly.
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The Atomic Nucleus
Michael F. L'Annunziata, in Radioactivity (Second Edition), 2016
20.8.2 Neutron Radioactivity
The term "neutron radioactivity" should not be confused with the emission of neutrons from heavy nuclei following nuclear fission reactions, or neutron emission as the result of the decay of nuclei by spontaneous fission. Somewhat analogous to proton radioactivity, described previously, neutron radioactivity refers to the direct emission of neutrons from light nuclei that possess an excess number of neutrons—that is, a number of neutrons beyond their neutron drip line.
20.8.2.1 Beta-Delayed Neutron Emission
Neutron radioactivity most commonly occurs as a beta-delayed process; that is, neutron emission from the atomic nucleus occurs following beta (β−) decay. Beta-delayed neutron emission by fission products is an important source of neutrons that contributes to the total number of neutrons responsible for the continuity of nuclear fission chain reactions in nuclear power reactors.
Beta-delayed neutron emission involves the emission of one or more neutrons from a neutron-rich nucleus when the neutrons exist in neutron-unbound states in a daughter nucleus at an elevated energy following beta decay (Birch et al., 2014). This decay process is abbreviated as β−n, β−2n, or β−3n, for the emission of one, two, or three neutrons following beta decay. The emission of four neutrons from a beta-decay daughter nucleus at an excited state is possible, but not yet confirmed. The half-lives of nuclei that exhibit neutron radioactivity are considerably shorter than those that undergo proton radioactivity, because a neutron, due to its lack of charge, does not encounter any coulomb barrier to hinder its emission from a nucleus. When half-lives are very short (<10−12 s), the process may be termed neutron emission rather than neutron radioactivity, as described by Thoennessen et al. (2013).
An example of beta-delayed neutron emission, namely β−n and β−2n, in the decay of 94Br is illustrated in Fig. 20.23. The element bromine has an atomic number Z = 35, whereby the isotope 94Br with a neutron number of 59 (N = 94 − 35 = 59) is very rich in neutrons. The beta decay of 94Br leaves neutrons of the daughter nuclei 94Kr at elevated unbound energy states, permitting their emission from the 94Kr nucleus to form 93Kr or 92Kr, respectively. Either one or two neutrons are emitted simultaneously from the 94Kr nucleus, and the neutron-emission decay sequences may be written as follows:

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Figure 20.23. Beta-delayed neutron emission (β−n and β−2n) from neutron-rich 94Br.
Adapted from Rykaczewski (2008), Holifield Radioactive Ion Beam Facility for the US Department of Energy: https://www.phy.ornl.gov/hribf/app/decay/neutrons.shtml.
[20.54]B3594r→t1/2=70msβ−K3694r+β−+ν¯→nK3693r+n
[20.55]B3594r→t1/2=70msβ−K3694r+β−+ν¯→2nK3692r+2n
Among the light nuclei (Z ≤ 28), there are numerous isotopes for which the beta-delayed neutron emission probabilities have been measured. By the year 2014 a total of 203 nuclei had been identified as potential precursors to beta-delayed neutron emission; among these, the probabilities of β−n emission have been measured experimentally for 109 nuclei, whereas probabilities of β−2n emission have been measured experimentally for only 19, and the probabilities of β−3n emission measured in four nuclei (Birch et al., 2014). The beta-delayed emission of four neutrons (ie, β−4n), is yet to be measured. The nuclides that exhibit neutron radioactivity are found in the neutron-rich region of the Chart of the Nuclides, which is a Z versus N plot of the nuclides. These are illustrated in Fig. 20.24 for nuclides in the region of Z ≤ 28. The radionuclides that decay by beta-delayed 1n emission (β−n) are marked in red. Nuclides decaying by β−2n and β−3n are fewer in number, as mentioned previously. Also, as noted by Birch et al. (2014), all nuclides that decay by β−2n also decay by β−1n precursors; and all β−3n precursors are also β−2n and β−1n. Nuclides that exhibit negatron emission and neutron radioactivity reside in the neutron-rich side of the Chart of the Nuclides, ie, the region to the right of the black stable line of nuclides; whereas nuclides that decay by positron emission and exhibit proton radioactivity are found in the region to the left of the black line. Nuclides that decay by proton emission are not identified in Fig. 20.24; however, they reside among the yellow boxes of proton-rich nuclides to the left of the black boxes of stable nuclides.

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Figure 20.24. Chart of nuclides for Z ≤ 28 nuclei. The coloring of the boxes indicates the measurement status of single-neutron decay emission probability, P1n. Small white boxes indicate the existence of an isomer for that isotope.
Adapted from the NUCLEUS display software of the Atomic Mass Data Center: http://amdc.in2p3.fr/web/nubdisp_en.html.
20.8.2.2 Direct Neutron Emission
The direct emission of neutrons from neutron-rich nuclei can occur; and in such cases the half-life is generally much shorter than proton emission, because neutrons will not encounter any coulomb barrier to their escape from the atomic nucleus. An example is the dineutron decay of neutron-rich 16Be measured by Spyrou et al. (2012), which decays with a 200 ns half-life according to the following decay sequence:
[20.56]B416e→t1/2=200nsB414e+2n
The emission of only one neutron or the sequential emission of two neutrons from 16Be is not possible energetically, because the ground state of 15Be is at a significantly higher energy than the parent 16Be nuclide (Spyrou et al., 2012).
20.8.2.3 Detection and Measurement of Neutron Radioactivity
The detection and measurement of neutron radioactivity require instrumental techniques that are different to the gas ionization detection and measurement techniques described previously for proton radioactivity. The neutral charge on the neutron will not permit its detection by gas ionization. However, certain inorganic and organic plastic scintillators are sensitive to neutron interactions such as proton recoil following elastic scattering in organic and plastic scintillators. A review of methods for the detection and measurement of neutrons is provided by the writer in an earlier book (L'Annunziata, 2012c).
A method proposed by Caesar et al. (2013) for the detection and measurement of neutron radioactivity and the very short half-lives involved is described in detail by Thoennessen et al. (2013). A diagram of the instrumental arrangement is illustrated in Fig. 20.25, with the decay scheme of 16B as an example. The neutron-rich nucleus of 16B emits a neutron with a half-life of <190 ps according to the decay sequence
[20.57]B516→t1/2<190psB515+n
The very short half-life requires the measurement of the neutron emissions from the onset of the production of the 16B. A method of half-life measurement illustrated in Fig. 20.25 is the measurement of the neutron distribution along position-sensitive scintillation detectors, known as the decay in a magnetic field method. As described by Thoennessen et al. (2013), the 16B is produced by accelerating a beam of 17C against a target material that knocks out a proton from the 17C, producing 16B. The reaction 17C(-p)16B occurs at the location identified by a red dot in Fig. 20.25. The decay sequence 16B → 15B + n at various times in flight is illustrated by three blue dots along the flight path of the 16B, bent by a deflecting magnetic field with a bend radius r and a deflecting angle θ after a possible drift distance d. The deflecting magnetic field will bend the path of the charged particles away from the paths of the emitted neutrons, permitting the detection and counting of 15B. The distribution of the neutrons measured along the distance x of the position-sensitive scintillation detectors (see Fig. 20.25) will provide a measure of the half-life of the nuclear decay. Decay times as short as 10 ps will show a concentration of neutrons detected near the zero-degree line (little deflection) at a distance l from the target. A longer half-life of the order of 500 ps to 1 ns will exhibit a distribution of neutrons further away from the zero-degree line along the distance x. Longer decay times of up to 100 μs will exhibit a spread further away from the zero-degree line.

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Figure 20.25. Schematics of the decay in a magnetic field technique. The incoming 17C beam produces 16B in the target (red dot), which enters a magnetic field (bend radius r and deflection angle θ) after a drift distance d. The 16B can then decay at different positions along the flight path (blue dots) into 15B and a neutron. The 15B fragments and neutrons are detected and identified in a set of charged-particle detectors and position-sensitive scintillation detectors (at a distance l of 1500 cm from the target), respectively.
Thoennessen, M., et al., 2013. Novel techniques to search for neutron radioactivity. Nuc. Instrum. Methods Phys. Res. Sect. A 729, 207–211, reprinted with permission from Elsevier © 2013.
For additional reading on neutron radioactivity the reader may peruse papers by Spyrou et al. (2012), Thoennessen et al. (2013), and Birch et al. (2014).
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Neutron-Spin-Echo Spectroscopy and Magnetism
C. Pappas, ... F. Mezei, in Neutron Scattering from Magnetic Materials, 2006
Abstract
Neutron-spin-echo (NSE) spectroscopy uses the precession of neutron spins in a magnetic field to measure the energy transfer at the sample and decouples the energy resolution from beam characteristics like monochromatisation and collimation. The resolution can therefore be improved substantially, up to the 10-5 range, while keeping the high intensity advantage of a beam, which is only 10–20% monochromatic. The typical time range covered by NSE, i.e., 10-12 s to about 2×10-7 s, spans from typical microscopic times to the mesoscopic time scale. For this reason NSE is in particular used for the study of a large variety of slow motion phenomena (critical slowing down, relaxation effects, disordered dynamics, soft matter), which occur on a mesoscopic time scale between microscopic collision times and macroscopic dynamics. NSE applications in the field of magnetism benefit from the unique combination of high energy resolution with polarisation analysis, which allows for a direct and unambiguous separation of the (often) weak magnetic scattering from all other structural contributions.
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Fusion Power Plants
Garry McCracken, Peter Stott, in Fusion (Second Edition), 2013
13.3 Radiation Damage and Shielding
Fusion neutrons interact with the atoms of the walls, blanket, and other structures surrounding the plasma. They undergo nuclear reactions and scatter from atoms, transferring energy and momentum to them. Neutrons have a number of deleterious effects. First, they damage the structural materials. Second, they cause the structure to become radioactive, which requires the material to be carefully recycled or disposed of as waste at the end of the power plant's life. However, the activity of materials in fusion power plants is confined to the structural materials, since the waste product of the fusion reaction is helium.
Radiation damage processes (Box 13.2) have been studied in considerable detail in fission reactors, providing a good basis for assessing the problems in fusion power plants. The fusion neutron spectrum consists of a primary component of 14 MeV neutrons from the DT reaction together with a spectrum of lower-energy neutrons resulting from scattering. The damage caused by fusion neutrons is expected to be more severe than that from fission neutrons because the spectrum of fusion neutrons extends to higher energies. The higher-energy neutrons cause reactions that deposit helium in the solid lattice, and this has a marked effect on the behavior of materials under irradiation. To fully characterize the materials for a fusion power plant requires the construction of a powerful test facility with neutrons in the appropriate energy range.
Box 13.2
Radiation Damage
When an energetic neutron collides with an atom of the first wall or blanket structure, it can knock the atom out of its normal position in the lattice. The displaced atom may come to rest at an interstitial position in the lattice, leaving a vacancy at its original lattice site (Figure 13.2). In fact, the displaced atom may have enough energy to displace other atoms before coming to rest, and so the damage usually occurs in cascades. The damage is quantified in terms of the average number of displacements per atom (dpa) experienced during the working life of the material. In places like the first wall, where the neutron flux is highest, the damage rate is expected to reach the order of hundreds of dpa. At these levels of damage, the strength of the material will be reduced significantly, and some of the wall components will have to be renewed several times during the lifetime of the power plant. In principle, the neutron damage can be reduced by reducing the neutron flux—but this requires a larger structure for the same output power and increases the capital costs. Thus, there has to be a careful optimization of the size of the plant to strike a balance between the capital and maintenance costs.
A neutron can also undergo a nuclear reaction with a lattice atom, leaving a transmuted atom (or atoms) in place of the original. Usually the new atom is radioactive, and this is the main source of radioactivity in a fusion power plant. The neutron can also eject a proton or an alpha particle from the target nucleus, and these reactions are referred to as (n,p) and (n,α) reactions. A typical (n,p) reaction is
F56e+n→p+M56n
The (n,α) reactions are particularly important for the 14 MeV neutrons from the DT fusion reaction. The protons and alpha particles produced in these reactions pick up electrons and form hydrogen and helium atoms in the lattice. Individual atoms tend to coalesce, forming bubbles of gas in the lattice, and can be deleterious to the structural strength. Further damage to the lattice can be produced by energetic recoil of the reaction product.
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Radioactivity and Our Well-Being
Michael F. L'Annunziata, in Radioactivity (Second Edition), 2016
1.1.5.d Boron Neutron Capture Therapy
Neutron capture therapy (NCT) involves the treatment of malignant tumors that cannot be treated effectively by other methods due to the particular invasive character of the tumor or its inoperable characteristics. The method encompasses two stages. A pharmaceutical or drug containing in its molecular structure an element with a high cross-section for the capture of thermal or slow neutrons, such as boron-10, is administered. The drug should ideally concentrate in the tumor cells—that is, it should have a specificity to settle after administration in tumor cells rather than healthy tissue. The patient and tumor are irradiated with epithermal neutrons, which have an energy in the range of 0.4–100 eV. While passing through tissue and before reaching the tumor, the epithermal neutrons lose energy via collisions, mainly with protons in the bodily tissue, and thus are slowed down to the thermal energy range of <0.4 eV. When the thermal neutrons reach the tumor cells, they are captured mainly by the boron-10 previously administered to the patient in a tumor-specific drug. The capture of thermal neutrons by the boron-10 isotopes distributed throughout the tumor cells results in the production of boron-11 isotopes, which in turn undergo fission into high-energy alpha particles (helium nuclei) and high-energy nuclei of lithium-7, as illustrated in Fig. 1.19. The range of travel of these ionizing particles is very short, ∼5–9 μm, which is the size of a single cell. Both particles possess high energy, a positive charge, and relatively high mass, which gives them great power to deposit much energy via ionization (ie, high linear energy transfer) over a short distance (a few μm); this causes much ionization damage within the cancerous cell and delivers a lethal blow. When boron-10 is used in this way to kill malignant tumor cells selectively, the method is known as boron neutron capture therapy (BNCT).

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Figure 1.19. Schematic of the boron neutron capture fission reaction. BNCT is based on the neutron capture reaction 10B(n,α)7Li, where a 10B atom captures a low-energy thermal neutron (Eth < 0.4 eV) and spontaneously decays to produce the linear recoiling particles 4He (α particle) and 7Li. In tissues these particles have short penetration ranges, approximately the width of a single cell (5 μm for 7Li and 9 μm for 4He). The average linear energy transfer is high (7Li, 162 keV/μm; 4He, 196 keV/μm), which results in densely ionizing radiation restricted to the track of each particle.
From Chandra, S., Lorey II, D.R., 2007. SIMS ion microscopy imaging of boronophenylalanine (BPA) and 13C15N-labeled phenylalanine in human glioblastoma cells: relevance of subcellular scale observations to BPA-mediated boron neutron capture therapy of cancer. Int. J. Mass Spec. 260, 90–101, reprinted with permission from Elsevier © 2007.
BNCT has a potential advantage over other therapeutic methods previously described, because it is both biologically and physically specific: it is biologically specific when sufficient boron-loaded drug can be administered to concentrate in tumor cells, so it is biologically focused at the cellular level, as illustrated in Fig. 1.20; and it is physically specific because the neutron capture fission reaction will occur only with boron-10 isotopes and not with other isotopes of the elements C, H, O, N, or P common in human tissue. As noted by Professor Akira Matsumura of the Faculty of Medicine, University of Tsukuba, Japan and then president of the International Society for Neutron Capture Therapy, in his announcement of the 15th International Congress on Neutron Capture Therapy in September 2012:

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Figure 1.20. Therapeutic mechanism of BNCT. The boron-10 residing within a tumor cell captures a thermal neutron and undergoes fission, with the release of high-energy 4He and 7Li nuclei which deal a lethal blow of ionization within the cell.
Figure from Evangelisti, L., et al., 2013. Boron neutron capture therapy and 18F-labelled borophenylalanine positron emission tomography: a critical and clinical overview of the literature. Appl. Radiat. Isot. 74, 91–101, reprinted with permission from Elsevier © 2013.
Boron Neutron Capture Therapy is attracting increasing interest and demand as the "next-generation charged particle therapy" after proton and carbon ion therapy since it is the only radiation therapy that can focus on the cellular level.
http://square.umin.as.jp/ICNCT15/general/.
Much research and clinical trials are now ongoing around the world to establish BNCT "as an accepted and mainstream modality for several cancers" (Kreiner et al., 2011), including head and neck cancers. Research is under way to determine the most efficient boron-loaded drugs that will settle in sufficient concentration in tumor cells, and the most efficient means of delivery of epithermal neutrons via research reactors or accelerator-created neutrons in the hospital setting. This was highlighted in the announcement of the 16th International Congress on Neutron Capture Therapy, held in Helsinki, Finland in June 2014, which included the statement by Dr. Leena Kankaanranta, oncologist and president of the International Society of Neutron Capture Therapy, noting that key developments to be discussed are "the development of hospital-deployable accelerator-based neutron sources and next-generation boron targeting agents for BNCT, as well as highlights of the latest biological, biophysical and clinical research" (http://icnct16.org/welcome-message/).
Another isotope receiving attention in NCT is gadolinium-157, which has a thermal neutron capture cross-section much higher than that of boron-10; however, research into the application of gadolinium-157 in NCT is far behind developments made with boron-10. An excellent review of NCT is provided by Issa et al. (2013). The IAEA reports that 20 countries now have BNCT facilities, although the technique is still in clinical trial stages