Woman
smallholder farmers in Kenya. In many parts of Africa and other parts
of the world, women are the primary smallholders. In many contexts,
women face unequal access to land, markets, knowledge, and other assets
needed to maintain their farms.Small vegetable farm in Hainan, China
A smallholding or smallholder is a small farm operating under a small-scale agriculture model.
Definitions vary widely for what constitutes a smallholder or
small-scale farm, including factors such as size, food production
technique or technology, involvement of family in labor and economic impact. Smallholdings are usually farms supporting a single family with a mixture of cash crops and subsistence farming.
As a country becomes more affluent, smallholdings may not be
self-sufficient, but may be valued for the rural lifestyle. As the sustainable food and local food
movements grow in affluent countries, some of these smallholdings are
gaining increased economic viability. There are an estimated 500 million
smallholder farms in developing countries of the world alone,
supporting almost two billion people.
Small-scale agriculture is often in tension with industrial agriculture, which finds efficiencies by increasing outputs, monoculture, consolidating land under big agricultural operations, and economies of scale. Certain labor-intensive cash-crops, such as cocoa production in Ghana or Côte d'Ivoire, rely heavily on small holders; globally, as of 2008 90% of cocoa is grown by smallholders. These farmers rely on cocoa for up to 60 to 90 per cent of their income. Similar trends in supply chains exist in other crops like coffee, palm oil, and bananas. In other markets, small scale agriculture can increase food system investment in small holders improving food security. Today some companies try to include smallholdings into their value chain, providing seed, feed or fertilizer to improve production.
According to conventional theory, economies of scale allow agricultural productivity,
in terms of inputs versus outputs, to rise as the size of the farm
rises. Specialization has also been a major factor in increasing
agricultural productivity, for example as commodity processing began to
move off the farm in the 19th century, farmers could spend more effort
on primary food production.
Although numerous studies show that larger farms are more productive than smaller ones, some writers state that whilst conventional farming creates a high output per worker, some small-scale, sustainable, polyculture farmers can produce more food per acre of land.
A smallholder coffee farmer in Colombia contributing her coffee to an agricultural cooperative.
Cooperatives give small farmers an opportunity to be more competitive
in markets, especially markets like coffee and cocoa where many of the
purchasers are large businesses with high market power.
Small farms have some economic advantages. Farmers support the local economy
of their communities. An American study showed that small farms with
incomes of $100,000 or less spend almost 95 percent of their
farm-related expenses within their local communities. The same study
took into comparison the fact that farms with incomes greater than
$900,000 spend less than 20 percent of their farm-related expenses in
the local economy.
Small-scale agriculture often sells products directly to consumers. Disintermediation
gives the farmer the profit that would otherwise go to the wholesaler,
the distributor, and the supermarket. About two-thirds of the revenue is
expended on product marketing. If farmers sell their products directly to consumers, they receive a higher percentage of the retail price, although they will spend more time selling the same amounts of product, which is an opportunity cost.
Food security
Because smallholding farms frequently require less industrial inputs and can be an important way to improve food security
in less-developed contexts, addressing the productivity and financial
sustainability of small holders is an international development priority
and measured by indicator 2.3 of Sustainable Development Goal 2. The International Fund for Agricultural Development has an ongoing program for Adaptation for Smallholder Agriculture.
During the global COVID-19 pandemic, and the attendant disruptions of food systems, their role has become more important.
Environmental and climate adaptation
While
the historical focus on smallholders has been increasing global food
supply under climate change and the role played by smallholder
communities, climate adaptation
efforts are still hindered by lack of information on how smallholder
farmers are experiencing and responding to climate change. There is lack
of detailed, context-specific information on what climate change
portends to smallholder farmers in different and widely varying
agroecological environments and socio-economic realities, and what
management strategies they are employing to deal with these impacts.
Especially for smallholders working in commodity crops, climate
change introduces an increasing amount of variability to farmer economic
viability; for example, coffee production globally is under increased threat, and smallholders in East Africa, such as in the Ugandan, Tanzanian or Kenyan industries, are rapidly losing both viable coffee land and productivity of plants.
In some cases, smallholders are an important source of deforestation. For example, smallholders are an important component of the oil palm
industry of Southeast Asia, contributing 40% of the production. Because
such farmers are less able to access financing than larger businesses,
they are unable to fund methods to increase the productivity of their
farms when yields decline, increasing their need to clear more land.
Increasing productivity, especially amongst smallholder farms, is an
important way to decrease the amount of land needed for farming and slow
environmental degradation through processes like deforestation.
Formats
The definition of a small farm has varied over time and by country. Agricultural economists have analyzed the distinctions among farm sizes since the field's inception. Traditional agricultural economic theory considered small farms inefficient, a stance that began to be challenged in the 1950s. An overview of research published by the World Bank in 1998 indicated that the productivity of small farms often exceeded that of larger ones.
Hobby farms
A hobby farm (also called a lifestyle block in New Zealand, or acreage living or rural residential in Australia) is a smallholding or small farm
that is maintained without expectation of being a primary source of
income. Some are held merely to provide recreational land for horses or
other use. Others are managed as working farms for secondary income, or
are even run at an ongoing loss as a lifestyle choice by people with the
means to do so, functioning more like a country home than a business.
Nucleus estate and smallholder
Nucleus estate and smallholder (NES) is a farming system for commodity crops, often oil palm, practised in different world regions. It is most famous today for its application in the palm oil sector in Indonesia.
The nucleus is the part of such a plantation that is under concession
and management of the company, while another part of the plantation is
operated by smallholders typically on their own land but planted by the
company. NES farming is a particular form of contract farming.
In many developing countries, smallholding is a small plot of land with low rental value, used to grow crops. By some estimates, there are 525 million smallholder farmers in the world. These farms vary in land sizes, production and labor intensities.
The distribution of farm sizes depends on a number of agroecological
and demographic conditions, as well as on economic and technological
factors.
Smallholders are critical to local and regional food systems, as well
as livelihoods, and especially so during periods of food supply chain
disruptions.
Smallholders dominate production in certain key sectors such as coffee
and cocoa. Various types of agribusinesses enterprises work with
smallholding farmers in a range of roles including buying crops,
providing seed, and acting as financial institutions.
In low-income countries, women make up 43 percent of smallholding agricultural labor but produce 60–80 percent of food crops.
India
In India, there is five sizes classification for smallholders. These are 'marginal' less than 1 hectare (2+1⁄2 acres), 'small' between 1 and 2 hectares (2+1⁄2
and 5 acres), 'semi medium' between 2 and 4 hectares (5 and 10 acres),
'medium' between 4 and 10 hectares (9.9 and 24.7 acres), 'large' above
10 hectares (25 acres). If we use 4 hectares (10 acres) (marginal +
small + medium) as a threshold, 94.3% of holdings are small and these
constitute 65.2% of all farmland.
The bulk of India's hungry and poor people are constituted of
smallholder farmers and landless people. 78% country's farmers own less
than 2 hectares (5 acres), which constitutes 33% of total farmland but
at the same time, they produce 41% of the country's food grains. 20% of
the world's poor live in India, although the country was self-sufficient
in food production in 2002 due to the first Green Revolution started in
the latter half of the twentieth century, numerous households lacked
resources to purchase food. Holdings less than 2 ha contributed 41% of
total food grain production in 1991 compared to 28% in 1971, which means
a substantial increase, whereas medium holdings registered a mere 3%
increase in the same period and large holdings registered a decline from
51 to 35%. This signifies the importance of smallholders in the Green
Revolution and the attainment of national food security. Smallholder
families are becoming more vulnerable and more disadvantaged due to the
expansion of international trade liberalisation. The needs and
aspirations of small farmers must feature prominently in policies of
market reform that seek to improve food and nutritional security.
India's total increase rate of productivity across the farming sector
was far less in 1990's when compared to previous decades.
Kenya
Kenyan smallholder dairy farmer
Kenya's smallholder means someone who owns, possess or produces
agricultural products in small-scale . smallholder production accounts
for 78 percent of total agricultural production and 70 percent of
commercial production. Majority of the smallholder population work in farm sizes averaging 0.47 hectares (11⁄4 acres). This represents the vast majority of Kenya's rural poor population who depend on agriculture for their livelihood.
Adverse risk events during the period 1980–2012 led to production
losses in smallholder farms resulting in a drop in agricultural gross
domestic product (GDP) of 2 percent or more.
Increasing the productivity of smallholder farmers is encouraged due to
its potential of improving food availability, increasing rural incomes,
lowering poverty rates, and growing the economy.
Diversification of crops in smallholder farms is one of the potential
strategies in sustaining agricultural productivity, and copping with
marketing risks. It is also a transitional step from subsistence to commercial agriculture.
Age, education of household head, type of crops, cropping system,
amount of credit, and irrigation facilities are some of the factors
influencing diversification in smallholder farms.
Tanzania
Along
the upper and middle reaches of the Nduruma River in the Pangani River
Basin, Tanzania, there is not enough water to go around. Smallholder
farmers address inequities in land and water distribution by enforcing
existing traditional local rules. Whilst larger estate farms may have
governmental licences guaranteeing rights to the water, a study
found that those large-scale farms which adhere to the traditional water
rights structures fare better in terms of social reputation, which
better ensures their access to water. Adhering to the water law in order
to enforce their permits is less effective, as regional Tanzanian local
governments generally attempt to avoid conflict with their populace. On
a larger scale, however, existing traditional rules are ineffective in
maintaining cooperation among users along the Nduruma River.
In 1975, there were 4.2 million smallholder farming households in
Thailand. In 2013, Thailand had 5.9 million smallholder farming
households. The average area of these smallholdings had shrunk from 3.7 to 3.2 hectares (9+1⁄4 to 8 acres) over that period. Instead of farms getting larger and less numerous, as has been the case in the Global North, the reverse happened: they got smaller and more numerous.
United States
Several definitions of small farm have been formulated in legislation. In 1977 the US Congress, via the Food and Agriculture Act of 1977, defined a small farm as one with sales under $20,000. At the time these comprised 70% of farms in the US. The Act sponsored additional research on small farming operations by US land grant universities and their extension services and mandated that an annual report on these activities be issued by the US Secretary of Agriculture. A 1997 study by the United States Small Farms Commission
defined small farms as those with less than $250,000 in gross receipts
annually on which day-to-day labor and management are provided by the
farmer and/or the farm family that owns the production, or owns or
leases the productive assets. In 2000, such farms accounted for about
90% of the more than 2.1 million U.S. farms, but only about 40% of U.S.
farm production.
The concentration of production on fewer and larger operations is
a longstanding concern among some segments of the agricultural
community. Others view these changes as inevitable, and even necessary
to maintain the efficiency and competitiveness of the sector.
Many farmers are upset by their inability to fix the new types of high-tech farm equipment.
This is due mostly to companies using intellectual property law to
prevent farmers from having the legal right to fix their equipment (or
gain access to the information to allow them to do it). This has encouraged groups such as Open Source Ecology and Farm Hack to begin to make open-source agricultural machinery.
European Union
The debate concerning the role of small farms within the European Union
is ongoing. The European Commission states that more than three
quarters of farm holdings in the European Union are less than 10
hectares, with a large number less than five hectares, although as of 2009 it had not established a formal definition of the term that could be used in its Common Agricultural Policy. The public perception of the possible benefits of small-scale farming has led to requests for further studies from the European Commission.
The quark
content of the neutron. The color assignment of individual quarks is
arbitrary, but all three colors must be present. Forces between quarks
are mediated by gluons.
The neutron is a subatomic particle, symbol n or n0 , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave similarly within the nucleus, and each has a mass of approximately one dalton, they are both referred to as nucleons. Their properties and interactions are described by nuclear physics. Protons and neutrons are not elementary particles; each is composed of three quarks.
The chemical properties of an atom are mostly determined by the configuration of electrons
that orbit the atom's heavy nucleus. The electron configuration is
determined by the charge of the nucleus, which is determined by the
number of protons, or atomic number. The number of neutrons is the neutron number. Neutrons do not affect the electron configuration, but the sum of atomic and neutron numbers is the mass of the nucleus.
Atoms of a chemical element that differ only in neutron number are called isotopes. For example, carbon, with atomic number 6, has an abundant isotope carbon-12 with 6 neutrons and a rare isotope carbon-13 with 7 neutrons. Some elements occur in nature with only one stable isotope, such as fluorine. Other elements occur with many stable isotopes, such as tin with ten stable isotopes, or with no stable isotope, such as technetium.
The properties of an atomic nucleus depend on both atomic and
neutron numbers. With their positive charge, the protons within the
nucleus are repelled by the long-range electromagnetic force, but the much stronger, but short-range, nuclear force
binds the nucleons closely together. Neutrons are required for the
stability of nuclei, with the exception of the single-proton hydrogen nucleus. Neutrons are produced copiously in nuclear fission and fusion. They are a primary contributor to the nucleosynthesis of chemical elements within stars through fission, fusion, and neutron capture processes.
An atomic nucleus is formed by a number of protons, Z (the atomic number), and a number of neutrons, N (the neutron number), bound together by the nuclear force. The atomic number determines the chemical properties of the atom, and the neutron number determines the isotope or nuclide. The terms isotope and nuclide are often used synonymously,
but they refer to chemical and nuclear properties, respectively.
Isotopes are nuclides with the same atomic number, but different neutron
number. Nuclides with the same neutron number, but different atomic
number, are called isotones. The atomic mass number, A,
is equal to the sum of atomic and neutron numbers. Nuclides with the
same atomic mass number, but different atomic and neutron numbers, are
called isobars.
The nucleus of the most common isotope of the hydrogen atom (with the chemical symbol1H) is a lone proton. The nuclei of the heavy hydrogen isotopes deuterium (D or 2H) and tritium (T or 3H)
contain one proton bound to one and two neutrons, respectively. All
other types of atomic nuclei are composed of two or more protons and
various numbers of neutrons. The most common nuclide of the common
chemical element lead, 208Pb, has 82 protons and 126 neutrons, for example. The table of nuclides comprises all the known nuclides. Even though it is not a chemical element, the neutron is included in this table.
The free neutron has a mass of 939565413.3 eV/c2, or 1.674927471×10−27kg, or 1.00866491588Da. The neutron has a mean square radius of about 0.8×10−15m, or 0.8 fm, and it is a spin-½fermion. The neutron has no measurable electric charge. With its positive electric charge, the proton is directly influenced by electric fields, whereas the neutron is unaffected by electric fields. But the neutron has a magnetic moment, so the neutron is influenced by magnetic fields. The neutron's magnetic moment has a negative value, because its orientation is opposite to the neutron's spin.
A free neutron is unstable, decaying to a proton, electron and antineutrino with a mean lifetime of just under 15 minutes (879.6±0.8 s). This radioactive decay, known as beta decay,
is possible because the mass of the neutron is slightly greater than
that of the proton. The free proton is stable. However, neutrons or
protons bound in a nucleus can be stable or unstable, depending on the nuclide. Beta decay, in which neutrons decay to protons, or vice versa, is governed by the weak force, and it requires the emission or absorption of electrons and neutrinos, or their antiparticles.
Nuclear
fission caused by absorption of a neutron by uranium-235. The heavy
nuclide fragments into lighter components and additional neutrons.
Protons and neutrons behave almost identically under the influence of the nuclear force within the nucleus. The concept of isospin,
in which the proton and neutron are viewed as two quantum states of the
same particle, is used to model the interactions of nucleons by the
nuclear or weak forces. Because of the strength of the nuclear force at
short distances, the binding energy of nucleons is more than seven orders of magnitude larger than the electromagnetic energy binding electrons in atoms. Nuclear reactions (such as nuclear fission) therefore have an energy density that is more than ten million times that of chemical reactions. Because of the mass–energy equivalence,
nuclear binding energies reduce the mass of nuclei. Ultimately, the
ability of the nuclear force to store energy arising from the
electromagnetic repulsion of nuclear components is the basis for most of
the energy that makes nuclear reactors or bombs possible. In nuclear
fission, the absorption of a neutron by a heavy nuclide (e.g., uranium-235)
causes the nuclide to become unstable and break into light nuclides and
additional neutrons. The positively charged light nuclides then repel,
releasing electromagnetic potential energy.
The neutron is classified as a hadron, because it is a composite particle made of quarks. The neutron is also classified as a baryon, because it is composed of three valence quarks. The finite size of the neutron and its magnetic moment both indicate that the neutron is a composite, rather than elementary, particle. A neutron contains two down quarks with charge −1/3e and one up quark with charge +2/3e.
The story of the discovery of the neutron and its properties is
central to the extraordinary developments in atomic physics that
occurred in the first half of the 20th century, leading ultimately to
the atomic bomb in 1945. In the 1911 Rutherford model,
the atom consisted of a small positively charged massive nucleus
surrounded by a much larger cloud of negatively charged electrons. In
1920, Ernest Rutherford
suggested that the nucleus consisted of positive protons and neutrally
charged particles, suggested to be a proton and an electron bound in
some way. Electrons were assumed to reside within the nucleus because it was known that beta radiation consisted of electrons emitted from the nucleus.
About the time Rutherford suggested the neutral proton-electron
composite, several other publications appeared making similar
suggestions, and in 1921 the American chemist W. D. Harkins first named the hypothetical particle a "neutron". The name derives from the Latin root for neutralis (neuter) and the Greek suffix -on (a suffix used in the names of subatomic particles, i.e. electron and proton). References to the word neutron in connection with the atom can be found in the literature as early as 1899, however.
Throughout the 1920s, physicists assumed that the atomic nucleus was composed of protons and "nuclear electrons" but there were obvious problems. It was difficult to reconcile the proton–electron model for nuclei with the Heisenberg uncertainty relation of quantum mechanics. The Klein paradox, discovered by Oskar Klein in 1928, presented further quantum mechanical objections to the notion of an electron confined within a nucleus.
Observed properties of atoms and molecules were inconsistent with the
nuclear spin expected from the proton–electron hypothesis. Both protons
and electrons carry an intrinsic spin of 1/2ħ.
Isotopes of the same species (i.e. having the same number of protons)
can have both integer or fractional spin, i.e. the neutron spin must be
also fractional (1/2ħ).
But there is no way to arrange the spins of an electron and a proton
(supposed to bond to form a neutron) to get the fractional spin of a
neutron.
In 1931, Walther Bothe and Herbert Becker found that if alpha particle radiation from polonium fell on beryllium, boron, or lithium,
an unusually penetrating radiation was produced. The radiation was not
influenced by an electric field, so Bothe and Becker assumed it was gamma radiation. The following year Irène Joliot-Curie and Frédéric Joliot-Curie in Paris showed that if this "gamma" radiation fell on paraffin, or any other hydrogen-containing compound, it ejected protons of very high energy. Neither Rutherford nor James Chadwick at the Cavendish Laboratory in Cambridge were convinced by the gamma ray interpretation.
Chadwick quickly performed a series of experiments that showed that the
new radiation consisted of uncharged particles with about the same mass
as the proton. These particles were neutrons. Chadwick won the 1935 Nobel Prize in Physics for this discovery.
Models
depicting the nucleus and electron energy levels in hydrogen, helium,
lithium, and neon atoms. In reality, the diameter of the nucleus is
about 100,000 times smaller than the diameter of the atom.
Models for an atomic nucleus consisting of protons and neutrons were quickly developed by Werner Heisenberg and others. The proton–neutron model explained the puzzle of nuclear spins. The origins of beta radiation were explained by Enrico Fermi in 1934 by the process of beta decay, in which the neutron decays to a proton by creating an electron and a (at the time undiscovered) neutrino. In 1935, Chadwick and his doctoral student Maurice Goldhaber reported the first accurate measurement of the mass of the neutron.
By 1934, Fermi had bombarded heavier elements with neutrons to
induce radioactivity in elements of high atomic number. In 1938, Fermi
received the Nobel Prize in Physics "for his demonstrations of the
existence of new radioactive elements produced by neutron irradiation,
and for his related discovery of nuclear reactions brought about by slow neutrons". In 1938 Otto Hahn, Lise Meitner, and Fritz Strassmann discovered nuclear fission, or the fractionation of uranium nuclei into lighter elements, induced by neutron bombardment. In 1945 Hahn received the 1944 Nobel Prize in Chemistry "for his discovery of the fission of heavy atomic nuclei". The discovery of nuclear fission would lead to the development of nuclear power and the atomic bomb by the end of World War II.
Neutrons are a necessary constituent of any atomic nucleus that
contains more than one proton. Interacting protons have a mutual electromagnetic repulsion that is stronger than their attractive nuclear interaction, so proton-only nuclei are unstable (see diproton and neutron–proton ratio). Neutrons bind with protons and one another in the nucleus via the nuclear force, effectively moderating the repulsive forces between the protons and stabilizing the nucleus.
"Free" neutrons or protons are nucleons that exist independently,
free of any nucleus. Since the neutron is slightly more massive than a
proton, the decay of a free neutron to a proton is allowed, while the
decay of a free proton is energetically disallowed. A high-energy
collision of a proton and an electron or neutrino can result in a
neutron, however.
Bound within a nucleus, however, both neutrons and protons can decay
by the beta decay process. The neutrons and protons in a nucleus form a
quantum mechanical system wherein each nucleon is bound in a
particular, hierarchical quantum state. Nucleon decay within a nucleus
can occur if allowed by basic energy conservation and quantum mechanical
constraints. The stability of nuclei and nuclide radioactivity are
consequences of these constraints.
The neutron and proton decay reactions are:
n0 → p+ + e− + ν e
where p+ , e− , and ν e denote the proton, electron and electron anti-neutrino decay products, and
p+ → n0 + e+ + ν e
where n0 , e+ , and ν e denote the neutron, positron and electron neutrino decay products.
The emitted particles, that is, the decay products, carry away
the energy excess as a nucleon falls from one quantum state to one with
less energy, while the neutron (or proton) changes to a proton (or
neutron). In these reactions, the original particle is not composed of the product particles; rather, the product particles are created at the instant of the reaction.
Outside the nucleus, free neutrons are unstable and have a mean lifetime of 879.6±0.8 s (about 14 minutes, 40 seconds); therefore the half-life for this process (which differs from the mean lifetime by a factor of ln(2) = 0.693) is 610.1±0.7 s (about 10 minutes, 10 seconds).
This decay is only possible because the mass of the proton is less than
that of the neutron. By the mass-energy equivalence, when a neutron
decays to a proton this way, a lower energy state is attained.
For the free neutron the decay energy for this process (based on the masses of the neutron, proton, and electron) is 0.782343MeV.
The maximal energy of the beta decay electron (in the process wherein
the neutrino receives a vanishingly small amount of kinetic energy) has
been measured at 0.782±0.013 MeV.
The latter number is not well-enough measured to determine the
comparatively tiny rest mass of the neutrino (which must in theory be
subtracted from the maximal electron kinetic energy) as well as neutrino
mass is constrained by many other methods.
A small fraction (about one in 1000) of free neutrons decay with
the same products, but add an extra particle in the form of an emitted
gamma ray:
n0 → p+ + e− + ν e + γ
This gamma ray may be thought of as an "internal bremsstrahlung"
that arises from the electromagnetic interaction of the emitted beta
particle with the proton. Internal bremsstrahlung gamma ray production
is also a minor feature of beta decays of bound neutrons (as discussed
below).
A schematic of the nucleus of an atom indicating β−
radiation, the emission of a fast electron from the nucleus (the
accompanying antineutrino is omitted). In the Rutherford model for the
nucleus, red spheres were protons with positive charge and blue spheres
were protons tightly bound to an electron with no net charge. The inset shows beta decay of a free neutron as it is understood today; an electron and antineutrino are created in this process.
A very small minority of neutron decays (about four per million) are
so-called "two-body (neutron) decays", in which a proton, electron and
antineutrino are produced as usual, but the electron fails to gain the 13.6 eV necessary energy to escape the proton (the ionization energy of hydrogen), and therefore simply remains bound to it, as a neutral hydrogen atom (one of the "two bodies"). In this type of free neutron decay, almost all of the neutron decay energy
is carried off by the antineutrino (the other "body"). (The hydrogen
atom recoils with a speed of only about (decay energy)/(hydrogen rest
energy) times the speed of light, or 250 km/s.)
While a free neutron has a half life of about 10.2 min, most neutrons within nuclei are stable. According to the nuclear shell model, the protons and neutrons of a nuclide are a quantum mechanical system organized into discrete energy levels with unique quantum numbers.
For a neutron to decay, the resulting proton requires an available
state at lower energy than the initial neutron state. In stable nuclei
the possible lower energy states are all filled, meaning each state is
occupied by a pair of protons, one with spin up, another with spin down. When all available proton states are filled, the Pauli exclusion principle
disallows the decay of a neutron to a proton within stable nuclei. The
situation is similar to electrons of an atom, where electrons that
occupy distinct atomic orbitals are prevented by the exclusion principle from decaying to lower, already-filled, energy states, with the emission of a photon.
Neutrons in unstable nuclei can decay by beta decay
as described above. In this case, an energetically allowed quantum
state is available for the proton resulting from the decay. One example
of this decay is carbon-14 (6 protons, 8 neutrons) that decays to nitrogen-14 (7 protons, 7 neutrons) with a half-life of about 5,730 years.
Similarly, a proton inside a nucleus can decay into a neutron, if
an energetically allowed quantum state is available for the neutron.
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 both
relatively rare in ordinary matter and quickly annihilate
when they encounter electrons (which are much less rare) and in any
case are repelled by the positive nucleus. Similar, but far more rare,
reactions involve the capture of a neutrino by a nucleon in inverse beta decay.
Competition of beta decay types
Three types of beta decay in competition are illustrated by the single isotope copper-64
(29 protons, 35 neutrons), which has a half-life of about 12.7 hours.
This isotope has one unpaired proton and one unpaired neutron, so either
the proton or the neutron can decay. This particular nuclide is almost
equally likely to undergo proton decay (by positron emission, 18% or by electron capture, 43%) or neutron decay (by electron emission, 39%).
Decay of the neutron by elementary particle physics
Within the theoretical framework of Standard Model
for particle physics, the neutron is composed of two down quarks and an
up quark. The only possible decay mode for the neutron that conservesbaryon number is for one of the neutron's quarks to changeflavour via the weak interaction. The decay of one of the neutron's down quarks into a lighter up quark can be achieved by the emission of a W boson.
By this process, the Standard Model description of beta decay, the
neutron decays into a proton (which contains one down and two up
quarks), an electron, and an electron antineutrino.
The decay of the proton to a neutron occurs similarly through the
weak force. The decay of one of the proton's up quarks into a down quark
can be achieved by the emission of a W boson. The proton decays into a
neutron, a positron, and an electron neutrino. This reaction can only
occur within an atomic nucleus which has a quantum state at lower energy
available for the created neutron.
Intrinsic properties
Mass
The mass of a neutron cannot be directly determined by mass spectrometry since it has no electric charge. But since the masses of a proton and of a deuteron
can be measured with a mass spectrometer, the mass of a neutron can be
deduced by subtracting proton mass from deuteron mass, with the
difference being the mass of the neutron plus the binding energy of deuterium (expressed as a positive emitted energy). The latter can be directly measured by measuring the energy () of the single 2.224 MeV
gamma photon emitted when a deuteron is formed by a proton capturing a
neutron (this is exothermic and happens with zero-energy neutrons). The
small recoil kinetic energy () of the deuteron (about 0.06% of the total energy) must also be accounted for.
The energy of the gamma ray can be measured to high precision by
X-ray diffraction techniques, as was first done by Bell and Elliot in
1948. The best modern (1986) values for neutron mass by this technique
are provided by Greene, et al. These give a neutron mass of:
Another method to determine the mass of a neutron starts from the
beta decay of the neutron, when the momenta of the resulting proton and
electron are measured.
Electric charge
The total electric charge of the neutron is 0 e. This zero value has been tested experimentally, and the present experimental limit for the charge of the neutron is −2(8)×10−22e, or −3(13)×10−41C. This value is consistent with zero, given the experimental uncertainties (indicated in parentheses). By comparison, the charge of the proton is +1 e.
Even though the neutron is a neutral particle, the magnetic moment of
a neutron is not zero. The neutron is not affected by electric fields,
but it is affected by magnetic fields. The value for the neutron's
magnetic moment was first directly measured by Luis Alvarez and Felix Bloch at Berkeley, California, in 1940. Alvarez and Bloch determined the magnetic moment of the neutron to be μn= −1.93(2) μN, where μN is the nuclear magneton.
The magnetic moment of the neutron is an indication of its quark substructure and internal charge distribution. In the quark model for hadrons, the neutron is composed of one up quark (charge +2/3 e) and two down quarks (charge −1/3 e). The magnetic moment of the neutron can be modeled as a sum of the magnetic moments of the constituent quarks.
The calculation assumes that the quarks behave like pointlike Dirac
particles, each having their own magnetic moment. Simplistically, the
magnetic moment of the neutron can be viewed as resulting from the
vector sum of the three quark magnetic moments, plus the orbital
magnetic moments caused by the movement of the three charged quarks
within the neutron.
In one of the early successes of the Standard Model in 1964 Mirza A.B. Beg, Benjamin W. Lee, and Abraham Pais
theoretically calculated the ratio of proton to neutron magnetic
moments to be −3/2, which agrees with the experimental value to within
3%. The measured value for this ratio is −1.45989805(34).
The above treatment compares neutrons with protons, allowing the
complex behavior of quarks to be subtracted out between models, and
merely exploring what the effects would be of differing quark charges
(or quark type). Such calculations are enough to show that the interior
of neutrons is very much like that of protons, save for the difference
in quark composition with a down quark in the neutron replacing an up
quark in the proton.
The neutron magnetic moment can be roughly computed by assuming a simple nonrelativistic, quantum mechanical wavefunction for baryons
composed of three quarks. A straightforward calculation gives fairly
accurate estimates for the magnetic moments of neutrons, protons, and
other baryons. For a neutron, the result of this calculation is that the magnetic moment of the neutron is given by μn= 4/3 μd − 1/3 μu, where μd and μu
are the magnetic moments for the down and up quarks, respectively. This
result combines the intrinsic magnetic moments of the quarks with their
orbital magnetic moments, and assumes the three quarks are in a
particular, dominant quantum state.
Baryon
Magnetic moment of quark model
Computed ()
Observed ()
p
4/3 μu − 1/3 μd
2.79
2.793
n
4/3 μd − 1/3 μu
−1.86
−1.913
The results of this calculation are encouraging, but the masses of
the up or down quarks were assumed to be 1/3 the mass of a nucleon. The masses of the quarks are actually only about 1% that of a nucleon. The discrepancy stems from the complexity of the Standard Model for nucleons, where most of their mass originates in the gluon fields, virtual particles, and their associated energy that are essential aspects of the strong force. Furthermore, the complex system of quarks and gluons that constitute a neutron requires a relativistic treatment. But the nucleon magnetic moment has been successfully computed numerically from first principles,
including all of the effects mentioned and using more realistic values
for the quark masses. The calculation gave results that were in fair
agreement with measurement, but it required significant computing
resources.
Spin
The neutron is a spin 1/2 particle, that is, it is a fermion with intrinsic angular momentum equal to 1/2ħ, where ħ is the reduced Planck constant. For many years after the discovery of the neutron, its exact spin was ambiguous. Although it was assumed to be a spin 1/2Dirac particle, the possibility that the neutron was a spin 3/2
particle lingered. The interactions of the neutron's magnetic moment
with an external magnetic field were exploited to finally determine the
spin of the neutron.
In 1949, Hughes and Burgy measured neutrons reflected from a
ferromagnetic mirror and found that the angular distribution of the
reflections was consistent with spin 1/2. In 1954, Sherwood, Stephenson, and Bernstein employed neutrons in a Stern–Gerlach experiment that used a magnetic field to separate the neutron spin states. They recorded two such spin states, consistent with a spin 1/2 particle.
As a fermion, the neutron is subject to the Pauli exclusion principle; two neutrons cannot have the same quantum numbers. This is the source of the degeneracy pressure which counteracts gravity in neutron stars and prevents them from forming black holes.
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.
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; but the main attraction between neutrons and protons is via
the nuclear force, which does not involve electric charge.
The simplified classical view of the neutron's charge
distribution also "explains" the fact that the neutron magnetic dipole
points in the opposite direction from its spin angular momentum vector
(as compared to the proton). This gives the neutron, in effect, a
magnetic moment which resembles a negatively charged particle. This can
be reconciled classically with a neutral neutron composed of a charge
distribution in which the negative sub-parts of the neutron have a
larger average radius of distribution, and therefore contribute more to
the particle's magnetic dipole moment, than do the positive parts that
are, on average, nearer the core.
The Standard Model of particle physics predicts a tiny separation of positive and negative charge within the neutron leading to a permanent electric dipole moment. But the predicted value is 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:
The antineutron is the antiparticle of the neutron. It was discovered by Bruce Cork in 1956, a year after the antiproton was discovered. CPT-symmetry
puts strong constraints on the relative properties of particles and
antiparticles, so studying antineutrons provides 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.
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.
In February 2016, Japanese physicist Susumu Shimoura of the University of Tokyo and co-workers reported they had observed the purported tetraneutrons for the first time experimentally.
Nuclear physicists around the world say this discovery, if confirmed,
would be a milestone in the field of nuclear physics and certainly would
deepen our understanding of the nuclear forces.
The dineutron is another hypothetical particle. In 2012, Artemis Spyrou
from Michigan State University and coworkers reported that they
observed, for the first time, the dineutron emission in the decay of 16Be.
The dineutron character is evidenced by a small emission angle between
the two neutrons. The authors measured the two-neutron separation energy
to be 1.35(10) MeV, in good agreement with shell model calculations,
using standard interactions for this mass region.
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.
The common means of detecting a chargedparticle 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.
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.
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 a light
nucleus such as hydrogen or helium than to a heavier nucleus. 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. Methods such as pulse shape
discrimination can be used in distinguishing neutron signals from
gamma-ray signals, although certain inorganic scintillator-based
detectors have been developed to selectively detect neutrons in mixed radiation fields inherently without any additional techniques.
Fast neutron detectors have the advantage of not requiring a
moderator, and are therefore capable of measuring the neutron's energy,
time of arrival, and in certain cases direction of incidence.
Free neutrons are unstable, although they have the longest half-life
of any unstable subatomic particle by several orders of magnitude. Their
half-life is still only about 10 minutes, so they can be obtained only
from sources that produce them continuously.
Natural neutron background. A small natural background
flux of free neutrons exists everywhere on Earth. In the atmosphere and
deep into the ocean, the "neutron background" is caused by muons produced by cosmic ray
interaction with the atmosphere. These high-energy muons are capable of
penetration to considerable depths in water and soil. There, in
striking atomic nuclei, among other reactions they induce spallation
reactions in which a neutron is liberated from the nucleus. Within the
Earth's crust a second source is neutrons produced primarily by
spontaneous fission of uranium and thorium present in crustal minerals.
The neutron background is not strong enough to be a biological hazard,
but it is of importance to very high resolution particle detectors that
are looking for very rare events, such as (hypothesized) interactions
that might be caused by particles of dark matter. Recent research has shown that even thunderstorms can produce neutrons with energies of up to several tens of MeV. Recent research has shown that the fluence of these neutrons lies between 10−9 and 10−13 per ms and per m2
depending on the detection altitude. The energy of most of these
neutrons, even with initial energies of 20 MeV, decreases down to the
keV range within 1 ms.
Even stronger neutron background radiation is produced at the
surface of Mars, where the atmosphere is thick enough to generate
neutrons from cosmic ray muon production and neutron-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 may
also produce 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.
Sources of neutrons for research. These include certain types of radioactive decay (spontaneous fission and neutron emission), and from certain nuclear reactions.
Convenient nuclear reactions include tabletop reactions such as natural
alpha and gamma bombardment of certain nuclides, often beryllium or
deuterium, and induced nuclear fission,
such as occurs in nuclear reactors. In addition, high-energy nuclear
reactions (such as occur in cosmic radiation showers or accelerator
collisions) also produce neutrons from disintegration of target nuclei.
Small (tabletop) particle accelerators optimized to produce free neutrons in this way, are called neutron generators.
In practice, the most commonly used small laboratory sources of
neutrons use radioactive decay to power neutron production. One noted
neutron-producing radioisotope, californium-252 decays (half-life 2.65 years) by spontaneous fission 3% of the time with production of 3.7 neutrons per fission, and is used alone as a neutron source from this process. Nuclear reaction sources (that involve two materials) powered by radioisotopes use an alpha decay source plus a beryllium target, or else a source of high-energy gamma radiation from a source that undergoes beta decay followed by gamma decay, which produces photoneutrons on interaction of the high-energy gamma ray with ordinary stable beryllium, or else with the deuterium in heavy water. A popular source of the latter type is radioactive antimony-124
plus beryllium, a system with a half-life of 60.9 days, which can be
constructed from natural antimony (which is 42.8% stable antimony-123)
by activating it with neutrons in a nuclear reactor, then transported to
where the neutron source is needed.
Experimental nuclear fusion reactors
produce free neutrons as a waste product. But 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 that
generate neutrons are likely to create radioactive waste, 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 due primarily to the long half-life of alpha-emitting transuranic actinides. Some nuclear fusion-fission hybrids are proposed to make use of those neutrons to either maintain a subcritical reactor or to aid in nuclear transmutation of harmful long lived nuclear waste to shorter lived or stable nuclides.
Neutron beams and modification of beams after production
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. But some effects may be
attained by use of inhomogeneous magnetic fields because of the neutron's magnetic moment. Neutrons can be controlled by methods that include moderation, reflection, and velocity selection. Thermal neutrons can be polarized by transmission through magnetic materials in a method analogous to the Faraday effect for photons. Cold neutrons of wavelengths of 6–7 angstroms can be produced in beams of a high degree of polarization, by use of magnetic mirrors and magnetized interference filters.
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.
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.
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,
in particular the hydrogen found in water molecules. When a fast
neutron collides with a light nucleus, it loses a large fraction of its
energy. By measuring the rate at which slow neutrons return to the probe
after reflecting off of hydrogen nuclei, a neutron probe may determine the water content in soil.
Because neutron radiation is both penetrating and ionizing, it can be
exploited for medical treatments. However, neutron radiation can have
the unfortunate side-effect of leaving the affected area radioactive. Neutron tomography is therefore not a viable medical application.
Fast neutron therapy uses high-energy neutrons typically greater than 20 MeV to treat cancer. Radiation therapy
of cancers is based upon the biological response of cells to ionizing
radiation. If radiation is delivered in small sessions to damage
cancerous areas, normal tissue will have time to repair itself, while
tumor cells often cannot. Neutron radiation can deliver energy to a cancerous region at a rate an order of magnitude larger than gamma radiation.
Beams of low-energy neutrons are used in boron neutron capture therapy
to treat cancer. In boron neutron capture therapy, the patient is given
a drug that contains boron and that preferentially accumulates in the
tumor to be targeted. The tumor is then bombarded with very low-energy
neutrons (although often higher than thermal energy) which are captured
by the boron-10 isotope in the boron, which produces an excited state of boron-11 that then decays to produce lithium-7 and an alpha particle
that have sufficient energy to kill the malignant cell, but
insufficient range to damage nearby cells. For such a therapy to be
applied to the treatment of cancer, a neutron source having an intensity
of the order of a thousand million (109) neutrons per second per cm2 is preferred. Such fluxes require a research nuclear reactor.
Protection
Exposure
to free neutrons can be hazardous, since the interaction of neutrons
with molecules in the body can cause disruption to molecules and atoms, and can also cause reactions that 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. But particular thought must be given to how
to protect from neutron exposure. For other types of radiation, e.g., alpha particles, beta particles, or gamma rays, material of a high atomic number and with high density makes 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 that has high
affinity for slow neutrons without causing secondary capture radiation,
such as lithium-6.
Hydrogen-rich ordinary water effects neutron absorption in nuclear fission
reactors: Usually, neutrons are so strongly absorbed by normal water
that fuel enrichment with a fissionable isotope is required. (The number
of neutrons produced per fission depends primarily on the fission
products. The average is roughly 2.5 to 3.0 and at least one, on
average, must evade capture in order to sustain the nuclear chain reaction.) 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.
Thermal neutrons are free neutrons whose energies have a Maxwell–Boltzmann distribution with kT = 0.0253 eV (4.0×10−21J)
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 neutron reactions show a much larger
effective cross-section than reactions involving faster neutrons, and
thermal neutrons can therefore be absorbed more readily (i.e., with
higher probability) 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.
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.
The use of cold and very cold neutrons (VCN) have been a bit
limited compared to the use of thermal neutrons due to the relatively
lower flux and lack in optical components. However, Innovative solutions
have been proposed to offer more options to the scientific community to
promote the use of VCN.
Cold neutron source providing neutrons at about the temperature of liquid hydrogen
Ultracold neutrons
Ultracold neutrons
are produced by inelastic scattering of cold neutrons in substances
with a low neutron absorption cross section at a temperature of a few
kelvins, such as solid deuterium or superfluid helium. An alternative production method is the mechanical deceleration of cold neutrons exploiting the Doppler shift.
A fast neutron is a free neutron with a kinetic energy level close to 1 MeV (1.6×10−13J), hence a speed of ~14000 km/s (~ 5% 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. Neutrons produced in fission, as noted above, have a Maxwell–Boltzmann distribution of kinetic energies from 0 to ~14 MeV, a mean energy of 2 MeV (for 235U fission neutrons), and a mode
of only 0.75 MeV, which means that more than half of them do not
qualify as fast (and thus have almost no chance of initiating fission in
fertile materials, such as 238U and 232Th).
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.
Fusion neutrons
The
fusion reaction rate increases rapidly with temperature until it
maximizes and then gradually drops off. The D–T 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.
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-fissileheavy 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 uses D–T fusion 14.1 MeV neutrons to cause more fission. Fusion neutrons are able to cause fission in ordinarily non-fissile materials, such as depleted 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 rest of the time. D–3He fusion produces no neutron.
Intermediate-energy neutrons
Transmutation flow in light water reactor, 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 resonance peaks 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-moderatedthermal reactor, where epithermal neutrons interact mostly with moderator nuclei, not with either fissile or fertileactinide
nuclides. But in a partially moderated reactor with more interactions
of epithermal neutrons with heavy metal nuclei, there are greater
possibilities for transient changes in reactivity that 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 that 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.
High-energy neutrons
High-energy neutrons have much more energy than fission energy neutrons and are generated as secondary particles by particle accelerators or in the atmosphere from cosmic rays. These high-energy neutrons are extremely efficient at ionization and far more likely to cause cell death than X-rays or protons.
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