From Wikipedia, the free encyclopedia
Neutron source is a general term referring to a
variety of devices that emit neutrons, irrespective of the mechanism
used to produce the neutrons. Depending upon variables including
the energy of the neutrons emitted by the source, the rate of
neutrons emitted by the source, the size of the source, the cost of
owning and maintaining the source, and government regulations
related to the source, these devices find use in a diverse array of
applications in areas of physics, engineering, medicine, nuclear
weapons, petroleum exploration, biology, chemistry, nuclear power
and other industries.
There are several kinds of neutron sources:
Small sized
devices
- Radioisotopes which undergo spontaneous fission
- Certain isotopes undergo
spontaneous fission with emission
of neutrons. The most commonly used spontaneous fission source is
the radioactive isotope californium-252. Cf-252 and all other
spontaneous fission neutron sources are produced by irradiating uranium or another transuranic element in a nuclear reactor,
where neutrons are absorbed in the starting material and its
subsequent reaction products, transmuting the starting material
into the SF isotope. Cf-252 neutron sources are typically 1/4" to
1/2" in diameter and 1" to 2" in length. When purchased new a
typical Cf-252 neutron sources emit between 1×107 to
1×109 neutrons per second but, with a half life of 2.6
years, this neutron output rate drops to half of this original
value in 2.6 years. The price of a typical Cf-252 neutron source is
from $15,000 to $20,000.
- Radioisotopes which decay with alpha particles packed in a
low-Z elemental matrix
- Neutrons are produced when alpha particles impinge upon any of
several low atomic weight isotopes including isotopes of beryllium,
carbon and oxygen. This nuclear reaction can be used to construct a
neutron source by intermixing a radioisotope that emits alpha
particles such as radium or polonium with a low atomic
weight isotope, usually in the form of a mixture of powders of the
two materials. Typical emission rates for alpha reaction neutron
sources range from 1×106 to 1×108 neutrons
per second. As an example, a representative alpha-beryllium neutron
source can be expected to produce approximately 30 neutrons for
every one million alpha particles. The useful lifetime for these
types of sources is highly variable, depending upon the half life
of the radioisotope that emits the alpha particles. The size and
cost of these neutron sources are also comparable to spontaneous
fission sources. Usual combinations of materials are plutonium-beryllium (PuBe), americium-beryllium (AmBe),
or americium-lithium
(AmLi).
- Radioisotopes which decay with high energy photons co-located
with beryllium or deuterium
- Gamma radiation with an energy exceeding the neutron binding
energy of a nucleus can eject a neutron. Two examples and their
decay products:
- 9Be +
>1.7 Mev photon → 1 neutron + 2 4He
- 2H (deuterium) + >2.26 MeV
photon → 1 neutron + 1H
- Sealed tube neutron generators
- Some accelerator-based neutron generators exist that work by
inducing fusion between beams of deuterium and/or tritium ions and metal hydride targets
which also contain these isotopes.
Medium
sized devices
- Plasma focus and plasma pinch devices
- The plasma focus neutron source (see Plasma focus, not to be confused with the
so-called Farnsworth-Hirsch fusor) produces
controlled nuclear
fusion by creating a dense plasma within which ionized deuterium and/or tritium gas is heated to
temperatures sufficient for creating fusion.
- Light ion accelerators
- Traditional particle accelerators with hydrogen (H), deuterium
(D), or tritium (T) ion sources may be used to produce neutrons
using targets of deuterium, tritium, lithium, beryllium, and other
low-Z materials. Typically these accelerators operate with voltages
in the > 1 MeV range,
- High energy bremsstrahlung photoneutron/photofission
systems
- Neutrons (so called photoneutrons) are produced when photons
above the nuclear binding energy of a substance are incident on
that substance, causing it to undergo giant dipole resonance after which it
either emits a neutron or undergoes fission. The number of neutrons
released by each fission event is dependent on the substance.
Typically photons begin to produce neutrons on interaction with
normal matter at energies of about 7 to 40 MeV, which means that megavoltage photon radiotherapy
facilities may produce neutrons as well, and require special
shielding for them. In addition, electrons of energy over about 50
MeV may induce
giant dipole resonance in nuclides by a mechanism which is the
inverse of internal conversion, and thus
produce neutrons by a mechanism similar to that of photoneutrons.
[1]
Large sized
devices
- Nuclear fission reactors
- Nuclear
fission which takes place within in a reactor produces very
large quantities of neutrons and can be used for a variety of
purposes including power generation and experiments.
- Nuclear fusion systems
- Nuclear
fusion, the combining of the heavy isotopes of hydrogen, also
has the potential to produces large quantities of neutrons. Small
scale fusion systems exist for research purposes at many
universities and laboratories around the world. A small number of
large scale nuclear fusion systems also exist including the National Ignition Facility
in the USA, JET
in the UK, and soon the recently started ITER experiment in France.
- High energy particle accelerators
- A spallation source is a high-flux source in
which protons that have been
accelerated to high energies hit a target material, prompting the
emission of neutrons.
Neutron
flux
For most applications, a higher neutron flux is always better (since it
reduces the time required to conduct the experiment, acquire the
image, etc.). Amateur fusion devices, like the fusor, generate only about 300 000 neutrons per
second. Commercial fusor devices can generate on the order of
109 neutrons per second, which corresponds to a usable
flux of less than 105 n/(cm² s). Large neutron beamlines
around the world achieve much greater flux. Reactor-based sources
now produce 1015 n/(cm² s),AND spallation sources
generate greater than 1017 n/(cm² s).
See also
References
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