The kinetic energy of an unbound neutron
The
neutron detection temperature
, also called the
neutron energy
, indicates a
free neutron
's
kinetic energy
, usually given in
electron volts
. The term
temperature
is used, since hot, thermal and cold neutrons are
moderated
in a medium with a certain temperature. The neutron energy distribution is then adapted to the
Maxwell distribution
known for thermal motion. Qualitatively, the higher the temperature, the higher the kinetic energy of the free neutrons. The
momentum
and
wavelength
of the neutron are related through the
de Broglie relation
. The long wavelength of slow neutrons allows for the large cross section.
[1]
Neutron energy distribution ranges
[
edit
]
Neutron energy range names
[2]
[3]
Neutron energy
|
Energy range
|
0.0 ? 0.025 eV
|
Cold (slow) neutrons
|
0.025 eV
|
Thermal neutrons (at 20°C)
|
0.025?0.4 eV
|
Epithermal neutrons
|
0.4?0.5 eV
|
Cadmium neutrons
|
0.5?10 eV
|
Epicadmium neutrons
|
10?300 eV
|
Resonance neutrons
|
300 eV?1 MeV
|
Intermediate neutrons
|
1?20 MeV
|
Fast neutrons
|
> 20 MeV
|
Ultrafast neutrons
|
But different ranges with different names are observed in other sources.
[4]
The following is a detailed classification:
Thermal
[
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]
A
thermal neutron
is a free neutron with a kinetic energy of about 0.025
eV
(about 4.0×10
?21
J
or 2.4 MJ/kg, hence a speed of 2.19 km/s), which is the energy corresponding to the most probable speed at a temperature of 290 K (17 °C or 62 °F), the
mode
of the
Maxwell?Boltzmann distribution
for this temperature, E
peak
=
k
T.
After a number of collisions with nuclei (
scattering
) in a medium (
neutron moderator
) at this temperature, those
neutrons
which are not absorbed reach about this energy level.
Thermal neutrons have a different and sometimes much larger effective
neutron absorption
cross-section
for a given
nuclide
than fast neutrons, and can therefore often be absorbed more easily by an
atomic nucleus
, creating a heavier, often
unstable
isotope
of the
chemical element
as a result. This event is called
neutron activation
.
Epithermal
[
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]
[
example needed
]
- Neutrons of energy greater than thermal
- Greater than 0.025 eV
Cadmium
[
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]
[
example needed
]
- Neutrons which are strongly absorbed by
cadmium
- Less than 0.5 eV.
Epicadmium
[
edit
]
[
example needed
]
- Neutrons which are not strongly absorbed by cadmium
- Greater than 0.5 eV.
Cold (slow) neutrons
[
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]
[
example needed
]
- Neutrons of lower (much lower) energy than thermal neutrons.
- Less than 5 meV.
- Cold (slow) neutrons are subclassified into cold (CN), very cold (VCN), and ultra-cold (UCN) neutrons, each having particular characteristics in terms of their optical interactions with matter. As the wavelength is made (chosen to be) longer, lower values of the momentum exchange become accessible. Therefore, it is possible to study larger scales and slower dynamics. Gravity also plays a very significant role in the case of UCN. Nevertheless, UCN reflect at all angles of incidence. This is because their momentum is comparable to the optical potential of materials. This effect is used to store them in bottles and study their fundamental properties
[5]
[6]
e.g. lifetime, neutron electrical-dipole moment etc... The main limitations of the use of slow neutrons is the low flux and the lack of efficient optical devices (in the case of CN and VCN). Efficient neutron optical components are being developed and optimized to remedy this lack.
[7]
Resonance
[
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]
[
example needed
]
- Refers to neutrons which are strongly susceptible to non-fission capture by
U-238
.
- 1 eV to 300 eV
Intermediate
[
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]
[
example needed
]
- Neutrons that are between slow and fast
- Few hundred eV to 0.5 MeV.
Fast
[
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]
- A
fast neutron
is a free neutron with a kinetic energy level close to 1
M
eV
(100
T
J
/
kg
), hence a speed of 14,000 km/
s
or higher. They are named
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:
Fast neutrons are usually undesirable in a steady-state nuclear reactor because most fissile fuel has a higher reaction rate with thermal neutrons. Fast neutrons can be rapidly changed into thermal neutrons via a process called moderation. This is done through numerous collisions with (in general) slower-moving and thus lower-temperature particles like atomic nuclei and other neutrons. These collisions will generally speed up the other particle and slow down the neutron and scatter it. Ideally, a room temperature
neutron moderator
is used for this process. In reactors,
heavy water
,
light water
, or
graphite
are typically used to moderate neutrons.
A chart displaying the speed probability density functions of the speeds of a few
noble gases
at a temperature of 298.15 K (25 C). An explanation of the vertical axis label appears on the image page (click to see). Similar speed distributions are obtained for
neutrons
upon
moderation
.
Ultrafast
[
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]
[
example needed
]
- Relativistic
- Greater than 20 MeV
Other classifications
[
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]
- Pile
-
- Neutrons of all energies present in nuclear reactors
- 0.001 eV to 15 MeV.
- Ultracold
-
- Neutrons with sufficiently low energy to be reflected and trapped
- Upper bound of 335 neV
Fast-neutron reactor and thermal-neutron reactor compared
[
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]
Most
fission reactors
are
thermal-neutron reactors
that use a
neutron moderator
to slow down ("
thermalize
") the neutrons produced by
nuclear fission
. Moderation substantially increases the fission
cross section
for
fissile
nuclei such as
uranium-235
or
plutonium-239
. In addition,
uranium-238
has a much lower capture cross section for thermal neutrons, allowing more neutrons to cause fission of fissile nuclei and propagate the chain reaction, rather than being captured by
238
U. The combination of these effects allows
light water reactors
to use
low-enriched uranium
.
Heavy water reactors
and
graphite-moderated reactors
can even use
natural uranium
as these moderators have much lower
neutron capture
cross sections
than light water.
[9]
An increase in fuel temperature also raises uranium-238's thermal neutron absorption by
Doppler broadening
, providing
negative feedback
to help control the reactor. When the coolant is a liquid that also contributes to moderation and absorption (light water or heavy water), boiling of the coolant will reduce the moderator density, which can provide positive or negative feedback (a positive or negative
void coefficient
), depending on whether the reactor is under- or over-moderated.
Intermediate-energy neutrons have poorer fission/capture ratios than either fast or thermal neutrons for most fuels. An exception is the
uranium-233
of the
thorium cycle
, which has a good fission/capture ratio at all neutron energies.
Fast-neutron reactors
use unmoderated
fast neutrons
to sustain the reaction, and require the fuel to contain a higher concentration of
fissile material
relative to
fertile material
(uranium-238). However, fast neutrons have a better fission/capture ratio for many nuclides, and each fast fission releases a larger number of neutrons, so a
fast breeder reactor
can potentially "breed" more fissile fuel than it consumes.
Fast reactor control cannot depend solely on Doppler broadening or on negative void coefficient from a moderator. However, thermal expansion of the fuel itself can provide quick negative feedback. Perennially expected to be the wave of the future, fast reactor development has been nearly dormant with only a handful of reactors built in the decades since the
Chernobyl accident
due to low prices in the
uranium market
, although there is now a revival with several Asian countries planning to complete larger prototype fast reactors in the next few years.
[
when?
]
See also
[
edit
]
References
[
edit
]
- ^
de Broglie, Louis.
"On the Theory of Quanta"
(PDF)
.
aflb.ensmp.fr
. Retrieved
2 February
2019
.
- ^
Carron, N.J. (2007).
An Introduction to the Passage of Energetic Particles Through Matter
. p. 308.
Bibcode
:
2007ipep.book.....C
.
- ^
"Neutron Energy"
.
www.nuclear-power.net
. Retrieved
27 January
2019
.
- ^
H. Tomita, C. Shoda, J. Kawarabayashi, T. Matsumoto, J. Hori, S. Uno, M. Shoji, T. Uchida, N. Fukumotoa and T. Iguchia,
Development of epithermal neutron camera based on resonance-energy-filtered imaging with GEM
, 2012, quote: "Epithermal neutrons have energies between 1 eV and 10 keV and smaller nuclear cross sections than thermal neutrons."
- ^
"Introduction"
,
Ultracold Neutrons
, WORLD SCIENTIFIC, pp. 1?9, 2019-09-23,
doi
:
10.1142/9789811212710_0001
,
ISBN
978-981-12-1270-3
,
S2CID
243745548
, retrieved
2022-11-11
- ^
Jenke, Tobias; Bosina, Joachim; Micko, Jakob; Pitschmann, Mario; Sedmik, Rene; Abele, Hartmut (2021-06-01).
"Gravity resonance spectroscopy and dark energy symmetron fields"
.
The European Physical Journal Special Topics
.
230
(4): 1131?1136.
arXiv
:
2012.07472
.
doi
:
10.1140/epjs/s11734-021-00088-y
.
ISSN
1951-6401
.
S2CID
229156429
.
- ^
Hadden, Elhoucine; Iso, Yuko; Kume, Atsushi; Umemoto, Koichi; Jenke, Tobias; Fally, Martin; Klepp, Jurgen; Tomita, Yasuo (2022-05-24).
"Nanodiamond-based nanoparticle-polymer composite gratings with extremely large neutron refractive index modulation"
. In McLeod, Robert R; Tomita, Yasuo; Sheridan, John T; Pascual Villalobos, Inmaculada (eds.).
Photosensitive Materials and their Applications II
. Vol. 12151. SPIE. pp. 70?76.
Bibcode
:
2022SPIE12151E..09H
.
doi
:
10.1117/12.2623661
.
ISBN
9781510651784
.
S2CID
249056691
.
- ^
Byrne, J.
Neutrons, Nuclei, and Matter
, Dover Publications, Mineola, New York, 2011,
ISBN
978-0-486-48238-5
(pbk.) p. 259.
- ^
Some Physics of Uranium. Accessed March 7, 2009
External links
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]