Neutron capture

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Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus.[1] Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, which are repelled electrostatically.[1]

Neutron capture plays a significant role in the cosmic

thermonuclear reactions (i.e., by nuclear fusion) but can be formed by neutron capture.[1]
Neutron capture on protons yields a line at 2.223 MeV predicted
solar flares
.

Neutron capture at small neutron flux

Decay scheme of 198Au

At small

thermal neutrons
are used, the process is called thermal capture.

The isotope 198Au is a beta emitter that decays into the mercury isotope 198Hg. In this process, the atomic number rises by one.

Neutron capture at high neutron flux

The

beta-stable
isotopes of higher-numbered elements.

Capture cross section

The absorption neutron cross section of an isotope of a chemical element is the effective cross-sectional area that an atom of that isotope presents to absorption and is a measure of the probability of neutron capture. It is usually measured in barns.

Absorption cross section is often highly dependent on

neutron moderation
slows the neutron from an original high energy.

The thermal energy of the nucleus also has an effect; as temperatures rise, Doppler broadening increases the chance of catching a resonance peak. In particular, the increase in uranium-238's ability to absorb neutrons at higher temperatures (and to do so without fissioning) is a negative feedback mechanism that helps keep nuclear reactors under control.

Thermochemical significance

Neutron capture is involved in the formation of isotopes of chemical elements. The energy of neutron capture thus intervenes[clarification needed] in the standard enthalpy of formation of isotopes.

Uses

Further information: Neutron activation and Neutron activation analysis

Neutron activation analysis can be used to remotely detect the chemical composition of materials. This is because different elements release different characteristic radiation when they absorb neutrons. This makes it useful in many fields related to mineral exploration and security.

Neutron absorbers

Main article: Neutron poison
Neutron cross section of boron (top curve is for 10B and bottom curve for 11B)

In engineering, the most important neutron absorber is 10B, used as boron carbide in nuclear reactor control rods or as boric acid as a coolant water additive in pressurized water reactors. Other neutron absorbers used in nuclear reactors are xenon, cadmium, hafnium, gadolinium, cobalt, samarium, titanium, dysprosium, erbium, europium, molybdenum and ytterbium.[4] All of these occur in nature as mixtures of various isotopes, some of which are excellent neutron absorbers. They may occur in compounds such as molybdenum boride, hafnium diboride, titanium diboride, dysprosium titanate and gadolinium titanate.

fuel rods. To use these elements in their respective applications, the zirconium must be separated from the naturally co-occurring hafnium. This can be accomplished economically with ion-exchange resins.[5]

See also

References

  1. ^
    Ahmad, Ishfaq; Hans Mes; Jacques Hebert (1966). "Progress of theoretical physics: Resonance in the Nucleus"
    . Institute of Physics. 3 (3): 556–600.
  2. ^ Morrison, P. (1958). "On gamma-ray astronomy". Il Nuovo Cimento. 7 (6): 858–865.
    S2CID 121118803
    .
  3. Bibcode:1973NASSP.342..285C
    .
  4. ^ Prompt Gamma-ray Neutron Activation Analysis. International Atomic Energy Agency
  5. ISBN 978-0-8031-0270-5
    . Retrieved 7 October 2012.

External links
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