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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 .
2. High-LET protons, produced by the scattering of fast neutrons and from the capture of thermal neutrons by nitrogen atoms [14 N(n,p) 14 C]; and 3. High-LET, heavier charged alpha particles (stripped down helium [4 He] nuclei) and lithium-7 ions, released as products of the thermal neutron capture and decay reactions with 10 B [10 B(n,α) 7 Li].
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.
Neutron capture therapy was first proposed in the literature in 1936 by Gordon L. Locher, who observed that isotopes with large neutron capture cross sections, such as boron-10, could be accumulated in cancerous tissue and bombarded with thermal neutrons to induce destruction of the cancerous cells. [4]
However, the neutron capture cross section of 236 U is low, and this process does not happen quickly in a thermal reactor. Spent nuclear fuel typically contains about 0.4% 236 U. With a much greater cross-section, 237 Np may eventually absorb another neutron and become 238 Np, which quickly beta decays to plutonium-238 (another non-fissile ...
Uranium-234 has a neutron-capture cross section of about 100 barns for thermal neutrons, and about 700 barns for its resonance integral—the average of neutrons having a range of intermediate energies. In a nuclear reactor non-fissile isotopes 234 U and 238 U both capture a neutron, thereby breeding fissile isotopes 235 U and 239 Pu, respectively.
These thermal neutrons are immensely more susceptible than fast neutrons to propagate a nuclear chain reaction of uranium-235 or other fissile isotope by colliding with their atomic nucleus. Water (sometimes called "light water" in this context) is the most commonly used moderator (roughly 75% of the world's reactors).
Although the thermal neutron fission cross section (σ f) of the resulting 233 U is comparable to 235 U and 239 Pu, it has a much lower capture cross section (σ γ) than the latter two fissile isotopes, providing fewer non-fissile neutron absorptions and improved neutron economy. The ratio of neutrons released per neutron absorbed (η) in 233 U