Effect Due to Neutron Capture
The effects of neutrons on materials arise hugely from the transfer of kinetic energy to atomic nuclei in one way or other. Therefore, highly energetic recoil nuclei might be indirectly generates through the absorption of a neutron and the subsequent emission of a γ. As earlier discussed, if the energy of the recoil nucleus is sufficient to permit it to be displaced from its normal (or equilibrium) position in the crystal lattice of solid, physical changes of a fundamentally permanent nature will be observed. An effects of fast neutrons in disrupting (or damaging) the crystal lattice through displacement of atoms are elaborates in the two earlier chapters, “Thermal and Displacement Spikes Due to Irradiation “and” Atomic Displacement Due to Irradiation." This damage is commonly referred to as radiation damage. An absorption or capture of lower energy thermal neutrons could generate two effects.
a.introduction of an impurity atom (this is used in the electronics industry to uniformly dope semiconductors) because of the transmutation of the absorbing nucleus
b.atomic displacement caused through recoil atoms or knock-ons
As remembered, the introduction of an impurity atom was elaborates previously, and atomic displacement is the result of (n,p) and (n, α ) reactions and (n, γ) reactions followed through radioactive decay.Thermal neutrons cannot generates atomic displacements straightly, but they could do so indirectly as the result of radioactive capture (n, γ) and other neutron reactions or elastic scattering.
Radioactive capture, or thermal neutron capture, generates several gamma rays (many times called photons) in the 5 MeV to 10 MeV energy range. While a gamma-ray photon is emitted through the excited compound nucleus formed through neutron capture, the residual atom suffers recoil (many times referred to as the shotgun effect). This recoil energy is often large enough to displace the atom from its equilibrium position and produce a cascade of displacements, or Frenkel defects, with a resultant property change of the material. The (n, γ) reaction with thermal neutrons can displace the atom since the gamma photon has momentum ( Eγ/c), that means in which the nucleus must have an equivalent and opposite momentum (conservation of momentum). Eγ is the gamma-ray (photon) energy, and here c is the velocity of light. It will recoil with a velocity υ If the recoil atom has mass A such that
Eγ/c =Aυ (5-1)
where all quantities are expressed in SI units. The recoil energy Er is equal to 1/2 Aυ2, so that
Er = E2γ/2Ac2. (5-2)
Upon converting the energies into MeV and A into atomic mass (or weight) units, the result is
Er = 5.4 x 10-4 E2γ/A. (5-3)
The maximum energy of a gamma ray accompanying a (n, γ) reaction is in the range among 6 MeV and 8 MeV. For an component of low atomic mass (about 10), the recoil energy could be 2 keV to 3 keV, that is much greater than the 25 eV essential to displace an atom.
In a thermal reactor, in that the thermal neutron flux commonly exceeds the fast neutron flux, the radiation damage caused through recoil from (n,γ ) reactions might be of the similar sequence as (or greater than) in which because of the fast neutrons in a material having an appreciable radioactive capture cross section for thermal neutrons. Another neutron reactions (for instance, (n,p), (n,γ )) will also generates recoil atoms, but these reactions are of little importance in thermal reactors. Thermal neutron capture effects are commonly confined to the surface of the material since most captures occur there, but fast-neutron damage is likely to extend by most of the material.
Impurity atoms are generated through nuclear transmutations. Neutron capture in a reactor generates an isotope which might be unstable and generates a whole new atom as it decays. For most metallic materials, long irradiations at high flux levels are essential to generates significant property changes since of the building of impurities. Thus, a semiconductor like as germanium (Ge) might have huge changes in conductivity because of the gallium and arsenic atoms which are introduced as the activated Ge isotopes decay. Within stainless steel, trace amounts of boron undergo a (n, α) reaction which produces helium bubbles that lead to the deterioration of mechanical properties.