Thermal and displacement spikes can cause distortion that is frozen as stress in the microscopic area. These spikes can cause a change in the material's properties.

EO 1.18 DEFINE the following terms:

a. Thermal spike

b. Displacement spike

EO 1.19 STATE the effect a large number of displacement spikes has on the properties of a metal.

Thermal Spikes

As mentioned previously, the knock-ons lose energy most readily when they have lower energies, because they are in the vicinity longer and therefore interact more strongly. A thermal spike occurs when radiation deposits energy in the form of a knock-on, which in turn, transfers its excess energy to the surrounding atoms in the form of vibrational energy (heat). Some of the distortion from the heating can be frozen as a stress in this microscopic area.

Displacement Spikes

A displacement spike occurs when many atoms in a small area are displaced by a knock-on (or cascade of knock-ons). A 1 MeV neutron may affect approximately 5000 atoms, making up one of these spikes. The presence of many displacement spikes will change the properties of the material being irradiated. A displacement spike contains large numbers of interstitials and lattice vacancies (referred to as Frenkel pairs or Frenkel defects when considered in pairs). The presence of large numbers of vacancies and interstitials in the lattice of a metal will generally increase hardness and decrease ductility. In many materials (for example, graphite, uranium metal) bulk volume increases occur.

Summary

The important information in this chapter is summarized below.

Thermal and Displacement Spikes Due To Irradiation Summary

Thermal spikes occur when radiation deposits energy in the form of a knock-on, which in turn, transfers its excess energy to the surrounding atoms in the form of vibrational energy (heat).

Displacement spikes occur when many atoms in a small area are displaced by a knock-on.

The presence of many displacement spikes changes the properties of the metal being irradiated, such as increasing hardness and decreasing ductility.

EFFECT DUE TO NEUTRON CAPTURE

Neutron radiation affects material because of neutrons being captured. This chapter discusses the effects that the neutrons being captured have on the material.

EO 1.20 DESCRIBE how the emission of radiation can cause dislocation of the atom emitting the radiation.

EO 1.21 STATE the two effects on a crystalline structure resulting from the capture of a neutron.

EO 1.22 STATE how thermal neutrons can produce atomic displacements.

Effect Due to Neutron Capture

The effects of neutrons on materials arise largely from the transfer of kinetic energy to atomic nuclei in one way or another. Thus, highly energetic recoil nuclei may be indirectly produced by the absorption of a neutron and the subsequent emission of a . As previously 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 a solid, physical changes of an essentially permanent nature will be observed. The effects of fast neutrons in disrupting (or damaging) the crystal lattice by displacement of atoms are discussed in the two previous chapters, "Atomic Displacement Due to Irradiation" and "Thermal and Displacement Spikes Due to Irradiation." This damage is commonly referred to as radiation damage. The absorption or capture of lower energy thermal neutrons can produce two effects.

a. introduction of an impurity atom (this is used in the electronics industry to uniformly dope semiconductors) due to the transmutation of the absorbing nucleus

b. atomic displacement caused by recoil atoms or knock-ons

As noted, the introduction of an impurity atom was discussed previously, and atomic displacement is the result of (n,p) and (n, ) reactions and (n, ) reactions followed by radioactive decay. Thermal neutrons cannot produce atomic displacements directly, but they can do so indirectly as the result of radioactive capture (n, ) and other neutron reactions or elastic scattering.

Radioactive capture, or thermal neutron capture, produces many gamma rays (sometimes called photons) in the 5 MeV to 10 MeV energy range. When a gamma-ray photon is emitted by the excited compound nucleus formed by neutron capture, the residual atom suffers recoil (sometimes 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,) reaction with thermal neutrons can displace the atom since the gamma photon has momentum (), which means that the nucleus must have an equal and opposite momentum (conservation of momentum). is the gamma-ray (photon) energy, and c is the velocity of light. If the recoil atom has mass A, it will recoil with a velocity such that

where all quantities are expressed in SI units. The recoil energy E_r is equal to 1/2 , so that

Upon converting the energies into MeV and A into atomic mass (or weight) units, the result is

The maximum energy of a gamma ray accompanying a (n, ) reaction is in the range between 6 MeV and 8 MeV. For an element of low atomic mass (about 10), the recoil energy could be 2 keV to 3 keV, which is much greater than the 25 eV necessary to displace an atom.

In a thermal reactor, in which the thermal neutron flux generally exceeds the fast neutron flux, the radiation damage caused by recoil from (n, ) reactions may be of the same order as (or greater than) that due to the fast neutrons in a material having an appreciable radioactive capture cross section for thermal neutrons. Other neutron reactions (for example, (n,p), (n, )) will also produce recoil atoms, but these reactions are of little significance in thermal reactors. Thermal neutron capture effects are generally confined to the surface of the material because most captures occur there, but fast-neutron damage is likely to extend through most of the material.

Impurity atoms are produced by nuclear transmutations. Neutron capture in a reactor produces an isotope that may be unstable and produce an entirely new atom as it decays. For most metallic materials, long irradiations at high flux levels are necessary to produce significant property changes due to the building of impurities. However, a semiconductor such as germanium (Ge) may have large changes in conductivity due to the gallium and arsenic atoms that are introduced as the activated Ge isotopes decay. In stainless steel, trace amounts of boron undergo a (n, ) reaction that generates helium bubbles which lead to the deterioration of mechanical properties.