Radiation displacement damage

Ionizing radiation Any radiation displacing electrons from atoms or molecules (e.g., alpha, beta, and gamma radiation). Ionizing radiation may produce severe skin and tissue damage. 

Electronic Excitation. The storage of energy in an insulating material by the absorption of ionizing radiation occurs via two major processes, namely, electronic excitation and displacement damage. Radiation damage by these processes in insulators has been a topic of intense research for decades and several excellent review articles already exist concerning a variety of material types (eg. oxides (1-5), alkali halides (6-8) and fluorites (9)). 

Behind Shield Rx + Space Neutron (DDD) Displacement Damage Dose (DDD) based on the limiting radiation dose to materials just behind the shield. 

For the JIMO mission, the unshielded external total ionizing dose and displacement damage dose was specified as 1x10 rads(Si) and 5.7x10 1-MeV equivalent neutrons per square centimeter, respectively (Reference 10- 9). The external environment would be dominated by charged particles, but it is described by a 1-MeV equivalent neutron. The radiation incident upon a component of interest is the sum of the external (shielded or unshielded) and internal radiation environment. 

Radiation IFull Mission Total Ionizing Dose [TID Displacement Damage Dose (DDD) Values fbrde n factor of 1 and exdudes reactor radiation 1x10 rads (Si) 5.7x10 T-MeV equiv. n/cm Levels not yet defined but considered to be less than JIMO mission... 

Radiation Damage. It has been known for many years that bombardment of a crystal with energetic (keV to MeV) heavy ions produces regions of lattice disorder. An implanted ion entering a soHd with an initial kinetic energy of 100 keV comes to rest in the time scale of about 10 due to both electronic and nuclear coUisions. As an ion slows down and comes to rest in a crystal, it makes a number of coUisions with the lattice atoms. In these coUisions, sufficient energy may be transferred from the ion to displace an atom from its lattice site. Lattice atoms which are displaced by an incident ion are caUed primary knock-on atoms (PKA). A PKA can in turn displace other atoms, secondary knock-ons, etc. This process creates a cascade of atomic coUisions and is coUectively referred to as the coUision, or displacement, cascade. The disorder can be directiy observed by techniques sensitive to lattice stmcture, such as electron-transmission microscopy, MeV-particle channeling, and electron diffraction. 

The process responsible for initiating RES follows from the earlier discussion of radiation damage in graphite. Specifically, in a displacement event a Frenkel pair... 

Fig. 6. Radiation damage in graphite showing the induced crystal dimensional strains. Impinging fast neutrons displace carbon atoms from their equilibrium lattice positions, producing an interstitial and vacancy. The coalescence of vacancies causes contraction in the a-direction, whereas interstitials may coalesce to form dislocation loops (essentially new graphite planes) causing c-direction expansion.

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