4.1 A radiation-hardness assurance program requires a methodology for relating radiation induced changes in materials exposed to a variety of particle species over a wide range of energies, including those encountered in spacecraft and in terrestrial environments as well as those produced by particle accelerators and nuclear fission and fusion reactors.
4.2 A major source of radiation damage in electronic and photonic devices and materials is the displacement of atoms from their normal lattice site. An appropriate exposure parameter for such damage is the damage energy calculated from NIEL by means of Eq 2. Other analogous measures, which may be used to characterize the irradiation history that is relevant to displacement damage, are damage energy per atom or per unit mass (displacement kerma, when the primary particles are neutral), and displacements per atom (dpa). See Terminology E170 for definitions of those quantities.
4.3 Each of the quantities mentioned in the previous paragraph should convey similar information, but in a different format. In each case the value of the derived exposure parameter depends on approximate nuclear, atomic, and lattice models, an on measured or calculated cross sections. If consistent comparisons are to be made of irradiation effects caused by different particle species and energies, it is essential that these approximations be consistently applied.
4.4 No correspondence should be assumed to exist between damage energy as calculated from NIEL and a particular change in a material property or device parameter. Instead, the damage energy should be used as a parameter which describes the exposure. It may be a useful correlation variate, even when different particle species and energies are included. NIEL should not be reported as a measure of damage, however, unless its correlation with a particular damage modality has been demonstrated in that material or device.
4.5 NIEL is a construct that depends on a model of the particle interaction processes in a material, as well as the cross section for each type of interaction. It is essential, when using NIEL as a correlation parameter, to ensure that consistent modeling parameters and nuclear data are used to calculate the NIEL value for each irradiation.
4.6 Damage energy deposited in materials can be calculated directly, without the use of NIEL, using the Monte Carlo codes mentioned in 3.2.4.7, if all the particles involved in atomic displacement are tracked. The utility of the NIEL concept arises in cases where some particles, especially recoiling heavy ions, do not need to be tracked. In the NIEL representation, these are treated instead by means of infinite homogeneous medium solutions of the type originated by Lindhard et al. (10).
