International Reactor Dosimetry File 2002 - IAEA Publications

10 2
Damage (MeV· mb)
10 1
10 0
10 –1
10 –2
10 –3 10 –10 10 –9 10 –8 10 –7 10 –6 10 –5 10 –4 10 –3 10 –2 10 –1 10 0 10 1 10 2
Neutron energy (MeV)
FIG. 8.2. Energy dependence of silicon displacement damage response function.
correlation depends on the assumption that displacement effects are the
dominant radiation damage mechanism and that equal numbers of initially
displaced atoms produce equal changes in device performance. Experimental
evidence indicates that displacement kerma is not a valid measure of changes in
the fundamental properties (carrier concentration, mobility and carrier
lifetime) that determine device performance [8.9, 8.10]. The reason that the
displacement kerma does not correlate with the property changes in GaAs over
the entire range of neutron energies of interest is attributed to variations in the
defect production efficiency for different sizes of displacement cascades. This
effect is also known to occur in other types of material, including structural
metals [8.11]. Despite these deficiencies (a lack of a strict proportionality
between the observed GaAs semiconductor damage and the calculated
displacement kerma), displacement kerma is still useful as an exposure
parameter, and is analogous to the use of dpa for exposures of iron.
Empirical efficiency factors that depend on the energies of the primary
knock-on atoms (PKA) have been proposed in order to remove the discrepancies
described above [8.9]. Figure 8.3 shows the shape of the empirical
damage efficiency factor for GaAs, and can be described by an empirical
function. As in Ref. [8.11], this PKA energy damage efficiency factor is used in
conjunction with a normalization factor of 2.2 to preserve the equivalence of
the GaAs damage function and the displacement kerma for 1 MeV neutrons.
The results of the calculation of GaAs displacement kerma factors
(displacement kerma per unit neutron fluence) are shown in Fig. 8.4 as a
function of neutron energy. Figure 8.5 shows the complete energy dependence
of the GaAs damage function. The unit of the kerma factor is megaelectronvolts
times millibarns (MeV·mb). The kerma factor can be multiplied by
95