Iron dpa

This iron displacement cross section corresponds to the 
ASTM E693-17 recommended displacement damage function for use in 
iron and low alloy steels (95% to 100% iron). The response 
function is useful as an exposure parameter and is used to 
correlate radiation-induced damage to iron that is related to 
microstructural changes in the metal, e.g. embrittlement and void 
swelling. A critical application is in reactor pressure vessel 
surveillance programs where this correlation is use to relate
radiation-induced changes in materials exposed in accelerated 
surveillance locations to the conditions of the pressure vessel. 

This response function has units of displacement per atom (dpa). 
The tabulation uses a histogram representation and is presented 
in the SAND-II 640-group energy structure. 

Because use of this functional form is required within the 
standard, ASTM E693-17 does not report an energy-dependent 
uncertainty for this response function. The absolute accuracy of 
the dpa calculation is not important when dpa is used as a 
correlation parameter for neutron irradiations, so long as the 
same response function is used by all laboratories in 
calculating dpa. The absolute uncertainty is estimated to be 
40% or more when applied to a light water reactor spectrum. 
For light water reactor applications, the relative accuracy 
of the dpa calculation estimated to be less than 10%. 

This response function was derived using the ENDF/B-VI cross 
sections (which are identical to the ENDF/B-VII.1 cross sections 
for the iron isotopes), the Robinson damage partition function, 
an effective displacement threshold energy of 40 eV, the 
Norgett-Robinson-Torrens (NRT) treatment in the displacement 
threshold region, and the beta=0.8 atomic scattering correction 
factor within the NRT formalism. The NRT equation is used to 
convert a damage energy into a number of displacements. It 
applies to naturally occurring iron, i.e. it represents a 
composite of the isotopic (Fe-54, Fe-56, Fe-57, and Fe-58) 
microscopic displacement kerma factors properly weighted by 
the natural abundances.