1-MeV(GaAs)-equivalent response

This 1-MeV(GaAs)-equivalent response function corresponds to the
ASTM E722-14 recommended displacement damage function used to
assess displacement damage effects in GaAs, e.g. Frenkel pair
production and changes in the minority carrier recombination 
lifetime in the bulk semiconductor material. This damage mode is
relevant to the gain degradation of GaAs heterojunction bipolar
transistors (HBTs) and the light output for GaAs light-emitting 
diodes (LEDs). The response function is useful as an exposure
parameter and has been validated to correspond to carrier 
lifetime for cases where the predominant source of displacement 
damage is from neutrons of energy between 10 keV and 20 MeV. The 
correlation with measured damage in many neutron fields has been 
confirmed with uncertainties no larger than 10%. This response 
function has not been validated for thermal neutron sources.

Because use of this functional form is required within the 
standard, ASTM E722-14 does not report an energy-dependent 
uncertainty for this response function. 

This response function has units of MeV-mb. Each factor can be
multiplied by 1.334E-13 to convert to units of rad(GaAs)*cm^2 or 
by 1.334E-19 to convert to J*m^2/kg or Gy(GaAs)*m^2. The 
tabulation uses a histogram representation and is presented in 
the SAND-II 640-group energy structure. 

The 1-MeV(GaAs)-equivalent neutron fluence is obtained by 
folding this response function with the neutron spectrum and 
dividing this by the reference value of 70 MeV-mb. See ASTM 
E722-14 for details. 

Experimental evidence indicates that, for GaAs, the microscopic 
kerma factor is not a valid measure of changes in the fundamental
properties (carrier concentration, mobility, and carrier 
lifetime) that determine device performance. Because of this, the 
displacement damage response function for GaAs does not 
correspond to the pure microscopic displacement kerma factor. The 
reason for this lack of correlation between observed damage and 
the displacement kerma is attributed to variations in the defect 
production efficiency within displacement cascades of different 
sizes. This type of effect is known to occur in other materials 
and is commonly addressed through the use of a recoil atom 
dependent correction term, e.g. the athermal recombination 
displacement per atom (arc-dpa) formalism. The damage function 
represented here reflects the use of empirical efficiency factors 
that depend upon the energies of the primary recoil atoms and 
were selected to match experimentally measured neutron damage in 
a variety of neutron fields. These recoil atom energy-dependent 
efficiencies are applied within a similar type of formalism as 
that used for calculating the microscopic kerma factors, i.e. 
the use of a displacement partition function and special 
treatment of the region near the displacement threshold value.

This response function was derived using the ENDF/B-VI cross 
sections, the Robinson damage partition function, a displacement 
threshold energy of 10 eV, and the Norgett-Robinson-Torrens 
(NRT) treatment in the displacement threshold region. It applies 
to naturally occurring GaAs, i.e. it represents a composite of 
the isotopic (Ga-69, Ga-71, and As-75) microscopic kerma factors 
properly weighted by the natural abundances and the chemical 
composition.