International Reactor Dosimetry File 2002 - IAEA Publications

more sensitive than volume effect devices to ionization radiation effects
produced by either a neutron field or a mixed neutron–gamma field.
The accepted methodology is to relate the damage caused by a specific
fluence of a given neutron spectrum to an equivalent fluence from a monoenergetic
spectrum at a reference energy that would produce the same level of
damage. 1 MeV is the reference energy used by the semiconductor radiation
effects community, and the ratio of the fluence from a specific neutron with
energy E to the fluence of a reference 1 MeV neutron required to cause the
same level of damage is referred to as the 1 MeV(Si) damage response
function. IRDF-2002 response functions include the results of the calculation
of silicon displacement kerma factors (displacement kerma per unit neutron
fluence) as a function of neutron energy over the range 10 –10 –20 MeV. The unit
of the displacement kerma factor is megaelectronvolts times millibarns
(MeV·mb). Each factor can be multiplied by 3.435 × 10 –13 to convert to
rad(Si)·cm 2 , or by 3.435 × 10 –19 to convert to J·m 2 /kg or Gy(Si)·m 2 .
An average value of the neutron displacement kerma factor near 1 MeV
is difficult to determine because of sharp neutron cross-section resonances in
that energy region. To avoid these difficulties, the semiconductor radiation
effects community has defined the displacement kerma of a reference 1 MeV
neutron to be exactly a reference displacement kerma level of 95 MeV·mb.
Values for the silicon displacement kerma are determined by calculating the
total kerma and then partitioning the data into ionization and displacement
fractions [8.7]. The correlation of the displacement kerma with the measured
damage in many neutron fields has been confirmed with integral uncertainties
no larger than 10% [8.8]. Figure 8.2 shows the neutron energy dependent
silicon displacement kerma.
For any given neutron spectrum, a 1 MeV(Si) equivalent fluence is
derived by convoluting the displacement kerma with the neutron spectrum and
dividing by 95 MeV·mb. The uncertainty in the specification of the neutron
spectrum should be propagated through this convolution and used to
determine the uncertainty in the resulting 1 MeV(Si) equivalent fluence. Note
that the displacement kerma is considered to be a radiation effects community
specified exposure metric and has no uncertainty (i.e. it represents a specified
response).
8.3. GALLIUM ARSENIDE dpa (ELECTRONICS DAMAGE)
The basis of the currently accepted protocol for the correlation of
neutron damage effects to a neutron fluence in a GaAs semiconductor device is
through the displacement kerma produced in bulk GaAs. However, this
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