Advanced nuclear data for radiation-damage calculations Page: 4 of 8
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tions such as ENDF/B-V, which were primarily
Intended for neutron-transport applications.
Fortunately, 1t 1s now becoming possible to
compute the spectra of particles emitted from
these high-energy reactions with reasonable
accuracy using modern preequllIblrum statlstl-
3
cal-model codes such as GNASH. A description
of the methods used In GNASH Is given 1n Sec-
tion 2. These spectra can be combined with
angular distributions based on Kalbach-Mann
a
systematics (see Section 3) to obtain distri-
butions In energy and angle for the recoil nu-
cleus. This step Is performed by the RECOIL
code described in Section 4. Instead of the
traditional damage cross section, RECOIL tabu-
lates all the particle and recoil distributions
directly using the new EHF/B File 6 format
(see Section 5). These tabulated spectra ran
then be used 1n a subsequent code to compute
damage 1n metals, or the same recoil data can
be used with a different partition function to
compute damage for non-metal lie materials such
as ceramics. Some results ano comparisons for
Iron are given in Section 6.
2. THE GNASH COPE
The GNASH daculation begins with the com-
pound system formed by the Interaction of a
neutron having , ipecIMed laboratory energy
with a particular target nucleus A set of
residjal nuclei Is then defined that can be
reached from the Initial compound system by
various sequences of particle emission. Each
nucleus Is characterized by a particular separa-
tion energy, a set of discrete levels, and a
set of continuum energy bins.
For every bin of each compound system, GNASH
computes the probability of particle or photon
•mission. At high enough incident energy, par-
ticle emissions from the first compound system
are affected by preequl1Iblrum processes that
•re calculated using the Hester Equation model
of Kalbath.^ As will be *een later, thv "pre-
equilibrium ratio" versus energy for each par-
ticle Is useful for determining Its angular
distribution. Particle emission probabilities
are determined from transmission coefficients
based on optical model parameters. For mater-
ials like Iron, these optical model parameters
can be adjusted6,7 to give good agreement with
measured cross sections. The good agreement
obtained gives confidence 1n the results for
unmeasured Isotopes or energy ranges.
Gamma emission 1s Important due to Its ef-
fect (through competition) on particle emis-
sion, as well as for Its direct effects. Con-
tinuum emission 1$ computed using transmission
coefficients based on the Brink-Axel giant dl-
Q
pole resonance model. For iron, the coeffi-
cients were normalized to give good fits to
capture cross sections.6,7 Detailed transition
probabilities can be Input for discrete photon
•mission.
GNASH represents continuum excitation energy
regions through use of a level density model.
g
For iron, the model of GIlLert and Cameron
with the parameters of Cook.1^ was employed.
Additionally, adjustments were made at lower
•xcl.stior, energies to match available discrete
level Information.
During a calculation, GNASH prepares a prin-
ter output file containing run parameters, a
preequilibrium summary, discrete-leva I data,
crosc '.actions, and various accumulated spectra
for particles and photons. Additionally, it
provides detailed transition probabilities in
the form of "population increments" un an
•uxIlHary binary output file. As discussed in
Section 4, these population Increments can be
used to derive particle spectre by reaction.
3. KALBACH-MANN SYSlEMATICS
By studying existing measurements of secon-
dary-particle spectre for incident particle
energies between 20 end 60 HeV, Kalbach end
a
Mann have derived u simple technique for pre-
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MacFarlane, R.E. & Foster, D.G. Jr. Advanced nuclear data for radiation-damage calculations, article, January 1, 1983; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1093956/m1/4/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.