Combining Semi-Classical and Quantum Mechanical Methodologies for Nuclear Cross-section Calculations Between 1 Mev and 5 Gev Page: 1 of 4
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Combining Semi-Classical and Quantum Mechanical Methodologies for Nuclear
Cross-Section Calculations Between 1 MeV and 5 GeV
C. Y. FU,1 F. B. GUIMARAES, and L. C. LEAL
Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
With a goal to develop a nuclear cross-section code usable over the wide energy range of 1 MeV to 5 GeV, one option is
to combine intranuclear cascade, pre-equilibrium, and Hauser-Feshbach models in existing codes. However, the first two
models are semi-classical while the third one is quantum mechanical, and combining them is not straightforward because
the third model requires spin and parity distributions for all excited states that cannot be supplied by either one of the first
two models. Approximations to overcome this difficulty are described in this paper. Success of this combined model will
allow nuclear data evaluations for a large number of materials whose cross sections are needed in a wide range of
applications, including the design, operation, and future upgrades of the SNS (1 GeV proton). The incident particles may
be neutrons, protons, charged pions, or photons. Though only partially completed at this time, the new model compares
well with experimental radionuclide production cross sections from thresholds to 2.6 GeV for proton-induced reactions
KEYWORDS: nuclear model, cross sections, calculations, iron, radioisotopes
Two existing model codes and additional developments de-
scribed in this paper are utilized to achieve the stated goal.
TNG') is a low-energy (1 MeV to 40 MeV) nuclear cross-
section model code developed as a tool for generating the
evaluated ENDF/B neutron files. TNG is based on a unified
Hauser-Feshbach (H-F) and pre-equilibrium (P-E) formalism
emphasizing the importance of discrete level structure. CEM>
is a high-energy (40 MeV to 5 GeV) nuclear cross-section
model code that uses an intranuclear cascade (INC) model, a
P-E model and an evaporation model. CEM is semi-classical,
hence it does not utilize discrete levels, their spins and pari-
ties, and their constraint on level density parameters. No mat-
ter how high the incident particle energy is, all residual
nuclides have part of their excitations in the low-MeV range
during the decay process, requiring partial wave analysis in the
H-F model for further particle emission and creation of final
radionuclides. The major challenge in developing a combined
model code is reconciling the semi-classical physics in CEM
with the quantum mechanical physics in TNG. An approxi-
mate method to solve this problem is described in this paper.
Thus, a cross section calculation starting with the INC model
for projectiles in the few-GeV range may pass through the P-E
model in the high-MeV range, the H-F model in the low-MeV
range, and finally with the gamma-ray cascade model in TNG,
reach the ground states of several hundred residual nuclides.
Also, the transition from a pure TNG calculation at an incident
energy of 40 MeV to a combined CEM-TNG calculation
above 40 MeV must be natural and smooth. Cross sections for
discrete levels and for gamma-ray production will be obtained
for incident energies between 40 MeV and 5 GeV.
Stated simply, the evaporation model and the low-energy
end of the P-E model in CEM will be replaced by the unified
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P-E/H-F model in TNG. This simple statement is hard to im-
plement because it involves a transition from semi-classical
physics to quantum mechanical physics. The required input
for TNG, such as spin and parity distributions for all excita-
tion energies in all residual nuclides, are simply not available
from the INC model and the high-energy P-E model in CEM.
This information is required for complete and realistic cross-
section evaluations for reactions induced by high energy par-
ENDF/B-VI, the most recent version of evaluated neutron
cross-section files in the U. S., allows discrete-level informa-
tion to be entered for precise descriptions of the cross sec-
tions exciting each discrete level, as well as associated
gamma-ray branching ratios for gamma-ray production cal-
culations in processing codes. This type of information can
only be generated by analyzing experimental data with P-
E/H-F codes such as TNG. And this type of discrete-level
information does exist for the 26 isotopic evaluations con-
tributed by ORNL to ENDF/B-VI, all done with the aid of
TNG analyses. This detailed information is needed by data
users and will be the type of information available from suc-
cessful completion of the present model up to incident ener-
gies of 5 GeV.
Since a large number of excited nuclides will be de-
excited by TNG, a code automating its input for each resid-
ual nuclide, including discrete levels, their spins and parities
and gamma-ray branching ratios, and reaction Q-values, has
been developed.' Some other elements important for com-
bining the two codes are described. Calculated results using
TNG and CEM are compared with experimental data for Fe
up to 300 MeV in order to understand the successes and fail-
ures of the two codes. Emphasis is being placed at energies
around 150 MeV, the upper energy of the LA150 library,) to
demonstrate the need for extending the LA150 library to
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Fu, C.Y. Combining Semi-Classical and Quantum Mechanical Methodologies for Nuclear Cross-section Calculations Between 1 Mev and 5 Gev, article, August 15, 2001; Tennessee. (digital.library.unt.edu/ark:/67531/metadc725422/m1/1/: accessed November 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.