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G. P. A. Nobre et al.
In nuclear reaction evaluations, however, predictive models are seldom employed
since the greater flexibility of parameter fitting of phenomenological models may
lead to better cross-section agreements [4]. In this work we aim to circumvent this
apparent deficiency of the HFB LD model by using experimental data from neutron
double-differential spectra cross sections on 56 Fe to impose direct constraints on
the HFB total LD for 56 Fe and 56 Mn. We show that we can obtain a more realistic
LD and at least equally good cross sections compared to a fitted Gilbert-Cameron
model, in particular for the 56 Fe(n,p) reaction which is of dosimetry interest. This
way we can combine the predictive power of a microscopic model with good
description of observed data, as required by a variety of applications. We also
investigate the consequences of this approach in the prediction of inelastic gamma
cross sections as compared to measured data.
2 Description of LD Models
LD models are crucial for Hauser-Feshbach and pre-equilibrium reaction mechanisms. Phenomenological models tend to better reproduce average behaviors while
missing detailed structure components. We will discuss the phenomenological GC
and the microscopic HFB models, as implemented in EMPIRE [5].
2.1 Gilbert-Cameron Model
Most phenomenological LD models are based in some form on the analytical
expression derived from the Fermi Gas Model [1]. We assume that the density of
intrinsic levels with spin J , parity π , and excitation energy E x can be factored in
terms of its state density and spin and parity dependence. The Gilbert-Cameron
model [1] splits the excitation energy range into two parts, with different functional
forms applied to each of them. Below a chosen matching energy U x a constanttemperature state density is employed while above U x the back-shifted Fermi Gas
state density is adopted, with pairing energy given by = n
12
√
A
, where A is the
nucleus mass number and n is 0, 1, or 2 for odd-odd, odd-even, and even-even
nuclei, respectively. Some model parameters are internally determined by imposing
that the total LD and its derivative are continuous at the matching point U x .
2.2 HFB Model
EMPIRE has implemented within its options the microscopic combinatorial
approach [2] developed for RIPL-3 [3]. It consists of using single-particle level
G. P. A. Nobre et al.
In nuclear reaction evaluations, however, predictive models are seldom employed
since the greater flexibility of parameter fitting of phenomenological models may
lead to better cross-section agreements [4]. In this work we aim to circumvent this
apparent deficiency of the HFB LD model by using experimental data from neutron
double-differential spectra cross sections on 56 Fe to impose direct constraints on
the HFB total LD for 56 Fe and 56 Mn. We show that we can obtain a more realistic
LD and at least equally good cross sections compared to a fitted Gilbert-Cameron
model, in particular for the 56 Fe(n,p) reaction which is of dosimetry interest. This
way we can combine the predictive power of a microscopic model with good
description of observed data, as required by a variety of applications. We also
investigate the consequences of this approach in the prediction of inelastic gamma
cross sections as compared to measured data.
2 Description of LD Models
LD models are crucial for Hauser-Feshbach and pre-equilibrium reaction mechanisms. Phenomenological models tend to better reproduce average behaviors while
missing detailed structure components. We will discuss the phenomenological GC
and the microscopic HFB models, as implemented in EMPIRE [5].
2.1 Gilbert-Cameron Model
Most phenomenological LD models are based in some form on the analytical
expression derived from the Fermi Gas Model [1]. We assume that the density of
intrinsic levels with spin J , parity π , and excitation energy E x can be factored in
terms of its state density and spin and parity dependence. The Gilbert-Cameron
model [1] splits the excitation energy range into two parts, with different functional
forms applied to each of them. Below a chosen matching energy U x a constanttemperature state density is employed while above U x the back-shifted Fermi Gas
state density is adopted, with pairing energy given by = n
12
√
A
, where A is the
nucleus mass number and n is 0, 1, or 2 for odd-odd, odd-even, and even-even
nuclei, respectively. Some model parameters are internally determined by imposing
that the total LD and its derivative are continuous at the matching point U x .
2.2 HFB Model
EMPIRE has implemented within its options the microscopic combinatorial
approach [2] developed for RIPL-3 [3]. It consists of using single-particle level