**64-Gd-154 Review**

By V.G. Pronyaev, NDS, IAEA

9 February 2003

**General Comment:**

Taking into account that the accuracy of nuclear data for fission
products is most important in neutron transport and nuclear
transmutation calculations, the following priority to the quality of the
data can be given when we compare different evaluations:

1.  Most important energy region is a region of spectrum with maximum
    neutron fluxes in thermal and fast fission reactors, namely around
    0.0253 eV and below 5 MeV.

2.  Most important cross sections in order of reducing of their
    importance are integral cross sections: capture, total, elastic,
    inelastic with excitation of levels and other threshold cross
    sections; differential cross sections: secondary neutron spectrum
    from inelastic scattering and elastic scattering differential cross
    sections, gamma production cross section and spectra.

##### **Files**

ENDF/B-VI -- 1994 revision (below 100 keV) of JENDL-3.2 evaluation. The
resolved resonance region with 162 MLBW s- resonances is given for En
10^-5^ to 2680 eV. The unresolved resonance region for s-, p- and d-
resonances is given for En from 2680 eV to 50 keV. Results of optical,
statistical and pre-equilibrium model calculations (as in JENDL-3.2) are
given for En above 50 keV with some revision of capture and elastic
cross sections below 1.152 MeV and total cross section below 100 keV.

JEF-2 -- taken from the ENDF/B-IV evaluation (1974). 20 MLBW s-
resonances are given up to 275.96 eV in the resolved resonance region
(RRR). These parameters were given as SLBW parameters in ENDF/B-IV. Same
background cross sections as in ENDF/B-IV were added for elastic
scattering and capture channels in the RRR. Results of optical and
statistical model calculations (one-step reactions only) are given for
En above RRR.

JENDL-3.2 (and JENDL-3.3) -- 1990 evaluation with 150 MLBW s- and 10 p-
resonances below 2.76 keV taken from Mughabghab84 and Macklin87. Spins
and parity of resonances were assigned when they were not known.
Unresolved resonance region for En between 2.76 and 100 keV is given for
s-, p-, and d- waves. Effective scattering radius is 8.0 fm for the
resolved and 6.802 fm for the unresolved resonance region. As seems low
R'=6.802 fm value leads to the underestimation of elastic scattering and
total cross section in the unresolved resonance region at about 30%.
Results of optical, statistical and pre-equilibrium model calculations
are given for the high neutron energy.

BROND-2 -- 1989 evaluation. 48 s-wave resonances from Mughabghab84 are
given in the resolved resonance region (10^-5^ to 968 eV, RM formalism).
Results of optical, statistical and pre-equilibrium models with account
of direct reaction mechanism contribution in the excitation of
collective levels in the inelastic scattering are given for high energy.

CENDL-3 -- 2000 evaluation. The data for resolved and unresolved
resonance regions were taken from JENDL-3.2. Results of optical,
statistical and pre-equilibrium models with account of direct reaction
mechanism contribution in the excitation of collective levels in the
inelastic scattering are given for high energy.

###### Thermal and resonance region

Table below shows the thermal cross section values (barns at 0.0253 eV)
and RI (barns) for all libraries:

  --------- ----------- --------- ----------- --------- ---------- --------------
            ENDF/B-VI   JEF-2     JENDL-3.2   BROND-2   CENDL-3    Mughabghab84
                                  (and 3.3)                        

  Total     92.30       88.62     92.39       97.35     92.39      

  Elastic   7.29        3.58      7.38        10.54     7.38       

  Capture   85.01       85.04     85.01       86.81     85.01      85±12

  RI        215.91      248.17    215\.       191.40    215\.      230±25
  --------- ----------- --------- ----------- --------- ---------- --------------

There is no new experimental data in the thermal energy region available
since Mughabghab84 evaluation. The most detailed presentation of the
cross sections in the RRR and URR is given in JENDL-3.2 (and CENDL-3,
ENDF/B-VI) libraries. But value of the elastic scattering cross sections
at thermal energy with no experimental data depends from position of the
resonances, effective scattering radius R' and formalism used for cross
section parametrization. Because the value R'=8 fm is a most probable
value according the R' systematics for non-spherical optical model, we
can consider the ENDF/B-VI, JENDL-3.2 and CENDL-3 as best evaluation for
thermal energy region and for RRR. There can be missed s-, p- and d-
resonances at the RRR, contribution of which can be evaluated and added
as background cross section or through simulated resonances. The cross
sections in the URR are best presented through the average parameters
given in ENDF/B-VI, JENDL-3.2 and CENDL-3. Unfortunately, R'=6.8 fm (and
not 8 fm) was used in all libraries, what led to the underestimation of
elastic scattering and total cross sections in the energy range 2.68 and
50 keV. The capture cross section in all 3 libraries in the URR is
fitted to the Beer88 data. Latest Wisshak95 precise measurements are at
10-20 % above. This important discrepancy should be analyzed and
resolved.

###### Fast region

Fast region covers the energy above 50 keV. The discrepancy in the
capture cross-section evaluation (ENDF/B-VI, JENDL-3.2 and CENDL-3) with
the latest precise experimental results mentioned above exists also in
the fast region. The use of the spherical optical model with the
parameters not fully adjusted to the fit of the average resonance
parameters in the RRR leads to the underestimation of the elastic and
total cross sections for En below 1 MeV in this case. JEF-2 and BROND-2
give under the total inelastic scattering cross section (MT=4) and
inelastic scattering with excitation of continuum of levels (MT=91) the
cross section close to the first neutron emission cross section or its
partial with excitation of levels in continuum. This allowed the use of
simple temperature formulae for presentation of the secondary neutron
spectra in wide energy region, but as result, the elastic scattering
cross section is wrong above the (n,2n) threshold. BROND-2 accounts the
contribution of direct processes in the inelastic scattering with
excitation of discrete levels. Large difference in capture cross section
in MeV region (up to 5 times) between CENDL-3, JEF-2 and BROND-2 from
one side (give low cross sections) and JENDL-3.2 and ENDF/B-VI (give
high cross sections) is probably due to different level densities in
inelastic scattering and neutron capture channels in these libraries.
Cross sections from CENDL-3, JENDL-3.2 and ENDF/B-VI are close, where
inelastic scattering at discrete levels gives major contribution in
competition to capture channel. Gilbert-Cameron model for level density
with parameters used in JENDL-3.2 evaluation can lead to the
overestimation of capture cross section at high-energies.
Pre-equilibrium decay and statistical models used for evaluation of
cross section in fast region give the realistic results for all
threshold reactions. Inelastic scattering and (n,2n) cross sections look
more out more realiable in CENDL-3 evaluation. The (n,p) and (n,a) cross
sections are much higher in CENDL-3 than in other libraries for
sub-coulomb region, what requires some explanation (Q-reaction values
given in the files are the same). In all libraries (for exclusion of
CENDL-3) they were normalized to systematics at 14 MeV point, but the
model with account of all reaction mechanisms and reliable parameters
can give more justified value than some systematics based on strong
simplifications of the physical models. Secondary neutron angular and
energy distributions are given with the same level of details in
CENDL-3, JENDL-3.2 and ENDF/B-VI libraries.

###### Recommendation and conclusion

The CENDL-3 can be recommended as the best in the whole energy region
10^-5^ eV -- 20 MeV.

###### Further possible improvements

The missed resonances should be simulated or background cross section
should be added in the RRR. R' and other average parameters in the URR
should be consistent with the averaged values for RRR. Total and elastic
cross section in the URR should be revised. Non-spherical optical model
should be used in the calculations. Parameters of the potential should
be adjusted (at low energy) to the fitting of the average parameters in
the URR. Capture cross section in keV energy region should be
re-evaluated with account of latest experimental data (Wisshak95). The
reason of difference in (n,p) and (n,α) sub-coulomb reaction cross
sections in different libraries should be clarified.