**64-Gd-158 Review**

By V.G. Pronyaev, NDS, IAEA

10 March 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 -- 1977 evaluation. Resolved Resonance Region (RRR) is given
up to 9.98 keV with 93 MLBW s- resonances from Mughabghab73. There is
possible missing of resonances for En above 3 keV. No URR is given.
Results of optical and statistical model calculations are given for fast
energy region (En above 9.98 keV).

JEF-2 -- 1982 evaluation. 65 MLBW s-resonances are given for En below
6.0376 keV in the RRR. No URR is given. Results of spherical optical and
statistical model calculations are given for neutron energy above 6.0376
keV.

JENDL-3.2 (as JENDL-3.3) -- 1990 modification of JENDL-2 (1984)
evaluation. 95 MLBW s- wave and 2 p-wave resonances are given up to 9.96
keV for the RRR between 10^-5^ -- 6.58 keV. There is possible missing of
resonances for En above 3 keV. Unresolved Resonance Region with average
s-, p- and d- wave resonances covers the energy region between 6.58 keV
and 100 keV. Neutron strength functions for s- and d- wave and average
gamma-widths for all states were taken from Mughabghab84. Neutron
strength function for p-wave was evaluated in optical model. Average
level density and effective scattering radius were adjusted to fit
experimental capture and total cross section. Spherical optical,
statistical and pre-equilibrium model calculations are given for fast
energy region above 100 keV.

BROND-2 -- 1989 evaluation. Resolved resonance region with 96 RM s-wave
resonances is given for En between 10^-5^ and 9.9804 keV. The parameters
were taken as given in Mughabghab84 for MLBW presentation. No URR is
given. Optical, statistical, pre-equilibrium and direct reaction models
were used for evaluation of cross sections at high-energy region.

CENDL-3 -- 2000 evaluation. JENDL-3.2 is taken for RRR and URR below 100
keV. Results of optical, statistical, pre-equilibrium and direct
reaction model calculations for En above 100 keV. Spline fit of
experimental data for neutron capture cross section in 100 keV -- 3.0
MeV energy region.

###### Thermal and resonance region

Table below shows the cross section at 0.0253 eV (barns), effective
scattering radius R' (fm), RI (barns) and fast rector spectrum averaged
capture cross section (capture fast) for all libraries:

  --------- ----------- --------- ----------- --------- ---------- ----------------
            ENDF/B-VI   JEF-2     JENDL-3.2   BROND-2   CENDL-3    Mughabghab2002
                                  (and 3.3)                        

  Total     5.303       6.011     6.368       5.388     6.368      

  Elastic   3.301       3.510     3.871       3.420     3.871      

  Capture   2.002       2.501     2.497       1.968     2.497      2.2±0.2

  R' (RRR)  6.5         6.522     6.5         6.5       6.5        

  R' (URR)  \-          \-        7.1192      \-        7.1192     

  RI        64.9        63.092    63.93       69.39     63.83      73±7

  Capture   0.33792     0.33358   0.34154     0.33685   0.34366    
  fast                                                             
  --------- ----------- --------- ----------- --------- ---------- ----------------

Latest Heft78 systematic and simultaneous measurements (X4=12866) of
thermal capture cross section (2.44±0.30 b) and RI (94±8 b) just
renormalized to new quantum yields for gamma-line used in activation
measurements (new values are 2.218 b and 85.4 b) gives ratio
R=94/2.44=85.4/2.218=38.52 which contradicts to Mughabghab2002 value
R=33.2 as well as to values given in different libraries. The first
level at 22.3 eV gives the major contribution in the RI. This
discrepancy gives very strong indication that the capture area under
22.3 eV resonance probably is strongly underestimated in all libraries.
Total cross section in the URR of ENDF/B-VI fits well Wisshak95 data for
Gd-157. Other libraries used total cross section for Gd-natural for
parameters adjustment. The effective scattering radiuses in JENDL-3.2
(CENDL-3), JEF-2 and BROND-2 for RRR and URR are too small and cannot be
justified for rather wide RRR with large number of resonances and
especially for URR.

###### Fast region

Fast region can include naturally the energy above 54.8811 keV (above
the threshold of inelastic scattering). All libraries underestimate the
isotopical ^157^Gd total (and elastic scattering) cross sections below
0.5 MeV. Due to required consistency between total and partial cross
sections and wrong inelastic scattering cross section (MT=4 is treated
not as inelastic scattering cross section but as one component of
neutron emission spectrum in temperature presentation) in ENDF/B-VI,
JEF-2 and BROND-2 libraries, elastic scattering cross section in these
libraries is distorted for energies above the threshold of (n,2n)
reaction. CENDL-3 spline fit of capture cross section below 2.5 MeV
gives the best description of experimental data but smoothes the
variation (cusps) in the cross section due to competition from inelastic
scattering channels. CENDL-3 capture cross section above 2.5 shows (as
practically for all Gd isotopes evaluations) that competition from
inelastic scattering is probably overestimated that leads to too strong
decrease of capture cross section. For inelastic scattering with
excitation of discrete levels, BROND-2 gives results of calculations for
40 levels accounting the direct processes only for two and CENDL-3 for
all low-laying collective levels. As seems, CENDL-3 and BROND-2 with
used optical model parameters slightly overestimates the reaction
(non-elastic, inelastic) and underestimate the elastic scattering cross
sections. Temperature spectra are given for secondary neutron emission
in ENDF/B-VI, JEF-2 and BROND-2 libraries and more realistic MF=5
tabulated spectra in JENDL-3.2 and CENDL-3 libraries. CENDL-3 uses MF=4
for presentation of angular distribution in continuum for case when they
are not isotropic and strong angular-energy correlations exist. No
secondary gamma production cross section and spectra are given in all
libraries.

###### Recommendation and conclusion

The ENDF/B-VI can be recommended for RRR and URR and CENDL-3 evaluation
for the energy above 54.8811 keV.

###### Further possible improvements

Without new experimental data no improvement can be done for RRR and
URR. ENDF/B-VI evaluation in this energy ranges up-to-date and
consistent with existing experimental data. Region of non-parametric
presentation of cross sections in CENDL-3 should be shifted from 100.
keV to 54.8811 keV. Optical model parameters should be adjusted to
describe the isotopic cross section in URR and hundreds keV energy
region and give the evaluation of nonelastic cross section close to what
can be expected from available experimental data for neighboring nuclei.
Secondary neutron energy angular distributions should be given in the
file MF=6. The calculations of capture cross section for En above 2.5
MeV should be done with the re-analysis of level densities used in
inelastic scattering and capture channels. Evaluation of
gamma-production cross sections and spectra should be done.