**64-Gd-157 Review**

By V.G. Pronyaev, NDS, IAEA

4 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 -- 1999 revision of the 1977 evaluation. The revision was done
for RRR and URR. Resolved Resonance Region (RRR) is up to 306.6 eV with
60 MLBW s- resonances from Mughabghab84; L and J assignment accordingly
by Bayesian approach and randomly. Given resonances show no missing.
Unresolved Resonance Region (URR) is given between 306.6 eV and 54.8811
keV, where average s-wave resonance parameters are consistent with RRR
and all parameters (including parameters of p-wave) are adjusted from
the fitting to available experimental data. Optical and statistical
model calculations are given for fast energy region.

JEF-2 -- 1982 evaluation. 56 MLBW s-resonances are given for En below
306.4 eV in the RRR from 10^-5^ to 215 eV. No URR is given. Results of
statistical model calculations based on average resonance parameters for
s-, p- and d- waves are given for En between 215 eV and 100 keV. Results
of optical and statistical model calculations for neutron energy are
given above 100 keV.

JENDL-3.2 (and JENDL-3.3) -- 1990 modification of JENDL-2 (1984)
evaluation. 60 MLBW s- wave resonances are given in the RRR for En
between 10^-5^ -- 303.7 eV. Spins of resonances with unknown values are
assigned randomly. Unresolved Resonance Region with average s-, p- and
d- wave resonances covers the energy region between 303.7 eV and 100
keV. Parameters for s-wave were taken from Mughabghab84 and p- and d-
wave - from fitting to the optical model results at 100 keV. 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 56 RM s-wave
resonances is given for En between 10^-5^ and 306.5 eV. The parameters
are taken from Mughabghab84. URR with parameters for s- and p-waves
adjusted to fit the capture cross section is used for En between 306.5
eV and 10.391 keV. Optical, statistical pre-equilibrium and direct
reaction models were used for evaluation of cross sections at high
energy region.

CENDL-3 -- 1999 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 -- 2.5
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       254774\.    255052\.   254690\.    254560\.   254690\.   

  Elastic     1008\.      1759.7     1009\.      1008\.     1009\.     1000\. (Mu84)

  Capture     253766\.    253292\.   253681\.    253552\.   253681\.   254000.±815.

  R' (RRR)    7.8         4.9        4.9         4.9        4.9        

  R' (URR)    7.8         \-         6.88        4.9        6.88       

  RI          755\.       763\.      761.3       \-         759.4      754.±20.
  (declar.)                                                            

  Capture     1.4182      1.8307     1.3589      1.6102     1.3627     
  fast                                                                 
  ----------- ----------- ---------- ----------- ---------- ---------- ----------------

Capture RI cross section was increased in Mughabghab2002 (same as given
in ENDF/B-VI) to 754. b. Total cross section in the URR of ENDF/B-VI
fits well Wisshak95 data for Gd-157. Other libraries used total cross
section of natural Gd for parameters adjustment. The effective
scattering radiuses in JENDL-3.2 (CENDL-3), JEF-2 and BROND-2 for RRR
and URR as seems are too small.

###### Fast region

Fast region can include naturally the energy above 54.8811 keV (above
the threshold of inelastic scattering). All libraries underestimate the
isotopic ^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 wrong 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 of cross
section due to competition from inelastic scattering channels. CENDL-3
capture cross section above 2.5 MeV shows (as practically for all Gd
isotopes evaluations) that competition from inelastic scattering is
overestimated that leads to strong decrease of capture cross section.
For inelastic scattering with excitation of discrete levels, BROND-2
gives the results of calculations for 40 levels with account of the
direct processes for two levels and CENDL-3 for all low-laying
collective levels. As seems, with used optical model parameters CENDL-3
and BROND-2 slightly overestimates the reaction (non-elastic, inelastic)
cross sections. CENDL-3 and BROND-2 gives best description of the
experimental data for (n,2n) reaction. Temperature spectra are used for
presentation of 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
can be recommended 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 is up-to-date and
consistent. Region of non-parametric presentation of cross sections in
CENDL-3 should be shifted down from 100. keV to 54.8811 keV. Optical
model parameters should be adjusted, that the model will describe the
isotopic cross section in URR and hundreds keV energy region and give
the evaluation of nonelastic cross section close to what is seen from
available experimental data for neighboring nuclei. Secondary neutron
energy angular distributions should be presented in the file MF=6. The
calculations of capture cross section for En above 2.5 MeV should be
done with re-analysis of level densities used in inelastic scattering
and capture channels. Evaluation of gamma-production cross sections and
spectra should be done.