**64-Gd-155 Review**

By V.G. Pronyaev, NDS, IAEA

13 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 -- 1999 revision (below 60.4 keV) of the ENDF/B-V evaluation.
92 s-level MLBW resonances are given in the RRR from 10^-5^ to 183.3 eV.
Average resonance parameters adjusted to describe experimental data by
Wisshak98 and Beer88 in the URR (183.3 eV to 60.4 keV) are given for s-,
p- and d- waves. 1977 results of optical and statistical model
calculations are given in fast energy region (60.4 keV -- 20 MeV). First
neutron emission cross section is given under inelastic scattering MT=4
(MT=91).

JEF-2 -- taken from JENDL-2 (1986). 98 MLBW s-resonances are given below
181.8 eV in the RRR. The average resonance parameters for s-, p- and d-
waves are given in the URR (181.8 eV -- 100 keV). Optical and
statistical models were used in fast energy region. No contribution from
direct and pre-equilibrium model is accounted.

JENDL-3.2 (and JENDL-3.3) -- 1990 evaluation. RRR parameters are the
same as in JENDL-2 (see also JEF-2). For the URR, the average neutron
width for s-wave were taken from Mughabghab84, and for p- and d- waves
-- from optical model calculations; the average level spacing was
adjusted to fit the capture cross section, and effective scattering
radius -- to fit the total cross section. Results of optical,
statistical and pre-equilibrium model calculations are used in fast
energy region. (n,p) and (n,a) cross sections are normalized to the
systematics at 14 MeV energy point.

BROND-2 -- 1989 evaluation. 92 RM s-resonances in the energy range
10^-5^ to 183.43 eV. S-, p- and d- waves average resonance parameters
are given in the URR (183.43 eV -- 10.391 keV). Results of optical,
statistical and pre-equilibrium model calculations with the account of
direct processes are given in the fast energy region. First neutron
emission cross section is given under inelastic scattering MT=4 (MT=91).

CENDL-3 -- 2000 evaluation. Evaluation in the RRR and URR is the same as
in JENDL-3.2 (and JENDL-2). Results of optical, statistical and
pre-equilibrium model calculations with the account of direct processes
are given in the fast energy region.

###### Thermal and resonance region

Table below shows the cross section values at 0.0253 eV (barns),
scattering radiuses in the RRR and URR (fm), RI (barns) and capture fast
reactor spectrum averaged cross section (barns):

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

  Total      60801\.     60800\.   60800\.     60974\.    60799\.   

  Elastic    60.69       59.01     59.00       59.34      59.01     60\.

  Capture    60740\.     60740\.   60741\.     60915\.    60740\.   60900±5000
                                                                    (~0~)

  RI         1538\.      1550\.    1540\.      191.40     1531\.    

  R', RRR    8.0         6.7       6.7         6.7        6.7       

  R', URR    8.0         3.59      6.83        6.7        6.83      

  Capture,   2.4603      2.8826    2.6371      2.8665     2.6447    
  fast                                                              
  ---------- ----------- --------- ----------- ---------- --------- --------------

There is no large difference in the values of the cross sections at the
0.0253 eV point for different libraries. The use of more physically
grounded effective scattering radius in the RRR and URR done for
ENDF/B-VI improves the evaluation of elastic scattering and total cross
sections. Capture cross section in the URR for ENDF/B-VI (the same is
true for BROND-2) follows Shorin74 data, which differ from latest Beer88
and Wisshak95 results. JENDL-3.2 (and CENDL-3) describes well the
capture cross section data in the URR with average parameters obtained
from RRR or taken from optical model fit of low energy cross sections.
JEF-2 data in the URR are above the experimental data.

###### Fast region

Fast region covers the energy above 100 keV. ENDF/B-VI, JEF-2 and
BROND-2 evaluations are based on rather old results of calculations
where inelastic scattering cross sections are presented through the
first neutron emission cross section (or part of it) in MT=4 and 91.
Consistency required between the total and partial cross sections leads
to the wrong presentation of elastic above the threshold of the (n,2n)
reaction. JENDL-3.2 and CENDL-3 give adequate to the required needs
presentation of the evaluated data in the fast energy region. The
capture cross section fits well the experimental data where CENDL-3
probably slightly underestimating the capture above 2.5 MeV. Inelastic
scattering cross section with excitation of discrete levels is most
consistently (with account of direct processes) evaluated in the CENDL-3
library. Unfortunately the used optical model potential gives a strong
overestimation (up to 30%) of the non-elastic scattering cross section
below 6 MeV. This leads to the overestimation of inelastic scattering
cross section at these energies. The secondary neutron angular
distributions for continuum of levels are given through MF=4, what is
adequate presentation for elastic and inelastic scattering with
excitation of discrete levels but not for reactions with excitation of
continuum of the levels. JENDL-3.2 evaluation of inelastic scattering
was done without account of direct process contribution. The optical
model parameters have energy dependence, which gives an overestimation
of total cross section for energy of neutrons above 6 MeV in comparison
with well-known cross section for natural gadolinium.

###### Recommendation and conclusion

The ENDF/B-VI can be recommended for the resolved resonance energy range
(below 183.3 eV), JENDL-3.2 (same as in JENDL-3.3, CENDL-3) can be
recommended for the unresolved resonance region (183.3 eV -- 100 keV)
and CENDL-3 for the fast neutron energy range (100 keV -- 20 MeV). Some
recommended parts should be revised (see below).

###### Further possible improvements

The number of missed resonances in the RRR should be evaluated and their
contribution should be accounted. Effective scattering radius for the
URR should be estimated from non-spherical optical model fit of the data
at low energy. Present value 6.83 fm is probably too low. Experimental
data for neighboring isotopes or natural mixture of isotopes can be used
with some standard (N-Z) charge asymmetry component of potential.
Optical model potential used in CENDL-3 gives too high reaction cross
section below 6 MeV. The possible reason could be too low imaginary part
of the potential for these energies. The secondary energy-angular
distributions in continuum of levels should be given in MF=6
presentation.